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ASME Conference Presenter Attendance Policy and Archival Proceedings

2011;():i. doi:10.1115/SMASIS2011-NS2.
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This online compilation of papers from the ASME 2011 Conference on Smart Materials, Adaptive Structures and Intelligent Systems (SMASIS2011) 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to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in the ASME Digital Library and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Modeling, Simulation and Control

2011;():1-6. doi:10.1115/SMASIS2011-4923.

Shape memory alloys (SMA) are thermally activated smart materials. Due to their ability to change into a previously imprinted actual shape through the means of thermal activation, they are suitable as actuators for mechatronical systems. Despite of the advantages shape memory alloy actuators provide (lightweight-actuators, lower costs[[ellipsis]]etc.) these elements are only seldom integrated by engineers into mechatronical systems. The reason for this phenomenon is the insufficiently described dynamic behavior, especially at different boundary conditions. Also the lack of empirical data (like fatigue behavior and thermal balances) is a reason why development projects with shape memory actuators lead often to failures. Therefore a need of developing methods, standardized testings of empirical properties and computer aided actuator development systems is motivated. Based on an analysis of energy fluxes into and out of the actuator, a numerical model, implemented in MATLAB/SIMULINK is presented. The numerical model includes also a configuration and design tool which allows simulating different solutions to a problem. Additionally, this paper describes a development method for SMA which is fitted to uniqueness of these smart materials. In conclusion, this paper compares the conventional developing process to the presented method applying a mechatronical SMA-device.

Commentary by Dr. Valentin Fuster
2011;():7-15. doi:10.1115/SMASIS2011-4924.

This paper presents the design, the prototype construction and the experimental testing of a shape memory actuator implementing the concept of elastic compensation put forward in a previous publication by the authors. A two-SMA actuator, compensated by a spring-assisted bistable rocker-arm, is designed theoretically to provide nearly-constant output forces, then it is built and characterized under laboratory conditions. The test results are in good agreement with the theoretical predictions and show that, for given output force, the compensated actuator produces net strokes from 2.5 to 22 times greater than an identical uncompensated actuator. The stroke improvement increases dramatically with the generated output force. Weaknesses of the compensated design are the heavier average stress sustained by the SMA springs, which could impair the fatigue life, and a higher response time.

Topics: Actuators , Design , Testing , Shapes
Commentary by Dr. Valentin Fuster
2011;():17-26. doi:10.1115/SMASIS2011-4930.

This paper reflects back on the author’s research in structural control, originating in the early 1970’s. Upon reflection, in the early 1970’s and even as far as into the 1990’s, it constituted near heresy to suggest that large civil engineering structures could be candidates for control. The previous techniques of the 1930’s of building structures with masonry and stone were then being phased out, being replaced with lighter and more cost-effective steel-framed structures. The newer designs then emerging in the late 1960’s and early 1970’s were steel-framed structures that functioned as cantilevered tubes. The characteristic designs emerging included an absence of stone or masonry especially around the steel pillars, glass or similar non-load bearing cladding. Additionally, interior walls were non-load bearing affording more spacious rooms as well as affording the occupants the ability to reposition interior walls as desired. Moreover, the newly emerging steel-framed structures with increased compliance properties were increasingly prone to wind excitations as compared to the prior generation of structures. As a consequence, a number of these newer structures exhibited increased sway and other related dynamic behaviors. My mechanical engineering servomechanisms background included groundings in observability, controllability, as well as control of spatially distributed systems. Therefore, I felt confident then that control systems theoretic methods held promise to produce favorable and cost-effective results if properly applied to problematic civil engineering structures. That confidence still remains. I also realized the critical importance of being able to design structures in advance to be controlled, as opposed to the less desirable situation of dealing with an after-the-fact retrofit of an existing problematic structure. This paper affords an opportunity for the author to provide his anecdotal recollections and afterthoughts. Because the story to be told is of personal recollections, it is presented in first person.

Commentary by Dr. Valentin Fuster
2011;():27-33. doi:10.1115/SMASIS2011-4935.

The properties of NiTi based Belleville springs have been analyzed by numerical simulations and experimental tests. In particular, the unique features of these components, mainly due to stress and/or thermally-induced phase transformation mechanisms, have been preliminary analyzed by finite element simulations, by means of a special constitutive model for NiTi alloys. Subsequently, Belleville washers have been manufactured from a commercial pseudoelastic NiTi alloy, by disk cutting and a successive shape setting by a thermo-mechanical treatment. Furthermore, the thermo-mechanical response of the washers, in terms of isothermal force-deflection curve and thermal cycles between phase transition temperatures, has been experimentally analyzed. The experimental observations confirm the numerical results, i.e. the springs show a significant effect of the temperature on the characteristic curve as well as a marked hysteretic behavior. Due to these properties NiTi Belleville washers can be used as smart elastic elements, i.e. with tunable stiffness and damping, as well as solid state actuators. Furthermore, the marked hysteretic behavior of the washers, in terms of force-deflection response, is particularly useful for the realization of vibration and/or seismic absorbers.

Commentary by Dr. Valentin Fuster
2011;():35-41. doi:10.1115/SMASIS2011-4939.

This paper presents the development of Prandtl–Ishlinskii hysteresis model and tracking control of piezoelectric stack actuator with severe hysteresis. Classic Prandtl–Ishlinskii model which is a linearly weighted superposition of many backlash operators with different threshold and weight values, inherits the symmetry property of the backlash operator at about the center point of the loop formed by the operator. To describe the asymmetric hysteresis of piezoelectric stack actuators, two sets of weighting parameters are proposed to modify the weight values of backlash operators in the ascending and descending branches. Hence, two weight values correspond to one operator. Each pair of the weight values slides smoothly from one to another when the output of their corresponding operator is at a desired threshold. A feedforward controller was designed based on the modified model, which can precisely describe the inverse of the hysteresis. Then the modified model and the hysteresis of the piezoelectric stack actuator cancelled each other. A feedback controller was design to compensate for actuator creep. Different types of signal are used to test the feedforward and feedback controllers. The results show that the proposed hysteresis control scheme which combines feedforward and feedback controllers greatly improves the tracking accuracy of the piezoelectric actuator and the error is less than 0.15 μm.

Commentary by Dr. Valentin Fuster
2011;():43-50. doi:10.1115/SMASIS2011-4942.

The goal of this study is to provide shock mitigation in an active (or semi-active) shock absorption system, typically comprising of a spring, and an adjustable stroking load element, such as an adaptive energy absorber (EA) or semiactive damper element, in which the stroking load can be electronically adjusted in real-time. Typically, there is a maximum limiting stroking load that can be accommodated by a payload. Thus, a Constant Stroking Load Regulator (CSLR) is developed that accepts sensor feedback, and then selects control gains that result in the energy absorber (EA) providing the required controllable stroking load. A key benefit of this regulator is that it is capable of adapting to a varying range of payload mass, impulse types, and impulse excitation levels. The payload mass is measured and used as a control input parameter. The measured impact velocity is used to determine the impulse acceleration level by assuming an impulse profile, which tends to be application-specific. Finally, the required constant stroking load is determined using a physics-based model. The CSLR is designed to achieve a “soft landing” such the payload comes to rest when the available stroke is used completely, in order to minimize the stroking load and thereby minimize the potential for payload damage. The CSLR methodology was then experimentally validated for a representative occupant protection system consisting of a seat suspension with an adaptive stroking element, which in this case was a magnetorheological energy absorber (MREA). A MREA was used as the stroking element because its stroking load can be adjusted electronically. To validate the CSLR strategy, experimental drop tests were conducted for two different payloads. The impact velocity was 10.3 ft/s (3.15 m/s) and the acceleration profile was a 50 ms duration half-sine pulse. The constant stroking load was pre-calculated as a function of payload mass and initial velocity. During each drop test, the required stroking load was supplied to the MREA in order to achieve a “soft landing.” The CSLR was successfully demonstrated under laboratory conditions. These tests demonstrated feasibility of using the CSLR, in conjunction with a MREA as the stroking element.

Commentary by Dr. Valentin Fuster
2011;():51-54. doi:10.1115/SMASIS2011-4943.

Many systems must operate in the presence of bounded perturbations. In this paper we present a new robustness result for nonlinear systems. We use this result to show that for bounded perturbations, a simple adaptive controller with leakage can produce output regulation to a neighborhood with radius dependent upon the size of the perturbation. This regulation occurs in the presence of persistent disturbances and the convergence is exponential.

Commentary by Dr. Valentin Fuster
2011;():55-63. doi:10.1115/SMASIS2011-4953.

Actuators based on dielectric elastomers are a promising technology in robotic and mechatronic applications. Up to now, the practical electro-mechanical response and controllability of actuators based on dielectric elastomers are limited by the inadequacy of the employed driving circuits, which are based on voltage-regulated converters. In order to circumvent the aforementioned activation issues, the design procedure of a novel activation strategy for controlling dielectric elastomer actuators is presented in this article. The proposed electronic driver derives from the flyback converter topology and it is able of delivering to the dielectric elastomer actuator middle-frequency, current-pulse trains dependent on the duty-cycle value. The driver’s transformer, switching and protection circuit components design and optimization are based on an estimation of the dielectric elastomer actuator’s electrical parameters. The design of the transformer is crucial for the actuator’s performance and energy efficiency, meanwhile the driver’s switching and protection circuit components are important for the appropriate driver functioning and safety operation. The reported experimental results show that the proposed electronic driver performances are in accordance with the driver’s design.

Commentary by Dr. Valentin Fuster
2011;():65-72. doi:10.1115/SMASIS2011-4970.

With low driving voltage (<5V) and the ability to be operated in aqueous environments, ionic polymer-metal composite (IPMC) materials are quickly gaining attention for use in underwater applications. There are, however, drawbacks to IPMCs, including the “back relaxation” effect. Specifically, when subjected to a DC input (or an excessively slow dynamic input), the IPMC actuator will slowly relax back toward its original position. There is debate over the physical mechanism of back relaxation, although one prevalent theory describes an initial current, caused by the electrostatic forces of the charging electrodes, which drives water molecules across the ion-exchange membrane and deforms the IPMC. Once the electrodes are fully charged, however, the dominant element of the motion is the osmotic pressure, driving the water molecules back to equilibrium, thus causing back relaxation. A new method to mitigate back relaxation is proposed, taking advantage of controlled activation of patterned (sectored) electrodes on the IPMC. Whereas previous approaches to correct back relaxation rested on an increase of input voltage which can lead to electrolysis, subsequently damaging the material, this method involves only proper control of isolated electrodes to compensate for the back relaxation and does not require sensor feedback. An electromechanical model of the actuator is used to guide the design of these input signals, and the feasibility of using electrode patterning to mitigate back relaxation is demonstrated. Without reaching electrolysis, an IPMC is able to maintain its position for approximately 30 seconds. Compared to a simple step response, the rate of relaxation is reduced by 94% and the maximum error is reduced by 64%.

Commentary by Dr. Valentin Fuster
2011;():73-78. doi:10.1115/SMASIS2011-4972.

Many recent advances in electroelastic energy harvesting have benefitted from the use of nonlinear effects that reshape the potential well of the device and lead to bandwidth improvement. In this letter, the merits of using impact barriers for potential well shaping and bandwidth improvement in nonlinear electroelastic energy harvesters are considered. A bistable piezoelectric cantilever beam with symmetrically placed impact barriers is developed and tested through numerical simulation and experiment. Preliminary results indicate that impact barriers prolong the stability of large-amplitude attractors across a wider frequency range than an equivalent nonimpacting device, albeit at the expense of a greater peak amplitude. A mathematical model and simulation parameters are provided, and the system is investigated through both numerical simulation and experiment. A nontrivial relationship between barrier placement and the linewidth of the frequency response was observed in experiments. Future work will seek to improve the mathematical model to more accurately capture the behavior seen in the experiment.

Commentary by Dr. Valentin Fuster
2011;():79-88. doi:10.1115/SMASIS2011-4974.

A novel three-dimensional robotic surface is devised using triangular modules connected by revolute joints that mimic the constraints of a spherical joint at each triangle intersection. The finite element method (FEM) is applied to the dynamic loading of this device using three dimensional (6 degrees of freedom) beam elements to not only calculate the cartesian displacement and force, but also the angular displacement and torque at each joint. In this way, the traditional methods of finding joint forces and torques are completely bypassed. An effiecient algorithm is developed to linearly combine local mass and stiffness matrices into a full structural stiffness matrix for the easy application of loads. An analysis of optimal dynamic joint forces is carried out in Simulink® with the use of an algebraic Ricatti equation.

Commentary by Dr. Valentin Fuster
2011;():89-95. doi:10.1115/SMASIS2011-4976.

Piezoactuators exhibit hysteresis and dynamic effects which often cause significant positioning error in a wide variety of motion control applications, especially in applications where the reference trajectory is periodic in time, such as the raster motion in scanning probe microscopy. A feedback-based approach known as repetitive control (RC) is well-suited to track periodic reference trajectories and/or to reject periodic disturbances. However, when an RC is designed with a linear dynamics model and subsequently applied to a system with hysteresis, stability and good tracking performance may not be guaranteed. In this work, the effect of hysteresis on the closed-loop stability of RC is analyzed. In the analysis, the hysteresis effect is represented by the Prandtl-Ishlinskii hysteresis model. Using this model, stability conditions are provided for an RC designed for piezoelectric actuators which commonly exhibit hysteresis. The approach is applied to a custom-designed piezo-driven nanopositioner for tracking periodic trajectories.

Commentary by Dr. Valentin Fuster
2011;():97-106. doi:10.1115/SMASIS2011-4987.

Shape Memory Alloy (SMA) actuator wires are often discussed in the context of multi-functional materials. This is because the temperature-induced phase transformation causes a significant (∼4%) contraction and a corresponding change in resistance. When a restoring force such as a pre-stretched spring is placed in series with an SMA wire, the contraction creates a repeatable actuation force, and the resistance provides measurement of the strain the wire, or structural (spring) deflection. Work has already been presented to demonstrate the stress, strain, and resistance characteristics of a single SMA actuator wire in series with a spring flexure under different pre-stresses and heating power input frequencies. Also, a method has been presented to linearly approximate the resistance vs strain characteristic, thus providing a direct mapping from SMA wire resistance to flexure deflection. This mapping method has been tested in the context of a simple PID controller used to position a flexure in series with a single SMA. This paper expands previous work by characterizing a system consisting of a spring flexure in series with two opposing SMA wires. Such an opposing SMA configuration is relevant to embedded SMA applications and gives the potential to increase cycling frequency by providing an active restoring force. The characterization results show that coupling SMA wires introduces alters the resistance vs strain characteristics of both wires because the second SMA wire essentially becomes a non-linear, hysteretic spring in series with the first. However, knowledge of the physics behind the complicated behavior enables sensible calibration schemes to be developed to accurately map resistance to strain for simultaneous sensing and actuating applications.

Commentary by Dr. Valentin Fuster
2011;():107-114. doi:10.1115/SMASIS2011-4992.

Buckling dielectric elastomer actuators are special type of electro-mechanical transducers that exploit electro-elastic instability phenomena to generate large out-of-plane axial-symmetric deformations of circular membranes made of non-conductive rubbery material. In this paper a simplified explicit analytical model and a monolithic finite element model are described for the coupled electro-mechanical analysis and simulation of buckling dielectric elastomer membranes which undergo large electrically induced displacements. Experimental data are also reported which validate the developed models.

Commentary by Dr. Valentin Fuster
2011;():115-122. doi:10.1115/SMASIS2011-4995.

Harvesting vibration energy using piezoelectric materials has gained considerable attention over the past few years. Typically, a piezoelectric energy harvester is a unimorph or bimorph cantilevered beam which undergoes base vibration. The focus of this paper is to compare the Euler-Bernoulli model and the Timoshenko model, which are both used for modeling the vibration-based energy harvester. Procedures of deriving the electro-mechanical equation of motion are provided, following exact expressions for the electrical output in two models. Parametric case studies are carried out in order to compare the frequency response of two models. Simulation results show that there is a great difference between Euler-Bernoulli model and Timoshenko model at low length-to-thickness aspect ratio. Such difference diminishes and becomes negligible as aspect ratio increases. It is shown that for the design of piezoelectric energy harvester with small aspect ratio, Timoshenko model can be more accurate than Euler-Bernoulli model in predicting the system behavior.

Topics: Modeling
Commentary by Dr. Valentin Fuster
2011;():123-132. doi:10.1115/SMASIS2011-4999.

The paper presents the design equations for an on-off shape memory alloy actuator under an arbitrary system of external constant forces. A binary SMA actuator is considered where a cursor is moved against both conservative and dissipative force which may be different during the push or pull phase. Three cases are analyzed and differentiated in the way the bias force is applied to the primary SMA spring, using a constant force, a traditional spring, or a second SMA spring. Closed-form dimensionless design equations are developed, which form the basis of a step-by-step procedure for an optimal design of the whole actuator.

Commentary by Dr. Valentin Fuster
2011;():133-138. doi:10.1115/SMASIS2011-5000.

Shape memory alloy (SMA) used as electrically controlled on-demand actuators provide engineers new opportunities to create lighter automated components and devices in vehicles due to their compact size, silent operation, and inherently low mass. Outstanding and critical issues are cost-effective and robust control and protection of the SMA actuator element within the device to achieve long lasting service. SMA responds autonomously to external conditions such as temperature and stress and exhibit many property changes during excitation, but many current devices only use SMA as compact actuators; not making use of their intrinsic sensing capabilities. Inherent SMA property changes during use can provide significant utility for improved optimal control strategies. The motivation for this work is to create a robust control method for electrically controlled SMA actuation to simplify device implementation and improve reliability by using intrinsic material property changes. The current work demonstrates the use of electrical resistance feedback in an integrated controller to allow reduction of parasitic mass, cost, and complexity in 2-position devices. Using signal processing and algorithm logic states, we create virtual sensors that successfully identify start of the actuation, end of actuation, reset, and stress overload events. Using electrical resistance to sense the start of actuation allows successful/repeatable performance over a wide range of environmental conditions. Sensing the end of actuation and reset readiness prevents overheating and allows for shorter actuation cycle times, respectively without additional position and state sensors. While many previous efforts have examined the use of resistance in control schemes, one critical need not addressed in previous controllers is the ability to detect stress overload of the SMA during excitation. To protect against unintentional blocked deployment, many current devices include bulky mechanical overload protection systems that prevent stress spikes and SMA damage accumulation. Using resistance feedback, we demonstrate the detection of stress overloads thus extending device lifetime without the need for external mechanisms. The time derivative of the electrical resistance, logic state of the controller, and detection and use of peak/valley widths and thresholds define control events. These events become software based sensors that can augment or replace dedicated external sensors. Software based sensors were successfully employed to control an SMA wire actuator under various environmental temperatures and stress conditions. The control algorithm is not affected to changes in electrical contact resistance, material degradation and other noise sources yielding a powerful method for simple control of two position SMA devices without the need of external sensors.

Commentary by Dr. Valentin Fuster
2011;():139-144. doi:10.1115/SMASIS2011-5005.

This paper presents power generation performance of unimorph PZT (lead zirconate titanate) cymbal harvesters optimally designed for the power requirements of a specific application. Proof-of-concept work has shown that the traditional cymbal design can be adapted to a new design that is capable of sustaining higher mechanical loads by replacing the piezoelectric plate with a unimorph circular piezoelectric diaphragm between the metal end caps. The unimorph circular diaphragm is constructed by bonding PZT to a steel substrate to provide increased strength. Additional work was performed to prepare the new cymbal design for large-scale implementation in a variety of applications. The parameters that affect energy harvesting performance for the cymbal structure are first optimized by parametric studies to produce optimum generated energy from a specific range of applied cyclic forces. Key parameters in the unimorph PZT cymbal design include the material properties and the dimensions of the end caps, the ratios of the diameters of the unimorph disc and the end cap cavity, and thickness ratio of the PZT layer and the substrate. Based on the optimized unimorph PZT cymbal structure, a specimen was then fabricated and tested on the load-frame to validate analytically predicted energy generating performance. The specimen was tested under a 1 Hz cyclic load of up to 2,100 N. The measured open circuit output voltages for two different load inputs were in accordance with the analytical prediction.

Commentary by Dr. Valentin Fuster
2011;():145-154. doi:10.1115/SMASIS2011-5008.

This study addresses the formulation of feedback controllers for stochastically-excited vibratory energy harvesters. Maximizing power generation from stochastic disturbances can be accomplished using LQG control theory, with the transducer current treated as the control input. For the case where the power flow direction is unconstrained, an electronic drive capable of extracting as well as delivering power to the transducer is required to implement the optimal controller. It is demonstrated that for stochastic disturbances characterized by second-order, bandpass-filtered white noise, energy harvesters can be passively tuned such that optimal stationary power generation only requires half of the system states for feedback in the active circuit. However, there are many applications where the implementation of a bi-directional power electronic drive is infeasible, due to the higher parasitic losses they must sustain. If the electronics are designed to be capable of only single-directional power flow (i.e., where the electronics are incapable of power injection), then these parasitics can be reduced significantly, which makes single-directional converters more appropriate at smaller power scales. The constraint on the directionality of power flow imposes a constraint on the feedback laws that can be implemented with such converters. In this paper, we present a sub-optimal nonlinear control design technique for this class of problems, which exhibits an analytically computable upper bound on average power generation.

Commentary by Dr. Valentin Fuster
2011;():155-162. doi:10.1115/SMASIS2011-5010.

This paper presents model reference adaptive control (MRAC) of underwater vehicle propelled by the Ionic polymer metal composite (IPMC) actuator. Trajectories of the vehicle are controlled by simultaneously controlling the bias and amplitude of the sinusoidal voltage applied to the IPMC actuator attached at the real end of the vehicle. It is assumed that the system parameters as well as high frequency gain matrix are unknown. Using Lyapunov stability theory and factorization of the high frequency gain matrix, an adaptive output feedback control is designed for trajectory control of a heading angle and a speed of the vehicle. In the proposed approach, SDU (Square Diagonal and Upper triangular matrix) decomposition of the high frequency gain (HFG) matrix is used. Only signs of the leading principle minors of the HFG matrix are assumed to be known. Simulations results are presented to show that precise trajectory control of the heading and speed is achieved in spite of the coupling between controlled variables.

Topics: Vehicles , Feedback
Commentary by Dr. Valentin Fuster
2011;():163-169. doi:10.1115/SMASIS2011-5011.

Ionic polymer-metal composites (IPMCs) are a novel class of soft sensing and actuation materials with promising applications in robotic and biomedical systems. In this paper we present a model for nonlinear electrical dynamics of IPMC actuators, by applying perturbation analysis on the dynamics-governing partial differential equation (PDE) around a given bias voltage. By approximating the steady-state electric field under the bias with a piecewise linear function, we derive a linear PDE for the perturbed charge dynamics, which has piecewise constant coefficients and coefficients linear in the spatial variable. Through power series expansion, we solve the PDE to get the charge distribution up to any prescribed order. The perturbed electric field and current are subsequently obtained, which results in a bias-dependent impedance model. This model captures the nonlinear nature of the IPMC electrical dynamics, and degenerates to the linear model when the bias is zero. Simulation results are presented to illustrate the modeling approach.

Commentary by Dr. Valentin Fuster
2011;():171-177. doi:10.1115/SMASIS2011-5025.

The Green Rotorcraft project (part of Clean Sky JTI) is studying the Gurney flap as a demonstrator of a smart adaptive rotorblade. Deployment systems for the Gurney flap need to sustain large centrifugal loads and vibrations while maintaining precisely the displacement under aerodynamic loading. Designing such a mechanism relies on both the actuation technology and the link which transmits motion to the control surface. Flexible beams and piezoelectric patch actuators have been chosen as components to design this mechanism. Flexible beams are providing an hinge-less robust structure onto which the piezoelectric actuators are bonded. A candidate topology is determined by investigating the compliance of a simple wire structure with beam elements. A parametrized finite element model is then built and optimized for displacement and force through surrogate optimization. The whole process does not requires many finite element analyses and quickly converge to an optimized mechanism.

Topics: Hinges , Optimization
Commentary by Dr. Valentin Fuster
2011;():179-188. doi:10.1115/SMASIS2011-5027.

Constant-Force actuators based on Dielectric Elastomers (DE) can be obtained by coupling a DE film with particular compliant frames whose structural properties must be carefully designed. In any case, the practical achievement of a desired force profile can be quite a challenging task owing to the time-dependent phenomena which affect the DE electromechanical response. Within this scenario, a hyper-viscoelastic model of a rectangular Constant-Force actuator is reported. The model, based on the Bond Graph formalism, can be used as an engineering tool when designing and/or controlling actuators which are expected to work under given nominal conditions. Numerical simulations are provided which predicts the system response to fast changes in activation voltage and actuator position as imposed by an external user.

Commentary by Dr. Valentin Fuster
2011;():189-198. doi:10.1115/SMASIS2011-5030.

In this manuscript, we propose a technique to harvest energy from excitation sources that possess two frequency components: a fundamental component with large energy content, and a super-harmonic component with smaller energy content at twice the fundamental component. Excitations of this nature are common in the environment due to inherent nonlinearities in the dynamics of the excitation source. Normally, two separate energy harvesters are needed to extract the energy at each frequency; however, this paper discusses a single cantilevered piezoelectric vibratory energy harvester (VEH) that exploits the parametric amplification phenomenon to scavenge energy from both frequencies by varying the tilt angle between the axis of the harvester and the direction of the excitation. To investigate the efficacy of the proposed concept, the equations governing the electromechanical dynamics of the harvester are derived. The resulting partial differential equations and associated boundary conditions are then reduced to a single-mode Galerkin based reduced-order model. Analytical expressions for the steady-state output power across a purely resistive load are obtained using the method of multiple scales. Results indicate that percentage improvement in the output power depends on the excitation’s parameters, the tilt angle, and the mechanical damping ratio. It is observed that there is an optimal tilt angle at which the flow of energy from the environment to the electric load is maximized. Furthermore, when the mechanical damping ratio is small, significant enhancement in the output power is attainable even when the magnitude of the super-harmonic is very small when compared to the fundamental component. Such findings reveal that, under certain conditions, parametric amplification can be utilized to enhance the output power of a VEH especially for micro-scale applications where the damping ratio can be easily controlled. Experimental results are presented to validate the theoretical concepts.

Commentary by Dr. Valentin Fuster
2011;():199-206. doi:10.1115/SMASIS2011-5042.

A nonlinear piezoelectric wind energy harvester is proposed which operates at low wind speeds and is not sensitive to the speed of the gusts. The piezoelectric transduction mechanism is used instead of DC generators to eliminate the gearbox in the windmill and thus reduces the friction. The reduced friction facilitates operation of the windmill at low wind speeds. Permanent magnets have been placed in the blade part of the windmill. The magnets axially repel another set of magnets which are positioned at the tip of the piezoelectric beams. As a result, when the rotating magnets pass over the piezoelectric beams they excite the beams and affect the type of their vibrations. The nature of excitations in the proposed design is therefore both parametric excitations and ordinary excitations. The nonlinear magnetic axial force makes the vibrations of the beams nonlinear and can make the beams bi-stable. This phenomenon is utilized to enhance the power output and to improve the robustness of the power production. Two designs are presented which incorporate parametric and ordinary excitations to generate electric power. The performance of each design is examined through experimental investigations.

Topics: Wind energy
Commentary by Dr. Valentin Fuster
2011;():207-214. doi:10.1115/SMASIS2011-5050.

In this paper, an analog velocity feedback controller is considered for active vibration suppression of a thin plate for attenuation of sound levels in the frequency range of 0–100 Hz. The active control methods can be applied to interior cavity noise reduction, as encountered for instance in automotive applications. For that purpose, a simplified experimental vibro-acoustic cabin model was built in our laboratory and developed methodologies are demonstrated on the set-up. The set-up includes a rectangular box (1 × 1 × 2 m) which is separated with a flexible thin plate (1 × 1 × 0.001 m) to obtain two enclosed cavities: the passenger compartment (PC) and the engine compartment (EC). The vibration control is applied only on the flexible plate since the walls enclosing the cavities are made of more rigid material (wood filled concrete). By employing piezoelectric patch as actuator and laser doppler vibrometer as vibration sensor, an analog proportional velocity feedback controller is designed and built experimentally for suppressing the low-frequency modes of the flexible plate. In order to attenuate only lower-frequency structural modes of the thin panel, pre-filters are also included in analog circuit. The vibration of thin plate and sound in the passenger compartment is measured for controller-inactive and active cases while disturbing the thin plate via shaker. By measuring vibration and sound response, closed and open loop experimental frequency responses are obtained and presented. The aim of this experimental study is to investigate performance of active vibration control applications on acoustic attenuation as the first step towards robust structural acoustic control.

Commentary by Dr. Valentin Fuster
2011;():215-222. doi:10.1115/SMASIS2011-5054.

Due to the recent advancement in wireless technology, the demand on power sources has increased dramatically. Personal electronics like cell phones, mp3 or electronics devices like wireless keyboard, both are dependent on batteries as the source of energy. However, with the ever-shrinking electronics, existing battery technology imposes a limitation on size, weight and often times maintenance. Technology like energy harvesting from ambient sources like solar or vibration is believed to play a critical role in these devices and can act as power source for uninterrupted operation. In this paper we discuss, energy harvesting from personal devices like cell phones or keyboards. Since, most of the signal generated is in the form of pulse, an energy harvesting circuit for energy extraction from pulse is discussed. Conventional energy harvesting circuits for this kind of signal are also evaluated in terms of efficiency.

Commentary by Dr. Valentin Fuster
2011;():223-229. doi:10.1115/SMASIS2011-5055.

In this paper network methods and Finite-Element methods are combined to optimize the design of a piezoelectric sound generator. The FE-model is used to determine network parameters of the transducer model and finally the behavior of the chosen design variant. For the design optimization the network model is used. One design variant reaches a nominal sound pressure level of 93 dB at a center frequency of 497 Hz and a bandwidth of 242 Hz in the simulation. The computation of the dynamic behavior of a single design variant using the network model was approx. 600k times faster than using the FE-model.

Commentary by Dr. Valentin Fuster
2011;():231-239. doi:10.1115/SMASIS2011-5063.

A class of active polymers that respond mechanically to ultraviolet and visible light is studied using an advanced fluid-structural interaction model that photomechanically couples three dimensional, unsteady Navier-Stokes equations with a plate model of liquid crystal networks. These materials have been shown to exhibit fast response times that scale with the resonant frequency of the polymer film structure. Moreover, these films are classified as glassy (modulus ∼ 1GPa) and thus have demonstrated flapping behavior on the order of 100Hz or greater in millimeter length films. A variety of applications include micro-propulsion systems for insect size aircraft and biomedical actuators for micro-manipulation of biological materials. A critical challenge in utilizing these materials is explored here by quantifying photomechanical performance and efficiency under unsteady, aerodynamic loads. To quantify the active material performance, we utilize detailed fluid-structural computational methods that couple light input energy, strain energy of the photomechanical film, and interactions with ambient air. The effect of ambient air pressure is found to have a significant impact on photomechanical performance.

Commentary by Dr. Valentin Fuster
2011;():241-249. doi:10.1115/SMASIS2011-5073.

Piezoelectric materials exhibit hysteresis in the field-strain relation at essentially all drive levels. Furthermore, this non-linear relation is dependent upon both prestresses and dynamic stresses generated during employment of the materials. The accurate characterization of this nonlinear and hysteretic material behavior is critical for material characterization, device design, and model-based control design. In this paper, we will discuss the characterization of hysteresis using the homogenized energy model (HEM) framework. At the mesoscale, energy relations characterizing field and stress-dependent 90 and 180 degree switching are used to develop fundamental kernels or hysterons. Material and field nonhomogeneities are subsequently incorporated by assuming that certain parameters are manifestations of underlying densities. This yields a macroscopic model that accurately characterizes the fundamental material behavior yet is sufficiently efficient for optimization and control implementation. Attributes of the model will be illustrated through comparison to experimental data.

Commentary by Dr. Valentin Fuster
2011;():251-260. doi:10.1115/SMASIS2011-5097.

Intelligent materials have been the subject of research for many years. Shape memory alloys (SMAs) are a type of intelligent material that has been targeted for many different uses; such as actuators, sensors and structural supports. SMAs are attractive as actuators due to their large energy density. Although a great deal of information is available on the axial load capacity and on the tip force for SMA tweezer-like devices, there is not enough information about the load capacity at mid-span, especially at the macro-level. Imposed displacement at mid-span experimental evaluation of an SMA beam in the austenitic and martensitic regimes has been studied. To this end, a specimen of near equi-atomic nitinol was heat-treated (shape set) into a ‘U’ shape and loaded into a custom test fixture such that the boundary conditions of the beam are approximated as roller-roller; and the sample was deformed at different temperatures while reaction forces were measured. The displacement is near maximum displacement of the U shape without causing a change in concavity, thus full-scale capacity is shown. Additionally, Unified Model (finite element) predictions of the experimental response are also presented, with good agreement. Due to the robust nature of the Unified Model, geometric parameter variations (wire diameter and radius of curvature) were then simulated to encompass the design envelop for such an actuator. The material properties needed as inputs to the Unified Model were obtained from constant temperature tensile tests of a specimen subjected to the same heat treatment (shape set straight). The resultant critical stresses were then extracted using the tangent method similar to the one described in ASTM F-2082. It is worth noting that the specimen was trained before the stress value extraction, but the transversely loaded specimen was not trained due to the difficulty involved (inherent uneven stress distribution). The contribution of this work is the presentation of experimental results for transverse (mid-span) loading of a nitinol wire and the simulation results allowing for design of a proper actuator with known constraints on force, displacement or temperature (2 of 3 needed). In other words, this work could be used as a type of 3D look-up table; e.g. for a desired force/displacement, the required temperatures are given. Future work includes developing a sensor-less control strategy for simultaneous force/displacement control.

Commentary by Dr. Valentin Fuster
2011;():261-270. doi:10.1115/SMASIS2011-5099.

A negative capacitance shunt is a basic, analog, active circuit electrically connected to a piezoelectric transducer to control vibrations of flexural bodies. The electrical impedance of the negative capacitance shunt modifies the effective modulus of the piezoelectric element to reduce the stiffness and increase the damping which causes a decrease in amplitude of the vibrating structure to which the elements are bonded. The negative capacitance circuit is built around a single operational amplifier using passive circuit elements. To gain insight into the electromechanical coupling, the power consumption of the op-amp and the power dissipated in the resistive element are measured. The power output of the op-amp increases for increasing control gain of the negative capacitance. The power characteristics of the shunt are compared to the reactive input power analysis developed in earlier work.

Commentary by Dr. Valentin Fuster
2011;():271-277. doi:10.1115/SMASIS2011-5103.

This research experimentally investigates the operation of several aeroelastic flutter energy harvesters in an array. In order for such a wind energy harvesting array to operate effectively, it is important to understand the interaction between neighboring power harvesters including downstream wake effects, and how these interactions can be leveraged to maximize the output of the system. The fluttering motion of the energy harvester imparts an unsteady wake into the flow downstream of the device. Wind tunnel experiments with a pair of flutter energy harvesters show that this wake structure has significant effects on the oscillation amplitude, frequency, and power output of the trailing device. These wake interaction effects are shown to vary with the stream-wise and cross-stream separation distance between the two devices. At some separations, an advantageous frequency lock-in between the two devices occurs. When this occurs, the wake of the leading device adds constructively with the trailing device, causing larger oscillation amplitudes and higher power output in the trailing device. Experiments to characterize this variation in power output due to these wake interaction effects and to determine the optimal spacing of the energy harvesters are presented and discussed.

Commentary by Dr. Valentin Fuster
2011;():279-288. doi:10.1115/SMASIS2011-5107.

Many multi-beam energy harvesters appearing in the literature require custom analytical or finite-element models to compute their eigensolutions and piezoelectric coupling effects. This paper discusses the use of the transfer matrix method to derive analytical solutions to beam structures with point-wise discontinuities or with lumped inertias between members or at the tip. In this method, transfer matrices are developed for the beam’s states (deflection, slope, shear force, and bending moment) analogously to the state transition matrix of a linear system. Euler-Bernoulli beam theory is used to derive transfer matrices for the uniform beam segments, and point transfer matrices are derived to handle discontinuities in the structure. The transfer matrix method is shown to be advantageous for analyzing complex structures because the size of the transfer matrix does not grow with increasing number of components in the structure. Furthermore, the same formulation can be used for a wide range of geometries, including arbitrary combinations of beam segments — single- or multi-layered — and lumped inertias. The eigensolution of the transfer matrix is shown to produce the natural frequencies and mode shapes for these structures. Subsequently, the electromechanical coupling effects are incorporated and the base excitation problem is considered. The electromechanical equations of motion are decoupled by mode and shown to be a generalization of existing analytical models. Parametric case studies are provided for beam structures with varying piezoelectric layer coverage.

Commentary by Dr. Valentin Fuster
2011;():289-293. doi:10.1115/SMASIS2011-5111.

Ultrasonic levitation bearings are a new kind of bearing system that offers the advantages of air-bearings combined with dynamically adjustable supporting force without the need for pressurized air. Such systems are typically driven close or in their resonance frequency, due to energetic reasons. In this contribution the two possible driving methods, namely forced- and self-excitation, are compared in sense of their transient amplitude behavior in the presence of nonlinearities. It is known that during amplitude changes at high vibration amplitudes the system’s resonance frequency varies with Duffing-characteristic due to the nonlinear stiffness of piezoelectric material. It will be shown that self-excitation is the preferable driving method in sense of obtaining a high bandwidth of amplitude.

Commentary by Dr. Valentin Fuster
2011;():295-301. doi:10.1115/SMASIS2011-5114.

Piezoelectric shunt damping is a well known technique to damp the vibrations of mechanical structures. The design of the electrical shunt aims to maximize the energy dissipation. Beside linear networks, switching circuits are a technique that can adapt to varying excitation frequencies. In this paper, the maximum damping performance of the SSDI technique is analyzed and compared to linear LR-shunts. It is shown that it can perform about 60% better.

Topics: Damping
Commentary by Dr. Valentin Fuster
2011;():303-313. doi:10.1115/SMASIS2011-5132.

Shape Memory Alloy (SMA) driven actuation devices offer the potential for dramatic improvements in flight vehicle performance. Such actuators are ideally suited for the light-weight, low-bandwidth, compact size requirements associated with small changes in the vehicle geometry to enhance performance. Over the last 10+ years SMA-based actuation concepts have been considered for use on commercial aircraft, military aircraft, rotorcraft, and spacecraft. Many of these actuation concepts are driven by twisting SMA tubes which are under variable shear loading. This work extends previous quasi-static modeling work to provide a time-domain coupled thermo-mechanical model for SMA torque tubes. The model includes states associated with the material and states associated with peripheral dynamic systems, such as the heater. Approaches for obtaining the key parameters required by the model directly from experimental data are then described. Steps for developing controllers using these models are then reviewed including linearization and linear quadratic regulator (LQR) based control synthesis. The controller is implemented and tested in closed-loop position tracking experiments. These are completed in a lab setting and the results indicate a robust (in terms of gain and phase margin) and high-performance (in terms of settling time) tracking controller. The complete sequence described in this work illustrates the potential of model based optimal control applied to Shape Memory Alloy torque tubes.

Commentary by Dr. Valentin Fuster
2011;():315-324. doi:10.1115/SMASIS2011-5133.

A force sensing resistor (FSR) is a conductive polymer that changes resistance with the application of pressure at its surface. FSR can be used for tactile applications. In this work, the use of FSR to measure the fingertip force within an electronic Braille reading device is considered. To achieve this goal, an experimental procedure to test the FSR’s response is proposed. In this experiment, the FSR is placed between a linear actuator and a load cell. The linear actuator generates different loading profiles to mimic various tactile forces. Identification process starts by applying static loadings at the FSR’s surface. These loads are used to calibrate the FSR and study its time drift. In the next phase of the process, an up-chirp signal is used to identify the dynamics of the FSR. The resulting data are modeled using system identification techniques to obtain possible dynamic models for the FSR. Both linear and nonlinear models are proposed. The linear model is compared to Hammerstein, Wiener, and Hammerstein-Wiener nonlinear models. The accuracy and robustness of the four models are assessed using various loading profiles. Numerical criteria are developed to compare these models with respect to the experimental results.

Topics: Force , Resistors
Commentary by Dr. Valentin Fuster
2011;():325-331. doi:10.1115/SMASIS2011-5138.

The focuses of this study are to design an adaptive tuned vibration absorber based on a smart material known as magnetorheological elastomers (MRE) and to test its dynamic performance. A primary system replicating a cryogenic cooler was designed and fabricated in order to test the effectiveness of the vibration absorber. A hybrid magnetic system (electromagnet and permanent magnets) was also designed and fabricated in order to actuate the MRE as an adaptive stiffness element in the vibration absorber. Vibration testing was conducted on both the primary system and vibration absorber individually in order to characterize the behavior and verify the design constraints. Further testing was performed on the two degree of freedom system to measure and assess the feasibility of the MRE material for use in an application requiring an adaptive vibration absorber. The results show that by using a hybrid design for the electromagnet within the vibration absorber, the stiffness of the MRE material can be both increased and decreased above its nominal value; therefore demonstrating the feasibility of this design as an alternative adaptive vibration absorber.

Commentary by Dr. Valentin Fuster
2011;():333-343. doi:10.1115/SMASIS2011-5150.

Energy harvesting technologies present a solution to decreasing battery size and to the in-situ recharging of batteries in mobile devices or wireless sensor nodes. Combining multiple energy harvesters to power one system can increase the viability of potential designs as solutions for many applications. This work uses LTspice circuit simulations to examine passive circuit topologies for a multiplicity of piezoelectrics as a multisource energy harvesting solution. The simulation results show increases in the maximum instantaneous power seen on the storage capacitor for both a series and parallel circuit topology. There is also an increase in the available voltage level for the series topology. However, these increases in output power and voltage are sensitive to differences in the harvester outputs. Therefore, passive multi-source topologies present a viable solution for combining the energy from a multiplicity of piezoelectric harvesters.

Commentary by Dr. Valentin Fuster
2011;():345-352. doi:10.1115/SMASIS2011-5151.

Bending is a common mode of application and operation of shape memory alloys (SMA). So far the coupled thermomechanical behavior of these alloys have been modeled with numerical methods such as finite element. The issue in developing exact solutions for a SMA beam in bending is because of the distributed and hysteric stress-strain profile. In this paper an analytical approach is developed to find the exact solution for the displacement due to the applied force on the SMA superelastic beam. The approach is based on the assumption of linear distribution of strain along the height of a cross section in the beam. The solution is validated by experimental data and the results of the solution for a superelastic beams for different cases are illustrated.

Commentary by Dr. Valentin Fuster
2011;():353-359. doi:10.1115/SMASIS2011-5167.

In this study we investigate the maximum sustainably harvestable vibrational power from a bird for the purpose of creating new long life tracking tags. We develop a model for harvestable power based on known maximum laden mass capabilities of various bird species and a standard power model for bird flight. Both maximum harvestable power and practically harvestable powers are derived. Practically harvestable power was derived assuming a payload mass and a given specific power for the vibrational harvester. Based on these models, data from various species ranging from 2.55 g to 11.6 kg in mass is used to show relationships between harvestable power and mass. Using estimates for a piezoelectric energy harvester specific power, the practically harvestable power ranges from tens of microwatts to hundreds of milliwatts, depending on the species.

Topics: Flight
Commentary by Dr. Valentin Fuster
2011;():361-367. doi:10.1115/SMASIS2011-5171.

Transforming aeroelastic vibrations into electricity for low-power generation has received growing attention over the past couple of years. The goal is to convert wind energy into electricity for powering small electronic components employed in wireless applications such as structural health monitoring. The potential applications of interest for aeroelastic energy harvesting range from lifting components in aircraft structures to several other engineering problems involving wireless electronic components located in high wind areas. This paper investigates linear and nonlinear aeroelastic energy harvesting using electromagnetic induction. A two-dimensional airfoil with plunge and pitch degrees of freedom (DOF) is considered. The electromagnetic induction is introduced to the plunge DOF by means of a coil-magnet combination and the nonlinearities are introduced through the pitch DOF. The governing dimensionless aeroelastic equations are given with electromagnetic coupling and a resistive load in the electrical domain. The effects of several dimensionless system parameters (electromechanical coupling, load resistance, and coil inductance) on the dimensionless electrical power as well as the dimensionless linear flutter speed are investigated. After considering the linear problem, combined nonlinearities are investigated to improve the electrical output. A cubic stiffness of the hardening type is combined with the free play nonlinearity to make the resulting nonlinear oscillations bounded with acceptable amplitude over a wide range of airflow speeds. The results and the dimensionless simulations presented in this work can be employed for designing and optimizing scalable aeroelastic energy harvesters for wind energy harvesting using electromagnetic induction.

Commentary by Dr. Valentin Fuster
2011;():369-375. doi:10.1115/SMASIS2011-5174.

Torsional behavior of shape memory alloys can be employed in different biomedical applications. The goal of this paper is to investigate the behavior of these alloys under torsional loading conditions. To this end a torsional model is developed in MATLAB, in which a uniaxial model is extended to predict the torque-angle behavior of superelastic wires/rods. Tensile and torsional testing are performed on NiTi wires to determine martial properties and to verify this model. The material properties are determined based on ASTM standards. The effect of different parameters such as lengths and radii on the torque-angle behavior are investigated with the model. Moreover, the effect of temperature on the torsional behavior of SMA wires are presented.

Topics: Wire , Modeling
Commentary by Dr. Valentin Fuster
2011;():377-385. doi:10.1115/SMASIS2011-5176.

Flexible piezoelectrics offer several advantages to use in energy harvesting and biomimetic locomotion. These advantages include ease of application, high power density, silent and effective operation over a range of frequencies as well as light weight. Piezoelectric materials exhibit the well-known direct and converse piezoelectric effects. The direct piezoelectric effect has received growing attention for low-power generation to use in wireless electronic applications while the converse piezoelectric effect constitutes an alternative to replace the conventional actuators used in biomimetic locomotion. In this paper, underwater thrust and electricity generation are investigated experimentally by focusing on biomimetic structures with macro-fiber composite piezoelectrics. Fish-like bimorph configurations with and without a passive caudal fin (tail) are fabricated and compared. The favorable effect of having a passive caudal fin on the frequency bandwidth is reported. The presence of a passive caudal fin is observed to bring the second bending mode close to the first one, yielding a wideband behavior in thrust generation. The same smart fish configuration is tested for underwater piezoelectric power generation in response to harmonic excitation from its head. Hydrodynamic loads resulting from base excitation yield considerably larger power output as compared to in-air base excitation at the same acceleration amplitude. This work also discusses the feasibility of thrust generation using the harvested energy toward enabling self-powered swimmer systems.

Commentary by Dr. Valentin Fuster
2011;():387-395. doi:10.1115/SMASIS2011-5179.

Design optimization-based techniques are presented for the minimization of biodynamic loads of a seated occupant subjected to a shock due to an initial velocity vertical impact. A 95th percentile male occupant was modeled using a multiple degree-of-freedom biodynamic lumped parameter model (BLPM) seated on a vertically stroking adaptive seat suspension with a semi-active magnetorheological energy absorber (MREA). The governing equations of motion of the adaptive MREA-based seat suspension with biodynamic lumped parameter model were developed. The variation of magnetorheological yield force with respect to the energy absorber stroke was shaped using cubic polynomial for the maximum shock attenuation. Three cost functions were devised with a common goal of minimizing biodynamic decelerations. Constraints were established based on limitation of MREA stroke and, yield force as well as stroking load. The MREA yield force and damper stroke were crucial parameters for improved biodynamic response mitigation to shock loads. A globally optimized biodynamic response was analyzed among several local optimum responses for better shock attenuation.

Commentary by Dr. Valentin Fuster
2011;():397-403. doi:10.1115/SMASIS2011-5181.

In a crash event, such as the crash of an aircraft or the collision of two ground vehicles, the impact dynamics are a function of the impact velocity and payload mass. A typical bumper system on a ground vehicle has passive viscous energy absorbers (PVEAs) that are optimally designed for a specific impact velocity and payload, so that off-design performance may be suboptimal, and may even be unacceptable for large perturbations in sink rate and payload mass from the designed values. This is because the load-stroke profile of the energy absorbing suspension system (EASS) is passive in that spring stiffness and damping of the energy absorbers is fixed. Therefore, in this study, the PVEA in an EASS is replaced by an active or semi-active energy absorber (SAEA), and the effects of time delay in achieving controllable semi-active damping is analyzed in the context of impact dynamics. To accomplish this, a three degree-of-freedom dynamic model of an EASS is presented, and the effect of the time delay in commanding the controllable force of the EA is analyzed. The asymptotic stability and Hopf bifurcation of the trivial steady state response are analyzed for a range of time delay. A technique to stabilize the impact dynamic is developed, and it is shown that the impact dynamics can be stabilized using appropriate feedback control.

Commentary by Dr. Valentin Fuster
2011;():405-409. doi:10.1115/SMASIS2011-5197.

An ankle foot orthosis (AFO) is a device that provides a controlled force to compensate for the muscle deficiencies in the ankle and helps normalize the gait of the patient. Evidence has indicated that there exists an optimal match correlating the patient’s gait related problems and the AFO stiffness. AFO ankle stiffness is measured by the moment around the ankle joint exerted by the AFO per degree of ankle joint rotation. To date, several testing devices and procedures have been developed to assess the stiffness characteristics of AFOs. Most of the devices are manually driven and may not exactly replicate human leg motion. Objective of developing an automated testing assembly is to identify stiffness characteristics of passive AFOs so as to develop an active AFO with shape memory alloy. We have developed an assembly using aluminum T-slotted profiles, single flange linear bearings, and living hinges from 80/20 Inc. Angle measurement was done by mounting Digital Protractor on the shank segment. The whole assembly was mounted on BOSE ElectroForce 3330 test instrument. dSPACE hardware-in-the-loop solution was used for real time data capture of force and angle sensor output. After assessing the characteristics of passive AFO we incorporated SMA wire in the AFO. Similar tests were conducted to evaluate effect of SMA wires on the overall stiffness of an AFO. The results confirm that SMA wires provide stiffness variation such a way that AAFO can be developed to achieve stiffness variation close to normal ankle stiffness.

Commentary by Dr. Valentin Fuster
2011;():411-417. doi:10.1115/SMASIS2011-5210.

In this paper, we study the behavior of shape memory alloy (SMA) nanowires subjected to multi-axial loading. We use the model developed in our earlier work to study the microstructure and mechanical properties of finite length nanowires. The phase field model with the Ginzburg-Landau free energy is used to model the phase transformation based on the chosen order parameter. The governing equations of the thermo-mechanical model are solved simultaneously for different loading cases. We observe that nanowire behaves in a stiff manner to axial load with complete conversion of the unfavorable martensite to the favorable one. The bending load aids the phase transformation by redistributing the martensitic variants based on the local axial stress sign. The nanowire behavior to multi-axial (axial and bending together) is stiffer axially than the axial loading case. The understanding of the behavior of nanowire to multi-axial loading will be useful in developing better SMA-based MEMS and NEMS devices.

Commentary by Dr. Valentin Fuster
2011;():419-425. doi:10.1115/SMASIS2011-5223.

Shape memory alloys (SMA) are well-known for their ability to transform into an imprinted shape by means of thermal activation (pseudoplasticity) or after a mechanical deformation (pseudoelasticity). The thermal effects can be used in a wide range of industrial applications like valves, unlocking devices or comfort applications in the field of automotive mechatronics. While there are many ideas concerning shape memory actuators, only few thoughts have been spent on service applications around these unique actuators. At present, product-related services are usually considered as an add-on to the actual product. But in future, industrialized countries are subject to a structural change toward service societies. For this reason, new concepts and methods which enable the companies to design the potential services in an optimal way are necessary already during the development of a product. This is a paradigm shift from the separated consideration of products and services to a new product understanding consisting of integrated products and services. In the case of shape memory technology, recycling processes present an interesting field for such integrated services. Starting with general ideas towards recycling concepts for and with shape memory components, this paper focuses on refresh-annealing as an example of an interesting recycling process. Finally, the paper is summed up by an outlook on future works on development methods for generic shape memory actuators and their service systems. The aim of this study is to show the possibilities and the importance of services in the field of shape memory technology. As a result, new applications and markets for SMA can be developed.

Topics: Shapes
Commentary by Dr. Valentin Fuster
2011;():427-433. doi:10.1115/SMASIS2011-5239.

Shunted piezoelectric transducers can be used to dissipate vibration energy of a host structure. The Synchronized Switch Damping on Inductance (SSDI) is a shunting technique featured by a nearly rectangular shape of the resulting voltage on the transducer being in anti-phase with the structure’s velocity. As for these systems, previous studies have reported the strong connection of the dissipated energy on the slope of the voltage signal occurring during the switch. This implies that any electrical losses have to be minimized in order to increase the slope of the voltage signal and, thus, the damping performance of the shunt. Moreover, the rate of change of the voltage signal represents a critical issue for autonomous shunts, where the switch can be inefficient because the power for switching the circuit is not supplied by an external source but is supplied by the vibrating structure itself. In this study, a new technique for improving the damping performance of autonomous switching shunt is proposed based on reducing the electrical losses of the switch. Finally, an experimental validation of the novel shunting technique is carried out.

Topics: Damping , Vibration
Commentary by Dr. Valentin Fuster
2011;():435-442. doi:10.1115/SMASIS2011-5254.

While the usefulness of small scale prototypes for harvesting the energy of waves have already been demonstrated, the utilization of the capability of flow energy in rivers based on electroactive polymers is still a significant challenge. To harvest the energy of flowing waters, a novel flow energy converter based on a simple and environmentally sustainable mechanical design is developed, consisting of an elastomeric tube with a closing mechanism at the outlet side. Because the stationary stretch of the tube is comparably small, the tube has to be operated in resonance, which offers a high resonant magnification of the tube deformation. The resonant operation can be obtained, when the tube is closed rapidly and a shock wave is induced into the tube, which is often referred to as water hammer. Based on a small-scale prototype, this expected mechanical behavior of the tube was demonstrated.

Commentary by Dr. Valentin Fuster

Structural Health Monitoring/NDE

2011;():443-446. doi:10.1115/SMASIS2011-4921.

High-pressure hydraulic hoses are used throughout industry to transmit fluid power. The current state of the art in hose replacement consists of two strategies; these are (1) replacement upon failure and (2) time-based replacement. For the replacement upon failure method, end users inspect hoses and either replace when there is obvious physical damage or the hose has burst and allowed the release of fluid under high pressure. Hose users that employ time-based replacement cycles often collect data and either subjectively or statistically choose a replacement frequency intended to prevent unexpected failures. Engineers at Eaton Corporation worked with Purdue University to develop an alternative. A novel hose construction using two conductors with an isolating layer provides a component in an electrical circuit which can be monitored to determine the status, or health, of a hose in operation. The first step in this development was the realization that hose failure is a process and not an event. By tracking a hose’s electrical signature and characterizing the change that occurs when the internal structure begins to break down, a user is alerted prior to a catastrophic hose failure. Eaton is developing notification systems capable of both monitoring the hose’s electrical signature and alerting an equipment user prior to unexpected failure. The system requires direct electrical connection to the hose fitting for monitoring. There are currently two strategies in development, a wired system and a wireless system. The wired system uses a remote diagnostic unit with cables running to each hose assembly to query the hose and alert an equipment user directly. The wireless system employs battery-powered sensors installed on a hose assembly which communicate with a gateway located nearby. When a hose approaches its end of life a warning is issued by illuminating a warning light or issuing a remote warning through a cellular or wireless network. There are significant gains in the ability to prevent hydraulic hose failures. These unexpected incidents lead to downtime, damage to equipment, environmental damage, and serious personal injury. Additionally, using this advanced warning system allows users to use nearly a hose’s entire life. This improves asset utilization considerable when compared to the useful life sacrificed by using time-based replacement schedules. This technology will reduce operating costs and prevent downtime, environmental incidents, and the threat of personal injury present when hydraulic hose fails.

Commentary by Dr. Valentin Fuster
2011;():447-453. doi:10.1115/SMASIS2011-4991.

Research at the AFRL Space Vehicles Directorate is being conducted to reduce schedule times for assembly, integration, and test, to make satellite-based capabilities more responsive to user needs. Structural Health Monitoring has been pursued as a means for validating workmanship and has been proven on PnPSat-1. Embedded ultrasonic piezoelectric wafer active sensors (PWAS) have been utilized with local and global inspection techniques, developed both in house and by collaborating universities, to detect structural changes that may occur during assembly, integration, and test. Specific attention has focused on interface qualification. It is now reasonable to believe that evaluation of interfaces through the use of such sensors can also be used to indirectly qualify the structure thermally and that tedious thermal-vacuum testing may be truncated or eliminated altogether. This paper focuses on the computational development of extracting thermal properties from ultrasonic transmission records. Methods are validated on simple bolted lap-joint cantilever beams.

Commentary by Dr. Valentin Fuster
2011;():455-463. doi:10.1115/SMASIS2011-4993.

A local damage at the tip of a composite propeller is diagnosed by properly comparing its impact-induced free coupled dynamics to that of a pristine wooden propeller of the same size and shape. This is accomplished by creating indirectly via collocated measurements distributed information for the coupled acceleration field of the propellers. The powerful data-driven modal expansion analysis delivered by the Proper Orthogonal Decomposition (POD) Transform reveals that ensembles of impact-induced collocated coupled experimental acceleration signals are underlined by a high level of spatio-temporal coherence. Thus they furnish a valuable spatio-temporal sample of coupled response induced by a point impulse. In view of this fact, a tri-axial sensor was placed on the propeller hub to collect collocated coupled acceleration signals induced via modal hammer nondestructive impacts and thus obtained a reduced order characterization of the coupled free dynamics. This experimental data-driven analysis reveals that the in-plane unit components of the POD modes for both propellers have similar shapes-nearly identical. For the damaged propeller this POD shape-difference is quite pronounced. The shapes of the POD modes are used to compute indices of difference reflecting directly damage. At the first POD energy level, the shape-difference indices of the damaged composite propeller are quite larger than those of the pristine wooden propeller.

Topics: Propellers , Signals
Commentary by Dr. Valentin Fuster
2011;():465-471. doi:10.1115/SMASIS2011-5031.

Because of high specific strength and many other benefits, the use of composites for the large lightweight structures such as modern aircrafts and wind turbines are increasing. However, one of the serious drawbacks of composites is that the structural failure occurs in complex patterns without yielding. Therefore, structural health monitoring has been intensively investigated for the early detection of any problems in structural integrity. One of the promising sensors for this purpose is fiber Bragg grating (FBG) sensor. They can be easily inserted into the layered-structure of the composite materials due to their small size. The excellent multiplexing capability enables measurement to be taken at multiple points along a single sensor line. As well as damage detection, the structural shape measurement also draws attention. Particularly for structures experiencing aerodynamic forces such as wind turbines or helicopter blades, the structural shape itself is important because the applied aerodynamic forces are affected by structural shape deflections. Therefore, the authors have conducted a series of studies on the structural shape estimation of various structures. We have also developed a wavelength division multiplexing (WDM) Bragg grating sensing system for high speed strain sensing as well as low frequency dynamic strains. In the case of high-speed sensing, the interrogator allows a sampling ratio of over 40 kHz for six linearly arrayed FBG sensors per channel. Utilizing the developed interrogator, this paper presents some experimental results for simultaneous measurement of deformation and fracture signals of composite structures. An array of FBG sensors were installed onto composite beam specimens and the acoustic emission (AE) signals due to structural failure was continuously monitored while the overall structural deflection shape was monitored in real time. The reconstructed shapes of the specimens were in good agreement with the shapes captured from photographs taken with a high-speed camera. In summary, it was demonstrated that both fracture signals and the overall deformation shape of composite structures could be simultaneously monitored.

Commentary by Dr. Valentin Fuster
2011;():473-479. doi:10.1115/SMASIS2011-5033.

This paper introduces a fiber Bragg grating (FBG)-based sensing system for the multi-MW scale wind turbine health monitoring, and describes the results of field tests of dynamic strain monitoring and the deformed shape estimation of the wind turbine tower structure. An FBG interrogator was developed with a spectrometer-type demodulator based on a linear photo detector. Real-time shape estimation of the wind turbine tower was accomplished using strain data gathered by surface mounted fiber Bragg grating sensors. The finite element model of the wind turbine tower was created and the displacement-strain transformation (DST) on the basis of the modal approach was obtained. The time histories of the strain were gathered for the cases of blade stop and start using the FBG sensors located at the upwind side of the tower. Finally, the full deflection shapes of the tower were successfully estimated using arrayed FBG sensors.

Commentary by Dr. Valentin Fuster
2011;():481-487. doi:10.1115/SMASIS2011-5065.

In this paper, a new design for microstrip patch antenna strain sensors is proposed. The new antenna sensor works based on the meandered microstrip patch antennas. It is threefold more sensitive than previously proposed circular microstrip patch antenna strain sensors. Also, the overall physical dimension of the new antenna sensor is reduced by the factor of five. The current sensor is able to detect strain in all directions. In order to design the antenna sensor, two available commercial FEM software packages ANSYS™ and HFSS™ are used. Both experimental and FEM results corroborate the multidirectional feature of the new antenna sensor. Also, the effect of the hole size in the structure (for coaxial connection to the antenna) on the antenna performance has been studied. Based on the results obtained, the antenna sensor can be recommended for use in structural health monitoring for strain-based damage detection in aerospace structures.

Topics: Strain sensors
Commentary by Dr. Valentin Fuster
2011;():489-493. doi:10.1115/SMASIS2011-5067.

In this paper, numerical models are constructed to analyze the magneto-mechanical interaction in a laminate embedded with terfenol-D (TD) for sensing purposes. The first model is a linear 3D model constructed in multi-physics finite element software and examines mechanical and magnetic sensing parameters of a sensing layer embedded in a composite laminate with a delamination along the sensing layer. The structural plies in the model are defined with the anisotropic properties of T300/ carbon fiber reinforced polymer (CFRP) plies. The results of the first model show that there is a local change in stress and strain in the region of the delamination. However, by assuming the magnetic permeability is constant in the constitutive sensing equation, the sensing parameter (magnetic flux density) does not change as a function of stress but only magnetic field intensity. The second model is a 2D boundary element model constructed in FADD2D that analyzes the stress intensity factors generated by a crack in a beam of similar geometry and loading configuration. It is loaded mechanically through two endpoints on a beam and the crack is offset from the center of the beam. The results of the second model show no significant stress increase in the crack region due to the Poisson’s effect creating crack closure on the crack. These models are used to analyze the mechanical and magnetic mechanisms that allow Terfenol-D to be used as an embedded sensor in composites.

Commentary by Dr. Valentin Fuster
2011;():495-501. doi:10.1115/SMASIS2011-5069.

Knowledge of the damage location in composite structures is a necessary output for both Non-Destructive Evaluation (NDE) and Structural Health Monitoring (SHM). Although several damage localization approaches using a triangulation method and Time-of-Flight (ToF) of guided waves have been reported in literature, the damage localization technique is still not mature for composite structures with complex material properties, varying thickness and complex geometries. This paper investigates the development of a new approach for SHM and damage localization using a guided wave based active sensing system. In contrast to the traditional ellipse method, the proposed method does not require the information of structural thickness, ToF, or the estimation of group velocities of each guided wave mode at different propagation angles, which is one of the main limitations of most current ToF methodologies involving composites. This approach uses time-frequency analysis to calculate the difference of the ToF of the converted modes for each sensor signal. The damage location and the group velocity are obtained by solving a set of nonlinear equations. The proposed method can be used for composite structures with unknown lay-up and thickness. To validate the proposed method, experiments were conducted on both composite plates and stiffened composite panels. Eight piezoelectric (PZT) transducers were surface-bonded on each composite specimen and used in four pairs. The PZT transducers in each pair were bonded close to each other. In the PZT array, one PZT transducer from one PZT pair was used as the actuator and the other three pairs were used as sensors. A windowed cosine signal was used as the excitation signal. The locations of the delaminations in the composite specimens were validated using a flash thermography system. The accuracy of the proposed method in localizing delaminations was examined through comparison with the experimental measurements.

Commentary by Dr. Valentin Fuster
2011;():503-512. doi:10.1115/SMASIS2011-5100.

The study of efficiency and safety for wind turbine structures under variable operating conditions is increasingly important for wind turbine design. Optimum aerodynamic performance of a wind turbine demands that serviceability effects and ultimate strength loads remain under safety design limits. From the perspective of wind turbine efficiency, variations in wind speed causes bluffing effects and vortex shedding that lead to vibration intensities in the longitudinal and transversal direction that can negatively impact aerodynamic performance of the turbine. From the perspective of wind turbine safety, variations in loading may lead to transient internal loads that threaten the safety of the structure. Inertial effects and asynchronous delays on rotational-force transmission may generate similar hazards. Monitoring and controlling displacement limits and load demands at critical tower locations can improve the efficiency of wind power generation, not to mention the structural performance of the turbine from both a strength and serviceability point of view. In this study, a probabilistic monitoring approach is developed to measure the response of the combined tower/nacelle/blade system to stochastic loading, estimate peak demand, and compare that demand to building code-derived estimates of structural resistance. Risk assessment is performed for the effects of along and across-wind forces in a framework of quantitative risk analysis with the goal of developing a near real-time estimate of structural risk that may be used to monitor safety and serviceability of the structure as well as regulate the aggressiveness of the controller that commands the blade angle of attack. To accomplish this goal, a numerical simulation of the aerodynamic performance of a wind turbine (including blades, the nacelle and the tower) is analyzed to study the interaction between the structural system and incoming flow. A model based on distributed-stationary random wind load profile for the combined along-wind and across-wind responses is implemented in Matlab to simulate full aero-elastic dynamic analysis to simulate tower with nacelle, hub, rotor and tower substructures. Self-weight, rotational, and axial effects of the blades, as well as lateral resistance of substructure elements are incorporated in the finite element model, including vortex-shedding effects on the wake zone. Reliability on the numerical solution is inspected on the tower structure by comparing the numerical solution with established experimental-analytical procedures.

Commentary by Dr. Valentin Fuster
2011;():513-518. doi:10.1115/SMASIS2011-5101.

In structural health monitoring (SHM) of aerospace components, such as stiffened panels, detection and localization of damage is an important issue. This paper presents a methodology for determining the existence and location of low velocity impact damage in a stiffened composite panel. Using a matching pursuit decomposition algorithm, converted modes due to damage were extracted in the time-frequency domain. The energy of the converted mode was then used in conjunction with a probabilistic tomography approach that was able to localize the damage with a high level of accuracy. The results obtained confirm the ability of this approach to detect and localize damage in complex composite structures.

Commentary by Dr. Valentin Fuster
2011;():519-523. doi:10.1115/SMASIS2011-5116.

The use of elastic wave based Structural Health Monitoring has shown its usefulness in both characterizing and diagnosing composite structures. Techniques using elastic wave SHM are being developed to allow for improved efficiency and assurance in all stages of space structure development and deployment. These techniques utilize precise understanding of wave propagation characteristics to extract meaningful information regarding the health and validity of a component, assembly, or structure. However, many of these techniques focus on the diagnostic of traditional, isotropic materials, and questions remain as to the effect of the orthotropic properties of resin matrix composite material on the propagation of elastic waves. As the demands and expectations placed upon composite structures continue to expand in the space community, these questions must be addressed to allow the development of elastic wave based SHM techniques that will enable advancements in areas such as automated build validation and qualification, and in-situ characterization and evaluation of increasingly complex space structures. This study attempts to aid this development by examines the effect of cross ply, off-axis fiber orientation on the propagation characteristics of lamb waves. This is achieved by observing the result of symmetric and anti-symmetric wave propagation across materials in cases containing both off-axis and axially-aligned elements. In both cases the surface plies of the test specimen are axially aligned with the wave propagation direction. Using these results, the relative effect of core ply orientation on lamb wave propagation, and lamb wave sensitivity to bulk properties, or alternatively, the dominance of surface properties on propagation characteristics, can be seen, and this information can be used to aid in future research and application of lamb waves for interrogation of advanced, high-strain composite space structures. It was found that the core orientation caused significant variation in the S0 wave velocity, while yielding little influence on the A0 wave velocity.

Commentary by Dr. Valentin Fuster
2011;():525-534. doi:10.1115/SMASIS2011-5190.

This paper presents an investigation of predictive modeling of space structures for structural health monitoring (SHM) with piezoelectric wafer active sensors (PWAS) transducers. The development of a suitable SHM system for complex space structure is not trivial; creating a robust SHM capability requires at least: (a) flexible accommodation of numerous configurations; (b) detection of damage in complex multifunctional structures; (c) identification if mechanical interfaces are properly connected. To realize this, we propose a predictive modeling approach using both analytical tools and finite element method (FEM) to study the health status of the structure, the power and energy transduction between the structure and the PWAS. After a review of PWAS principles, the paper discusses the modeling and the power and energy transduction between structurally guided waves and PWAS. The use of guided wave (GW) and the capability of embedded PWAS to perform in situ nondestructive evaluation (NDE) are explored. FEM codes are used to simulate GW of 2D and 3D space structure using the commercials software ABAQUS. PWAS transducers placement at different location on a flat plate and on an isogrid panel was simulated. The signal scattered by a crack emerging from the hole is simulated. Predictive modeling of power and energy transduction is discussed using an analytical approach. This model of 2-D power and energy transduction of PWAS attached to structure allows examination of power and energy flow for a circular crested wave pattern. Wave propagation method for an infinite boundary plate, electromechanical energy transformation of PWAS and structure, and wave propagation energy spread out in 2-D plate are considered. The parametric study of PWAS size, impedance match gives the PWAS design guideline for PWAS sensing and power harvesting applications.

Commentary by Dr. Valentin Fuster
2011;():535-543. doi:10.1115/SMASIS2011-5191.

Current satellite validation tests involve numerous procedures to qualify the space vehicle for the vibrations expected during launch and for exposure to the space environment. Structural Health Monitoring methods are being considered in an effort to truncate the number of validation tests required for satellite checkout. The most promising of these monitoring techniques uses an active wave-based method in which an active piezoelectric transducer propagates a Lamb wave through the structure, where it is then received by a second sensor and evaluated over time to detect structural changes. Thus far, this method has proven effective in locating structural defects in a complex satellite panel; however, the attributes associated with the first wave arrival change significantly as the wave travels through ribs and interfaces. This study establishes a method to identify important features in the sensor signal that may otherwise be missed. In this work, an FE model of a plate is developed, and variability is introduced for each state observation. Different states are modeled as masses distributed at 12 locations. Signals are obtained through an explicit time-domain analysis. Matching Pursuit Decomposition is used to extract physically significant parameters from the signal, the features are reduced to a more manageable subset, and Support Vector Machines are utilized to identify important features in the sensor signal. The results of the classification indicate that for a plate specimen the extracted feature corresponding to anti-symmetric wave mode interaction with damage does the best job of differentiating damage states.

Commentary by Dr. Valentin Fuster
2011;():545-553. doi:10.1115/SMASIS2011-5219.

Many structural damage detection methods utilize piezoelectric sensors. While these sensors are efficient in supporting many structural health monitoring (SHM) methodologies, there are a few key disadvantages limiting their use. The disadvantages include the brittle nature of piezoceramics and their dependence of diagnostic results on the quality of the adhesive used in bonding the sensors. One viable alternative is the utilization of Magneto-Elastic Active Sensors (MEAS). Instead of mechanically creating elastic waves, MEAS induce eddy currents in the host structure which, along with an applied magnetic field, generate mechanical waves via the Lorentz force interaction. Since elastic waves are generated electromagnetically, MEAS do not require direct bonding to the host structure and its elements are not as fragile as PWAS. This work explores the capability of MEAS to detect damage in aluminum alloy. In particular, methodologies of detecting fatigue cracks in thin plates were explored. Specimens consisted of two identical aluminum plates featuring a machined slot to create a stress riser for crack formation. One specimen was subjected to cyclic fatigue load. MEAS were used to transmit elastic waves of different characteristics in order to explore several SHM methodologies. Experiments have shown that the introduction of fatigue cracks created measurable amplitude changes in the waves passing through the fatigued region of the aluminum plate. The phase indicated sensitivity to load conditions, but manifestation in the cracked region lacked stability. Nonlinear effects were studied using plate thickness resonance, which revealed birefringence due to local stresses at the site of the fatigue crack. The resonance spectrum has also shown a frequency decrease apparently due to stiffness loss. Preliminary results suggest opportunities for fatigue damage detection using MEAS. Application of MEAS for the diagnosis of complex structures is currently being investigated.

Commentary by Dr. Valentin Fuster
2011;():555-562. doi:10.1115/SMASIS2011-5225.

This paper presents experimental investigations of the effect of Lamb wave excitation frequency on detection of a given delamination in composite plates. Typical aerospace type composite plates are used and integrated piezoelectric transducers function as both actuator and sensor. Also, a scanning Laser Doppler Vibrometer (LDV) is used for preliminary sensing of structural responses when excited by a single PZT actuator. Results in time domain are quantified by a damage index calculation based on modified L2 error norm. Phase difference calculations based on complex continuous wavelet transform (CWT) and Hilbert-Huang transform (HHT) are presented. Experimental results show a significant effect of incident Lamb waves on delamination signature.

Commentary by Dr. Valentin Fuster
2011;():563-570. doi:10.1115/SMASIS2011-5235.

In this paper the experimental activities that were performed at Thales Alenia Space (TAS-I) System, Turin, Italy by Acellent Technologies Inc is presented. The final objective was defining a Vehicle / Vehicle Subsystem, built-in Health Management System which embeds self diagnosis and prognosis functions. Under this program a Composite Overwrapped Pressure Vessel (COPV) for space applications was monitored under pressure cycling (mechanical loading). The subscale demonstrator consisted of an aluminium metallic liner over wrapped by a CFRP layer. The metallic liner is seamless and manufactured by spin-forming. The liner material is aluminum AA6061 T6, with Yield Strength of 286 MPa, as declared by the bottle supplier (US Hydrospin); expected elongation to rupture is around 10%. The test was conducted for 3 days on a water filled COPV and at the end of three days the metal liner inside the propellant tank was cracked and caused water leakage. Acellent used a statistical data interpretation technique via feature extraction and data modeling approach to demonstrate that the system was able to generate the early alarm and also capable of localizing the damage which appeared at two hot spot locations.

Commentary by Dr. Valentin Fuster

Bio-Inspired Smart Materials and Structures

2011;():571-580. doi:10.1115/SMASIS2011-4931.

Ionic polymer metal composites (IPMC) are a new class of smart materials that have attractive characteristics such as muscle like softness, low voltage and power consumption, and good performance in aqueous environments. Thus, IPMC’s provide promising application for biomimetic fish like propulsion systems. In this paper, we design and analyze IPMC underwater propulsor inspired from swimming of Labriform fishes. Different fish species in nature are source of inspiration for different biomimetic flapping IPMC fin design. Here, three fish species with high performance flapping pectoral fin locomotion is chosen and performance analysis of each fin design is done to discover the better configurations for engineering applications. In order to describe the behavior of an active IPMC fin actuator in water, a complex hydrodynamic function is used and structural model of the IPMC fin is obtained by modifying the classical dynamic equation for a slender beam. A quasi-steady blade element model that accounts for unsteady phenomena such as added mass effects, dynamic stall, and the cumulative Wagner effect is used to estimate the hydrodynamic performance of the flapping rectangular shape fin. Dynamic characteristics of IPMC actuated flapping fins having the same size as the actual fins of three different fish species, Gomphosus varius, Scarus frenatus and Sthethojulis trilineata, are analyzed with numerical simulations. Finally, a comparative study is performed to analyze the performance of three different biomimetic IPMC flapping pectoral fins.

Commentary by Dr. Valentin Fuster
2011;():581-590. doi:10.1115/SMASIS2011-4934.

Morphing aircraft and other shape-changing structures are well suited to McKibben-like flexible composite actuators. These actuators, made from fiber-reinforced elastomeric composites, are extremely efficient in converting potential energy (pressurized air) into mechanical energy. Such actuators are promising for use in micro air vehicles, prosthetics and robotics because they offer excellent force-to-weight ratios and behave similar to biological muscle. Use of an incompressible pressurizing fluid instead of compressible air may also offer higher actuator stiffness, better control, and compatibility with existing actuation systems. Using incompressible fluids also allows the actuator to serve as a variable stiffness element which can be modulated by opening and closing valves that constrain or allow fluid flow. The effect of an incompressible fluid (water) on the performance of Rubber Muscle Actuators (RMA), with varying diameters, lengths and segment lengths, was experimentally investigated in the current work. Upon pressurization with air or water, past an activation threshold, overall force and stroke increased with increasing actuation length and diameter. Actuation force when pressurized with water is slightly greater than with air. Both air and water-pressurized actuation force and strain decrease significantly when segment length is less than a minimum critical length. Closed valve actuator stiffness (modulus) of actuators at full length, when pressurized with an incompressible fluid is up to 60× greater than the open valve stiffness of the same actuator. Air-filled RMAs with equal parameters only see a 10× increase. Incompressible fluid-filled RMAs have great potential to provide needed high actuation forces within adaptive material systems. Design guidelines are given to aid additional RMA use.

Commentary by Dr. Valentin Fuster
2011;():591-598. doi:10.1115/SMASIS2011-4946.

This paper proposes a fully coupled three-scale finite element model for the mechanical description of an alumina/magnesium alloy/epoxy composite inspired in the mechanics and architecture of wood cellulose fibres. The constitutive response of the composite (the large scale continuum) is described by means of a representative volume element (RVE, corresponding to the intermediate scale) in which the fibre is represented as a periodic alternation of alumina and magnesium alloy fractions. Furthermore, at a lower scale the overall constitutive behavior of the alumina/magnesium alloy fibre is modelled as a single material defined by a large number of RVEs (the smallest material scale) at the Gauss point (intermediate) level. Numerical material tests show that the choice of the volume fraction of alumina based on those volume fractions of crystalline cellulose found in wood cells results in a maximisation of toughness in the present bio-inspired composite.

Commentary by Dr. Valentin Fuster
2011;():599-606. doi:10.1115/SMASIS2011-4959.

Self-healing in fibre reinforced polymer (FRP) composites is an active area of research, principally aimed at restoring the losses in mechanical strength associated with impact induced damage. This bioinspired function may be imparted upon a composite structure via the embedment of a vasculature that is capable of delivering functional agents from an external reservoir to regions of internal damage. A simple segregated vasculature design incorporated into a FRP via a ‘lost wax’ process was found to facilitate a self-healing function which resulted in an outstanding recovery (≥97%) in post-impact compression strength. The process involved infusion of a healing resin through the vascule channels. Resin egress from the backface damage, ultrasonic C-scan testing and microscopic evaluation all provide evidence that sufficient vascule-damage connectivity exists to confer a reliable and efficient self-healing function.

Commentary by Dr. Valentin Fuster
2011;():607-612. doi:10.1115/SMASIS2011-4962.

An analytical model of a composite structure consisting of an F2 MC tube embedded in epoxy is used to study the impact of confining structural media on the ability of F2 MC tubes to pump fluid and change stiffness. The surrounding stiff epoxy reduces the performance relative to an isolated F2 MC tube, but this disadvantage can be minimized by tailoring the tube wall thickness and fiber angle. The composite can pump 250 times more fluid than a piston of the same diameter if the F2 MC tube has a thick shell wall and a near-axial fiber orientation. With a moderately thick wall and low fiber angle, the closed valve axial stiffness can be increased by a factor of 2.2 relative to the open valve stiffness. The maximum pumping factor and modulus ratio, however, are 80% and 2.6% of the F2 MC tube without the surrounding epoxy.

Commentary by Dr. Valentin Fuster
2011;():613-620. doi:10.1115/SMASIS2011-5015.

Conducting polymer actuators and sensors utilize electrochemical reactions and associated ion transport at the polymer-electrolyte interface for their engineering function. Similarly, a bioderived active material utilizes ion transport through a protein and across a bilayer lipid membrane for sensing and actuation functions. Inspired by the similarity in ion transport process in a bilayer lipid membrane (BLM) and conducting polymers, we propose to build an integrated ionic device in which the ion transport through the protein in the bilayer lipid membrane regulates the electrolytic and mechanical properties of the conducting polymer. This article demonstrates the fabrication and characterization of a DPhPC planar BLM reconstituted with alamethicin and supported on a polypyrrole bridge measuring 100 μm × 500 μm and formed across micro-fabricated gold pads. The assembly is supported on silicon dioxide coated wafers and packaged into an electronic-ionic package for electrochemical characterization. The various ionic components in the integrated ionic device are characterized using electrical impedance spectroscopy (EIS), cyclic voltammetry (CV), and chronoamperometry (CA) measurements. The results from our experimental studies demonstrate the procedure to fabricate a rugged electro active polymer supported BLM that will serve as a platform for chemical, bioelectrical sensing and VOC detection.

Topics: Membranes
Commentary by Dr. Valentin Fuster
2011;():621-627. doi:10.1115/SMASIS2011-5028.

This study experimentally shows that an oscillatory behavior observed in a trim flight of an ornithopter has a stable limit-cycle oscillation (LCO) characteristics and that the magnitude of the LCO in body pitch dynamics can be suppressed by active tail motion. A free flight of the tested ornithopter is emulated in the wind tunnel using a specially devised tether that provides the minimal mechanical interference to the flight of ornithopter. Due to the symmetric wing motion in forward trim flight, the longitudinal flight dynamics is more focused than the lateral one. The non-contact type sensors are used to measure the time histories of the flight state variables such as wing and tail motions, body pitch angle, and altitude. The tail motion for the pitch LCO reduction is achieved by two actuators: 1) Servo motor for the rigid-body motion of the tail elevation angle, and 2) Macro-Fiber Composite strain actuator for the elastic deformation of the tail camber. The performances of the LCO suppressions are compared in the root-mean-square-error sense and the harmonically activated in-phase tail motion linked to wing motion is observed to effectively reduce the pitch LCO.

Commentary by Dr. Valentin Fuster
2011;():629-635. doi:10.1115/SMASIS2011-5038.

Conducting polymers possess similarity in ion transport function to cell membranes and perform electro-chemo-mechanical energy conversion. In an in vitro setup, protein-reconstituted bilayer lipid membranes (bioderived membranes)perform similar energy conversion and behave like cell membranes. Inspired by the similarity in ionic function between a conducting polymer membrane and cell membrane, this article presents a thin-film laminated membrane in which alamethicin-reconstituted lipid bilayer membrane is supported on a polypyrrole membrane. Owing to the synthetic and bioderived nature of the components of the membrane, we refer to the laminated membrane as a hybrid bioderived membrane. In this article, we describe the fabrication steps and electrochemical characterization of the hybrid membrane. The fabrication steps include electropolymerization of pyrrole and vesicle fusion to result in a hybrid membrane; and the characterization involves electrical impedance spectroscopy, chronoamperometry and cyclic voltammetry. The resistance and capacitance of BLM have the magnitude of 4.6×109 Ω-cm2 and 1.6×10−8 F/cm2 .The conductance of alamethicin has the magnitude of 6.4×10−8 S/cm2 . The change in ionic conductance of the bioderived membrane is due to the electrical field applied across alamethicin, a voltage-gated protein and produces a measurable change in the ionic concentration of the conducting polymer substrate.

Commentary by Dr. Valentin Fuster
2011;():637-646. doi:10.1115/SMASIS2011-5053.

Turbulence-induced vibration is generally considered undesirable, and is a phenomenon that if not properly anticipated can lead to catastrophic structural failure. From an energy harvesting perspective however, these types of vibrations have been found to be quite valuable. Turbulence can greatly diminish the performance of traditional fluid flow energy harvesting devices such as turbine, or propeller type designs. Even more recently developed nontraditional harvesters that take advantage of vortex shedding, flutter, or related phenomena in fluid dynamics become extremely inefficient, and in some cases, completely fail to generate useful power in turbulent flow. Motivation for this work comes from the fact that very little research has been done on methods for harvesting energy from turbulence. The primary objective of this research is to design and develop a deploy-and-forget energy harvesting device for use in low velocity (∼0.5 m/s) highly turbulent water flow environments i.e. small rivers and streams. The work presented here focuses on a novel, lightweight, highly robust, energy harvester design referred to as “piezoelectric grass”. This biologically inspired design consists of an array of cantilevers, each constructed with piezoelectric material. When exposed to proper turbulent flow conditions, these cantilevers vibrate vigorously due to rapidly varying pressure fields along their faces. Electrical power is generated directly from these vibrations via the piezoelectric effect. Small-scale wind tunnel tests were carried out to validate the concept. Included in this paper are the results of a direct comparative study performed on two types of harvesters in air. The generating elements or “blades of grass” of one design are made of PVDF cantilevers (type-1), and those of the other design are made with PZT QuickPacks™ mounted to the base of spring steel cantilevers (type-2). Results from these tests show very clearly that optimum harvesting conditions exist. The maximum power output per cantilever was ∼1μW for the type-1 harvester, and increased up to ∼ 1mW for the type-2 harvester which is among the highest found in literature for similar harvesting methods.

Commentary by Dr. Valentin Fuster
2011;():647-652. doi:10.1115/SMASIS2011-5072.

Dielectric elastomers are known for their electro-mechanically coupled constitutive behavior which has demonstrated the potential for developing a number of novel adaptive structures. Despite the advances in understanding these materials using nonlinear field theory and experimental characterization, several questions remain regarding how to effectively integrate these materials in adaptive robotic structures. Here, a new design is proposed to integrate these materials into legged robotic structures that can achieve relatively large and rapid stiffness changes for enhanced mobility and agility of a field demonstrated hexapod robot. A set of resonance test are performed to quantify changes in effective stiffness as a function of the applied electric field. The results are incorporated into a bi-layer beam model to estimate changes in the effective stiffness of a robotic C-shaped leg. The results show promise for developing adaptive robotics legs for multi-terrain mobility.

Commentary by Dr. Valentin Fuster
2011;():653-662. doi:10.1115/SMASIS2011-5092.

Recently, flexible matrix composite (FMC) actuators were demonstrated in a robotic fish for swimming. When actuated at a specific frequency in the experiments, the sinusoidal component of the thrust was eliminated, leaving only a constant thrust. This elimination of the sinusoidal component of the thrust is due to the hydroelastic tailoring of the tail stiffness with the actuation frequency. The FMC actuators are pressure-driven muscle-like actuators capable of large displacements as well as large blocking forces. The FMC actuators can also exhibit a large change in stiffness through simple valve control when the working fluid has a high bulk modulus. Several analytical models have been developed that capture the geometrical and material nonlinearities, the compliance of the inner liner, and entrapped air in the fluid. This paper focuses on the inter fiber compaction in the composite laminate, which is shown to reduce the effective closed-valve stiffness. In this paper, a new analytical model considering the inter fiber compaction effect as well as the material and geometric nonlinearities is developed. Analysis and experimental results demonstrate that the new compaction model can improve the prediction of the response behavior of the actuator.

Commentary by Dr. Valentin Fuster
2011;():663-671. doi:10.1115/SMASIS2011-5095.

Recent research in our group has shown that artificial cell membranes formed at the base of a hair-like structure can be used to sense air flow in a manner similar to the mechanotransduction processes found in mammalian hair cells. Our previous work demonstrated that a single artificial hair cell can be formed in an open substrate. However, that study also motivated the need to develop fully-encapsulated devices that feature arrays of hair-cells. Since the transduction element in this concept is an artificial cell membrane, or lipid bilayer, this work investigates two parallel substrate designs for providing encapsulation and a method for forming arrays of bilayers. In one effort, a flexible substrate with internal compartments for hosting the biomolecules and mating cap are constructed and experimentally characterized. The regulated attachment method (RAM) is used to form interface bilayers within the sealed device. Capacitance measurements of the sealed interface bilayer show that the sealing cap slightly compresses the bottom insert and reduces the size of the enclosed bilayer. Single channel measurements of alamethicin peptides further verify that the sealed device, which is also leak-proof under water, can be used to detect the insertion and gating activity of transmembrane proteins in the membrane. The second effort pursued herein is the fabrication and initial testing of a method to form arrays of interface bilayers by using anchored hydrogel pads that support curved aqueous lenses in oil. In this fashion, the configuration of the array does not require manipulating droplets, but instead depends on the arrangement of the built-in gels used to support the aqueous lenses. As with RAM, mechanical force is used to promote contact of adjacent aqueous lenses held in the flexible substrate. Initial tests show that gel-supported lenses can be used for forming multiple lipid bilayers within the device and that these interfaces can be interrogated individually or collectively using an electrode switching circuit.

Topics: Sensors , Membranes
Commentary by Dr. Valentin Fuster
2011;():673-680. doi:10.1115/SMASIS2011-5096.

We investigate the electromechanical sensing capabilities of aligned carbon nanotube (CNT) arrays as a means for a lightweight and simple electromechanical transduction element. CNT array heights of 25 and 350 μm are examined using a modified dynamic mechanical analyzer (DMA) to impart multiple strain-based test configurations while simultaneously measuring electrical resistance. Observed gauge factors range from 12.5 for the 25μm array to greater than 190 for the 350 μm array, with a peak sensitivity of 13,750 ohms/gram force achieved for the 350 μm array. The electromechanical response observed was independent of the examined frequency range, suggesting high fidelity sensory capabilities. Results of this study serve as a preliminary proof of concept for using CNT arrays as a transduction mechanism for a proposed artificial hair sensor for small air vehicles.

Commentary by Dr. Valentin Fuster
2011;():681-690. doi:10.1115/SMASIS2011-5104.

The outer ears (pinnae) of many bat species are smart structures that undergo non-rigid deformations controlled through an intricate muscular actuation system. It is hypothesized that such non-rigid changes in the physical shape of the pinnae provide a substrate for adaption of the spatial sensitivity (reception beampattern) of the animals’ biosonar system on a short time scale. In the research presented here, a simplified biomimetic baffle shape was developed to investigate the functional properties of non-rigidly deforming baffles. This prototype had the shape of an obliquely truncated cone that was augmented with local shape features that aided in achieving a biomimetic deformation pattern and may also have direct acoustic effects on the device beampattern. The prototype was manufactured from a thin sheet of rubber and actuated parsimoniously through a single linear actuator. Despite its comparative simplicity, the prototype device was able to reproduce the deformation pattern seen in the ears of horseshoe bats qualitatively. Biomimetic baffle deformations resulted in profound, qualitative, and quantitative changes to the beampattern. Future research will investigate how the time-variant beampatterns relate to the specifics of the deformation patterns and how they could be controlled and used in an engineering context.

Commentary by Dr. Valentin Fuster
2011;():691-698. doi:10.1115/SMASIS2011-5105.

This paper presents the design, fabrication, and characterization of a second generation biomimetic jellyfish robot that uses ionic polymer metal composites (IPMCs) as flexible actuators for propulsion. The shape and swimming style of this underwater vehicle are based on the Aurelia aurita jellyfish, which has an average swimming speed of 13 mm/s and which is known for a high swimming efficiency. The critical components of the vehicle include the flexible bell that provides the overall shape and dimensions of the jellyfish, a central hub used to provide electrical connections and mechanical support to the actuators, and flexible IPMC actuators that extend radially from the central hub. In order to provide increased shape holding ability and reduced weight, the bell is fabricated from a commercially available heat-shrinkable polymer film. A new lightweight hub has been designed and was fabricated by 3D printing using ABS plastic material. The hub features internal electrical contacts for providing voltage to the individual IPMC actuators. Finally, a new set of IPMC actuators are manufactured using the Direct Assembly Process (DAP). The IPMC actuators constructed for this study demonstrated peak-to-peak strains of ∼ 0.7% in water across a frequency range of 0.1–1.0Hz. By tailoring the applied voltage waveform and the flexibility of the bell, the completed robotic jellyfish swam at maximum speed of 1.5 mm/s.

Commentary by Dr. Valentin Fuster
2011;():699-705. doi:10.1115/SMASIS2011-5109.

On avian wings, significant flow control is accomplished using localized control loops, both active and passive, between leading- and trailing-edge feathers. Conversely, most man-made flight control systems respond to perturbations in inertial measurements (global states) rather than the flow itself (local states). This paper presents the design of a distributed, biomimetic flow control system and a characterization of its performance compared to a wing with traditional control surfaces relying on inertial measurements. This new design consists of a skeletal wing structure with a network of feather-like panels installed on the upper and lower surfaces, extending beyond the trailing edge and replacing leading- and trailing-edge flaps/ailerons. Each feather is able to deform into and out of the boundary layer, thus permitting local airflow manipulation and transpiration through the wing. For this study, two airfoil sections are compared — a standard wing section with a trailing-edge flap, and section with multiple trailing-edge feathers. COMSOL Multiphysics is used to model the flow field under various flight conditions and flap deflections. A dynamics model of the wing is also simulated in order to compute the disturbances caused by wind gusts. Continuous gusts are simulated, and the disturbance rejection capabilities of the baseline and feathered wing cases are compared.

Commentary by Dr. Valentin Fuster
2011;():707-713. doi:10.1115/SMASIS2011-5117.

Alamethicin is an antibiotic peptide from the fungus Trichoderma viride that forms ion channels in bilayer lipid membranes. Each peptide consists of 20 amino acids that can form larger channels with the congregation of multiple monomers of the peptide. These formed ion channels have some voltage dependent characteristics when a potential is induced across the bilayer. This potential can be from an applied voltage source or from an ion concentration gradient inducing a transmembrane potential across the membrane. The peptide alamethicin can be modeled as a conductor that allows the flow of ions through the membrane. The formed channels have distinct conductance level states caused by accumulation of additional alamethicin monomers being added to an individual ion channel. The voltage dependence of the accumulation of multiple ion channels can be modeled for the average response. A probabilistic model is used to capture the statistics of the state changes of individual channels. This type of model can be summed to simulate the conductance of multiple channels within a bilayer. This work focuses on obtaining the statistic for individual ion channels and using those statistics to show that a probabilistic model of the peptide’s conductance can capture some of the dynamics seen in aggregated responses. The Nernst equation is used to estimate the transmembrane potential caused by an ion gradient of a bilayer in equilibrium. This potential is used in the model to assist in determining the current conductance states of an individual channel of the peptide in the presence of an ion gradient. This paper will show the experimental results of ion currents across a bilayer induced by membrane potentials and the ion currents induced by ion gradients. The statistics of the measurements are used in a probabilistic conductance model of the peptide alamethicin.

Commentary by Dr. Valentin Fuster
2011;():715-723. doi:10.1115/SMASIS2011-5134.

A flexible leg (FlexLeg) design using BiFlex actuators was designed, fabricated and characterized. BISMAC actuators are unidirectional flexible actuators capable of exhibiting high curvature. These actuators were modified to achieve bidirectional deformation. The new bidirectional actuators termed as “BiFlex” actuators, have proven the capability to achieve large deformation in two directions. The FlexLegs consist of six segments which can be actuated individually. Two different sets of legs were constructed to determine the effect of size. The small legs measure 35.8 mm in height and 63.2 mm in width and the large legs were 97.4 mm in height and 165.4 mm in width. The small FlexLegs achieved a maximum deformation of 12% and 4% in the x- and y-direction respectively using a power of 0.7 W while producing a maximum force of 0.023 N. They were also able to withstand a load of 1.18 N. The large FlexLegs had a maximum deformation of 57% and 39% in the x- and y-direction respectively using a power of 3 W while producing a force of 0.045 N. They were able to withstand a load of 0.25 N. The legs were also able to perform several walking algorithms consisting of stepping, crabbing and yawing.

Commentary by Dr. Valentin Fuster
2011;():725-732. doi:10.1115/SMASIS2011-5168.

Carbon-based flow sensors can be made by embedding carbon nanotubes (CNT) into a polymeric substrate. Specifically, when a conductive aqueous solution flows over the surface of the exposed CNT, a flow-dependent voltage is generated. The carbonaceous flow sensors fabricated in our work were all tested in salt water (5% NaCl). In order to measure the surface coverage of the CNT coated sensors, the electrical resistance across the surface of each sample was measured. Electrical Impedance Spectroscopy (EIS) measurements were also carried out in order to understand the electrical relationship between the sensor and the salt water. In order to study the surface topology and morphology of the flow sensors, scanning electron microscopy (SEM) was used. Voltage measurements of sensors with different levels of resistance were tested in varying fluid velocities. The least resistive sensor showed small, but detectable changes in voltages, while higher resistance sensors showed less response. On the other hand, the average current did not change with varying flow conditions for any of the sensors.

Commentary by Dr. Valentin Fuster
2011;():733-742. doi:10.1115/SMASIS2011-5198.

Flapping wing Unmanned Aerial Vehicles (UAVs) or ornithopters are proliferating in both the civil and military markets. Ornithopters have the potential to combine the agility and maneuverability of rotary wing aircraft with excellent performance in low Reynolds number flight regimes. These traits promise optimized performance over multiple mission scenarios. Nature achieves this broad performance in birds using wing gaits that are optimized for a particular flight regime. The goal of this work is to improve the performance of ornithopters during steady level flight by passively implementing the Continuous Vortex Gait (CVG) found in natural avian flyers. In this paper we present new experimental results for a one degree of freedom (1DOF) compliant spine which was inserted into an experimental test ornithopter leading edge wing spar in order to achieve the desired kinematics. The lift and thrust along with electric power metrics at different flapping frequencies were measured using a six-channel load cell and a current senor, respectively. These metrics were determined for the test ornithopter both with and without the compliant spine insert. Initial results validate the ability of our compliant spine design to withstand the loads seen during flight at flapping frequencies of up to and including 5 Hz. For the ornithopter test platform used in the study, inserting the compliant spines into the wing leading edge spar accurately simulates the CVG increasing the mean lift by 16%, and reducing the power consumed by 45% without incurring any thrust penalties.

Topics: Testing , Wings
Commentary by Dr. Valentin Fuster
2011;():743-752. doi:10.1115/SMASIS2011-5207.

Ornithopters or flapping wing Unmanned Aerial Vehicles (UAVs) have potential applications in civil and military sectors. Amongst the UAVs, ornithopters have a unique ability to fly in low Reynolds number regions and also have the agility and maneuverability of a rotary wing aircraft. In nature, birds achieve such special characteristics by morphing their wings. The compliant spine (CS) design concept presented here represents a novel method of achieving wing morphing passively. In this paper, an optimal design method is developed that incorporates dynamic finite element analysis. To solve the CS design problem a new multi-objective optimization problem is formulated with three objective functions. The first objective function seeks to minimize the mass of the compliant spine. The second objective function seeks to maximize the deflection of the compliant spine for a particular dynamic loading condition. Finally, the third objective function seeks to minimize the stress in the design observed under the dynamic loading conditions experienced during flight. The deflections and stresses in the CS design are based on measured wing loads and are calculated by applying a sinusoidal forcing function at a prescribed forcing frequency. The optimization, performed via a controlled elitist genetic algorithm which is a variant of NSGA-II, is used to design CSs operating under dynamic conditions. Modal analysis and frequency response of an optimal compliant spine during the upstroke are also shown.

Topics: Design , Optimization
Commentary by Dr. Valentin Fuster
2011;():753-758. doi:10.1115/SMASIS2011-5238.

The concept of self-healing materials has gained widespread acceptance in the research community. Over recent years a diverse array of bio-inspired self-healing concepts, from solid-state diffusion to liquid-phase healing in a broad range of engineering materials, embracing ceramics, polymers and fibre reinforced polymer composite materials have been proposed in the open literature. In this research study the liquid-phase healing of operational damage, namely impact damage, is being addressed. The challenge of self-healing advanced fibre reinforced polymer composites is ensuring healing success without degrading the host composite’s performance, a problem not encountered in the self-healing of generic polymeric systems. In the genre of self-healing fibre reinforced composite materials, autonomous healing has been undertaken by a healing medium already located within the damage zone and released through the damage site either passively or actively through human invention. This approach requires the ‘engineering’ control of the storage medium’s toughness for release and the development of bespoke resin chemistries to be compatible with the manufacturing route, to remain active whilst latent and then to recover full mechanical performance once a damage event occurs. This study has generated a proof of concept whereby the healing medium is only deployed to the damage site once a sensor has been triggered. In essence this study aims to develop stimuli triggered deployment of a healing medium held remotely in a storage reservoir to repair impact damage to a composite material. The principle of the concept is revolves around the ability of a reservoir to deliver a healing medium to a damage site via a network of vessels contained in the centerline of the composite laminate. A Labview controlled peristaltic pressure rig containing the reservoirs for the resin and hardener, their independent pumps, pressure gauges, control switches and indicators was developed. Through the application of an impact event successfully deliver and subsequent healing of the damage event was achieved showing the potential of this concept for minimising parasitic mass and maximising healing potential in fibre reinforced composite materials.

Topics: Biomimetics
Commentary by Dr. Valentin Fuster

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