ASME Conference Presenter Attendance Policy and Archival Proceedings

2012;():i. doi:10.1115/SMASIS2012-NS1.

This online compilation of papers from the ASME 2012 Conference on Smart Materials, Adaptive Structures and Intelligent Systems (SMASIS2012) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Development and Characterization of Multifunctional Materials

2012;():1-9. doi:10.1115/SMASIS2012-7911.

The prorogation of electro-magneto-elastic coupled shear-horizontal waves in one dimensional infinite periodic piezoelectric waveguides is considered within a full system of the Maxwell’s equations. Such setting of the problem allows to investigate the Bloch-Floquet waves in a wide range of frequencies. Two different conditions along the guide walls and three kinds of transmission conditions at the interfaces between the laminae of waveguides have been studied. Stop band structures have been identified for Bloch-Floquet waves both at acoustic and optical frequencies. The results demonstrate the significant effect of piezoelectricity on the widths of band gaps at acoustic frequencies and confirm that it does not affect the band structure at optical frequencies. The results show that under electrically shorted transmission conditions Bloch-Floquet waves exist only at acoustic frequencies. For electrically open interfaces the dynamic setting provides solutions only for photonic crystals. In this case the piezoelectricity has no effect on band gaps.

Commentary by Dr. Valentin Fuster
2012;():11-17. doi:10.1115/SMASIS2012-7922.

Direct write (DW) technology offers a simple method of rapid manufacturing technology for printing electronic, optoelectronic devices, and complex functional devices. The key component of DW technology is the functional inks, which are colloidal suspensions of functional nanoparticles in various solvents such as aerosol or liquid form. With a DW approach, patterns or structures can be easily deposited on flexible substrates such as paper, plastics, and composites, once the solvent volatilizes or is driven off via conventional, laser, or microwave sintering. In this work, polymer-assisted silver (Ag) nanoinks were synthesized by silver salt and polymer in the water solution at relatively high silver precursor concentrations and relatively low concentration of polymers. The silver nanoparticle dispersion and morphology was examined by dynamic light scattering (DLS) and transmission electron microscopy (TEM). The results showed that the size of Ag nanoparticles was in nanoscale (∼20 nm) with a narrow distribution of Ag nanoparticle sizes. The viscosity and thermal properties of synthesized silver nanoinks were characterized to determine their applicability and the lifetime. It has been shown that the synthesized silver nanoink can be printed on a flexible plastic substrate or glass substrate. The morphology of the Ag nanoink line printed on the substrate was observed by optical microscopy and scanning electron microscopy (SEM) to understand the relationship between the microstructure and wettability. Uniaxial tension tests of silver nanoink line on a Kapton film indicate that the ink can be stretched ∼20% without failure. The resistance of silver nanoink line printed on the Kapton films was also measured by four probe conductivity measurement system to assess the electrical performance. The resistivity is about 7.5 × 10−5 Ω-cm by thermal treatment at 250°C for 30 min, which is about half that of bulk silver (1.6 × 10−6 Ω-cm). Overall, the performance of the synthesized silver nanoink is comparable to a commercially available ink with lower Ag weight content at relatively low cost.

Topics: Silver
Commentary by Dr. Valentin Fuster
2012;():19-25. doi:10.1115/SMASIS2012-7936.

In the present study, a series of novel linear polyaspartimide-based silane endcapped (cross-linked) polymers are synthesized using 4-4′ bismaleimidodiphenylmethane, Jeffamine D-400 (BMI-JA-400), and (3-Aminopropyl) trimethoxysilane. To add strength to these systems, the trimethoxysilane moiety is cross-linked with the addition of water to create a thermosetting material with both improved toughness and variable cross-link densities. Thermal analysis is done to evaluate the developed shape-memory polymer (SMP) resin for composite processing feasibility. The solvent content in the resin and thermal stability is monitored using thermogravimetric analysis (TGA) while advanced rheometric expansion system (ARES) with parallel plate geometry is used to measure viscosity variation with temperature.

The resin BMI-JA-400-Si-70/30 is chosen for making the composite based on its viscosity, weight change, and kinetic results. Differential scanning calorimetry (DSC) is performed to determine the cure kinetics including the temperatures at which the cure reaction initiates and completes in order to develop the cure cycle for composite fabrication. The selected SMP resin is hand-impregnated with T-300 plain-weave and T-700 uni-weave carbon fabric. Six-ply composites are successfully fabricated with < 2% void content using both fabric weaves. The thermo-mechanical properties of the SMP resin are measured using dynamic mechanical analysis (DMA). In addition, the shape memory cycle with free recovery is conducted on the SMP resin and composites.

Commentary by Dr. Valentin Fuster
2012;():27-34. doi:10.1115/SMASIS2012-7968.

Shape memory polymers (SMPs) are a widely studied class of materials due to their numerous applications in various fields of engineering. They find applications in deployable structures, biomedical devices, adaptive optical devices, sensors and actuators, in textiles etc. Recent studies have shown shape memory behavior in many polymers. Sulfonated poly ether ether ketone (SPEEK) is an ionic polymer which is being extensively studied for its application in fuel cells as a Proton Exchange Membrane (PEM) polymer due to its relatively higher thermal and mechanical stability over other PEMs in addition to proton transport. Recent studies on a sulfonated ionomer, Nafion® which has only one broad reversible phase transition, can show tunable, multiple shape memory effects by deforming the polymer at different temperatures without compromising the shape fixity (Rf). This paper reports, for the first time, the swelling (in solvents) induced shape memory behavior observed in SPEEK. The study was motivated by the preliminary observations of the response of SPEEK to solvent stimulus. SPEEK samples of varying degrees of sulfonation (DS) were prepared by the sulfonation of poly ether ether ketone (PEEK). The shape fixation and recovery rates (Rr) of the polymer under different temperatures and solvent conditions are reported. A comparative study of the shape memory response of the material with varying DS was also carried out. We also report for the first time the potential use of the parallel plate geometry of a rheometer for estimating the force during the shape recovery process. Visual demonstration of the shape memory effect is carried out using solvents at different temperatures.

Commentary by Dr. Valentin Fuster
2012;():35-42. doi:10.1115/SMASIS2012-7975.

Ionic polymer-metal composites (IPMCs) have intrinsic sensing and actuation capabilities. However, IPMCs require ionic hydration to operate. As the most commonly used solvent, water content contained in the polymer changes with the humidity level of the ambient environment, which affects the sensing behavior of an IPMC in air. Motivated by the need to ensure consistent sensing performance of IPMCs under different ambient environments, in this paper we propose thick (up to 10 micrometers) parylene C coating for IPMC sensors, develop effective coating processes, and evaluate the stability of the encapsulated sensors in air. During the process of parylene coating, water molecules would evaporate inside the deposition chamber, resulting in the encapsulated IPMCs’ losing sensing capability. To address this challenge and control the hydration level of an encapsulated IPMC, the proposed fabrication process comprises major steps of parylene deposition, water absorption, and SU-8 seal. The influence of hydration level controlled by the water absorption step is studied to improve the sensitivity of the IPMC sensor. The water impermeability of the proposed encapsulation technique is tested in different media. Experiments have also been conducted to evaluate the performance of the encapsulated IPMC sensor. The sensing consistency and the lifetime of an encapsulated sensor in air are studied in an environment with changing humidity, along with the comparison with an uncoated IPMC sensor. Experimental results show that the proposed thick parylene coating can effectively maintain the water content inside the IPMC and reduce the interference due to the ambient humidity change, which allows IPMC sensors to be used in many practical applications.

Commentary by Dr. Valentin Fuster
2012;():43-48. doi:10.1115/SMASIS2012-7986.

A nanocomposite of Multi-walled Carbon nanotubes (MWCNT) and Polypolypyrrole (PPy) is fabricated and characterized for supercapacitor application. PPy is uniformly coated on the MWCNT surface by mean of in-situ chemical polymerization. MWCNT content is varied to control the thickness of deposited Pyrrole layer. Ferric chloride solution (FeCl3.6H2O) is used as oxidant to polymerize Pyrrole. Highly conductive nickel foam is used as a current collector for the electrode. Scanning Electron Microscopy (SEM) and Transmission Electron (TE) imaging were used in characterizing composite surface morphology. Electrochemical behavior is studied by mean of Cyclic Voltammetry (CV) and AC Impedance Spectrometry. The effect of varying monomer to MWCNT weight ratio in composite electrical properties was studied in this paper.

Commentary by Dr. Valentin Fuster
2012;():49-53. doi:10.1115/SMASIS2012-8002.

Piezoelectric materials are the active elements in e.g. ultrasound transducers and thus widely used in sensor and actuator systems. The rather recent addition to ultrasonic transducer materials are the so-called ferroelectrets, voided piezoelectric-active polymer films. One type among the most studied ferroelectrets is cellular polypropylene. The properties of ferroelectrets, such as low elastic stiffness, high piezoelectric activity, good matching with air makes them suitable as ultrasonic transducer material. Here, we demonstrate the controlled adjustment of the piezoelectric properties, especially of the resonance frequency of cellular polypropylene films by means of transducer-structure modifications.

Commentary by Dr. Valentin Fuster
2012;():55-61. doi:10.1115/SMASIS2012-8004.

Different routes for electrode processing which fulfill the requirements of piezoelectric transducer will be presented. One attempt is the electrode deposition via inkjet printing, another is the deposition via an air-brush technique. For the preparation of electrodes via inkjet printing, different inks such as a silver composite or the semiconducting poly(3,4-ethylene-dioxythiophene): polystyrenesulfonate (PEDOT:PSS) are used. A further attempt is the deposition of carbon nano tubes (CNT’s) via an air-brush technique. For all three systems the ink or solution formulation, the deposition techniques, suitable parameter and partly additional encapsulation steps will be discussed in detail accompanied by a description of electrode properties, e.g. the conductivity, as well as by the characterization of the materials poling behavior in particular.

Commentary by Dr. Valentin Fuster
2012;():63-68. doi:10.1115/SMASIS2012-8009.

In this paper, novel ultra-long aligned array of Barium Titanate (BaTiO3) nanowires (NWs) was used to fabricate piezoelectric sensors and investigate their vibration sensing and energy harvesting potential. The acceleration sensing characteristics of the piezoelectric BaTiO3 NWs based sensor was presented by conducting vibration excitation experiments induced from an electromagnetic shaker. Two different top electrode configurations which include a melted Indium and a short cantilever Indium beam located over the BaTiO3 NWs surface were utilized to study the acceleration sensing and energy harvesting scope respectively. The results shown validate their excellent energy conversion capabilities and demonstrate linear behavior over a wide frequency spectrum which elucidates their potential to be developed as advanced sensors and high efficiency vibrational energy harvesting devices.

Commentary by Dr. Valentin Fuster
2012;():69-74. doi:10.1115/SMASIS2012-8021.

Hybrid nanocomposites with single walled carbon nanotubes (SWNT) and graphene oxide (GO) as nanofillers and polyvinylidene fluoride (PVDF) as a polymer were synthesized as potential electronic active polymers (EAPs) with high breakdown strength. A co-solvent method was developed to achieve exfoliation and dispersion of GO in PVDF. The microstructure of the PVDF was found to be predominantly γ phase. Percent crystallinity of PVDF increased due to the addition of the hybrid nanofillers. And, at room temperature, the storage modulus is increased by 56.26% over the pure PVDF. The dielectric constant increased from ∼7 to ∼25 for the hybrid nanocomposites as compared to pure PVDF at 1KHz measurement frequency. Dielectric loss of the hybrid nanocomposite is found less than 0.6 for the frequency range from 20 Hz–1MHz. Electrical conductivity of the hybrid nanocomposite increase by nearly two orders of magnitude at 1KHz when compared to pure PVDF. The effect of the presence of these hybrid nanofillers on microstructure and properties of PVDF are discussed.

Commentary by Dr. Valentin Fuster
2012;():75-79. doi:10.1115/SMASIS2012-8030.

This study seeks to assess the morphology of Nafion nanofiber membranes produced by electrospinning technology. The effects of solution concentration, electrode separation and applied voltage on diameter are investigated. The morphology of Nafion nanofibers measurements for different concentration and operating conditions are acquired by scanning electron microscopy (SEM). The thermal performance is also analyzed by TGA and DSC. Results show that the bead-free nanofibers are electrospun from 0.3 wt%∼0.7 wt% PEO in 5 wt% Nafion solution at the electrode separation of 16 cm ∼ 20 cm, an applied voltage of 20 kV ∼ 50 kV and an electrode bar rotation rate of 0.9 rpm. The mean dimeters of Nafion increase with increasing PEO content and electrode separation. The mean dimeters of Nafion decrease with increasing applied voltage. The formation of continuous smooth nanofiber is related to solution viscosity and electrospinning conditions. In addition, glass transition temperature of the Nafion nanofibers increases with increasing PEO concentration and Nafion nanofibers show a good thermal stability.

Commentary by Dr. Valentin Fuster
2012;():81-88. doi:10.1115/SMASIS2012-8036.

Here we present a suitable tag prototype with phase-segregated poly(ester urethane) (PEU) as base material for effectively switching a quick response (QR) code in its surface from non-readable to readable. In comparison with recently introduced tags (different geometry) we minimized the thickness from plaque (2 mm) to foil size (0.5 mm) and reduced the lateral QR code length from 15 to 5 mm. Subsequent to surface-dyeing by means of guest diffusion, the QR code was laser-engraved. The implementation of thermo-mechanical functionalization via tensile deformation and cooling resulted in the formation of stable shapes, which exceeded the barrier of QR code readability at an elongation of 20%. Once functionalized, tags were switched on demand by heating. As such the recovery of the PEU was accomplished and the QR code could again be read out. QR code carriers based on shape memory polymer can be used in product and brand protection applications.

Commentary by Dr. Valentin Fuster
2012;():89-96. doi:10.1115/SMASIS2012-8038.

The surface of a shape memory poly(ester urethane) (PEU) was either black- or blue-colored to obtain switchable quick response (QR) codes after laser engraving and thermo-mechanical functionalization (programming). The investigation of dye and functional stability against UVA and hydrolytic aging (at 23 and 60 °C) gave that contrast decline due to dye decolorization (in case of UVA aging) or distinct dye diffusion (in case of hydrolytic aging) finally inhibited the QR code readability. By contrast, PEU as marked base material could be adequately fixed and recovered even when the Michelson contrast in the QR code region was falling in course of aging below a crucial value of 0.1, whereupon the QR code was no longer readable. Hence, we concluded that under the given experimental conditions the decisive parameter for tag applicability was the surface contrast.

Commentary by Dr. Valentin Fuster
2012;():97-104. doi:10.1115/SMASIS2012-8040.

Processing of Nickel-Titanium (NiTi) shape memory alloys (SMAs) is challenging because smallest compositional variances and all types of microstructural features strongly affect the elementary processes of the martensitic transformation and thus the functional properties of the material. Against this background, powder metallurgical near net shape methods are attractive for the production of NiTi components. Especially additive manufacturing technologies (AM) seem to provide high potential, although they have received only little attention for processing NiTi so far. This work is the first to report on pseudoelastic properties of additive manufactured Ni-rich NiTi. We show how to establish pseudoelasticity in NiTi samples prepared by the additive manufacturing technique Selective Laser Melting (SLM). Therefore, we analyze phase transformation behavior, mechanical characteristics and functional properties of our materials subjected to different heat treatments. The obtained results are compared to the behavior of conventional NiTi. The presented results clearly indicate that SLM provides a promising processing route for the fabrication of high quality NiTi parts.

Commentary by Dr. Valentin Fuster
2012;():105-109. doi:10.1115/SMASIS2012-8042.

Over the past decade ferromagnetic shape memory alloys (FSMA) have been the subject of intensive R&D work due to their potential in actuators, sensors, and intelligent systems. These smart materials can elongate and contract up to 10% when subjected to moderate magnetic fields, generating thus motion and force. Typically FSMA materials are alloys of Ni-Mn-Ga with various off-stoichiometric compositions. A new production approach based on commercially available machinery is introduced in this paper. Large single crystals have been produced demonstrating homogeneous structure and excellent magneto-mechanical properties. The properties of actuator elements (sticks) are presented as well as the influence of the alloy microstructure. The FSMA elements are used to develop prototype actuator and sensor devices for industrial and automotive applications. In this respect a benchmark between an FSMA actuator and a commercially available solenoid actuator has indicated that the FSMA technology offers potential to replace several of today’s electromagnetic actuators by advanced FSMA solutions, especially when temperature requirements are met.

Commentary by Dr. Valentin Fuster
2012;():111-116. doi:10.1115/SMASIS2012-8069.

In this paper, we describe preliminary analysis for a method of constructing homogeneous actuated electromechanical composite structures from many smaller electromechanical “active cells”. The preliminary cell design consists of a contractile shape memory alloy (SMA) elements (Nitinol) connecting two electrically conductive terminals. The composite structure is actuated by applying a gross electrical potential across the entire structure. This causes connected cells to activate through resistive heating. The material properties of the ensemble structure can be varied by changing the properties of the binding material, enabling a wide range of stiffness and damping properties. A structure made from these cells can be easily fabricated in a nearly arbitrary shape — simply by producing an appropriate mold — and would be highly redundant and robust to localized cell failures. The primary focus of this paper is to perform a static analysis of the connectivity of arrays of active cells and the associated power dissipation in the individual cells.

Commentary by Dr. Valentin Fuster
2012;():117-121. doi:10.1115/SMASIS2012-8070.

Ionic Polymer Transducers (IPTs), also known as Ionic Polymer Metal Composites (IPMCs), are a promising group of intelligent materials which exhibit electromechanical coupling behavior in both actuation and sensing. They are composed of an electroactive ionomer, inserted between metallic electrodes. In this study, IPTs are experimentally examined as a sensor in the aspect of electrode composition optimization. Sensors with electrodes having several volumetric percentages of metallic powder, ruthenium dioxide -RuO2-, are tested in bending under different step displacement inputs to explore the output current response. Optimum metallic powder content in the electrode solution for a sensor generating maximum current output is determined. Furthermore, the magnitude of the tip deflection’s effect on sensitivity is examined. The IPTs used in the experiments are fabricated via Direct Assembly Process which enables direct control over the electrode architecture.

Commentary by Dr. Valentin Fuster
2012;():123-132. doi:10.1115/SMASIS2012-8072.

Experimental investigations of different architectures made of pure, as produced carbon nanotubes (CNTs) are the main focus of this presented article. Different types of experimental setups are used to analyze the free strain of the CNT-based architectures. According to their build-up different experimental setups like actuated tensile tests, in-plane and out-of-plane strain measurements are realized to investigate the actuation mechanism and possible dependencies. The first analyzed architecture can be characterized as a 2D paper of randomly oriented, entangled single walled CNTs, also called Bucky-paper. In contrast the second investigated architecture consists of highly oriented, vertically aligned multi walled CNTs grown on a substrate of glassy carbon. The results are evaluated according to findings of various other material quality tests in order to find a significant statement for their possible actuation mechanisms.

Commentary by Dr. Valentin Fuster
2012;():133-137. doi:10.1115/SMASIS2012-8085.

Polymer materials have been proposed to be good candidates for the development of new actuators. Due to their tunable mechanical and electrical properties, they can be used as electro-active devices. In this contribution, we focus on dielectric elastomers based actuators, and word toward establishing innovative and alternative integration/miniaturization processes inspired from microelectronics and MEMS technology. Dielectric elastomer actuators are made of an elastomer dielectric layer sandwiched between two conductive electrodes. Upon voltage application attraction forces between the electrodes generates a mechanical displacement correlated with the elastomer Young modulus and permittivity. Here, we propose to use the polydimethylesiloxane (PDMS) due to its high elasticity and its permittivity made adjustable by addition of ceramic nanoparticles. An original process for structuring PDMS layers is developed to overcome the technological challenges encountered during the integration of such materials in a micro-actuator. In this paper, we present several results of characterization that allowed us to better understand the physicochemical mechanisms involved at different technological steps for both the material alone or mixed with Titanate of Barium (TiO3Ba) nanoparticles. We also measured the permittivity and the elasticity modulus of these materials at the end of the manufacturing process thereby verifying the conservation and the enhancement of the initial properties that set our choice. These results are very promising for increasing the electrostatic pressure or to lower the actuation voltage. To make a prediction of permittivity by a mixing rule, we inspect some theories in this aim. Finally, we demonstrate that the actuation response of charged elastomer with TiO3Ba nanoparticles follows a hyperelastic behavior. This result is particularly helpful for the design of a micro-actuator in a given application.

Commentary by Dr. Valentin Fuster
2012;():139-148. doi:10.1115/SMASIS2012-8100.

Recent work on multifunctional materials has shown that a functionally graded interface between the fiber and matrix of a composite material can lead to improved strength and stiffness while simultaneously affording piezoelectric properties to the composite. However the modeling of this functional gradient is difficult through micromechanics models without discretizing the gradient into numerous layers of varying properties. In order to facilitate the design of these multi-phase piezoelectric composites, accurate models are required. In this work, multi-inclusion models are extended to predict the effective electroelastic properties of multiphase piezoelectric composites. The presented formulation will provide a general framework for modeling other coupled fields of heterogeneous materials. To evaluate the micromechanics modeling results, a three dimensional finite element model of a four-phase piezoelectric composite was created in the commercial finite element software ABAQUS with different volume fractions and aspect ratios. The simulations showed excellent agreement for predicting the electroelastic properties of the multiphase piezoelectric composites.

Commentary by Dr. Valentin Fuster
2012;():149-155. doi:10.1115/SMASIS2012-8103.

The healing process exhibited by biological structures has inspired the creation of engineered materials capable of mimicking this behavior, providing adaption to impeding crack propagation and subsequently healing it. Recently, a new approach to self-healing was devised in which a sensing network was combined with shape memory polymers (SMPs) to allow the controlled response of the material to damage. The system was designed such that in the presence of a crack the polymer locally modified its modulus to toughen the damaged region and arrest crack growth. This process is followed by the shape memory response, closing the crack and healing the system. This paper will study the mechanics of the toughening portion of this self-healing system and specifically develop models to predict the stress intensity factor of a crack tip in a nonhomogeneous inclusion. The models will be formulated using finite element analysis (FEA) and a single inclusion model based on Eshelby’s equivalent theory with the elastic gradient defined by a point source thermal load. It will be shown that as the temperature of the crack tip passes the glass transition temperature of the polymer, the stress intensity factor at crack tip decreases to 95% of the original material stress intensity factor. This is due to the formed elastic gradient and deflection of the stress concentration away from the crack tip into the bulk polymer.

Commentary by Dr. Valentin Fuster
2012;():157-161. doi:10.1115/SMASIS2012-8126.

As new compositions of single crystal relaxor ferroelectrics are developed, the full thermo-electro-mechanical characterization of each composition and the limitations on linear behavior imposed by field driven phase transformations requires a highly specialized set of experiments. This characterization currently requires two crystal cuts which must each be subjected to five electric field-electric displacement cycles at different stress levels, five stress-strain cycles at different electric field levels, and this process must be repeated at five temperatures. In this document, a new approach to characterizing the linear behavior and phase transformation behavior is proposed based on a combination of a work-energy based model of the driving forces for the phase transformation together with data measured through electrical loading while measuring strain and electric displacement, and a measurement of mechanical compliance.

Commentary by Dr. Valentin Fuster
2012;():163-168. doi:10.1115/SMASIS2012-8127.

Back-relaxation — a phenomenon, where the ionic electro-active polymer actuator in its excited state decays back towards its initial shape — is commonly associated with the aqueous IPMC and explained with leak of water. Regardless of the absence of the fluent liquid, the dry actuators with electrodes made of carbon and ionic liquid as electrolyte, exhibit similar side effect. We show that by means of their long-term transient spatial actuation, moment of force, and back-relaxation, the behavior of the carbon-based actuators is comparable to the water-based IPMC actuators.

Commentary by Dr. Valentin Fuster
2012;():169-175. doi:10.1115/SMASIS2012-8131.

For morphing wing skin applications, low in-plane stiffness is advantageous to reduce the cost of actuation and high out-of-plane stiffness is required to withstand the aerodynamic loads. A proposed solution is to engineer a composite material made of a honeycomb support combined with a multi-state infill that can reduce the Young’s modulus for a low in-plane stiffness. Assuming thin beam theory and using the potential energy formulation, equivalent in-plane Young’s moduli can be calculated for a range of honeycomb cell geometries. The out-of-plane deflection of a representative plate fixed on all edges is calculated using flat plate theory and used to assess the performance of the skin system. To optimize the cell geometry for a given application, the out-of-plane deflection is constrained and the honeycomb cell geometry varied to investigate the design space. Results show that a skin can be designed to have in-plane Young’s moduli similar to the polymer infill and still have a low out-of-plane deflection. However, these results come at the expense of increased skin weight. Further analysis to obtain a more realistic design is done by imposing weight and geometric constraints.

Commentary by Dr. Valentin Fuster
2012;():177-184. doi:10.1115/SMASIS2012-8143.

Magnetorheological elastomers (MREs) are an emerging class of smart materials whose mechanical behavior varies in the presence of a magnetic field. Historically MREs have been comprised of soft-magnetic iron particles in a compliant matrix such as silicone elastomer. Numerous works have experimentally cataloged the MRE effect, or increase in shear stiffness, versus the applied field. Several other researchers have derived constitutive models for the large deformation behavior of MREs. In almost all cases the arrays of embedded particles, and or the particles themselves, are assumed magnetically symmetric with respect to the external magnetic field, i.e. the bulk materials exhibit magnetic symmetry in the given experimental or analytical configuration.

In this work the author presents results of dynamic shear experiments, Lagrangian dynamic analysis, and static shear simulations on MRE material systems that exhibit broken magnetic symmetry. These new materials utilize barium hexaferrite powder as the magnetically anisotropic filler combined with a compliant silicone elastomer matrix. Simulations of representative laminate structures comprised of varied arrays of magnetic particles exhibit novel actuation behaviors including reversible shearing deformation, variable magnetostriction, and most surprisingly, piezomagnetism. Results of dynamic shear experiments and analytical modeling support predicted shearing actuation responses in MREs having broken symmetry and only in those systems.

Commentary by Dr. Valentin Fuster
2012;():185-192. doi:10.1115/SMASIS2012-8148.

This paper describes some of the recent results of an ongoing U.S. Army research program examining the electronic behavior of hyperelastic stretchable capacitor, resistor, and inductor networks for which the conductor material employed is stretchable. As with traditional rigid analog components, stretchable electronic components exhibit frequency-dependant behavior. Unlike their rigid counterparts, stretchable electronic components may also exhibit dramatic strain-dependent behavior. In this way stretchable circuit networks may be viewed as controllable spatio-temporal filters. Resistance, capacitance, and inductance all change to varying degrees depending on the specific set of spatio-temporal inputs. These variations may be harnessed to create an adaptive circuit element that is controllable. This paper describes the results of integrating stretchable components into a tunable band-pass filter. Center frequency, bandwidth, and gain can be varied in a controllable way by varying the capacitance or resistance of specific circuit elements by stretching them. Biaxially stretchable components are described that are subjected to equibiaxial strain-states as high as 100% area strain. We examine the influence that the type of compliant conductor has on tunable circuit properties and on control authority.

Commentary by Dr. Valentin Fuster
2012;():193-200. doi:10.1115/SMASIS2012-8176.

Vibration suppression in flexible structures is becoming an important design problem to develop energy-autonomous systems powered using the harvested ambient energy. Reduced energy control laws are developed to address the trend towards autonomous ultra-light weight aerospace structures with limited energy supply. Experiments build upon recent advances in harvester, sensor and actuator technology that have resulted in thin, light-weight multi-layered composite wing spars. These beam like multifunctional spars are designed to be capable of alleviating wind gust of small Unmanned Aerial Vehicles (UAVs) using the harvested energy. Experimental results are presented for cantilever wing spars with micro-fiber composite transducers controlled by reduced energy controllers with a focus on two vibration modes. A reduction of 16dB and 11dB is obtained for the first and the second mode using the harvested ambient energy. This work demonstrates the use of reduced energy control laws for solving gust alleviation problems in small UAVs, provides the experimental verification details, and focuses on applications to autonomous light-weight aerospace systems.

Commentary by Dr. Valentin Fuster
2012;():201-206. doi:10.1115/SMASIS2012-8178.

This study focuses on the characterization of a porous multifunctional elastomer-CNT nanocomposites for potential use as pressure sensors. A thermoplastic polyurethane (TPU) was chosen as an elastomeric matrix, which was reinforced with multiwall carbon nanotubes (0–10 wt%) by high shear twin screw extrusion mixing. Porosity was introduced to the composites through the phase separation of a single TPU-CO2 solution. Interactions between MWNT and TPU were elucidated through calorimetry, gravimetric decomposition, conductivity measurements and microstructure imaging. The piezoresistance (pressure-resistance) behavior of the nanocomposites was investigated and found to be dependent on MWNT concentration and nanocomposite microstructure.

Commentary by Dr. Valentin Fuster
2012;():207-212. doi:10.1115/SMASIS2012-8180.

Dielectric Electroactive Polymers belong to a new class of smart materials, whose functional principle is based on electrostatic forces. They can either be used as actuators to provide considerable stretch ratios or as generators to convert mechanical strain energy into electrical energy by use of an initial amount of energy. Since the polymer material and also the covering compliant electrodes show non-ideal electrical properties, like finite resistivity and conductivity respectively, design rules have to be derived, in order to optimize the devices. The electrode conductivity in connection with the polymer resistivity causes a voltage drop along the electrode surface, resulting in a reduced actuation strain or energy conversion. To minimize its parasitic effects, the influence of this effect is studied by the in-plane field propagation based on a model obtained with the equivalent network method. It is shown that the proposed model provides accurate results, which can be used to study the effect of contacting electrodes, especially in case of point contacts.

Commentary by Dr. Valentin Fuster
2012;():213-217. doi:10.1115/SMASIS2012-8194.

In this project, fully generalised analytical expressions for longitudinal Young’s modulus are developed for zigzag Single-Walled Nanotubes (SWNTs), based on a hexagonal honeycomb motif, using an energy equivalent approach. Honeycomb geometry (bond lengths and angles), and bond-stretch and bond-angle force constants are considered as variable parameters and deformation is assumed due to simultaneous bond stretching and bond angle variation in response to an applied axial load. A parametric study is then performed to explore the structure-property relationships in auxetic (negative Poisson’s ratio) nanotube.

Topics: Modeling , Nanotubes
Commentary by Dr. Valentin Fuster
2012;():219-225. doi:10.1115/SMASIS2012-8195.

In this contribution we present a new method as a “basic toolbox” for proper design of active composite structures. The characterization of the complete integrated active component is described, including the properties of the hosting composite material, the proper choice and characterization of the active material which is to be integrated and the interaction of both. The finite element model which was used to design the active component is presented. In order to improve prediction accuracy and functionality of this phenomenological modeling approach the behavior of the integrated active material, namely Shape Memory Alloy (SMA), is analyzed separately. New opportunities for additional functionalities are investigated: Two-way actuation due to the stiffness of the hosting composite structure is investigated as well as the possibility to introduce different maximum strain for actuation due to different pre-strains in the actuating material. An application-oriented finite element model able to predict the structure shape in hot and cold states enables more complex designs and demonstrates the potential of this new technology for various applications.

Commentary by Dr. Valentin Fuster
2012;():227-236. doi:10.1115/SMASIS2012-8257.

Shape memory composites (SMCs) based on shape memory alloys (SMAs) and shape memory polymers (SMPs) are interesting due to their controllable temperature-dependent mechanical properties. The complementary characteristics of SMAs and SMPs can be used to create materials or systems with shape recovery created by the SMA and shape fixity provided by the SMP. In this research, three SMC operating regimes are identified and the behavior of SMC structures is analyzed by focusing on composite fixity and interfacial stresses. Analytical models show that certain SMPs can achieve sufficient shape fixing. COMSOL Multi-Physics simulations are in agreement with analytical expressions for shape fixity and interfacial stresses. Analytical models are developed for an end-coupled linear SMP-SMA two-way actuation system.

Commentary by Dr. Valentin Fuster

Modeling, Simulation and Control of Adaptive Systems

2012;():237-246. doi:10.1115/SMASIS2012-7906.

The paper presents a modular architecture for SMA actuators elastically compensated by thin beams loaded axially beyond their buckling limit. Starting from the exact equations for the elastic curve of the beams, an approximate procedure is developed for the engineering design of the entire compensating system. The theory of the compensator is validated successfully against a finite element model and experimental results. The experimental characterization of a complete prototype actuator shows that the forces generated by the compensated actuator are constant for both instroke and outstroke over the full range of displacements. The actuator concept proposed lends itself to modular assembly to multiply either the stroke covered (series combination) or the force generated (parallel combination).

Commentary by Dr. Valentin Fuster
2012;():247-257. doi:10.1115/SMASIS2012-7908.

This paper investigates the transduction of a piezoaeroelastic energy harvester under combined base and aerodynamic loadings. The harvester consists of a typical rigid airfoil supported by hardening flexural and torsional springs. The airfoil is placed in an incompressible air flow and subjected to a harmonic base excitation in the plunge direction. Considering a nonlinear quasi-steady aerodynamic model, the response behavior and electric output of the harvester are analyzed near the flutter instability. A center manifold reduction is implemented to reduce the original five-dimensional system into one nonlinear first-order ordinary differential equation. Subsequently, the normal form of the reduced system is derived to study slow modulation of the voltage amplitude and phase. Several case studies are presented indicating a considerable improvement in the output voltage of the harvester under the combined loading even when the air speed is below the flutter velocity, i.e., even when the harvester cannot maintain steady-state periodic oscillations in the absence of the harmonic base excitation. It is also shown that, when the base-excitation amplitude is sufficiently large and its frequency is close to the frequency of the self-sustained limit-cycle oscillations emanating from the flutter instability, the periodic solution resulting from the base excitation entrains the self-sustained oscillations yielding a periodic output voltage. However, when the excitation frequency is far from the limit-cycle frequency, or the amplitude of base excitation is small, the voltage is two-period quasiperiodic.

Commentary by Dr. Valentin Fuster
2012;():259-270. doi:10.1115/SMASIS2012-7921.

The goal of this paper is to investigate the use of a very simple direct adaptive controller in the guidance of a large, flexible launch vehicle. The adaptive controller, requiring no on-line information about the plant other than sensor outputs, would be a more robust candidate controller in the presence of unmodeled plant dynamics than a model-based fixed gain linear controller. NASA’s seven-state FRACTAL academic model for ARES I-X was employed as an example launch vehicle on which to develop the controller.

To better understand the difficult dynamic issues, we started with a simplified model that incorporated the inherent instability of the plant and the nonminimum phase nature of the dynamics: an inverted pendulum with an attachable slosh tank. We formulated controllers for this simplified plant with slosh dynamics using control algorithms developed only on a reduced–order model consisting of the rigid body dynamics without slosh. The controllers must be designed to reject three different persistent input disturbances: persistent pulse, step, and sine. We assumed that only position feedback was available, and that rates would have to be estimated.

For comparison, a fixed gain linear controller was developed using the well-known Linear Quadratic Gaussian methodology employing state estimation to obtain rate estimates. For a stable adaptive controller, we used direct adaptive control theory developed by Balas, et al. For this theory we need CB > 0 and a minimum-phase open-loop transfer function. We employed a new transmission zero selection method to develop a blended output shaping matrix which would satisfy these conditions robustly. We used approximate differentiation filters to obtain rates for the adaptive controller. Again for comparison, we redesigned the LQG controller to use the same blended output matrix and filters.

Following the work on the pendulum, the same method was applied to develop an adaptive controller for the FRACTAL launch vehicle model. An adaptive controller stabilizes a rigid body version of FRACTAL over a very long timeline while exceeding all reasonable state and output limits.

Commentary by Dr. Valentin Fuster
2012;():271-279. doi:10.1115/SMASIS2012-7924.

Many systems must operate in the presence of delays both internal to the system and in its inputs and outputs. In this paper we present a robustness result for mildly nonlinear systems. We use this result to show that, for small unknown time varying input delays, a simple adaptive controller can produce output regulation to a neighborhood with radius dependent upon the size of an upper bound on the delay. This regulation occurs in the presence of persistent disturbances and the convergence is exponential. We conclude with an example to illustrate the behavior of this adaptive control law.

Commentary by Dr. Valentin Fuster
2012;():281-294. doi:10.1115/SMASIS2012-7925.

This work presents an approach for developing the model of a smart fin dynamics that is activated by a fully-enclosed piezoelectric (PZT) bimorph actuator, which is created by bonding two Macro Fiber Composites (MFCs). Observing the dynamics of the fin indicates that the use of a linear dynamic model does not adequately describe its behavior. An earlier work proposed incorporating a proportional damping matrix as well as Bouc-Wen hysteresis model and backlash operators to create a more accurate model. However, the number of parameters describing the expanded model is large, which limits its use. Therefore, there is a need for a different approach for developing an alternative model of the fin. In this work, a hybrid master-slave Genetic Algorithm (GA)-Neural Network (NN) model is proposed to identify the optimal set of parameters for the damping matrix constants, the Bouc-Wen hysteresis model and the backlash operators. A total of nine sinusoidal input voltage cases that resemble a grid of three different amplitudes excited at three different frequencies are used to train and validate the model. Three input cases are considered for training the NN architecture, connection weights, bias weights and learning rules using GA. The NN consists of three layers: an input layer that has two nodes for the amplitude and the frequency of the input voltage, an output layer that has seven nodes for the backlash, hysteresis, and damping operators, and a hidden layer that is free to have any number of nodes between two and nine. The GA constantly performs natural selection of chromosomes that propagate best compilation of NN parameters. Simulation results show that the proposed model can predict the damping, hysteresis and backlash of the smart fin–actuator system under various operational conditions.

Commentary by Dr. Valentin Fuster
2012;():295-303. doi:10.1115/SMASIS2012-7930.

This paper presents the development of an indirect intelligent sliding mode controller (IISMC) for shape memory alloy (SMA) actuators. The controller manipulates applied voltage, enabling temperature control in one or more SMA tendons, which are offset to produce bending in a flexible beam tip. Hysteresis compensation is achieved using a hysteretic recurrent neural network (HRNN), which maps the nonlinear, hysteretic relationships between SMA temperatures and bending angle. Incorporating this HRNN into a variable structure control architecture provides robustness to model uncertainties and parameter variations. Single input, single output and multivariable implementations of this control strategy are presented. Controller performance is evaluated using a flexible beam deflected by single and antagonistic SMA tendons. Experimental results demonstrate precise tracking of a variety of reference trajectories for both configurations, with superior performance compared to an optimized PI controller for each system. Additionally, the IISMC demonstrates robustness to parameter variations and disturbances.

Commentary by Dr. Valentin Fuster
2012;():305-310. doi:10.1115/SMASIS2012-7938.

Rather than address morphing in the conventional way of applying forces to a fixed-compliance structure, this work addresses morphing in a structure that can exhibit variable compliance. Specifically, this work addresses the shape morphing of a simply-supported elastic beam from a forward approach perspective, that is, determining beam shape due to applied forces. The focus on this study is on the spatial distribution of the structure due to local/total modulus change in its steady state condition. The resulting morphed shapes are quantified in terms of the characteristic parameters, such as spatial distribution of the peaks of the shapes, deviation from the axis of the symmetry and, in case of flat-top shapes, the width of the flat portion.

Commentary by Dr. Valentin Fuster
2012;():311-320. doi:10.1115/SMASIS2012-7940.

Vanadium dioxide (VO2) undergoes a thermally induced solid-to-solid phase transition. A VO2-coated silicon cantilever demonstrates large change in its bending curvature across its phase transition. Due to phase transition and thermal expansion effects, the curvature – temperature hysteresis in VO2 actuators comes with a non-monotonic hysteretic behavior, introducing new challenges in its modeling. Motivated by the underlying physics, in this paper we present a novel model that combines a monotonic Preisach hysteresis operator with a linear operator. A constrained least square scheme is proposed to estimate the model parameters. For comparison purposes, we also consider a Preisach operator with a signed weighting function, and a hybrid model consisting of a monotonic Preisach operator for the curvature within the transition and linear operators outside the transition. Experimental results confirm the effectiveness of the proposed model.

Commentary by Dr. Valentin Fuster
2012;():321-327. doi:10.1115/SMASIS2012-7942.

Macro Fiber Composites (MFCs), comprised of PZT fibers, are being considered for a variety of applications due to their flexibility and relatively low production costs. Like other PZT actuators, MFCs also exhibit hysteresis and constitutive nonlinearities that must be characterized in models and control designs to achieve the full potential. Here we use an Euler-Bernoulli beam model coupled with the homogenized energy strain model to predict the structural/hysteretic response of a thin cantilever beam with an MFC patch attached during a series of frequency sweep experiments. Optimization routines are employed to optimized both MFC parameters and beam parameters using a subset of displacement data. The posterior probability distribution of each model parameter is estimated using Markov Chain Monte Carlo simulations. Finally, we present model predictions with quantified uncertainties.

Commentary by Dr. Valentin Fuster
2012;():329-333. doi:10.1115/SMASIS2012-7944.

In this paper we introduce an Adaptive Disturbance Tracking Control (ADTC) Theory and make some modifications to implement it to address Region II control problem of large wind turbines. Since ADTC requires measurement of wind speed, a wind speed and partial state estimator based on linearized lower-order model of wind turbine at Region II operating point was developed. The estimated wind speed was then used with the adaptive controller and the states were used for state feedback. The combination of partial state feedback and adaptive disturbance tracking control is implemented in National Renewable Energy Laboratory (NREL)’s 5 MW offshore wind turbine model and simulated in MATLAB/Simulink. The simulation result was then compared with existing fixed gain controller.

Commentary by Dr. Valentin Fuster
2012;():335-344. doi:10.1115/SMASIS2012-7945.

Ferroelectric (e.g., PZT), ferromagnetic (e.g., Terfenol-D) and ferroelastic (e.g., shape memory alloy (SMA)) materials offer unique capabilities for a range of present and emerging control applications. To fully realize the capabilities these materials offer, model-based control designs must account for the nonideal effects (e.g., creep, rate-dependent hysteresis, and constitutive nonlinearities) that the materials exhibit. In this paper, we employ the homogenized energy model (HEM) to characterize rate-dependent hysteresis behavior, construct an approximate inverse algorithm to compensate the material nonlinearities, and combine this with a sliding mode controller to accommodate uncertainties in the model. We illustrate this in the context of an actuator employing the ferroelectric material PZT but note that the general framework is also applicable to magnetic and shape memory alloy transducers. Through numerical examples, we illustrate the effectiveness of the HEM inverse-based sliding mode design for tracking a reference trajectory in the presence of modeling and inversion errors.

Commentary by Dr. Valentin Fuster
2012;():345-351. doi:10.1115/SMASIS2012-7946.

This paper presents the development of a bimorph piezoelectric cymbal energy harvester that is particularly useful for extracting energy from the vibrating systems of relatively high compressive load. The bimorph cymbal harvester can be used to charge a capacitor or a battery through the piezoelectric layers fitted within the metal end caps under repeated compression or deformation.

In this work, feasibility of a bimorph piezoelectric cymbal harvester in series operation is investigated through theoretical analysis and experimental validation. The bimorph cymbal uses a composite disc of two piezoelectric layers and a steel substrate between metal end caps.

Theoretical modeling to quantify the generated energy by using bimorph cymbal design is first conducted. A parametric study is then performed to optimize generated energy with the dominant design parameters influencing energy harvesting performance for the cymbal structure. The parameters such as thickness of the end caps, radius ratio of the apex to the cavity of the end caps, cavity depth, and thickness ratio of the piezoelectric to the steel substrate are considered. Based on the optimized dimension, a cymbal harvester was fabricated and tested to validate analytically predicted open-circuit voltage on a hand jack type test rig.

Experimental result indicates that the measured open-circuit voltage from the bimorph cymbal harvester is less than that of analytically predicted. However, it shows that the bimorph piezoelectric cymbal structure is an alternative cymbal design that is useful for harvesting energy from the source of relatively high load.

Commentary by Dr. Valentin Fuster
2012;():353-361. doi:10.1115/SMASIS2012-7964.

This paper explores the merits of shape memory Negator springs as powering elements for solid state actuators. A Negator spring is a spiral spring made of strip of metal wound on the flat with an inherent curvature such that, in repose, each coil wraps tightly on its inner neighbour. The unique characteristic of Negator springs is the nearly-constant force needed to unwind the strip for very large, theoretically infinite deflections. Moreover the flat shape, having a high area over volume ratio, grants improved bandwidth compared to any solution with solid wires or helical springs. The SMA material is modelled as elastic in austenitic range while an exponential continuum law is used to describe the martensitic behaviour. The mathematical model of the mechanical behaviour of SMA Negator springs is provided and their performances as active elements in constant-force, long-stroke actuators are assessed.

The SMA Negator spring is also simulated in a commercial finite element software, ABAQUS, and its mechanical behaviour is estimated through FE analyses. The analytical and the numerical prediction are in good agreement, both in martensitic and in austenitic range.

Commentary by Dr. Valentin Fuster
2012;():363-372. doi:10.1115/SMASIS2012-7973.

An innovative shape memory alloy actuated cage has been developed for spinal fusion surgery. Spinal fusion surgery is performed on people suffering from low back pain. The viscoelastic spinal disc between the two vertebrae can degenerate in some fashion, such as herniation, and the space needs to be restored to relive the pressure on the nerves within the lower back. There are two main parts to a spinal disc, the annulus fibrosis and the nucleolus. The annulus fibrosis is a cartilaginous structure and is of interest to preserve. Therefore a minimally invasive cage utilizing superelastic elements has been developed. Furthermore, the cage safety and efficacy has been proven and will be presented here. Within this work, the efficacy and longevity of the cage will be presented. To this end, ASTM testing for spinal implants has been conducted on an electromechanical test system capable of inducing simultaneous axial and torsional forces.

Commentary by Dr. Valentin Fuster
2012;():373-381. doi:10.1115/SMASIS2012-7985.

We study flexural vibrations of a thin rectangular cross section cantilever beam submerged in a quiescent viscous fluid. The cantilever is subject to base excitation and undergoes oscillations whose amplitude is comparable with its width. The structure is modeled as an Euler-Bernoulli beam and the fluid-structure interaction is captured through a nonlinear complex-valued hydrodynamic function which accounts for added mass and fluid damping. Results from a parametric 2D computational fluid dynamics analysis of an oscillating rigid lamina, representative of a generic beam cross section, are used to establish the dependence of the hydrodynamic function on the governing flow parameters. It is found that, as the frequency and amplitude of the vibration increase, vortex shedding and convection phenomena are enhanced, thus promoting nonlinear hydrodynamic damping. We derive a computationally efficient reduced order modal model for beam oscillations incorporating the non-linear hydrodynamic function and we validate theoretical results against experiments on underwater vibrations flexible beams.

Commentary by Dr. Valentin Fuster
2012;():383-390. doi:10.1115/SMASIS2012-7988.

An original sliding mode controller is designed, based on an existing mathematical model for response control of the human vestibular system. The human vestibular system is located in the inner ear and significantly contributes to the functions of detecting head motion, maintaining balance and posture, and realizing gaze stabilization. The vestibular system sends signals to the brain to tell it how the head and body are moving, and the brain reacts by changing eye position accordingly. The nonlinearities of the vestibular system are not completely understood. The biggest nonlinearity is the nystagmus, a bouncing of the eyes to compensate for quick head movement. Another nonlinearity is that the quick phase does not start until head movement reaches a certain frequency. Considering these nonlinearities as well as the uncertainties of the system, sliding mode control a good choice for controlling the system. Several mathematical models of the human vestibular system are considered for use in the control design. The best model of those considered is chosen based on the models’ consideration of nonlinearities and their levels of complexity. The mathematical model used in this paper is a nonlinear transfer function. The output is controlled with a robust sliding mode controller. Results demonstrate the need to increase control parameters as frequency of the sinusoidal input increases to minimize overshoot error. However, since the human head cannot tolerate an infinitely large frequency input, control parameters also will necessarily be limited. Therefore, results show that the designed sliding mode robust controller is an effective mechanism for controlling the mathematical model of the human vestibular system.

Commentary by Dr. Valentin Fuster
2012;():391-400. doi:10.1115/SMASIS2012-7989.

In this paper, we present a systematic approach to developing robust control algorithms for a single-tendon shape memory alloy (SMA) bending actuator. Parameter estimation and uncertainty quantification are accomplished using Bayesian techniques. Specifically, we utilize Markov Chain Monte Carlo (MCMC) methods to estimate parameter uncertainty. The Bayesian parameter estimation results are used to construct a sliding mode control (SMC) algorithm where the bounds on uncertainty are used to guarantee controller robustness. The sliding mode controller utilizes the homogenized energy model (HEM) for SMA. The inverse HEM compensates for hysteresis and converts a reference bending angle to a reference temperature. Temperature in the SMA actuator is estimated using an observer, and the sliding mode controller ensures that the observer temperature tracks the reference temperature. The SMC is augmented with proportional-integral (PI) control on the bending angle error.

Commentary by Dr. Valentin Fuster
2012;():401-410. doi:10.1115/SMASIS2012-7999.

In the recent years, using piezoelectric material as sensors and actuators has drawn significant attention in vibration analysis and control of structures. In the present paper, bonded piezoelectric sensors and actuators have been used to control the aeroelastic oscillations of a cantilever wing under the effects of three-dimensional unsteady subsonic aerodynamic loading. An aerodynamic model using a numerical panel method is developed and validated to calculate the three-dimensional unsteady aerodynamic loading and finite element formulation is applied to model the wing structure as a cantilever plate undergoing small transverse oscillations. The structural and aerodynamic models are combined to simulate the aeroelastic oscillations and interchange the data simultaneously. An active feedback control method to suppress the oscillations is presented and investigated. Finally, an analysis is performed to examine the effects of actuator placement on the wing surface in suppression of oscillations.

Commentary by Dr. Valentin Fuster
2012;():411-418. doi:10.1115/SMASIS2012-8016.

In this paper, a single degree of freedom system was used to model a beam with a breathing crack. Then analytical approximate solution approach was employed to solve this nonlinear vibration equation based on the Homotopy Perturbation Method (HPM). Nonlinear free vibration frequencies under different crack depth to thickness ratios were calculated and compared to the results that were predicted by the bilinear oscillator model. Both predictions were validated experimental data. It has been clearly shown that the fundamental frequency of a beam with a breathing crack is lower than the case in which the crack is assumed to be open and remains open. Nonlinear forced vibration responses containing all harmonics were determined analytically. Numerical simulation results were used to validate our analytical approximation solutions. A damage detection scheme was proposed to relate the crack depth to thickness ratio with a damage index, which is derived from the nonlinear forced responses. This damage index is able to accurately assess the breathing crack condition even for a very small crack depth.

Commentary by Dr. Valentin Fuster
2012;():419-427. doi:10.1115/SMASIS2012-8017.

In this paper, a semi-analytical analysis of the pseudoelastic response of shape memory alloy rods and tubes subjected to combined axial and torsional loading is proposed. A three-dimensional phenomenological SMA constitutive model is simplified to obtain the corresponding two-dimensional constitutive relations. The rod is partitioned into a finite number of narrow annular regions and the equilibrium equations are found in each annular region for both loading and unloading paths. Several numerical examples are presented to demonstrate the efficiency of the proposed method, and the results are compared with three-dimensional finite element simulations.

Commentary by Dr. Valentin Fuster
2012;():429-435. doi:10.1115/SMASIS2012-8044.

The nonlinear, hysteretic stress-strain characteristic of superelastic shape memory alloys (SMA) results in energy dissipation and therefore in high damping capacities. Due to the nonlinearity the damping capacity strongly depends on the amplitude of the applied excitation. In this work, a rheological non-smooth model is used to describe the principle behavior of superelastic SMA undergoing harmonic displacements. The equivalent mechanical model consists of a spring representing the elastic deformation of the superelastic SMA in austenitic and detwinned martensitic state. A friction element represents the stress plateaus for forward and backward transformation between austenitic and martensitic state. A constant force is applied to the system to generate an offset which shifts the hysteresis to positive force values. Two mechanical stops are implemented to describe the end of the stress plateaus and therefore correspond to the strain differences of the stress levels for forward and backward transformation. Thus, the system behavior is highly amplitude-dependent.

A harmonic approximation of the force generated by the superelastic SMA element during one excitation period is calculated by applying the Harmonic Balance Method to the nonlinear force signal of the rheological model. In this context the Fourier coefficients are calculated by performing a piecewise integration of the force signal. The Integrals are being calculated for each steady interval. The equivalent stiffness and damping coefficients are given for this approximation as functions of excitation amplitude and the system parameters. Based on these results, the damping capacity of a superelastic shape memory element undergoing harmonic displacements is presented using an analytical expression for the damping ratio.

Commentary by Dr. Valentin Fuster
2012;():437-446. doi:10.1115/SMASIS2012-8050.

A strategy is presented to identify the optimal localized activation and actuation for a morphing thermally-activated SMP structure or structural component to obtain a targeted shape change subject to design objectives such as minimal total required energy and time. This strategy combines numerical representations of the SMP structure’s thermo-mechanical behavior subject to activation and actuation with nonlinear optimization methods to efficiently solve the morphing inverse problem that includes minimizing cost functions which address thermal and mechanical energy, morphing time, and damage. The details of this strategy are presented along with simulated examples to display the capabilities and limitations, as well as potential future directions for improving these techniques.

Topics: Design
Commentary by Dr. Valentin Fuster
2012;():447-452. doi:10.1115/SMASIS2012-8051.

Cylindrical shells, very commonly used in aerospace applications, are susceptible to buckling when subjected to static and dynamic or transient loads. Bucking load enhancement with minimum weight addition is an important requirement in space structures. Buckling control of space structures using piezoelectric actuators is an emerging area of research. The earlier work on enhancement of buckling load on columns reported a 3.8 times enhancement theoretically and 123% experimentally [1–2]. The enhancement was (25%) when buckling control was implemented on plates [3] using PZT actuators. Buckling control of cylindrical shells is challenging because of the uncertainties in the location of buckling and the coupling between bending and membrane action. Earlier attempt to improve the buckling load carrying capacity of the cylindrical shell did not result in a considerable increase in the buckling load [4]. This is because the buckling modes of cylindrical shell are very close to each other when compared to structures like column and plate. An optimized actuator location is hence necessary to improve the load carrying capacity of the cylindrical shells. Unlike vibration control problems where the actuators locations are optimized to minimize the structural Volume Displacement (SVD) or to maximize the energy dissipation, buckling control is aimed at controlling the critical modes of buckling and hence improving the load carrying capacity of the shells [5]. Numerical analyses are carried out, comparing different configurations used in buckling control of thin shells. Experiments are performed to support the numerical analysis as the behavior of cylindrical shells under axial compression is highly sensitive to geometric imperfections. Load – Axial shortening graphs are used to compare the performance of cylindrical shell for the various actuator configurations.

Topics: Actuators , Pipes , Buckling
Commentary by Dr. Valentin Fuster
2012;():453-460. doi:10.1115/SMASIS2012-8082.

Legged robotics exhibit mobility over complex terrains that overcomes many of the challenges experienced with traditional wheeled robots. This includes the ability to traverse rough terrain, climb obstacles, and in some robotic platforms, even scale walls. It is known from bio-locomotion research that humans and other animals adjust the stiffness of their muscles to accommodate differences in the terrain. Methods to implement changes in the passive mechanical stiffness on a legged robotic platform have included geometric changes to the leg configuration, complex mechanical linkages and gears, or thermally induced modulus changes in polymers. Each of these cases are limited in their dynamic response and efficiency. As an alternative, we have developed a leg module that changes its stiffness by application of an electric field. This is achieved by applying a large voltage to the dielectric elastomer VHB. Previous studies have demonstrated up to a 92% stiffness reduction. The goal of this work is to identify the electromechanical dynamic responses of those elements to understand limits in adaptability from abrupt changes in terrain. We quantify the structural dynamic behavior and electromechanical limits governing rapid stiffness changes in our legged VHB module. Structural vibration characterization is presented to illustrate transient dynamic changes when the VHB material is exposed to a step input voltage change. The results are analyzed and compared to the system dynamics required for the iSprawl legged robot.

Commentary by Dr. Valentin Fuster
2012;():461-471. doi:10.1115/SMASIS2012-8084.

Pneumatic artificial muscles (PAMs) are lightweight, flexible actuators capable of higher specific work than comparably-sized hydraulic actuators at the same pressure and electric motors. PAMs are composed of an elastomeric bladder surrounded by a helically braided sleeve. Lightweight, compliant actuators are particularly desirable in portable, heavy-lift robotic systems intended for interaction with humans, such as those envisioned for patient assistance in hospitals and battlefield casualty extraction. However, smooth and precise control remains difficult because of nonlinearities in the dynamic response. The objective of this paper is to develop a control algorithm that satisfies accuracy and smooth motion requirements for a two degree-of-freedom manipulator actuated by pneumatic artificial muscles and intended for interaction with humans, such as lifting a human. This control strategy must be capable of responding to large, abrupt variations in payload weight over a high range of motion. In previous work, the authors detailed the design and construction of a proof-of-concept PAM-based manipulator. The present work investigates the feasibility of combining output feedback using proportional-integral-derivative control or fuzzy logic control with model-based feedforward compensation to achieve improved closed-loop performance. The model upon which the controller is based incorporates the internal airflow dynamics, the geometric parameters of the pneumatic actuators, and the arm dynamics. Simulations were performed in order to validate the control algorithm, guide controller design, and predict optimal gains. Using real-time interface software and hardware, the controller was implemented and experimentally tested on the manipulator. Performance was evaluated for several trajectories, and different payload weights. The effect of varying the feedforward gain was also analyzed. Model refinement further improved performance.

Commentary by Dr. Valentin Fuster
2012;():473-480. doi:10.1115/SMASIS2012-8107.

Wave propagation and energy diffusion in smart structures with shunted piezoelectric patches are examined in this study. The dynamic behavior of a structure can be modified through piezoelectric shunts with negative capacitance. This technique is extremely interesting, as it controls the dynamic behavior of the structure in a large frequency range. The effects of this piezoelectric shunt are studied via a wave propagation approach, and energy diffusion properties of specific wave modes in the structure can be obtained. However, for a proper design of the overall structure, a finer analysis of the real-life circuit is required. The aim of the present work is indeed to establish some essential rules that will guide one to choose more suitable design parameters for the actual system.

Commentary by Dr. Valentin Fuster
2012;():481-487. doi:10.1115/SMASIS2012-8125.

Active materials, due to their intrinsic multi-functional material characteristics, have shown great promise for the improvement of agility and the mitigation of adverse turbulence effects for micro-air-vehicles (MAVs). One such subsection of active materials known as dielectric elastomers have demonstrated this multifunctional role by functioning as the wing surface and a boundary layer control device. In the past this material has shown that an increase in overall lift of 20% is possible and has the ability to delay stall by up to 5 degrees for an elliptical wing at a chord based Reynolds number of 63000. In order to better understand the effect of these fluid structure interactions, simultaneous time dependent structural deformation, aerodynamic loads, and flow velocities were measured and compared. These responses showed direct correlation between the applied electric field on the membrane wing and the aerodynamic loads and flow responses.

Commentary by Dr. Valentin Fuster
2012;():489-496. doi:10.1115/SMASIS2012-8137.

Ferromagnetic materials exhibit rate-dependent hysteresis, creep and constitutive nonlinearities due to their inherent domain structure. For model-based control applications, these non-linear attributes must be incorporated in a models in a manner that facilitates model calibration and real-time control implementation. In this paper, we present a homogenized energy model for these materials. This is a multiscale framework that quantifies energy at the domain level and then employs stochastic homogenization techniques to provide macroscopic models that are highly efficient to implement. The accuracy of models will be validated using a variety of experimental data.

Commentary by Dr. Valentin Fuster
2012;():497-506. doi:10.1115/SMASIS2012-8144.

Dielectric Elastomers (DE) seem to be a promising technology for the implementation of light and compact Variable Stiffness Actuators (VSAs), thanks to their large power densities, low costs and shock-insensitivity. Nonetheless, the development of DE-based VSA is not trivial owing to the relevant dissipative phenomena that affect the DE when subjected to rapidly changing deformations. In this context, the purpose of the present paper is to investigate the practical feasibility of DE-based VSA. As a case study, two conically-shaped actuators, in agonistic-antagonistic configuration, are modeled accounting for the visco-hyperelastic nature of the DE films. The model is then linearized and employed for the design of a stiffness controller. The control algorithm requires the knowledge of the actuator configuration (via a position sensor) and of the force exchanged with the environment (via a force sensor). An optimum full-state observer is then implemented, which enables both accurate estimation of the DE time-dependent behavior and adequate suppression of sensor measurement noise. At last, experimental results are provided together with the description of an effective electronic driver that allows an independent activation of the agonistic-antagonistic DE membranes.

Commentary by Dr. Valentin Fuster
2012;():507-516. doi:10.1115/SMASIS2012-8164.

The microstructure of magnetic shape memory alloys (MSMAs) is comprised of tetragonal martensite variants, each with their preferred internal magnetization orientation. In the presence of an external magnetic field, the martensite variants tend to reorient so that the preferred internal magnetization aligns with the external magnetic field. As a result, MSMAs exhibit the shape memory effect when there is a magnetic field in the vicinity of a material point. Furthermore, the tetragonal nature of the martensite variants allows for a compressive stress to cause variant reorientation. This paper studies the magneto-mechanical behavior of MSMAs under various load paths, including complex loading conditions where both the applied magnetic field and compressive stress vary simultaneously.

Typically, MSMAs have been studied experimentally and modeled mathematically with either axial compressive stress or transverse magnetic field varying and the other remaining constant. For each load case, the mathematical models are calibrated with a set of experimental data that mimics those to be predicted. Model parameters have been found to be quite different when the calibration was performed with experimental results from different load cases.

This work investigates if current models, namely the Kiefer and Lagoudasmodel or the Waldauer et al. model, are capable of predicting both of the typical loading configurations mentioned above with a single calibration. Furthermore, this work uses the Waldauer et al. model to simulate more complex loading, where an MSMA element is subject to simultaneously varying stress and field; this type of loading might occur if an actuator is being designed to displace a variable load over a controlled distance.

Topics: Stress
Commentary by Dr. Valentin Fuster
2012;():517-525. doi:10.1115/SMASIS2012-8175.

Piezoelectric systems and structures have been used for decades in a variety of applications ranging from vibration control and sensing to morphing and energy harvesting. Conventional piezoelectric ceramics with uniform electrodes typically employ the 31-mode of piezoelectricity in bending, where the 3- and 1-directions are the directions of poling and strain, respectively. In order to employ the more effective 33-mode of piezoelectricity, Interdigitated Electrodes (IDEs) have been used recently in the design of the Macro-Fiber Composite (MFC). In this paper, an investigation into the two-way electroelastic coupling in bimorph cantilevers (in the sense of direct and converse piezoelectric effects) that employ IDEs for 33-mode operation is conducted. To this end, distributed-parameter electroelastic models are developed for the dynamic scenarios that involve two-way coupling, namely piezoelectric power generation and shunt damping as well as the problem of dynamic actuation. Various interdigitated MFC bimorph cantilevers are tested against the model under dynamic actuation, power generation, and shunt damping to identify their modal electromechanical coupling terms. Detailed investigations are conducted by decoupling the system dynamics to keep the direct and converse effects separately pronounced for parameter identification. Additionally, this work sheds light on the literature comparing the electrical power generation performances of 33-mode (interdigitated electrodes) and 31-mode (uniform electrodes) piezoelectric bimorphs of the same volume based on extensive experiments and distributed-parameter electroelastic modeling.

Commentary by Dr. Valentin Fuster
2012;():527-536. doi:10.1115/SMASIS2012-8191.

Ionic conducting polymer-metal composites (abbreviated as IPMC) are interesting actuators that can act as artificial muscles in robotic and microelectromechanical systems. The electrochemical-mechanical behavior of these materials has been modeled by various black or gray box models. In this study, the governing partial differential equation of the behavior of IPMC is solved using finite element methods to find the critical actuation parameters such as strain distribution, maximum strain, and response time. 1D results of the FEM solution are then extended to 2D to find the tip displacement of a flap actuator. Model of a seven-degree of freedom biped robot, actuated by IPMC flaps, is then introduced to study. Possibility of fast and stable bipedal locomotion using IPMC artificial muscles is the main motivation of this study. Taking the actuator limits into account, joint path trajectories are generated to achieve a fast and smooth motion. The stability of the proposed gait is then evaluated using ZMP criterion and motion simulation. Fabrication parameters of each actuator such as length, platinum (or gold) plating thickness and installation angle are also studied using the generated trajectories.

Commentary by Dr. Valentin Fuster
2012;():537-544. doi:10.1115/SMASIS2012-8197.

Synthetic jets offer new capabilities for localized active cooling of electronics due to their compact size, low cost and substantial cooling effectiveness. The design of devices to create synthetic jets and optimize active cooling performance is challenging due to the strong, two way, fluid-structure interaction (FSI) between the working fluid and the flexible structure that moves the fluid driven with piezoelectric actuators. Previous modeling efforts relied on lumped parameter approaches or electrical analogs. Although computationally less intensive, these approaches may not be accurate in all regions of the design space of interest and trade off fidelity for ease of use. In this effort, a 3D finite element model of the structure is coupled with a 3D computational fluid dynamics model of the fluid to explore the viability of such an approach. The motion of the structure moves the fluid grid, and the fluid feeds back pressure forces onto the structure that are required to converge at each iteration. Transient response of the deflection, pressure and exit velocity will be presented. Validation of the FSI model with experimental data for the frequency response of these quantities will also be presented.

Commentary by Dr. Valentin Fuster
2012;():545-551. doi:10.1115/SMASIS2012-8203.

The thermo-mechanical properties of Nickel-Titanium based Belleville washers have been analyzed by numerical simulations. In fact, these components exhibit unique mechanical and functional features due to the reversible stress-induced and/or thermally-induced phase transition mechanism of NiTi alloys. The numerical simulations have been carried out by using a commercial finite element software code and a special constitutive model for SMAs. The effects of the geometrical configuration of the washers as well as of the operating temperature, under fully austenitic conditions, have been analyzed. The results highlighted a marked hysteretic response, in terms of force-deflection curve, due to the hysteresis in the stress-strain behavior of NiTi alloys. In addition, a marked influence of the geometry, as well as of the temperature, has been observed on the thermo-mechanical response of the washer, i.e. in terms of both mechanical and functional properties.

Topics: Deflection , Springs
Commentary by Dr. Valentin Fuster
2012;():553-558. doi:10.1115/SMASIS2012-8226.

Army combat operations have placed a high premium on reconnaissance missions for micro air vehicles (MAVs). An analysis of insect flight indicates that in addition to the bending excitation (flapping), simultaneous excitation of the twisting degree-of-freedom is required to manipulate the control surface adequately. By adding a layer of angled piezoelectric segments to a Pb(Zr,Ti)O3 (also referred to as PZT) bimorph actuator, a bend-twist coupling may be introduced to the flexural response of the layered PZT, thereby creating a biaxial actuator capable of driving wing oscillation in flapping wing MAVs. The present study presents numerical investigation of the response of functionally–modified bimorph designs intended for active bend-twist actuation of cm-scale flapping wing devices. The relationships of geometry and orientation of the angled segments with bimorph bend-twist response will be presented using results of finite-element analyses.

Topics: Actuators
Commentary by Dr. Valentin Fuster
2012;():559-563. doi:10.1115/SMASIS2012-8230.

Numerous vibrating electromechanical systems miss a rigid connection to the inertial frame. An artificial inertial frame can be generated by a shaker which compensates for vibrations. In this paper we present an encapsulated and perforated unimorph bending plate for this purpose. As basis for system simulation and optimization a new 3-port multi domain network model was derived. An extension of the network allows the simulation of the acoustical behavior inside the capsule. Network parameters are determined using Finite Element simulations. The dynamic behavior of the network model agrees with the Finite Element simulation results up to the first resonance of the system. The network model was verified by measurements on a laboratory setup, too.

Commentary by Dr. Valentin Fuster
2012;():565-569. doi:10.1115/SMASIS2012-8232.

Two-layer piezomagnetic elements within a homogeneous magnetic field have been well studied. In this paper the effect of an inhomogeneous magnetic field distribution in the magnetic layer on the electromechanical properties of a two-layer element with planar conductor arrangement on top is investigated. Based on static Finite Element (FE) simulations the parameters of its network model are determined. The inductance of the arrangement with and without magnetic layer allows the calculation of the reluctances of the magnetic system. Magnetic field strength and moment of the fixed-fixed beam give the magnetomechanical transduction coefficient, moment and deflection the bending compliance. The dynamic behavior of the electromechanical transducer can be calculated efficiently by the completed network model in sensing as well as actuation direction.

Commentary by Dr. Valentin Fuster
2012;():571-575. doi:10.1115/SMASIS2012-8243.

In order to simulate the torsional behavior of NiTi torque tubes, two different 3D thermo-mechanical constitutive models are utilized. Firstly, an available incremental constitutive model is used in which a return mapping algorithm is implemented to numerically calculate the strains for any applied stresses. Secondly, Microplane theory is employed based on which 1D constitutive laws are considered for associated stress and strain components on any arbitrary plane passing through a material point followed by a homogenization process to generalize the 1D equations to a 3D macroscopic model. Both of the constitutive models are implemented in ABAQUS by developing UMAT. In order to compare the two approaches, torque-angle of rotation and shear stress-shear strain responses for torsion of thin-walled Nitinol torque tubes with different thicknesses are studied. The numerical results of these two approaches show to be in a good agreement indicating the capability of Microplane theory in constitutive modeling of shape memory alloys. This theory provides explicit relationships to calculate strains in terms of stresses, and this makes it very beneficial in obtaining the SMA responses in a fast and easy manner.

Commentary by Dr. Valentin Fuster

Structural Health Monitoring

2012;():577-585. doi:10.1115/SMASIS2012-7904.

Delamination is a frequent and potentially serious damage that can occur in laminated polymer composites due to the poor inter-laminar fracture toughness of the matrix. Vibration based detection methods employ changes caused by loss of stiffness in dynamic parameters such as frequencies and mode shapes to detect and assess damage. Because it is a whole field method, and can be applied instantaneously and remotely, vibration monitoring using frequency measurements offers great potential for implementation in online structural health monitoring systems. However, one of the disadvantages of using frequency measurements is that while the presence of damage is easily identified through a shift in measured frequency, the determination of the location and the severity of the damage is not easy to accomplish. To determine the location and severity of damage from measured changes in frequency, it is necessary to solve the inverse problem, which requires the solution of a set of non-linear simultaneous equations.

In this paper, we have compared the performance of three different inverse algorithms for delamination detection in the fibre-reinforced composite laminates: direct of solution using a graphical method, artificial neural network (ANN) and surrogate-based optimization. In particular, the graphical method which was earlier proposed for problems of two variables has been extended to solution of three variables, the interface, location along the beam length and size of delamination in laminated composite beams. The three inverse algorithms have been compared using numerical validation data generated from the theoretical model of delaminated beam with and without artificial errors. All three algorithms can predict the delamination parameters accurately using the validation data directly generated from theoretical model. However, if artificial errors are introduced in the numerical data to simulate uncertainties in measurement of frequencies, ANN does not fare as well as the the other two methods as it is more sensitive to the artificial discrepancies. Also, ANN requires the network to be retrained if the measured frequency modes do not match the input modes in the existing network. The graphical technique and the surrogate based optimization performed equally well in the validations. However, the graphical technique is only applicable to no more than three variables, while the surrogate-based optimization algorithm can be applied to inverse problems with several unknown parameters such as in the case of delaminations in composite plates.

Commentary by Dr. Valentin Fuster
2012;():587-594. doi:10.1115/SMASIS2012-7905.

Singular Spectrum Analysis (SSA) is a novel technique and has proven to be a powerful tool for time data series analysis. It takes singular value decomposition (SVD) of Hankel matrix embedded by analyzed time data series and decomposes the data into several simple, independent and identifiable components. In this paper, first, the coupling degree of the 1st and 2nd singular values through the composition of the analyzed signal in SSA is used as two important values to detect damage. Besides, based on the extracted sub-space or null-space from SVD of analytic matrix, damage detection algorithm is developed by considering the orthonormality between the sub-space and null-space. The proposed algorithms are verified using non-stationary response data of a model bridge (data from scouring test of a bridge) and field experiment of a bridge during abnormal weather condition. Discussion on the proposed methods with different assessment method to identify the occurrence of damage using SSI-DATA and SSI-COV to identified the system dynamic characteristics are also made.

Commentary by Dr. Valentin Fuster
2012;():595-600. doi:10.1115/SMASIS2012-7910.

This work reports on the modelling and experimental validation of a bi-axial vibration energy harvesting approach that uses a permanent-magnet/ball-bearing arrangement and a wire-coil transducer. The harvester’s behaviour is modelled using a forced Duffing oscillator, and the primary first order steady state resonant solutions are found using the homotopy analysis method (or HAM). Solutions found are shown to compare well with measured bearing displacements and harvested output power, and are used to predict the wideband frequency response of this type of vibration energy harvester. A prototype harvesting arrangement produced a maximum output power of 12.9 mW from a 12 Hz, 500 milli-g (or 4.9 m/s2) rms excitation.

Commentary by Dr. Valentin Fuster
2012;():601-607. doi:10.1115/SMASIS2012-7915.

In order to obtain a more accurate finite element (FE) model for a built structure, experimental data collected from the actual structure can be used to update the FE model. This process is known as FE model updating. Numerous FE model updating algorithms have been developed in the past few decades. However, most existing algorithms suffer computational challenges, particularly when applied to a large structure with dense measurements. The reason is these approaches usually operate on a relatively complicated model for the entire structure. To address this issue, a substructure updating approach is presented in this paper. The Craig-Bampton theory is adopted to condense the entire structural model into a substructure (currently being analyzed) and a residual structure. Dynamic response of the residual structure is approximated using only a limited number of dominant mode shapes. To improve the convergence of this substructure approach for model updating, an iterative convex optimization procedure is developed and validated through numerical simulation with a 200 degrees-of-freedom spring-mass model. The proposed substructure model updating is shown to successfully detect the locations and severities of simulated damage.

Topics: Optimization
Commentary by Dr. Valentin Fuster
2012;():609-614. doi:10.1115/SMASIS2012-7916.

Advanced signal processing approaches such time-frequency analysis are widely used for online evaluation, damage detection, and wear state classification. The idea of this paper is to introduce a new methodology for online examination of wear phenomena in metallic structure by means of acoustic emission (AE), Short-Time Fourier Transform (STFT) and Wavelet Transform (WT). The proposed novel low-cost system is developed for analyzing and monitoring specific signals indicating tribological effects with focus on field programmable gate array (FPGA) implementation of discrete WT (DWT). In addition, experimental results obtained from each approach are given showing the success of the introduced approach.

Commentary by Dr. Valentin Fuster
2012;():615-623. doi:10.1115/SMASIS2012-7917.

In this paper, the detection for two kinds of cracks is studied: (1) linear notch crack; (2) nonlinear breathing crack. A pitch-catch method with piezoelectric wafer actives sensors (PWAS) is used to interrogate an aluminum plate with a linear notch crack and a nonlinear breathing crack respectively as two cases. The inspection Lamb waves generated by the transmitter PWAS, propagate into the structure, interact with the crack, acquire crack information and are picked up by the receiver PWAS. The linear notch crack case is investigated through: (1) analytical model developed for Lamb waves interacting with a general linear damage; (2) finite element simulation. The breathing crack, which acts as a nonlinear source, is simulated using two approaches: (1) element activation/deactivation technique; (2) contact model. The theory and solving scheme of the proposed element activation/deactivation approach is discussed in detail. The signal features of different damage severities are analyzed. Crack opening, closing, stress concentration, surface collision phenomena are noticed for the breathing cracks. Mode conversion is noticed for both crack cases. The generation mechanism and mode components of the new wave packets are investigated by studying the particle motion through the plate thickness. A damage index is proposed based on the spectral amplitude ratio between the second harmonic and the excitation frequency for the breathing crack. The damage index is found capable of estimating the presence and severity of the breathing crack. The paper finishes with summary and conclusions.

Commentary by Dr. Valentin Fuster
2012;():625-632. doi:10.1115/SMASIS2012-7923.

This paper presents the design, simulation, and preliminary measurement of a passive (battery-free) frequency doubling antenna sensor for strain sensing. Illuminated by a wireless reader, the sensor consists of three components, i.e. a receiving antenna with resonance frequency f0, a transmitting antenna with resonance frequency 2f0, and a matching network between the receiving and transmitting antennas. A Schottky diode is integrated in the matching network. Exploiting nonlinear circuit behavior of the diode, the matching network is able to generate output signal at doubled frequency of the reader interrogation signal. The output signal is then backscattered to the reader through the sensor-side transmitting antenna. Because the backscattered signal has a doubled frequency, it is easily distinguished by the reader from environmental reflections of original interrogation signal. When one of the sensor-side antennas, say receiving antenna, is bonded to a structure that experiences strain/deformation, resonance frequency of the antenna shifts accordingly. Through wireless interrogation, this resonance frequency shift can be measured by the reader and used to derive strain in the structure. Since operation power of the diode is harvested from the reader interrogation signal, no other power source is needed by the sensor. This means the frequency doubling antenna sensor is wireless and passive. Based on simulation results, strain sensitivity of this novel frequency doubling antenna sensor is around −3.84 kHz/με.

Topics: Sensors
Commentary by Dr. Valentin Fuster
2012;():633-641. doi:10.1115/SMASIS2012-7935.

This paper develops uncertainty models for various an estimator used in computing the frequency-domain transmissibility function for single-input-multiple-output (SIMO) linear systems. The uncertainty is assumed to come from both internal (estimator) and external (environmental, noise, etc.) sources, and it is propagated through the estimation process to arrive at closed-form probability density functions for the estimation magnitude. The models is validated on a bolted plate in the laboratory, and receiver operating characteristics (ROCs) are computed to evaluate performance in terms of meaningful metrics such as probability of detecting a structural change within the context of structural health monitoring.

Commentary by Dr. Valentin Fuster
2012;():643-651. doi:10.1115/SMASIS2012-7939.

In this paper, a spectral finite element model (SFEM) is developed for an n-layered elastic beam and subsequently used to investigate its dynamic response and wave propagation characteristics. Each layer of the beam is idealized by a Timoshenko beam, in which shear deformation as well as rotational inertia are included. This higher order theory is critical to capture high frequency response of the multi-layered beam structures. Semi-analytical solutions were determined for the governing equations in order to construct the SFEM. Our frequency predictions were validated by the results of two and three-layer beams in the literature and good correlations were achieved. Fewer elements were used in our SFEM compared to conventional finite element based approaches, which substantially benefits the ultrasonic frequency simulations. Wave propagation responses were calculated for a two-layer beam, in which a notch in the top layer was assumed to represent the damage case. Wave reflection from the notch was observed to indicate the existence of damage. This newly developed SFEM can be served as a platform to conduct comprehensive simulations in order to capture wave propagation characteristics in multi-layered beam structures.

Commentary by Dr. Valentin Fuster
2012;():653-664. doi:10.1115/SMASIS2012-7948.

Structural health monitoring (SHM) is the process of damage identification in structural systems which have been an area of interest and a well-recognized field of technology in the past decade. Such systems involve the integration of smart materials, sensors and decision-making algorithms into the structure to detect damage, evaluate the structural integrity and predict the remaining life time. These systems have the potential to replace traditional non-destructive evaluation (NDE) of structures.

This study focuses on presenting an automated structural health monitoring (SHM) system based on detecting shifts in natural frequencies of the structure. The damage detection technique is implemented on a cracked composite beam vibrating in coupled bending-torsion where the crack is assumed open. Modal analysis is conducted on the composite beam in order to predict the natural frequency and the associated mode shapes. Based on this analysis, a database of information related to the specific composite beam being analyzed such as layups and natural frequencies are stored. The natural frequency will be measured and compared to that database for damage detection. A finite element model is also presented and compared with the analytical results. It is observed that the variation of natural frequencies in the presence of a crack is affected by the crack ratio, crack location and fiber orientation. In particular, the variation pattern is different as the magnitude of bending-torsion coupling changes due to different fiber angles.

A simple circuit containing a microcontroller is implemented to simulate the automated SHM concept. The microcontroller serves as the data storage device as well as the decision maker based on the instantaneous comparison between the healthy and the damaged structure. The proposed system may be implemented in many structural components such as aircraft frames and bridges. This SHM technology may help replace the current time-based maintenance scheme with a condition-based one. The condition-based maintenance scheme relies on the ability to monitor the condition of the system and supply information of damage detection to allow a corrective action to be taken.

Commentary by Dr. Valentin Fuster
2012;():665-669. doi:10.1115/SMASIS2012-7954.

The concept of wireless passive strain sensors has been introduced in the last few years for applications such as structural health monitoring. This study investigates the use of circular microstrip patch antenna (CMPA) sensors for wireless passive measurement of strain. The strain induced in an aluminium plate was measured wirelessly up to 5 cm away from the sensor using a CMPA made from commercial FR4 substrate, and at a distance up to 20 cm using a CMPA made from Rogers® RT/duroid 6010™. These results show the substrate of antennas is one of the factors affecting the interrogation distance. The interrogation distance between the sensor and the patch antenna was improved significantly using the Rogers® substrate.

Topics: Strain sensors
Commentary by Dr. Valentin Fuster
2012;():671-678. doi:10.1115/SMASIS2012-7958.

Fiber-reinforced polymer (FRP) composites have become a primary structural material in many new structures, particularly in the aerospace, wind turbine, automobile, and marine industries, due to their higher strength-to-weight ratios, corrosion resistance, and ease of manufacturing. However, these composite materials have complex damages modes that are different from typical monolithic metallic alloys, such as delamination, fiber breakage, matrix cracking, and fiber-matrix debonding. These avenues of damage tend to manifest internally to the composite structure, making them nearly invisible to visual inspection. Several damage detection approaches have been introduced for the purpose of in situ non-destructive evaluation (NDE) of composites; however, many of these approaches require complex analysis methods, data interpolation for achieving spatial sensing, and/or embedding invasive sensors into the composites themselves. To allow for widespread implementation of a next-generation NDE approach for composites, an easily discernible, highly visual, and fast approach that does not adversely affect the structural performance of the composite laminate is needed. This study introduces the use of a spatially distributed electrical conductivity distribution mapping method called electrical impedance tomography (EIT). EIT reconstructs a material’s 2D or 3D electrical conductivity within a series of boundary electrodes. A 100 mA current is injected between two opposing electrodes while the adjacent differential voltages are measured at the remaining electrodes; this process is repeated for all opposing electrode pairs. Using a linear reconstruction algorithm, changes in electrical conductivity are spatially resolved and plotted for easy detection, localization, and evaluation of damage. This approach is validated by applying EIT to a set of carbon fiber-reinforced polymer composite laminates. First, damage has been simulated in composite parts by selectively removing portions of the structure and then verifying that EIT has captured this occurrence. After validation of the EIT method, pristine composite laminates have been subjected to low velocity impact damage. Before and after impact EIT readings have been taken. The differential conductivity reconstruction is presented. This work demonstrates the value of adopting electrical impedance tomography for in situ NDE of FRP composites.

Commentary by Dr. Valentin Fuster
2012;():679-686. doi:10.1115/SMASIS2012-7959.

Reusable and on-orbit space structures endure harsh operating conditions and can sustain damage due to impact by micrometeoroids and orbital debris. Thus, it is necessary to develop and implement suitable sensing technologies for monitoring space structure performance, detecting damage, and preventing catastrophic failure from occurring. The challenge however is to tailor these sensors for operating in such unique environments and constraints. One such limitation is the amount of energy available for powering onboard sensing systems. Thus, the objective of this study is to design and characterize the properties of a novel self-sensing photoactive thin film. Self-sensing is encoded by designing the films to generate a photocurrent in response to illuminated light so that no electrical energy is needed for powering the sensor, and photocurrent generated varies with applied strain. First, the self-sensing thin films were fabricated using poly(3-hexylthiophene) (P3HT) and double-walled carbon nanotubes (DWNT). Two different sets of films, ones with and without DWNTs, were fabricated. Second, photocurrent generation was validated. Then, tensile tests were conducted for characterizing their strain sensing performance. Lastly, current-voltage measurements were also obtained for characterizing thin film shunt and series resistance relationships to applied strains. The results showed that photocurrent varied linearly with applied tensile strains, and this was mainly due to the effects of P3HT alignment and shunt resistance changes of the photoactive thin films.

Commentary by Dr. Valentin Fuster
2012;():687-696. doi:10.1115/SMASIS2012-7998.

An approach is presented to incorporate a multi-objective genetic algorithm (GA) optimization strategy for the evaluation of damage within a solid continuum. Through simulated test problems based on the characterization of internal pipe surface geometry (as could potentially be affected by a damage process) from steady-state dynamic measurements of outer surface displacement, the multi-objective GA is shown to provide substantial computational improvement over single-objective strategies. Furthermore, the multi-objective approach consistently traversed the optimization search space to efficiently produce more accurate characterization results and exhibited consistently better tolerance to measurement noise in contrast to the single-objective strategies. In general, the multi-objective approach maintains a high level of diversity in the solution population during the search process, thus being potentially better equipped to avoid local minima during the search process and identify multiple solutions where they exist.

Commentary by Dr. Valentin Fuster
2012;():697-706. doi:10.1115/SMASIS2012-8005.

This paper studies the aeroelastic oscillations of wing-like structures with the aim to detect at an incipient stage the presence of structural cracks. Such oscillations occur normally in certain flight evolutions of aircraft or can be excited by piezoelectric actuators bonded on the wing structure. These oscillations can be used to detect at an early stage the presence of cracks by monitoring the response of several piezoelectric sensors bonded on both sides of the structure during the aeroelastic oscillations. The proposed method of crack detection uses pairs of piezoelectric strip sensors bonded on the opposite sides of the structure and is based on the fact that the presence of a crack causes a difference between the strains measured by the two sensors of a pair. The structural analysis presented in this paper uses a nonlinear model for the cracks and a finite element formulation for the piezoelectric strips coupled with the structure. A 3D panel method developed by the authors is used to determine the unsteady aerodynamic loads acting on the oscillating wing structure. The dynamic analysis in the time domain is performed for the oscillating structures with piezoelectric strips subjected to unsteady aerodynamic loads. In the present work, the efficiency of this crack detection method is studied in realistic situations, by considering the aeroelastic oscillations in flexion and torsion of a wing-like structure which are excited in one of the following modes: (i) the aeroelastic oscillations excited by a pair of piezoelectric actuators bonded on the opposite sides of the structure; (ii) the aeroelastic oscillations excited by the harmonic oscillation of the angle of attack corresponding to the flight in atmospheric turbulence (harmonic gust); (iii) the aeroelastic oscillations generated by a sudden change in the angle of attack or in the airplane velocity due to a pilot control input. The numerical simulations for these cases have been performed by the simultaneous solution of the coupled equations of unsteady fluid flow and of the structure deformation motion, by using a finite element method for the dynamic of the structures with cracks and bonded piezoelectric strips, and a 3D panel method developed by the authors for the calculation of the unsteady aerodynamic loads. These numerical simulations have shown that the presence of a crack in the structure can be efficiently detected at an early stage by monitoring the response of the pairs of piezoelectric sensors.

Commentary by Dr. Valentin Fuster
2012;():707-711. doi:10.1115/SMASIS2012-8010.

In this paper, the experimental sensing results of damage testing using magnetostrictive particulate sensors, embedded in fiber reinforced polymer laminates, are presented. Carbon fiber reinforced polymer (CFRP) laminates (Hexcel AS4/3501-6) are embedded with terfenol-d particles and the ply count is varied to observe the change in the sensing. Sensing is observed using a non-contacting magnetostrictive strain sensor setup. The sensing parameter observed is the voltage induced in the secondary circuit. Two of the three batches presented have laminates that are embedded with .5″×.5″, release agent coated patches that prevent bonding between the terfenol-d and the CFRP layer. The laminate ply count ranges from 2–14 unidirectional plies. Two fabrication methods are used to distribute the particles in the laminate. The experimental results from the three batches reveal that the fabrication technique has a significant effect on the sensing signal. The effect of particle accumulation near the sensor dominates the sensing signal and makes the presence of a delamination difficult to assess. The experiments also show that when the ply count is varied, there is not much variation in the sensing signal.

Commentary by Dr. Valentin Fuster
2012;():713-721. doi:10.1115/SMASIS2012-8013.

Lamb waves are dispersive and multi-modal. Various wave modes make the interpretation of Lamb wave signal very difficult. It is desired that different modes can be separated for individual analysis. In the this paper, we present our studies on the multimodal Lamb wave propagation and wave mode extraction using frequency-wavenumber analysis. Wave spectrum in the frequency-wavenumber domain shows clear distinction among Lamb wave modes being present. This allows separating them or extracting a desired Lamb wave mode through a novel filtering strategy. Thus a single mode Lamb can be identified and extracted for certain types of damage detection in structural health monitoring (SHM). These concepts are illustrated through experimental testing. A scanning laser Doppler vibrometer is used to acquiring the time-space wavefield regarding the multimodal Lamb wave propagation. Then the recorded wavefield was analyzed in frequency-wavenumber domain and decomposed into different wave modes.

Commentary by Dr. Valentin Fuster
2012;():723-729. doi:10.1115/SMASIS2012-8023.

Lamb waves provide arguably the best prospect for achieving a structural health monitoring (SHM) capability with broad diagnostic coverage at sensor densities that are not impractically high. The traditional approach in Lamb wave SHM is to employ a single mode, typically one of the fundamental modes, in a non-dispersive and easily excited regime, which is done largely to simplify the interpretation of the elastic wave dynamics. However, the diagnostic value of an interrogation conducted using only the fundamental modes is limited. In general, higher order modes offer potential for greater sensitivity to structural damage and greater scope for discriminating between different failure mechanisms. This paper reports on experimental work demonstrating an in-situ fibre Bragg grating (FBG) sensing capability for Lamb waves at frequencies of up to 2MHz, an achievement that represents an important step toward developing a more robust and versatile approach to Acousto-Ultrasonic SHM.

Commentary by Dr. Valentin Fuster
2012;():731-737. doi:10.1115/SMASIS2012-8024.

The Australian Defence Science and Technology Organisation (DSTO) is developing Structural Health Monitoring (SHM) approaches for use on air vehicles. This work describes a potential method for measuring quasi-static strains by monitoring the mechanical-load induced capacitive changes in a piezoelectric sensor. This approach may be combined with the well-documented capability of piezoelectric material to measure dynamic-strain, and may hence allow piezoelectric transducers to be used as low-power, single-solution strain sensors. DSTO has experimentally confirmed that the electrical impedance of a Macro Fiber Composite (MFC) piezoelectric transducer changes with varying strain. In particular, a sensitivity of 1.7 mΩ per με has been observed. With accurate transducer modelling, these changes could be used as an indication of the quasi-static strain in the underlying vehicular structure.

Topics: Sensors , Stress
Commentary by Dr. Valentin Fuster
2012;():739-748. doi:10.1115/SMASIS2012-8039.

Strategically located Fiber Bragg Grating (FBG) Sensors have been proposed as an in situ method to increase the signal to noise ratio (SNR) for metallic and composite components. This paper presents a systematic study that investigates the viability of FBG Sensors under high strain rate loading by initially measuring 1D-strains in a compression Hopkinson bar experiment, followed by 2D full-field strain-tensor in impact and blast experiments on plates. Specifically, high strain rates from commercialized FBG Sensors are compared to traditional resistive and semi-conductor based strain gages under various levels of 1D high strain rate loading. In the projectile-plate impact experiments, full-field back-surface strain measured using FBG Sensor arrays are compared with that measured from 3D surface Digital Image Correlation (3D-sDIC) strain measuring technique. Finally, strains in welded steel plates subjected to high explosive discharge are monitored with mounted FBG Sensors on the back surface. From this study, potential improvements in the SNR of FBG Sensors are recommended, and the survivability of these sensors under more complex, dynamic loading is evaluated.

Commentary by Dr. Valentin Fuster
2012;():749-756. doi:10.1115/SMASIS2012-8052.

This paper presents a novel data-driven approach for detecting cracks in reciprocating compressor valves by analyzing vibration data. The main idea is that the time-frequency representation will show typical patterns, depending on the fault state and other variables. The problem of detecting these patterns reliably is solved by taking a detour via two dimensional autocorrelation. This emphasizes the patterns and reduces noise effects, thus identifying appropriate features becomes easier. The features are then classified using well known pattern recognition approaches. The methods performance is validated by analyzing real world measurement data.

Commentary by Dr. Valentin Fuster
2012;():757-764. doi:10.1115/SMASIS2012-8055.

In this paper, a Guided Lamb Wave (GLW) technique using piezoelectric transducers is presented to detect and monitor corrosion damage in an aluminum panel. An electro-chemical etching method was applied to simulate corrosion damage in the test article. A signal processing approach using time-based analysis was demonstrated to measure amplitude change and phase shift of GLW signals captured in the aluminum specimen. Damage Index (DI) for each sensor network was calculated based on the time-domain signal processing method. From the DI information, damage profile maps were produced by using a tomographic algorithm. Corrosion damage location and its size were identified from an integrated damage profile map. The severity of the corrosion damage was examined based on the phase shift of a fundamental anti-symmetric mode (A0) in the GLW signals. The experimental results show the capability of the proposed corrosion monitoring technique based on the GLW approach.

Topics: Waves , Corrosion
Commentary by Dr. Valentin Fuster
2012;():765-771. doi:10.1115/SMASIS2012-8061.

This paper presents an investigation of predictive power and energy modeling of space structures for structural health monitoring (SHM) with piezoelectric wafer active sensors (PWAS). After a review of PWAS principles, the paper developed the multi-physics modeling of pitch-catch PWAS transfer function and discusses and the power and energy transduction between structurally guided Lamb waves and PWAS. The focus is a power and energy transduction analysis between the PWAS and a structure containing multimodal ultrasonic guided Lamb waves. The use of multimodal Lamb waves solution for power modeling is an extension of our previously presented simplified model that considered axial and flexural waves with low frequency approximation. Comparison between the axial and flexural approach and Lamb waves approach was evaluated. Frequency response functions are developed for voltage, current, complex power, active power, etc. The paper ends with summary, conclusion, and suggestion for further work.

Commentary by Dr. Valentin Fuster
2012;():773-782. doi:10.1115/SMASIS2012-8083.

Several ensembles of sweeping diagnostic impulsive forces were measured in a unidirectional carbon-epoxy composite beam modified locally with a soft viscoelastic patch. The spatial uniformity of the typical ensemble of diagnostic signals is addressed by a systematic spatio-temporal coherence analysis in terms of proper orthogonal decomposition (POD) modes. All samples of spanning ensembles are strongly dominated by the same POD mode characterized by a nearly uniform spatial modulation and a sharp triangular pulse time modulation. The higher POD modes have small amounts of energy. They possess an important statistics property: their spatial modulation mean value is nearly zero with standard deviation nearly identical to the nearly uniform value of the dominant POD mode. The nearly uniform spatial distribution of the dominant POD mode is a fuzzy picture of the ideal or nominal one where the impact-generated diagnostic forces should have a time waveform independent of the site of impact. Despite this energy content deficiency, the ensemble of acceleration signals acquired at a fixed point while the beam is excited by an ensemble of sweeping diagnostic forces has very robust POD modal structure. The POD modes show in a clear manner the presence of a soft viscoelastic patch simulating mass modifications. The POD-based coherence analysis of ensembles of diagnostic forces generated in this practical problem is potentially useful for a real-time verification-inspection of the integrity of networks of embedded and surface-mounted actuators and sensors.

Commentary by Dr. Valentin Fuster
2012;():783-790. doi:10.1115/SMASIS2012-8094.

Electromechanical impedance (EMI) method is an effective and powerful technique in structural health monitoring (SHM) which couples the mechanical impedance of host structure with the electrical impedance measured at the piezoelectric wafer active sensor (PWAS) transducer terminals. Due to the electromechanical coupling in piezoelectric materials, changes in structural mechanical impedance are reflected in the electrical impedance measured at the PWAS. Therefore, the structural mechanical resonances are reflected in a virtually identical spectrum of peaks and valleys in the real part of the measured EMI.

Multi-physics based finite element method (MP-FEM) has been widely used for the analysis of piezoelectric materials and structures. It uses finite elements taking both electrical and mechanical DOF’s into consideration, which allows good differentiation of complicated structural geometries and damaged areas. In this paper, MP-FEM was then used to simulate PWAS EMI for the goal of SHM. EMI of free PWAS was first simulated and compared with experimental result. Then the constrained PWAS was studied. EMI of both metallic and glass fiber composite materials were simulated. The first case is the constrained PWAS on aluminum beam with various dimensions. The second case studies the sensitivity range of the EMI approach for damage detection on aluminum beam using a set of specimens with cracks at different locations. In the third case, structural damping effects were also studied in this paper.. Our results have also shown that the imaginary part of the impedance and admittance can be used for sensor self-diagnosis.

Commentary by Dr. Valentin Fuster
2012;():791-798. doi:10.1115/SMASIS2012-8096.

Monitoring of fatigue cracking in steel bridge structures using a combined passive and active scheme has been approached by the authors. Passive acoustic emission (AE) monitoring is able to detect crack growth behavior by picking up the stress waves resulting from the breathing of cracks while active ultrasonic pulsing can quantitatively assess structural defect by sensing out an interrogating pulse and receiving the structural reflections. The dual-mode sensing functionality is pursued by using the R15I ultrasonic transducers.

In the paper, we presented the subject dual-mode sensing on steel compact tension (CT) specimens in a laboratory setup. Passive AE sensing was performed during fatigue loading and showed its capability to detect crack growth and location. At selected intervals of loading cycles, the test was paused to allow for active sensing by pulsing the transducers in a round-robin pattern. Plate waves were excited, propagated and interacted within the structure. Several approaches were proposed to analyze the interrogation data and to correlate the data features with crack growth. Root means square deviation (RMSD) damage index (DI) was found as a good indicator for indicating the overall crack development. Short time Fourier transform (STFT) provided both time and frequency information at the same time. Moreover, wave velocity analysis showed interesting results when crack developed across the transmitter-receiver path.

Commentary by Dr. Valentin Fuster
2012;():799-805. doi:10.1115/SMASIS2012-8109.

The National Research Council of Canada has developed Structural Health Monitoring (SHM) test platforms for load and damage monitoring, sensor system testing and validation. One of the SHM platform consists of two 2.25 meter long, simple cantilever aluminium beams that provide a perfect scenario for evaluating the capability of a load monitoring system to measure bending, torsion and shear loads. In addition to static and quasi-static loading procedures, these structures can be fatigue loaded using a realistic aircraft usage spectrum while SHM and load monitoring systems are assessed for their performance and accuracy. In this study, Micro-Electro-Mechanical Systems (MEMS), consisting of triads of gyroscopes, accelerometers and magnetometers, were used to compute changes in angles at discrete stations along the structure. A Least Squares based algorithm was developed for polynomial fitting of the different data obtained from the MEMS installed in several spatial locations of the structure. The angles obtained from the MEMS sensors were fitted with a second, third and/or fourth order degree polynomial surface, enabling the calculation of displacements at every point. The use of a novel Kalman filter architecture was evaluated for an accurate angle and subsequent displacement estimation. The outputs of the newly developed algorithms were then compared to the displacements obtained from the Linear Variable Displacement Transducers (LVDT) connected to the structures. The determination of the best Least Squares based polynomial fit order enabled the application of derivative operators with enough accuracy to permit the calculation of strains along the structure. The calculated strain values were subsequently compared to the measurements obtained from reference strain gauges installed at different locations on the structure. This new approach for load monitoring was able to provide accurate estimates of applied strains and loads.

Commentary by Dr. Valentin Fuster
2012;():807-815. doi:10.1115/SMASIS2012-8110.

According to U.S. Nuclear Regulatory Commission (NRC) Generic Letter 2008, the gas accumulation in the nuclear emergency core cooling systems is concerned since it may critically damage pipes, pumps and valves. There is a need to detect the inside gas accumulation including the quantification of gas location and volume. In this paper, we propose a in-situ technique for gas detection in a gas tank by using Lamb waves. Lamb wave propagation in a plate-like structure is affected by the boundary conditions. For structures in air or submerged in liquid, wave propagations are different. When the structure is in contact with liquid such as water, wave energy leaks into it from the solid material. Therefore, the way of gas detection is related to the detection of change in wave propagation characteristics. Experimental tests in a steel water tank were conducted and shown the Lamb wave’s response to the water presence. Theoretical study of Lamb waves propagation on a free plate in air and on a plate with one surface submerged in liquid were then conducted and compared. Further investigation to understand the change in Lamb wave propagation when water is present was conducted with frequency-wavenumber analysis. In the frequency-wavenumber space, it was found that a new plate wave mode, quasi-Scholte wave showed up. A0 Lamb mode showed a decreased propagation while S0 Lamb wave showed no changes. The change in the Lamb wave propagation is found to be frequency dependent.

Topics: Waves , Water
Commentary by Dr. Valentin Fuster
2012;():817-821. doi:10.1115/SMASIS2012-8113.

Recent improvements in technology has enabled the use of very sophisticated sensors such as embedded fiber bragg gratings (FBGs) to obtain strain measurements from a variety of structural types. Conventional strain gauges tend to be heavy and bulky. Because of their accuracy, light weight, small size and flexibility these fiber optic sensors have big potential to be used in space exploration and the aerospace industry especially for flying aircraft that have strict weight and size limitations. These strain measurements can be used to predict the deformation shape of aircraft during real-time flights. The development of such methods for monitoring and control can potentially reduce the risk of in-flight breakups, such as that of the Helios Wing.

The Structures, Propulsion, And Control Engineering (SPACE) NASA sponsored University Research Center (URC) of excellence has concentrated in the development of small, lightweight Uninhabited Air Vehicles (UAVs) that have excelled in the area of endurance. Today, the UAV project is focused on the design of a multi-mission multipurpose air system that can operate autonomously. The configuration is a twin boom, pusher, and conventional wing design. In this paper, methods developed by the National Aeronautic and Space Administration (NASA)’s Dryden Flight Research Center for real-time deformation shape prediction of lightweight unmanned flying aerospace structures for the purposes of Structural Health Monitoring (SHM) and condition assessment are investigated. SHM may allow for useful monitoring that would prevent such an event by providing wing shape information and structural monitoring to either a pilot or the flight system, allowing for evasive maneuvers before the breakup would occur. These methods also have the potential for increasing safety, allowing monitoring of structural integrity, detecting damages, and providing real-time flight control feedback. These methods are applied to the SPACE Center UAV for the purpose of assessing the effectiveness of the method and the potential for both SHM and control applications. In this paper, a computational finite element model of the SPACE Center UAV is developed and used to examine the method.

Commentary by Dr. Valentin Fuster
2012;():823-831. doi:10.1115/SMASIS2012-8118.

Horizontal axis wind turbine (HAWTs) structures, throughout the years, have presumed to be of relatively simple construction, but wind-induced aerodynamic vibrations, wind-field conditions, and power requirements tend to lead to the need for increasingly complicated designs. One phenomenon that requires special attention is the gyroscopic or Coriolis effect. In general, blades design codes are written to optimize for lightness and slenderness, but also to withstand excitations at high frequency. As a result, gyroscopic motion derives as a nonlinear dynamic condition in the out-of-plane direction that is difficult to characterize by means of the well-known vibrational theory that has been established for their design and analysis. The present study develops and presents a probabilistic analysis of the precession — gyroscopic — effects of a wind turbine model developed for tapered-swept cross-sections of nt degree with nonlinear variations of mass and geometry along the body of the blade. A dynamic orthogonal decoupling method is utilized to successfully perform the aeroelastic analysis by decoupling the damped-gyroscopic equations of motion, as a result of the addition of Rayleigh damping — symmetric proportional mass and stiffness — within the linear system in study. Results are valid for yaw-free rotor configurations by means of unknown and random (though bounded) yaw rates. Simultaneously, those results can easily be expanded for yaw-controlled mechanisms. The yaw-free assumption presents a higher risk of potential reliability expectations, given the stochastic impairment of the gyroscopic nature that is present for out-of-plane axis motions, requiring special attention at higher frequencies. This impairment becomes particularly troublesome for blade profiles with tapered-swept cross-section variations. This uncertainty can be minimized by incorporating a mathematical framework capable of characterizing properly the yaw action such that gyroscopic effects can be fully interpreted and diagnosed. In summary, the main goal is to decipher the complexity of gyroscopic patterns of flexible rotor blades with complex shape configurations, but also to provide substantial elements to successfully approach yaw-mechanics of tapered-swept rotor blades.

Commentary by Dr. Valentin Fuster
2012;():833-836. doi:10.1115/SMASIS2012-8139.

The University of California at San Diego (UCSD), under a Federal Railroad Administration (FRA) Office of Research and Development (R&D) grant, is conducting research to develop a system for in-situ measurement of the rail Neutral Temperature in Continuous-Welded Rail (CWR). It is known that CWR can break in cold weather and can buckle in hot weather. Currently, there is a need for the railroads to know the current state of thermal stress in the rail, or the rail Neutral Temperature (rail temperature with zero thermal stress), to properly schedule slow-order mandates and prevent derailments.

UCSD has developed a prototype for wayside rail Neutral Temperature measurement that is based on non-linear ultrasonic guided waves. Numerical models were first developed to identify proper guided wave modes and frequencies for maximum sensitivity to the thermal stresses in the rail web, with little influence of the rail head and rail foot. Experiments conducted at the Large-scale Rail NT Test-bed indicated a rail Neutral Temperature measurement accuracy of a few degrees. Field tests are planned at the Transportation Technology Center (TTC) in Pueblo, CO in June 2012 in collaboration with the Burlington Northern Santa Fe (BNSF) Railway.

Commentary by Dr. Valentin Fuster
2012;():837-844. doi:10.1115/SMASIS2012-8145.

In structural health monitoring (SHM), impact detection and characterization techniques often focus on identifying parameters of impact such as the location and velocity of an impacting object. A distributed network of sensors is used to passively detect the mechanical wave created by the impact. Various techniques are used to analyze the signals based on time of arrival, amplitude and phase. A simpler architecture could be used to determine whether an impacting event was benign or caused damage and requires further evaluation. This research focuses on detecting attributes of impact-generated elastic wave signals that are indicative of local damage at the impact site. Waveforms deviate insignificantly for undamaged materials, however, when a material is stressed to plastic deformation or damaged the waveform of propagation through the material is noticeably affected. This change in wave speed may be detectable by SHM sensors, and can be used as an indicator of damage. Low velocity impact experiments were conducted on thin aluminum plates instrumented with piezoelectric and magneto-elastic sensors at various locations. The sensors acquired the initial passage of the impact wave signal before reflections off the boundaries became a significant element. By inspecting the signal for deviations induced by damage (such as plastic deformation), a routine for evaluating damage can be inferred. Further work may correlate features of the signal with damage severity providing an extra level of information in determining the next step in evaluating the damage. Using this approach, it may be possible to evaluate impact damage using limited numbers of passive sensors.

Commentary by Dr. Valentin Fuster
2012;():845-854. doi:10.1115/SMASIS2012-8151.

This paper summarizes the experimental investigations for smart embedded sensing in rotorcraft composite components. The overall objective of this effort was to develop smart embedded sensor technologies for condition based maintenance (CBM) for composite components in Army rotorcraft. This paper presents the results of experimental investigations related to development and maturation of different types of embedded sensing solutions for structural health monitoring of composite components including Fiber Bragg Grating (FBG) sensors, phased and discrete piezoelectric sensor arrays. A discussion is provided relative to embedment of optical fibers into composites, and the results from embedded FBG sensors in a rotorcraft flexbeam subcomponent test specimen with seeded delamination subjected to dynamic loading. Likewise, results are analyzed of surface mounted phased array and embedded smart piezoelectric sensors in the flexbeam subcomponent test specimen with embedded delamination, subjected to fatigue cyclic loading. The paper also summarizes the lessons learned from efforts to nucleate and propagate delamination within composite components under dynamic cyclic loading.

Commentary by Dr. Valentin Fuster
2012;():855-860. doi:10.1115/SMASIS2012-8153.

Deterioration of concrete structures has become a widespread problem with high repair costs. The corrosion of rebar is one of the major causes. GPR has potential for rebar corrosion detection. But the rapid survey generates a large amount of data, which require an automatic approach for effective data processing and information extraction. This paper proposes an automatic process to effectively extract the rebar reflection in the radargram image and estimate the concrete condition above the rebar. The process uses template matching to locate the hyperbola position, image processing to extract hyperbolic region, and algebraic fitting to rapidly estimate hyperbola parameters. The estimated parameters can be used to calculate the wave propagation velocity and relative permittivity in concrete above the rebar, which can be used to further evaluate the concrete condition. The effectiveness of the proposed method is validated using experiment testing.

Commentary by Dr. Valentin Fuster
2012;():861-867. doi:10.1115/SMASIS2012-8154.

Ground penetrating radar (GPR) is one of the most extensively used nondestructive evaluation methods with rapid development in civil engineering in the past few years for its efficiency and high resolution. The evaluation mechanism of concrete materials using GPR is generally based on the reflected signals from the rebar. According to rebar reflections, the corrosion level of the rebar and the material deteriorating condition above the rebar can be deduced. GPR has been successfully applied to shallow subsurface rebar detection in concrete structures. However, very few literatures have addressed the detectability of reflected signal from the deeper layer of rebar. As the result, only a small shallow section of the bridge deck can be evaluated by GPR data so far. In this paper, the detectability of the deeper rebar layer in concrete bridge decks using GPR is investigated with the help of data processing and image processing techniques. The GPR data collected from both a simulation model and a test slab are used to demonstrate the proposed methods and the preliminary results show the reflected signals from the second layer of rebar can be extracted using proposed methods.

Commentary by Dr. Valentin Fuster
2012;():869-877. doi:10.1115/SMASIS2012-8177.

The next-generation design of structural components involves combining multiple functions. The goal of such Multi-functional structures (MFS) is to incorporate various tasks and functions such as structural, electrical and thermal features within a structural housing. The performance and behaviour characteristics of the multi-functional structures can be affected by degradation of any of the sub-components. This can have consequences on the safety, cost, and operational capability. Therefore, the timely and accurate detection, characterization and monitoring of the degradation in these sub-components are major concerns in the operational environment. This calls for Structural Health Monitoring (SHM) as a possible method to improve the safety and reliability of structures and thereby reduce their operational cost. As the application of SHM systems to monitor the status of the MFS increase, it will be increasingly important to determine the durability, reliability, and reparability of the components of SHM system such as sensors. The sensors themselves must be reliable enough so that they do not require replacement at intervals less than the economic lifetime of the structures and components they are monitoring. This is especially important when the deleterious structural changes in the sensor occurs without any discernible change in the structure being monitored In the present work, an assessment is carried out to quantify the degradation in the electric and electromechanical characteristics of polymer composite PZT sensors, under fatigue loading. Changes in the electrical properties of these sensors such as capacitance and inductance have been measured. The strain measurements of the sensor have also been compared to the theoretically calculated strain. The results show that the delineation of structural damage from sensor degradation is possible by monitoring the changes in the key electrical properties of the sensor components such as electrodes and PZT fibers as well as the comparison of experimentally measured and theoretically calculated strain values.

Commentary by Dr. Valentin Fuster
2012;():879-884. doi:10.1115/SMASIS2012-8200.

A novel preparation method of solid state photovoltaic carbon nanotubes (CNT) yarns has been successfully developed by depositing and grafting TiO2 thin films on CNT yarn substrates using a simple sol–gel method and designed for use in structural health monitoring (SHM) applications. The interaligned, ultrastrong and flexible CNYs display excellent electrical conductivity, mechanical integrity and their catalytic properties have been successfully used as working and counter electrodes. The TiO2 nanoparticles have been found to form a homogeneous thin film on the yarn surface, which shows efficient photovoltaic properties with remarkable stability when exposed to simulated solar light (AM 1.5). The yarns’ structure is not altered upon sol-gel treatment and light exposure. The TiO2 film is firmly anchored and the photovoltaic performance is retained even after multiple irradiation cycles. This preparation technique can also be applied to CNT yarn reinforced composite for an innovative in-situ and real-time self damage-sensing properties with infused triboluminescent (TL) materials.

Commentary by Dr. Valentin Fuster
2012;():885-890. doi:10.1115/SMASIS2012-8206.

This paper presents the implementation and characterization of a low power wireless vibration sensor that can be powered by a flash light. The wireless system consists of two components, namely the wireless sensor node and the wireless interrogation unit. The wireless sensor node includes a wireless strain gauge that consumes around 6 mW, a signal modulation circuit, and a light energy harvesting unit. To achieve ultra-low power consumption, the signal modulation circuit was implemented using a voltage-controlled oscillator (VCO) to convert the strain gauge output to an intermediate frequency (IF) signal, which is then used to alter the impedance of the sensor antenna and thus achieves amplitude modulation of the backscattered antenna signal. A generic solar panel with energy harvesting circuit is used to power the strain sensor node continuously. The wireless interrogation unit transmits the interrogation signal and receives the amplitude modulated antenna backscattering, which can be down-converted to recover the IF signal. In order to measure the strains dynamically, a Phase Lock Loop (PLL) circuit was implemented at the interrogator to track the frequency of the IF signal and provide a signal that is directly proportional to the measured strain. The system features ultra-low power consumption, complete wireless sensing, solar powering, and portability. The application of this low power wireless strain system for vibration measurement is demonstrated and characterized.

Topics: Vibration , Batteries
Commentary by Dr. Valentin Fuster
2012;():891-895. doi:10.1115/SMASIS2012-8216.

In this study, we investigate the interactions of Lamb wave A0 mode with different sizes of delaminations in composites using finite element code Abaqus®. According to Lamb wave dispersion curves, the group velocity of A0 mode increases rapidly as the frequency-thickness increases in the relatively low frequency region. In the delamination region, the frequency-thickness product decreases compared to the healthy laminate since the damage causes ply separation at the lamina interface. In the current study this observation is investigated in detail using finite element simulations. The resulting phase delay is analyzed by Empirical Mode Decomposition (EMD) and instantaneous phase approach. Finite element simulations are performed using Abaqus® and signal processing is performed in joint time-frequency domain using Hilbert-Huang Transform (HHT) method. The unwrapped instantaneous phase difference is correlated with the extent of delamination (quantitative level of damage).

Commentary by Dr. Valentin Fuster
2012;():897-902. doi:10.1115/SMASIS2012-8231.

Fiber reinforced composites (FRP) for industrial applications face constantly increasing demands regarding efficiency, reliability and economy. Furthermore, it was shown that FRP’s with tailored reinforcements are superior to metallic or monolithic materials. However, a trustworthy description of load-specific failure behaviour and damage evolution of composite structures can hardly be given, because these processes are very complex and are still not entirely understood. Amongst other things, several research groups have shown that material damages like fiber fracture, delamination, matrix cracking or flaws can be discovered by analyzing the electrical properties of conducting composites, e.g. carbon fiber reinforced plastics (CFRP). Furthermore, it was shown that this method could be used for structural health monitoring or non-destructive testing (NDT) [8–12].Within this work, Magnetic Induction Tomography (MIT), which is a new imaging approach, is introduced into the topic of NDT of CFRP’s. This non-contacting imaging method gains the inner spatial distribution of conductivity of a specimen and depicts material inhomogeneity, like damages, in 2D or 3D images. Numerical and experimental investigations are presented and give a first impression of the performance of this technique. It is demonstrated that MIT is a promising approach for NDT and could be used for fabrication quality control of conductive FRP’s and could potentially be used as a health monitoring system using an integrated setup.

Commentary by Dr. Valentin Fuster
2012;():903-906. doi:10.1115/SMASIS2012-8239.

The paper presents the design, development, and assembly of Structural Health Monitoring (SHM) experiments intended to be launch in space on a sub-orbital rocket flight as well as a high altitude balloon flight. The experiments designed investigate the use of both piezoelectric sensing hardware in a wave propagation experiment and piezoelectric wafer active sensors (PWAS) in an electromechanical impedance experiment as active elements of spacecraft SHM systems. The list of PWAS experiments includes a bolted-joint test and an experiment to monitor PWAS condition during spaceflight. Electromechanical impedances of piezoelectric sensors will be recorded in-flight at varying input frequencies using an onboard data acquisition system. The wave propagation experiment will utilize the sensing hardware of the Metis Design MD7 Digital SHM system. The payload will employ a triggering system that will begin experiment data acquisition upon sufficient saturation of g-loading. The experiment designs must be able to withstand the harsh environment of space, intense vibrations from the rocket launch, and large shock loading upon re-entry. The paper discusses issues encountered during design, development, and assembly of the payload and aspects central to successful demonstration of the SHM system during both the sub-orbital space flight and balloon launch.

Commentary by Dr. Valentin Fuster
2012;():907-916. doi:10.1115/SMASIS2012-8241.

The work presented in this paper provides an insight into the current challenges to detect incipient damage in complex metallic structural components. The goal of this research is to improve the confidence level in diagnosis and damage localization technologies by developing a robust structural health management (SHM) framework. Improved methodologies are developed for reference-free localization of fatigue induced cracks in complex metallic structures. The methodologies for damage interrogation involve damage feature extraction using advanced signal processing tools and a probabilistic approach for damage detection and localization. Specifically, piezoelectric transducers are used in pitch-catch mode to interrogate the structure with guided Lamb waves. A novel time-frequency (TF) based signal processing technique based on the matching pursuit decomposition (MPD) algorithm is developed to extract time-of-flight damage features from dispersive guided wave sensor signals, followed by a Bayesian probabilistic approach used to optimally fuse multi-sensor information and localize the crack tip. The MPD algorithm decomposes a signal using localized TF atoms and can provide a highly concentrated TF representation. The Bayesian probabilistic framework enables the effective quantification and management of uncertainty. Experiments are conducted to validate the proposed detection and localization methods. Results presented will illustrate the usefulness of the developed approaches in detection and localization of damage in aluminum lug joints.

Commentary by Dr. Valentin Fuster
2012;():917-924. doi:10.1115/SMASIS2012-8242.

Impact damage has been identified as a critical form of defect that constantly threatens the reliability of composite structures, such as those used in aircrafts and naval vessels. Low energy impacts can introduce barely visible damage and cause structural degradation. Therefore, efficient structural health monitoring methods, which can accurately detect, quantify, and localize impact damage in complex composite structures, are required. In this paper a novel damage detection methodology is demonstrated for monitoring and quantifying the impact damage propagation. Statistical feature matrices, composed of features extracted from the time and frequency domains, are developed. Kernel Principal Component Analysis (KPCA) is used to compress and classify the statistical feature matrices. Compared with traditional PCA algorithm, KPCA method shows better feature clustering and damage quantification capabilities. A new damage index, formulated using Mahalanobis distance, is defined to quantify impact damage. The developed methodology has been validated using low velocity impact experiments with a sandwich composite wing.

Commentary by Dr. Valentin Fuster
2012;():925-933. doi:10.1115/SMASIS2012-8255.

Increasing complexity of aerospace structures facilitates a growing need for structural health monitoring (SHM) systems capable of real-time active damage detection. A variety of sensing approaches have been demonstrated using embedded ultrasonic sensors such as piezoelectric wafer active sensors (PWAS) and magneto-elastic active sensors (MEAS). Common methodologies consider wave propagation (pitch-catch or pulse-echo) and standing wave (vibration or impedance) techniques with damage detection capabilities dependent upon structural geometry, material characteristics, distance to damage and damage size/orientation. While recent studies have employed damage detection and classification approaches that are dependent on cumulative statistics, this study explores the contribution of sensor parameters and experimental setup variability on the damage detection scheme. The impact of variability in PWAS and MEAS are considered on sensor use in ultrasonic and magneto-mechanical impedance damage detection.

In order to isolate sensor parameters, measurements were conducted with PWAS in free-free boundary conditions. Variability of PWAS parameters was evaluated by measuring the sensors impedance response. An analytical model of PWAS was used to estimate sensor parameters and to determine their variability. Additionally, experiments using MEAS were performed that demonstrate variation of magneto-mechanical impedance during structural dynamic tests. From these experiments the importance of sensor setup is discussed and its contribution into the overall detection scheme is explored.

Topics: Sensors
Commentary by Dr. Valentin Fuster

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