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ASME Conference Presenter Attendance Policy and Archival Proceedings

2017;():V009T00A001. doi:10.1115/IMECE2017-NS9.
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This online compilation of papers from the ASME 2017 International Mechanical Engineering Congress and Exposition (IMECE2017) 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presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Congress-Wide Symposium on Additive Manufacturing: Failure of Additively Manufactured Materials

2017;():V009T12A001. doi:10.1115/IMECE2017-71844.

Topological interlocking is an effective joining approach in both natural and engineering systems. Especially, hierarchical/fractal interlocking are found in many biological systems and can significantly enhance the system mechanical properties. Inspired by the hierarchical/ fractal topology in nature, mechanical models for Koch fractal interlocking were developed as an example system to better understand the mechanics of fractal interlocking. In this investigation, Koch fractal interlocking with different number of iterations N were designed. Theoretical contact mechanics model was used to analytically capture the mechanical behavior of the fractal interlocking. Then finite element (FE) simulations were performed to study the deformation mechanism of fractal interlocking under finite deformation. It was found that by increasing the number of iterations, the contact area increases and the interlocking stiffness and strength also significantly increase. The friction coefficient of contact plays an important role in determining the mechanical properties of fractal interlocking.

Topics: Modeling , Fractals
Commentary by Dr. Valentin Fuster
2017;():V009T12A002. doi:10.1115/IMECE2017-72623.

This paper presents an experimental investigation into the effects of the application of carbon nanotube (CNT) based nanopolymer, and thin film buckypaper, to the interface of stiffened carbon fiber reinforced polymer (CFRP) composite joints. Bonded CFRP composite T-joints, were manufactured with dispersed CNT epoxy nanopolymer mixture, and buckypaper films, applied at the joint interface, and tested under pull-off loading. The presence of the nanomaterial at the interface causes a localized out-of-plane reinforcement, which resists pull-off loads, leading to superior performance compared to composite bonded joints without nano-reinforcements, however, the introduction of substantial voids, in the case of the buckypaper samples, lead to faster structural failure. Digital image correlation (DIC) was used to map the strain contours of the T-joint specimen during testing, which revealed damage initiation and hot-spot zones. Fluorescent optical microscopy of the joint sections was also performed to investigate these hot-spot zones and damage initiation areas, at the mesoscale, to study the possible causal mechanisms of the failure process in the tested composite bonded joints.

Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Dynamic Failure of Advanced Materials

2017;():V009T12A003. doi:10.1115/IMECE2017-70354.

This research study highlights the testing method and relevant results for assessing impact performance of a carbon fiber composite front bumper crush can (FBCC) assembly subjected to full frontal crash loading. It becomes extremely important to study the behavior of lightweight composite components under a crash scenario in order to apply them to automotive structures to reduce the overall weight of the vehicle. Computer-aided engineering (CAE) models are extremely important tools to virtually validate the physical testing by assessing the performances of these structures. Due to lack of available studies on carbon fiber composite FBCCs assemblies under the frontal crash scenario, a new component-level test approach would provide assistance to CAE models and better correlation between results can be made. In this study, all the tests were performed by utilizing a sled-on-sled testing method. An extreme care was taken to ensure that there is no bottoming-out force for this type of test while adjusting the impact speed of sled. Full frontal tests on FBCC structures were conducted by utilizing five high-speed cameras (HSCs), several accelerometers and a load wall.

Excellent correlation was achieved between video tracking and accelerometers results for time histories of displacement and velocity. The standard deviation and coefficient of variance for the energy absorbed were very low suggesting the repeatability of the full frontal tests. The impact histories of FBCC specimens were consistent and in excellent agreement with respect to each other. Post-impact photographs showed the consistent crushing of composite crush cans and breakage of the bumper beam from middle due to the production of tensile stresses stretched caused by straightening of the bumper curvature after hitting the load wall.

Commentary by Dr. Valentin Fuster
2017;():V009T12A004. doi:10.1115/IMECE2017-70357.

This research article presents the crashworthiness response of carbon fiber composite front bumper crush can (FBCC) assembly subjected to 40% offset frontal impact loading. Automobile manufacturers continue to strive for overall vehicle weight reduction while maintaining or enhancing safety performance. Therefore, the physical testing of lightweight materials becomes extremely important under a crash scenario in order to apply them to automotive structures to reduce the overall weight of the vehicle. In this study carbon fiber/epoxy lightweight composite material is chosen to develop frontal bumper beam crush can assemblies. Due to lack of available studies on carbon fiber composite FBCCs assemblies under frontal offset crash scenario, a new component-level experimental study is conducted in order to develop data that will provide assistance to CAE models for better correlation. A sled-on-sled testing method was utilized to perform tests in this study. 40 % offset frontal tests on FBCC structures were conducted by utilizing three high-speed cameras (HSCs), several accelerometers and load wall.

Impact histories i.e. crash pulse, force-time history, force-displacement, impact characteristics and deformation patterns from all FBCC tests were consistent. The standard deviation and coefficient of variance for the energy absorbed were very low suggesting the repeatability of the 40% offset tests. Excellent correlation was achieved between video tracking and accelerometers results for time histories of displacement and velocity. Post-impact photographs showed the progressive crushing of composite crush cans, bumper beam/crush can adhesive joint failure located on unimpacted side and breakage of the bumper beam due to the production of shear stresses as it is stretched due to its curvature after hitting the sled.

Commentary by Dr. Valentin Fuster
2017;():V009T12A005. doi:10.1115/IMECE2017-70428.

Phononic crystals are composites with architected microstructure that exhibit superior shock mitigation properties which cannot be achieved through natural materials. In this study, the capability provided by layered phononic crystals for protection of structures subjected to near-contact detonation is investigated. To evaluate the protective performance of the layered composite, finite element simulation of a reinforced concrete (RC) column, with a layered composite attached to its surface, subjected to near-contact detonation is performed. As a reference case, the same RC column under the same near-contact detonation, without the layered composite, is also studied. Contours of damage and residual load carrying capacity of the RC column are analyzed for both cases. It is observed that due to optimized band-gap in the composite, high frequency components of the shock wave are filtered, while the low frequency components of the shock front are highly scattered. Therefore, the intense shock front with large peak overpressure and short duration gets dispersed and transforms into a wave with a longer duration and lower peak overpressure. Comparing the damage pattern in the protected RC column with the bare column, high level of protection provided by the layered composite is demonstrated. This study provides insight on how stress waves can be controlled through microstructural design of phononic crystals through topology optimization to achieve a desired dynamic and structural response.

Commentary by Dr. Valentin Fuster
2017;():V009T12A006. doi:10.1115/IMECE2017-70517.

The wheel is one of the important safety components of the vehicle. So, it is required to pass the dynamic rotating bending test, the dynamic radial fatigue test and the impact test. The 90-degree impact test represents the driving performance of a vehicle when the vehicle drives through the road pits, or drives in other harsh conditions. As for the steel wheel, there are no mandatory requirements for the impact test. In recent years, some steel wheel enterprises bring up 90-degree impact test for steel wheels in order to ensure the quality of their products. In this paper, a finite element simulation model of the steel wheel impact test bed under the case of 90-degree was established according to an enterprise’s impact test requirement. The software “ABAQUS” was used to simulate the 90-degree impact test. A wheel / tire overall model was assembled, considering the impacts of tire inflation and the tire preloading process. Then the deformation state of the rim under 90-degree impact load was analyzed to predict whether it could pass the requirements of relevant impact test successfully. The results show that the steel wheel does not meet the requirements of the impact test, which makes it necessary to study the steel wheel’s impact test and optimize the structure of the rim. This paper also provides a reference method for the impact simulation of the steel wheel.

Topics: Steel , Simulation , Wheels
Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Fatigue and Fracture of Active Materials

2017;():V009T12A007. doi:10.1115/IMECE2017-70729.

Changes in surface morphology have long been thought to be associated with crack propagation in materials. In this paper, we study the changes in the surface profile of the crack-tip plastic zone with an attempt to understand the relationship between the plasticity-induced surface profile changes and the crack growth behavior. Center crack specimens were electropolished and etched to reveal the grain structure for white light interferometer (WLI) imaging prior to and during fatigue testing. After growing the crack to a predetermined pre-crack length, a viewing zone was selected outside of the plastic zone of the pre-crack. The surface profile of the viewing zone was imaged using a WLI microscope at selected fatigue cycle intervals. An image processing algorithm was developed to evaluate the changes of the surface profile. We observed that the crack growth rate is not uniform at the microscopic scale; the crack growth was retarded at crack pinning points and the crack grows at a faster rate while propagating between the pinning points. Relatively large surface topology changes were observed to be constrained to the area surrounding the tip of pinned cracks. However, there was an avalanche of surface changes covering the entire monotonic zone upon the crack being released from a pinned location. Interestingly enough, minor or no measurable surface changes could be seen for propagating cracks. These results indicate a surface roughness change threshold may exist for predicting the duration during which a crack is pinned. Results suggest the threshold and crack propagation rate between pinning locations may be functions of the amplitude of the stress intensity factor.

Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Fatigue and Fracture of Joining Materials

2017;():V009T12A008. doi:10.1115/IMECE2017-71711.

With the advantage of having a high strength to weight ratio, composite materials are frequently being implemented as alternatives to steel and aluminum in military vehicles. To perform satisfactorily, joined composite laminates on a vehicle must be able to absorb a significant amount of energy under high strain rate loading events such as ballistic impact. In this paper the dynamic behavior and failure modes of adhesively bonded S2-glass/epoxy laminate joints are investigated. For this experiment, two structural adhesives are selected for comparison: a brittle methacrylate and a more compliant epoxy. The tests are conducted on an in-house assembled gas-gun to achieve the high strain rates necessary to break the adhesive bonds in two configurations, Mode I and II. Results obtained from the ballistic impact tests are compared to quasi-static test results to emphasize the rate-sensitivity of the bonded joints. Irrespective of the material configuration, the failure load of the adhesively bonded joint is seen to increase with the loading rate. Overall, epoxy appears to be 35–50% stronger than methacrylate by most measures. Under bending loading (mode I), most cases exhibit some amount of damage within the composite surrounding the bonded area, demonstrating a fiber-tear failure rather than a cohesive failure. The failure strength of the composite joint is thus not always proportional to the adhesion strength of the adhesive due to the weakness of delamination of the composite material, especially when loaded through the thickness of the composite. As compared with metal adherends, the composites are shown to absorb three times more energy per unit area.

Commentary by Dr. Valentin Fuster
2017;():V009T12A009. doi:10.1115/IMECE2017-71721.

Adhesive joint technology has been developed gradually, and the application fields of this type of joints have been expanded increasingly since they reduce the weight of the applications, provide uniform stress distribution across the joints, allow to bond similar, and dissimilar materials, and contribute to dampen the shock, and vibration. However, the performance of the adhesive joints under high loading rate such as blast or ballistic loading has been studied by few researchers. In this study, fully laminated plates consisting of 6061 aluminum plates (15” in diameter and 1/16” thick) and FM300K epoxy film adhesive were tested under shock wave loading. Full displacement field over the testing plates were obtained by TRC-SDIC technique, and the strain on the plates were computed by classical plate theory for large deflections. FEM model was analyzed and the results were compared with experimental results.

Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Mechanical Characterization in Extreme Temperature Environments

2017;():V009T12A010. doi:10.1115/IMECE2017-71540.

Despite the significant progress in the development of modern alloys, low alloy steels continue to be the materials of choice for large structural components at elevated temperature for extended periods of time. The resistance of these alloys to deformation and damage under creep and/or fatigue at elevated temperature make them suitable for components expected to endure decades of service. The material 2.25Cr-1Mo is commonly applied in boilers, heat exchanger tubes, and throttle valve bodies in both turbomachinery and pressure-vessel/piping applications alike. It has an excellent balance of ductility, corrosion resistance, and creep strength under moderate temperatures (i.e., up to 650°C). In the present work, a life prediction approach is developed for situations where the material is subjected conditions where creep and fatigue are prevalent. Parameters for the approach are based on regression fits in comparison with a broad collection experimental data. The data are comprised of low cycle fatigue (LCF) and creep fatigue (CF) experiments. The form of the life prediction model follows the cumulative damage approach where dominant damage maps can be used to identify primary microstructural mechanism associated with failure. Life calculations are facilitated by the usage of a non-interacting creep-plasticity constitutive model capable of representing not only the temperature- and rate-dependence, but also the history-dependence of the material. For the inelastic response, both the Garofalo and Chaboche models for creep and plasticity are employed, respectively.

Commentary by Dr. Valentin Fuster
2017;():V009T12A011. doi:10.1115/IMECE2017-72139.

In this study, interrupted creep and creep-fatigue tests of Alloy 617, which is a candidate alloy for boiler tubes and pipes of A-USC (advanced ultra-supercritical) power plants of the 700°C-class, were conducted to investigate damage evolution process. Also, the change of the micro texture of the alloy was continuously observed at a fixed area to elucidate the mechanism of damage evolution under creep and creep-fatigue loading from the viewpoint of the change of the order of atom arrangement using EBSD (Electron Back-Scatter Diffraction) analysis. The conditions of the creep test were a temperature of 800°C and the stress of 150 MPa in inert gas (99.9999% Ar). The stress-controlled creep-fatigue tests were carried out at 800°C in Ar using stress ratio R = −1 and hold time of 10 minutes at peak tension. IQ (Image Quality) values, which are the average sharpness of the obtained diffraction pattern, were used for evaluating the change of the micro texture during the tests. In both creep and creep-fatigue test, intergranular cracks appeared. The IQ value decreased monotonically in the vicinity of grain boundaries with the decrease of fracture life, indicating that the crystallinity of grain boundaries degraded faster than that of grains. This localized damage around grain boundaries was attributed to the intergranular crack propagation in the creep and creep-fatigue test. In addition, all the grain boundaries with IQ value lower than 85% of IQ value in as-received specimen were found to be cracked during both creep and creep-fatigue test. Therefore, there was the critical IQ value around grain boundaries at which intergranular cracks occurred under creep or creep-fatigue loading condition.

Topics: Creep , Fatigue , Heat , Nickel , Alloys , Damage
Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Mechanical Metamaterials

2017;():V009T12A012. doi:10.1115/IMECE2017-70854.

Kinematic motion structures having a reconfigurable property appear to be a potential candidate for the programmable matter. Motion structures with N-fold rotational symmetry show a reconfigurable pattern transformation, resulting in providing tunable mechanical properties, which deserves to explore more for their unique properties of transformation and the corresponding structural behaviors. The objective of this work is to synthesize motion structures from a bar-and-joint framework, investigating their transformability, linear structural properties - modulus and Poisson’s ratio, and nonlinear structural behaviors with kinematic bifurcation. Two-dimensional (2D) motion structures are synthesized by central scissor links with revolute joints, connected with binary links in the radial direction. They possess an N-fold rotational symmetry (MS-N), and their transformed patterns are investigated. Five 2D motion structures — MS-4, MS-6, MS-8, MS-10, and MS-12, are generated for investigating their mechanical properties together with their transformability. Analytical models of the motion structures are constructed for obtaining relative density, moduli, Poisson’s ratios, volume at each transformed state, and the strain energy required to transform from one state to another. This study integrates kinematics and structural mechanics, expanding the design space of light-weight structural materials with pattern transformation.

Commentary by Dr. Valentin Fuster
2017;():V009T12A013. doi:10.1115/IMECE2017-70858.

Programmable matter, a material whose properties can be programmed to achieve desired density with volume change, shapes or structural properties (stiffness, strength, Poisson’s ratio, etc.) upon command, is an important technology for intelligent materials. Recently emerging soft robotics-based pneumatic control can be potentially used for the design of programmable matter due to its several advantages — quick response for actuation, stiffening effect with internal air pressure, easy to manufacture, inexpensive materials, etc. The objective of this work is to construct programmable two-dimensional (2D) cellular structures with pneumatic actuators, investigating the effect of local deformation of the pneumatic actuators on the macroscopic pattern generation and mechanical properties of cellular structures. We synthesize 2D soft triangular structures with pneumatic actuators embedding dual air channels wrapped with fiber reinforcement. The local deformation modes provide different macroscopic deformations of cellular structures. We build an analytical model integrating the deformation of a single actuating member with nonlinear deformation of cellular structures. Finite element based simulations and experimental validation are followed. This study integrates soft robotics with cellular structures for intelligent materials design, expanding the design space of materials with programming. The fast response of the tunable soft cellular structures may be an ideal for the application of acoustic metamaterials with tunable band gaps.

Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Mechanics and Design of Cellular Materials

2017;():V009T12A014. doi:10.1115/IMECE2017-70409.

Thin wall structures are primarily deployed in automotive chassis to increase the energy absorption capacity of the automobiles in the event of an accident. Researchers have delved into developing lighter structures for improving automobiles’ fuel efficiency with a challenge of maintaining or preferably exceeding the energy absorption properties of the structure. In this study, the work presented is a continuation of research conducted on exploring the effects of the introduction of cellular core in tubular structures under axial compressive loading. The crushing response of cellular core hybrid tube was numerically studied using ABAQUS/Explicit module. The characteristics such as deformation or collapsing modes, crushing/ reactive force, locking strain, energy curves, and specific energy absorbed were studied. The cellular core hybrid tube shows significant potential for reducing the weight of automobile structure while giving positive indication towards enhancing the specific energy absorption capacity.

Commentary by Dr. Valentin Fuster
2017;():V009T12A015. doi:10.1115/IMECE2017-71006.

Porous materials are of interest for a number of applications one of them being energy absorption. These materials offer the ability to absorb more energy than a typical metallic solid and thus provide an opportunity to improve the performance of structures that endure blast loads. These structures undergo very large loads in very short periods of time and therefore maximizing energy absorption is paramount. This study seeks to improve the understanding of the response of porous materials by developing both analytical and finite element models for a liquid filled porous cylinder exposed to a dynamic compression loading.

The poroelastic cylinder consists of a porous metallic solid phase and a viscous liquid phase. These two phases provide for two mechanisms of energy dissipation which are that of the deformation of the solid and the viscous flow of the liquid. The theories of elasticity and porous media were used to formulate the governing equations for the liquid filled porous cylinder. These equations describe the coupling between the displacements of the solid cylinder and the pressure distribution of the liquid. Analytical and finite element models were developed to predict the cylinders response in order to determine the amount of energy absorbed when the cylinder is exposed to a dynamic compression load. Analytical models were developed to validate the finite element results. As more complexity is added to this problem an analytical approach becomes unviable and a finite element approach must be used. One such complexity that can be considered is the effect of utilizing a non-constant liquid viscosity, which requires developing a non-linear finite element model to account for the viscositys dependence on strain rate. This added non-linear effect should allow for additional viscous energy to be absorbed and thus can further enhance the performance of the system.

Topics: Pressure , Stress , Cylinders
Commentary by Dr. Valentin Fuster
2017;():V009T12A016. doi:10.1115/IMECE2017-71845.

In this investigation, mechanical behavior of periodic cellular solids with diamond-shaped inclusions connected via wavy network were explored. Two families of cellular solids within this category were designed based on two different geometric constraints. Auxetic effects and snap-through instability were observed for each family, respectively. The mechanical properties, including the stress-strain behavior, stiffness and Poisson’s ratio, were systematically quantified via finite element (FE) simulations. The parametric space for auxetic effects and snap-through instability was numerically identified. This study demonstrates the connection and transition between mechanical auxeticity and snap-through instability. The materials designed have potential engineering applications, such as lightweight supporting and protective foams, biomedical devices, smart composites or fabrics with switchable properties responsive to external environments.

Topics: Diamonds
Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Mechanics of Adhesion and Friction

2017;():V009T12A017. doi:10.1115/IMECE2017-70218.

The present study belongs to a broader investigation aiming to reduce noise emissions in nail guns. This noise reduction objective may be achieved by nail gun concept design improvements. However, modifying the tool design requires precise understanding of it dynamics. Therefore a dynamic model of the system including accurate predictions of the tribo-dynamic interactions at the wood-nail interface generating the penetration resistance forces (PRF) appears to be essential. Since different wood products possess different structural/material properties, PRF is first evaluated for various types of wood product individually. Ref. [1] develops the PRF modeling strategy and examines the nail penetration process for plywood samples. The present paper proposes an empirical model predicting PRF imposed on nails when penetrating particle board (PB) at quasi-static velocities (20–500 mm/min range). A universal testing machine (MTS) is used to drive the nails into the wood samples. Each wood sample is composed of five panels PB screwed together. The sample size is chosen to reduce the boundary influence on the penetration process and to avoid the complete perforation of the sample. To eliminate the possible acceleration influence, the penetration speed is maintained at constant amplitudes. The MTS machine measured PRF as a function of the position. The objective is to prepare a formulation predicting PRF as a function of nail position. In order to extend the prediction formula application range, the analysis reduces the studied factors to dimensionless parameters. The analysis shows that the PB fabrication process results in panels presenting three regions of different hardness modulus. As a result, at the region transitions the PRF curves show large amplitude fluctuations. This layered heterogeneity makes the development of a high precision prediction model representing various nail sizes very difficult. Nevertheless, the final model produces PRF evaluations with overall precision greater than 88% for most of the nail penetration.

Commentary by Dr. Valentin Fuster
2017;():V009T12A018. doi:10.1115/IMECE2017-70472.

In order to study the dry rough line-contact mechanism between two longitudinally rough metallic surfaces, the measured profile is mathematically described by quadratic functions for the application of the existing micro-contact models. The mechanical parameters are determined using the different approximating criteria. Next, based on these deterministic parameters, different micro-contact models are employed and extended to predict the characteristics of a line-contact. Comparison of different theoretical calculation results reveals that the greater maximum values of the contact deformation and the ratio of real to nominal contact area are predicted by the Hertz model as compared to the micro-contact models considering the elastoplastic deformation, and that the KE (Kogut and Etsion) and JG (Jackson and Green) models predict closer results. It is also found that when the rough surfaces are described by quadratic functions according to the same area criterion or same root mean square (RMS) criterion, the line-contact responses between them prescribed by any micro-contact models have the same tendency.

Commentary by Dr. Valentin Fuster
2017;():V009T12A019. doi:10.1115/IMECE2017-70626.

The growth of lightweight components and need for non-destructive fastening techniques leads to the use of adhesives in many industries. Modeling the behavior of adhesives in fastening joints can help in the design process to make an optimized joint, with minimal waste. However, in available material properties provided by manufactures of adhesives there is a gap in what is sufficient to accurately model and predict the behavior of real-world adhesive conditions. An adhesive joint may be loaded in mode I, mode II, mode III, or a combination of these in service. In components with outdoor application the ambient temperature outside in many regions can vary to below freezing to over 40 °C. The environmental conditions at these temperatures may influence the adhesive material properties. This body of research presents the results of adhesive properties subject to temperature testing. The needed material properties to compose an accurate model have been shown to be the mode I cohesive strength, mode I cohesive toughness, mode II cohesive strength, and mode II cohesive toughness. These properties can be measured with a test specimen designed to isolate that loading mode and condition. The specimens used are the Dog Bone Tensile Specimen (DBTS), the Double Cantilever Beam (DCB), Shear Loaded Dual Cantilever Beam (SLDCB), and Double Lap Shear (DLS). The effect of temperature will be tested by testing each specimen at −30°C, 20°C, and 45°C. Triplicates of each specimen at the respective temperature were tested. These results will be used in a cohesive zone model that will be validated with additional testing. The results from the two tested adhesives, Plexus MA832 and Pliogrip 7779/220, indicate these temperature conditions can change the cohesive strength in mode I by −60 to −40 % and mode II by −13 to 2% when at high temperatures (HT). The cohesive toughness in mode I by −40 to −20% and mode II by −40 to −2% when at high temperatures. The cohesive strength in mode I by −50 to 15% and mode II by 8% to 60% when at low temperatures (LT). The cohesive toughness in mode I by −70 to −20% and mode II by 30 to 60% when at low temperatures. As compared with those tested at room temperature (RT). The ranges here represent the response for both adhesives.

Commentary by Dr. Valentin Fuster
2017;():V009T12A020. doi:10.1115/IMECE2017-71200.

An involute spur gear pair meshing model is firstly provided in this study to achieve relevant data such as rolling velocity, sliding velocity, curvature radius etc. These data are needed in a transient, Newtonian elastohydrodynamic lubrication (EHL) model which is provided later. Based on these two models, the behavior of an engaged spur gear pair during the meshing process is investigated under dynamic conditions, film thickness, pressure, friction coefficient etc. could be achieved through the models. Then, power loss under certain operating condition is calculated. Relationship between power loss and lubrication performance is also analyzed.

Commentary by Dr. Valentin Fuster
2017;():V009T12A021. doi:10.1115/IMECE2017-71466.

In recent years, micro surface texturing for friction and adhesion control has gained momentum in a wide range of applications, such as MEMS devices, punches, and tools used metal forming processes, and injection molding machines.

In this study, air hardened tool steel, A2, with micro hexagonal dimples of different sizes and densities but constant depth, have been modeled and tested under dry sliding contact. Three-dimensional finite element models depict sliding dry contact between a rigid indenter and elastic-plastic textured surfaces are simulated. Coefficients of friction have been determined and compared for different texturing sizes and densities. In addition, these hexagonal patterns were fabricated on tool steel (A2) samples using photolithography. Coefficients of friction were experimentally measured using micro scratch tribometer. Both simulation and experimental results show there is a strong correlation between micro-texturing parameters and coefficient of friction. The results demonstrate that under dry sliding contact, coefficient of friction can be controlled through optimization of micro texturing parameters, specifically the spatial texture density (D/L) which is equal to the ratio of the size of the dimple (D) to the distance between the centers of two consecutive dimples (L). A minimum coefficient of friction exits at values of spatial texture densities (D/L) that range between 0.25 and 0.5 for this specific material.

Topics: Friction
Commentary by Dr. Valentin Fuster
2017;():V009T12A022. doi:10.1115/IMECE2017-71537.

Friction and wear of viscoelastic materials like rubbers are topics of extreme practical importance such as the construction of tires, shoe heels and soles, rubber O-ring seals, and wiper blades. Friction of viscoelastic materials differs from the frictional properties of the elastic solids as friction is directly related to energy dissipation via the internal damping of such materials while purely elastic materials do not dissipate energy. Based on hysteresis properties of viscoelastic materials, physics based multiscale models were developed by Persson for fiction [1, 2] and powdery wear [3] of rubbers sliding on rough surfaces. In this research, these theories were studied and the theoretical results were compared with experimental results obtained from a dynamic friction/wear tester. The inputs to the theoretical models were the fractal properties of the rough surface, the dynamic modulus, and the fatigue behavior of the viscoelastic material. The fractal properties of the rough surface was obtained from the 3D profile of the surface measured using an optical profilometer. The dynamic modulus of the rubber samples was characterized via dynamic mechanical analysis at different frequencies and temperatures. The fatigue crack growth behavior of the samples were found from experimental results of crack propagation versus tearing energy obtained from the fatigue test. Then, the friction coefficient between different rubber samples and rough surfaces was calculated as a function of sliding velocity using both analytical model and experimental approach. In the dynamic friction/wear tester, normal force was adjusted and measured accurately, in addition, the frictional force was measured using a load cell in longitudinal direction along the sliding axis. The experimental sliding friction coefficient was calculated as the ratio of longitudinal force at a constant velocity to the normal force. The mass loss of rubber sample was measured by weighting the sample before and after each test to obtain the wear rate. The comparison between experimental and analytical results showed that the friction model could predict the friction coefficient accurately while the theory of powdery wear is unable to capture all the physics involved in rubber wear on rough surfaces.

Commentary by Dr. Valentin Fuster
2017;():V009T12A023. doi:10.1115/IMECE2017-71791.

Inspired by biological suture joints with wavy morphology, wavy adhesive joints were designed and the shear resistance of the designs were explored via finite element (FE) simulations. The influences of waviness and material properties of the layer on the mechanical behaviors of the adhesive joints were quantified. Both adhesive and cohesive failure mechanisms were explored: (1) delamination along the interface between the softer layer and the harder substrates, and (2) layer material failure.

In the FE models, both cohesive interaction and ductile damage mechanics models were used to capture the two failure mechanisms. The effects of Young’s modulus and damage evolution parameters on the force-displacement relation were studied. Both failure mechanisms were observed by varying the material properties in the adhesive layer. It was found that, the stiffness, strength and the failure mechanisms of the wavy adhesive joints are largely dependent on the geometry and material properties of the layer.

Commentary by Dr. Valentin Fuster
2017;():V009T12A024. doi:10.1115/IMECE2017-71950.

A method to computationally determine the interfacial contact pressure and shear stress in bilayers has been evaluated. The method uses “Tied-Contact Pair” conditions between two shell surfaces of a coating and a substrate in Abaqus, which constitutes a bilayer circular disk. This method was evaluated for three cases: free expansion (simply supported at the center), fixed edge, and pinned outer edge. The results were validated with previous work. The displacements obtained were typically in the range of 2–3% of the theoretical values for the linear simulations. Comparisons were made between the theoretical stress values and the radial, tangential stresses obtained at the interfacing surfaces of the bilayer from the tied model. This provided a method to perform a preliminary investigation of the interfacial stresses at the interfacing shell surface, namely the normal contact pressure and shear stress as well as the radial & tangential stresses of the constituent layers for these two shell surfaces. The non-linear SAX2 shell element type was used in this simulation. A second model which uses “Mesh-Tie Constraints” in Abaqus was also investigated. The values obtained for both these models was found to match each other. However, the difficulty with the “Mesh-Tie Constraints” method was that it did not allow the interfacial stresses to be computed explicitly; a problem not encountered with the “Tied-Contact Pair” approach. A convergence study was performed to establish convergence of the model. This method holds promise to study the complex interfacial stress developed in multi-layered coatings typically encountered for Physical Vapor Deposition (PVD) processes.

Topics: Stress , Shells
Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Mechanics of Deformation and Failure of Energy Materials

2017;():V009T12A025. doi:10.1115/IMECE2017-70464.

High-speed and heavy-loaded rotating machinery require accurate prediction of rotor’s response and stability, which can be characterized by the static and dynamic coefficients of the bearing support. In this paper, a theoretical study has been done to investigate the performance of a fixed-tilting pad journal bearing with ball-in-socket pivot. The analytical model is established with the flexibility of the pad pivot and turbulent effect of the oil film both taken consideration. Under such situation, the pad pivot elastic deformation and its stiffness are calculated using Hertz Contact Theory for various operating points of the rotor-bearing system. The finite element method is adopted to simulate the static coefficients of the fixed-tilting pad bearing, obtaining its oil film pressure distribution varied with the bearing eccentricity ratio. The corresponding dynamic stiffness and damping of the oil film are solved using partial derivative method. In addition, a special interest is put in investigating the effect of the series complex stiffness of the oil film and pad pivot, according to which, the equivalent dynamic characteristics are obtained. The results show that the relation between these two factors are complex and interactive, both of which have a significant influence on the static and dynamic performance of the bearing.

Commentary by Dr. Valentin Fuster
2017;():V009T12A026. doi:10.1115/IMECE2017-71400.

Understanding the transport of hydrogen within metals is crucial for the advancement of energy storage and the mitigation of hydrogen embrittlement. Using nanosized palladium particles as a model, recent experimental studies have revealed several highly nonlinear phenomena that occur over a long period of time. The time scale of these phenomena is beyond the capability of established atomistic models. In this work, we present the application of a new model, referred to as diffusive molecular dynamics (DMD), to simulating long-term diffusive mass transport at atomistic length scale. Specifically, we validate the model for the long-term dynamics of a single hydrogen atom on palladium lattice. We show that the DMD result is in satisfactory agreement with the result of the classical random walk model. Then, we apply the DMD model to simulate the absorption of hydrogen by a palladium nanocube with an edge length of 16 nm. We show that the absorption process is dominated by the propagation of a sharp, coherent α/β hydride phase boundary. We also characterize the local lattice deformation near the dynamic phase boundary using the mean positions of the palladium and hydrogen atoms.

Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Mechanics of Soft Materials

2017;():V009T12A027. doi:10.1115/IMECE2017-71088.

Oil seals or radial lip seals are widely used in reciprocating, oscillating and rotating shaft applications. The sealability and durability of a lip seal greatly depends on the contact load and contact pressure distribution. It is challenging to find these contact parameters of the seal due to non-linear material behavior and small contact width, therefore numerical simulation can prove to be a viable method. In this paper, to address these challenges and to develop a robust numerical methodology, a Finite Element Model of a lip seal is created in ANSYS APDL. This model includes contact elements to model the lip seal’s contact-fit with certain interference, nonlinear material properties of elastomer and effect of the finger spring molded in the rubber body of the seal. The parameters for two term Mooney Rivlin Model for elastomer are obtained from simple uniaxial tension test. The numerical results demonstrate that the contact load exerted by the composite seal (with spring) is higher than the contact load exerted by elastomer portion of seal alone. It can be implied that the spring augments the radial load and increases the stiffness of the lip, which improves the lip’s sealability and durability. Experimental study is carried to validate the numerical results. The experimental results correspond well with the numerical results.

Commentary by Dr. Valentin Fuster
2017;():V009T12A028. doi:10.1115/IMECE2017-71380.

The modeling of material behavior is an important challenge in structural dynamics. While some materials can be well represented by a linear constitutive law, this becomes more complex when dealing with viscoelastic components. In this paper we investigate a fractional viscoelastic material model and present our results of research, focusing on its parametrical characteristics. We compare the results to a classical linear viscoelastic standard model and highlight advantages of the particular approach: we conduct monofrequent sinusoidal excitations using a DMTA (Dynamic Mechanic Thermal Analysis) machine. We use a viscoelastic TPU (Thermoplastic Polyurethane) sheet as sample and apply varying excitation frequencies and amplitudes. In a first modeling step we reproduce the experimental results with a fractional single degree-of-freedom system with promising results.

Commentary by Dr. Valentin Fuster
2017;():V009T12A029. doi:10.1115/IMECE2017-71962.

This study seeks to develop novel multi-material and multi-layer pads that are comfortable to wear and effective in protecting body parts that are subject to blunt impact. The proposed body protection pad will address a safety issue prominent in elderly people, industry workers, law enforcement/military personnel, and sport players. Among the population of those people, blunt impact due to various causes such as falls, bullets, and blast waves reduce quality of life, increase the possibility of early death, and cause extremely high medical costs to incur. Protector pads represent a promising strategy for reducing impact force and preventing injuries in high-risk individuals. However, clinical efficacy has been limited by poor user compliance. Currently available protectors are made of either hard shells or soft thick pads. Some of them are made of Non-Newtonian materials that are believed to be very efficient but their effectiveness hasn’t been proved yet. Even though some available protectors can be effective if worn, most people who need protection are reluctant to wear bulky and heavy garments or rigid shells. Therefore, it is important to develop new body protectors that best combine each individual’s requirements of wearing comfort (flexible, light weight), ease of fitting (customized), ensured protection, and cost-effectiveness.

The authors brought up many different design ideas and the most promising ones were selected and their effectiveness is investigated in detail. One of those pads utilizes dome shape top layer and thin fabric membrane component, such as Kevlar, that is very strong in tension but flexible in bending. Such design will make the pads excellent in dissipating shock energy and converting normal shock force to lateral direction to minimize the shock force transmitted to the body parts. Through computational simulations, these pads were proved to be very flexible in bending and torsion while strong and rigid in compression. In addition, suitable materials were identified, and it has been verified that such materials can be used to design a viable product(s) that is thin, light, and flexible for wearing comfort but strong in normal impact direction to protect the body.

This paper reports a parametric study using computational analyses (finite element analyses) conducted for dome-shaped structures with various materials such as thermoplastic polyurethane (Ninjaflex® and Semiflex®), polyethylene, resin polyester, polylactic acid (PLA), resin epoxy, epoxy S-glass, and epoxy E-glass. Parametric 3D CAD models of the dome-shape structures were created with various combinations of layers such as dome shell only, dome with fabric (such as Kevlar) membrane, dome with fabric membrane and solid filler, and dome with fillers of auxetic structure. Then, key structural characteristics of protectors such as normal (compression), bending, and torsional stiffness were evaluated through static analyses of FEA models. Then, impact/shock analyses were conducted using multiphysics finite-element-analysis models to validate the results obtained from the static analyses. Advanced additive manufacturing techniques (3D printers) were used to build prototypes of the pads for tests. Dimensions and materials of the multi-layer pads are optimized for light weight and flexibility while keeping excellent shock absorption capability. The mechanism for ideal input force distribution or shunting are explained and suggested for designing protectors using various combinations of materials and layers to reduce the risk of injury. The results show that the dome-shape structure can be an effective component of optimized body protection pads using a combination of various materials.

Commentary by Dr. Valentin Fuster
2017;():V009T12A030. doi:10.1115/IMECE2017-72097.

Rapid deployment of marine structures is of growing importance to U.S. naval forces. Surface-based inflatable structures including Rigid Inflatable Boats (RIBs), inflatable causeways and bridging, and launch and recovery systems provide unique solutions for temporary structures during sea-based missions. When performance specifications demand minimal weight and stowage, rapid deployability and temporary rigidity, solutions are limited to inflatable structures constructed of flexible materials. Driven by air pressure, today’s inflatables provide significant load-carrying capacities per unit weight (or stowed volume) utilizing technical textiles, elastomers or “soft” composites. Overloading of inflatable structures produces unique fail-safe behaviors (reversible wrinkling) that allow the structures to assume rigidity and load-carrying capacity upon load removal.

Design standards are virtually nonexistent for inflatable structures involving shapes constructed of spheres, beams, arches and most recently flat panels using 3D woven drop-stitch panels. Predictive performance tools (analytical and numerical) for static applications lag significantly behind those for conventional structures. Nonlinear system behaviors (material and geometric), thermo-mechanical coupling and fluid-structure interactions (FSI’s) pose significant challenges when applying existing design tools to inflatable structures. This gap is further exacerbated for dynamic applications as inflatable structures exhibit pressure-dependent natural frequencies and mode shapes. Surface-based structures must be designed with consideration given to operational sea state frequencies and wave periods so that the onset of structural instabilities (wrinkling, buckling) and loss of load-carrying capacities can be prevented.

The present research establishes the validity of physics based models using the Ideal Gas Law as an equation of state (EOS) to predict the natural frequencies and corresponding mode shapes of air-inflated drop-stitch fabric panels as functions of inflation pressure. Particular concern is given to the breathing modes for inflation pressures ranging from 5.0 to 30.0 psig. The presence of breathing modes can negatively impact the riding performance of RIBs vessels constructed with drop-stitch fabric hulls by amplification of the panel’s skin separation displacements and vertical accelerations, and are not seen in this material system for the pressures considered. Both numerical and experimental methods are pursued; the results of laboratory modal experiments are used to validate the numerical models. Predicted and experimental natural frequencies and mode shapes are compared and excellent correlation was observed. Increasing inflation pressures produced increasing in-plane and through-thickness normal stresses and modal frequencies of the drop-stitch fabric panels.

Commentary by Dr. Valentin Fuster
2017;():V009T12A031. doi:10.1115/IMECE2017-72138.

The classical phenomenological compressible Hart-Smith model expressed in exponential-logarithmic terms of stretch invariants is compared with substantial hyperelastic models available in Abaqus. It is implemented in Abaqus Explicit using a customized user subroutine. Compressible Hart-Smith model together with selected acclaimed models available in Abaqus are evaluated under uniaxial tension, equi-biaxial extension and planar tension modes of deformations. The required material constants are determined from a simple uniaxial tension test. In order to investigate mode-independent characteristics of considered models, predictive planar tension and biaxial extension simulations are performed using the material constants derived from a uniaxial tensile test. Obtained numerical results are validated with respect to classical experimental data for natural rubber reported by Treloar.

Commentary by Dr. Valentin Fuster
2017;():V009T12A032. doi:10.1115/IMECE2017-72455.

Field of composites is rapidly growing in many industries such as aviation, energy and automotive industries. Composites are known to have a high strength to low weight ratio. Significant improvement in the performance of coatings used in the protection of military and civil aircraft has been achieved the last thirty years. Composite coatings are exposed to many environmental conditions, which can significantly affect their properties. In this research, UV light treatment on the surface of composite was introduced to examine its effects on the adhesion properties between the coating and substrate. A cross-cut test was conducted on the composite panels to assess the adhesion of paint to the substrate after the treatments. Coating performance analyses were also carried out using a Fourier transform infrared spectrometer, water contact angle, and optical microscopic images. The first set of panels was treated with UV radiation for 0, 2, 4 and, 8 days, and the surface wettability was also assessed using the contact angle test. Two coats of paints, including a primer and top coat, were used, and the panels were exposed to UV radiation and immersed in water for 500 hrs and 1000 hrs. It was found that untreated panels showed a much higher contact angle of 106°, whereas the contact angle of panels treated with UV radiation was reduced to 47°. The cross-cut tests showed considerable flaking of the coating along the edges and squares of panels that were not treated, and very small flakes along the edges and parts of the grid square on panels that were UV treated, thus confirming the enhancement of coating adhesion between composite and coating surfaces by UV treatments.

Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Mechanics of Soft Materials and Soft Robots

2017;():V009T12A033. doi:10.1115/IMECE2017-70709.

Super elastic alloy (SEA) has good flexibility with moderate rigidity and then is widely used in eyeglasses, bras and so on. SEA’s flexibility is based on the phase transformation between austenite and martensite structures of material metallurgy, and it is known that the transformation can be controlled by heating and cooling the material. The active deformation of SEA is already reported as controllable by the heating and cooling under pre-stressed condition, but it is known that the deformation is on / off control because the material has very strong nonlinearity in their stress-strain relationship. Then the development of smooth control of SEA’s active deformation is studied to realize soft actuator in this report. The smoothness of the control is realized by the multiplying the wires of SEA in actuating unit of 1DOF actuator. The theory of SEA controller is formulated by using the constitutive equation of the metallo-thermo-mechanics which represents the kinetics of phase transformation between austenite and martensite structures. The developed controller with the formulation is applied to actuate 1DOF actuator with multiple wires of SEA for the verification of the realization. The qualitatively match between theory and experimental results is observed here but quantitatively mismatch has occurred in this report. In the discussion of this paper, it is shown that the mismatch is occurred because the formulation is based on the ideal condition without the deformation of actuator unit. Then this report shows that it is important to design the soft actuator not only their actuating element but also the other elements which affect the deformation behavior of the actuator as a total unit.

Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Mechanics of Thin Film and Multi-Layer Materials

2017;():V009T12A034. doi:10.1115/IMECE2017-70302.

The quality of grains and grain boundaries of polycrystalline copper thin films was analyzed by using image quality (IQ) value obtained from the observed Kikuchi pattern by applying electron back-scatter diffraction (EBSD) analysis. It is considered that the IQ value strongly correlates with the order of atomic configuration in the observed area, in other words, density of various defects, and thus, the area with high IQ value was defined as the area with high crystallinity. The yield strength of a grain was measured by using micro tensile test system in a scanning electron microscope. A bicrystal structure which had two grains with different IQ values was cut from a copper thin film by using focus ion beam (FIB) and the sample was fixed to a single-crystalline silicon beam and a micro probe, respectively, by tungsten deposition. Finally it was thinned to 1μm and stretched to fracture at room temperature. In this micro tensile test, however, the tungsten deposition on the side surface of the test samples caused serious error on the measured strength. Therefore, in this study, the experimental method was improved by the development of an effective method for elimination the excess tungsten deposition. During the tensile test, a mass of plastic deformation and necking phenomenon were obviously observed. Ductile fracture always occurred in the grain with higher Schmidt factor. It was found that the yield strength of a copper grain decreased monotonically with the increase in the IQ value when the IQ value at the grain boundary was larger than 3500.

Commentary by Dr. Valentin Fuster
2017;():V009T12A035. doi:10.1115/IMECE2017-70681.

The stable working condition of high speed, heavy loaded rotating machinery depends strongly on the stability provided by the journal bearing. Tilting pad journal bearings (TPJB) are widely used under such situation due to their inherent stability performance. However, because of the complexity of the TPJB structure, obtaining a reliable prediction of the journal bearing’s dynamic characteristics has always been a challenging task. In this paper, a theoretical analysis has been done to investigate the dynamic performance of a 4 pad TPJB with ball-in-socket pivot, emphasizing on the frequency dependency due to pivot flexibility. The analytical model containing the complete set of dynamic coefficients of the TPJB is built and the pivot stiffness is calculated and used to evaluate the equivalent dynamic coefficients of the bearing. In general, at lower perturbation frequency, the equivalent stiffness and damping increase with frequency. While for higher perturbation frequency, the dynamic coefficients are nearly independent of the frequency. Moreover, the results also show the limit value of the dynamic characteristics of the TPJB when the perturbation frequency is set to 0+ and ∞.

Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Modeling of the Fracture, Failure and Fatigue in Solids

2017;():V009T12A036. doi:10.1115/IMECE2017-70275.

The structural integrity of multi-layered material depends on the mechanical properties and the fracture behaviour at the interface. The sudden jump in mechanical properties across the interface is the major source of failure in layered materials. An accurate evaluation of mixed-mode SIFs becomes essential for safe design of layered structure components. In this work, extended finite element method (XFEM) has been used to analyze interfacial cracked three-dimensional structures under mechanical loading. In XFEM, partition of unity enrichment concept is used to model a crack e.g. a crack surface is modeled by Heaviside enrichment function whereas a crack front is modeled by branch enrichment functions. Discontinuity due the presence of bi-material interface is modeled by the signed distance function. Modified domain based interaction integral approach has been used to evaluate the individual stress intensity factors. Three-dimensional cylindrical domain having an interfacial crack is taken for the simulations. A comparative analysis has been performed with and without an interface for an embedded penny shape crack. The effect of material interface on the SIFs has been analyzed in detail. Finally, a three-dimensional interfacial crack growth simulation has been performed for arbitrary shape crack.

Commentary by Dr. Valentin Fuster
2017;():V009T12A037. doi:10.1115/IMECE2017-70582.

This paper agglomerates an Internal State Variable (ISV) model for polymers (Bouvard et al., 2010, 2013) with damage evolution (Horstemeyer and Gokhale, 1999: Horstemeyer et al., 2000; Francis et al., 2014) into a multiphase ISV framework (Rajagopal and Tao, 1995; Bammann et al., 1996) that features a finite strain theoretical framework for Fiber Reinforced Polymer (FRP) composites under various stress states, temperatures, strain rates, and history dependencies. In addition to the inelastic ISVs for the polymer matrix and interphase, new ISVs associated with the interaction between phases are introduced. A scalar damage variable is employed to capture the damage history of such material, which is a result of three damage modes: matrix cracking, fiber breakage, and deterioration of the fiber-matrix interface, and each damage model was well calibrated to the experimental data from Rolland et al., (2016). The constitutive model developed herein arises employing standard postulates of continuum mechanics with the kinematics, thermodynamics, and kinetics being internally consistent.

Commentary by Dr. Valentin Fuster
2017;():V009T12A038. doi:10.1115/IMECE2017-70621.

The current research represents a preliminary investigation of the cold spray metal deposition process which is a form of additive manufacturing and coating repair. It relates the deformations produced from a single particle impact with a substrate surface to the post-impact interfacial fracture behaviors resulting from separate globally applied mode-I and mode-II loadings. The particle and substrate materials were Al 5056 and Al 6061-T6, respectively. A description of the modeling process is presented and its inherent difficulties are discussed. A two-step numerical modeling process was pursued. First, the particle and substrate impact deformations were obtained using the strain rate-dependent Johnson-Cook Flow Stress Material Model for three particle velocities of interest. Second, the modes-I, II and III strain energy release rates (GI, GII, and GIII, respectively) were characterized along the curvilinear impact surfaces using linear-elastic fracture mechanics (LEFM). Crack initiation, crack growth and ultimate fracture loads of the particle-to-substrate interfaces were determined using the Virtual Crack Closure Technique (VCCT). The predicted results were compared for each particle velocity. The influence of mesh discretization, element distortions, interfacial contact surfaces, etc. are elaborated on for consideration in future modeling efforts. This research, in conjunction with future validation tests, is a fundamental step towards the numerical modeling of cold spray fracture behaviors across multiple material length scales; beginning with a single particle/substrate interface, extending to multiple particles and progressing towards the bulk material scale. Results of such models and experiments will identify specific cold spray processing parameters that may be optimized to improve the interface strengths and fracture resistance levels.

Commentary by Dr. Valentin Fuster
2017;():V009T12A039. doi:10.1115/IMECE2017-70694.

In the present work, an experimental study to analyze the role of friction between the crack faces and crack inclination in compressive fracture of wood has been conducted. Different orientations of (parallel to grain) central slit cracks in wood samples were put under quasi-static uniaxial compression to measure the peak stress carried by the sample. In order to extend the analysis to different friction coefficients at the crack faces without altering other properties, the wood was kept unchanged while the crack faces were coated with paint and polish. The peak stress was largeley observed to increase with an increase in the crack inclination (wrt horizontal) and decrease in the friction coefficient between the crack faces.

Commentary by Dr. Valentin Fuster
2017;():V009T12A040. doi:10.1115/IMECE2017-70977.

Present work deals with the investigation of fracture toughness and modeling parameters need in FEA application for steel use in shipbuilding structure. The investigated steel was 12.5mm thick low carbon high strength steel. Two types of tests were performed, tensile test and fracture test to evaluate mechanical properties and fracture toughness respectively. Cohesive zone model (CZM) was used because it is very computer effective and requires only two parameters, which can be determined in experiments with relative ease. Cohesive zone model with trapezoidal traction law found suitable for the investigated steel. To simulate CZM, bulk section with plane stress elements and bulk section with plane stress with plane strain core scheme are found suitable however bulk section with plane stress with plane strain core scheme gives accurate numerical results.

Topics: Shipbuilding , Steel
Commentary by Dr. Valentin Fuster
2017;():V009T12A041. doi:10.1115/IMECE2017-70998.

It is estimated that more than 70% of failures in engineering components are associated with fatigue loading. Therefore, fatigue is a major design tool for mechanical components. These components are usually subjected to multiaxial cyclic loading. In fact, multiaxial state is very common as tension specimen is under triaxial strain state even though its stress state is uniaxial.

There are three approaches to modeling fatigue damage: stress, strain and energy. Critical plane concept is established based on the fact that fatigue cracks initiate at specific plane(s), therefore, multiaxial fatigue damage parameter is evaluated at these plane(s). Critical plane fatigue models such as Fatemi-Socie is among the popular strain-based models. Because it was shown to provide estimation mostly within two factors of life for different materials and different multiaxial loading conditions.

This paper presents a new method for analyzing critical plane damage parameters. Using plane stress-strain transformation, maximum values of normal and shear stresses and strains from hysteresis loops are obtained at 360 planes. Plotting these values on polar diagrams shows that multiaxial cyclic responses represent polar curves that can successfully be fitted with definitive known polar equations. In principle, this means that both critical plane and fatigue damage can be determined analytically for a given loading path. However, fitting constants must first be determined.

A systematic analysis is performed on different experimental data that were obtained by testing two extruded magnesium alloys at proportional and 90° out of phase loading paths. A closed-form solution for Fatemi-Socie damage parameter is presented for these two loading paths.

Commentary by Dr. Valentin Fuster
2017;():V009T12A042. doi:10.1115/IMECE2017-71321.

In this work, molecular dynamics simulations have been used to study the brittle fracture behaviour of vitreous silica in mixed mode loading at room temperature. An implementation of the BKS potential with the coulombic term was used along with Lennard-Jones modification to model initial cristobalite. Ewald summation was used to obtain long-range coulombic contribution to the total potential energy of the system. A recipe (Huff et. al., Journal of Non-Crystalline Solids, 253, 133–142, 1999) was used to obtain the vitreous silica using a combination of different molecular dynamics runs which were done initially as NVT ensemble and at the end as NPT ensemble. Uniaxial tensile tests for uncracked specimen was carried out to validate the microscopic and macroscopic properties with that in the literature. Further, slit center cracks of different orientations were introduced in the vitreous silica and subjected to mixed mode loading by moving the boundaries slowly. Studies of mechanical behaviour were made to derive the variation of fracture stress and stiffness with the mode-mixity in amorphous solids.

Commentary by Dr. Valentin Fuster
2017;():V009T12A043. doi:10.1115/IMECE2017-71391.

API rated flanges [1] are used in oil and gas industries frequently. The current practice is to select a flange and its seal/gasket from a tabulated range of load and area of application. Some applications may require a unique flange or seal for a pressure and temperature that is not listed on the table. To design such non-API flange (geometry such as thickness, number of bolts, bolts pattern, etc. are different from API flange) that meets system requirements is a challenge. Currently there is no standard available as per authors knowledge to design non-API flange. Therefore, in this study, a finite element analysis with new acceptance criterion for the 5”-15-ksi non-API flange connection for different pressure and temperature has been performed. Elastic-plastic finite element analysis method is used to check the leak tightness [2]. A fatigue calculation based on elastic-plastic analysis with twice yield method as per ASME Section VIII, Division 2 [3], sec. 5.5.4, has also been performed.

A new acceptance criterion based on industry practices are used for leak tightness. The contact pressure and contact pressure band width are used to check the leak tightness. The results showed that for all load cases seals have enough contact pressure and contact pressure band width to prevent leakage and they meet the acceptance criteria. The calculated fatigue life was found more than the required life for an application.

Commentary by Dr. Valentin Fuster
2017;():V009T12A044. doi:10.1115/IMECE2017-71510.

Life prediction of turbine engines is crucial part of the management and sustainment plan to aircraft jet engine. Fretting is one of the primary phenomena that leads to damage or failure of blade-disk attachments. Fretting is often the root cause of nucleation of cracks at attachment of structural components at or in the vicinity of the contact surfaces. It occurs when the blade and disk are pressed together in contact and experience a small oscillating relative displacement due to variations in engine speed and vibratory loading. It is a significant driver of fatigue damage and failure risk of disk blade attachments. Fretting is a complex phenomenon that depends on geometry, loading conditions, residual stresses, and surface roughness, among other factors. These complexities also go beyond the physics of material interactions and into the computational domain. This is an ongoing effort, and the Author has been working on computationally modeling the fretting fatigue phenomenon and damage in blade-disk attachment. The model has been evolving in the past few years, and it has been addressing various fretting conditions. The present effort includes the thermal effect and temperature fluctuation during engine operation, and it models the effects of blade to disk attachment’s thermal conditions and its influence on fretting fatigue damage. It further extends the earlier model to include a coupled fatigue damage model. It allows modeling higher speeds and longer durability associated with blade disk attachments. Finally, to demonstrate its capabilities and taking advantage of experimental validation model, the most recent numerical simulations will be presented.

Commentary by Dr. Valentin Fuster
2017;():V009T12A045. doi:10.1115/IMECE2017-71515.

The response of quasi-brittle materials is greatly influenced by their microstructural architecture and variations. To model such statistical variability, Statistical Volume Elements (SVEs) are used to derive a scalar fracture strength for domains populated with microcracks. By employing the moving window approach the probability density function and covariance function of the scalar fracture strength field are obtained. The Karhunen-Loève method is used to generate realizations of fracture strength that are consistent with the SVE-derived statistics. The effect of homogenization scheme, through the size of SVE, on fracture pattern is studied by using an asynchronous spacetime discontinuous Galerkin (aSDG) finite element method, where cracks are exactly tracked by the method’s adaptive operations.

Commentary by Dr. Valentin Fuster
2017;():V009T12A046. doi:10.1115/IMECE2017-71763.

A Crystal Plasticity Finite Element Method (CPFEM) investigation was performed to study the effect of the grain orientation on the surface profile changes in FCC polycrystals undergoing plastic tensile deformation. A grain embedded in an isotropic sample was used to ensure that grain profile is influenced only by the grain orientation. Simulation with multiple sets of grain orientations has determined that strain compatibility is the key criterion dictating the relative sinking and rising of grains. As a consequence, the grains with axial orientations close to [001] and [111] orientations, i.e. the most compatible orientations, sink irrespective of the normal orientation. However, grains with other axial orientations may relatively rise or sink depending on the orientation normal to the surface.

Commentary by Dr. Valentin Fuster
2017;():V009T12A047. doi:10.1115/IMECE2017-72132.

Hollowness of loaded members has been investigated using rings and square plates with central hole under normal compressive loads. The objective is to define the relative size of the central hole or optimum hollowness that will not compromise the load carrying capability of the rings and plates. Hollowness is thus defined as the volume percentage of measurable void in load carrying member. The work presented here uses finite element analysis to plot the radial and transverse stresses of a ring and a square plate with central hole versus the radius the hole or hollowness. The observations from the numerical analysis are further confirmed with experimental verification using strain gages. Seven rings and seven plates are normally loaded between two plates while the radial and transverse strains are measured. The derived stress are plotted with respect to hollowness. The plots show a moderate increase in stresses up to about 35% to 40% hollowness in both finite element and experimental results. Above these values, which represent the optimum hollowness, the stresses increase along a steep curve. These results indicate that some load carrying or transmitting machine components can be fabricated with measurable voids leveraging recent advances in Additive Manufacturing.

Commentary by Dr. Valentin Fuster
2017;():V009T12A048. doi:10.1115/IMECE2017-72252.

A drill collar is a thick-walled tubular component that provides a passage to pumping drilling fluids and a mechanical protection for sensing, power supply, communication, and control devices. Multiple collars can be screwed together along with other downhole tools to make a bottomhole assembly (BHA). Radially oriented ports are often used in the wall of a collar for various reasons. These ports could be susceptible to fatigue-induced failures when a BHA has to undergo a large number of revolutions in a curved well. A cracked port could result in leakage, thereby causing flood damage to the internal devices, which are supposed to be protected from drilling fluid. Understanding the risk of fatigue cracking of a collar port is an important part of BHA design and well planning.

The total fatigue life of a port can be considered as a summation of the crack initiation life, which is consumed to nucleate a dominant crack with a minimum detectable size, and the crack growth life, which is measured as the crack grows from the minimum detectable size until it reaches the seal. Prediction of the initiation life is expected to be conservative due to the many uncertainties involved. As a result, solely relying on the predicted initiation life to retire a port and the entire collar is not cost effective. A more economical way of port fatigue management is to compute the crack growth life based on a minimum detectable crack size and use this life as the inspection interval. If a crack is detected during an inspection, a port is declared as failed because a cracked port cannot be repaired with the same strength. Otherwise, the port can last at least until the next scheduled inspection.

In this study, a fracture-mechanics-based method is developed to predict the fatigue crack growth (FCG) life of a collar port subjected to constant-amplitude cyclic bending. It is assumed that a prescribed corner crack with a minimum detectable size lies in a plane perpendicular to the collar axis. It intersects with the collar outside surface and the port wall surface. The crack front follows an elliptical function. The stress intensity factors (SIFs) along the crack front are numerically computed with finite element analysis (FEA) at the two endpoints, respectively. A response surface of the SIF is generated by assigning a set of predetermined crack fronts based on incrementally advancing positions of the two endpoints. It is then used to determine the SIFs at these points throughout all crack growth increments. The Paris law is utilized to describe the FCG rate of the collar material, whereby, along with the SIFs computed, the crack growth life and the associated crack front shape are incrementally determined. To validate the newly developed method, a test apparatus is developed to apply constant-amplitude cyclic bending to a collar specimen that contains a through-hole in the middle. The predicted growth rate for the crack on the collar outside surface agrees favorably well with the test data. The computed crack front before rupture is also in good agreement with the experimental measurement.

Commentary by Dr. Valentin Fuster
2017;():V009T12A049. doi:10.1115/IMECE2017-72349.

The objective of the current work is to conduct a systematic analysis on the effects of manufacturing induced defects such as random distribution of fibers and presence of voids in matrix on the damage initiation in polymeric composites. Upon infusing resin, the initial fiber configuration undergoes perturbation and results in a random distribution with pockets of resin rich areas and fiber clusters. In addition, this could result in micro voids (between the fibers in the bundle) and macro voids (between the fiber bundles). A novel methodology has been put forward to generate random distributions of fibers that would simulate different levels of perturbations in the manufacturing process resulting in different configurations of fiber clusters. An embedded Representative Volume Element (RVE) approach has been adopted in a finite element model to calculate the stress fields without artificial effects of the RVE boundary. Damage initiation is then analyzed using a previously proposed energy based criterion for cavitation in polymers.

Commentary by Dr. Valentin Fuster
2017;():V009T12A050. doi:10.1115/IMECE2017-72468.

The evaluation of the mutual effect of non-aligned multiple cracks is a prerequisite in applying fitness-for-service codes. For non-aligned parallel cracks, during on-site inspection, one needs to decide whether the cracks should be treated as coalesced or separate multiple cracks for Fitness-for-Service. In the existing literature, criteria and standards for the adjustment of multiple nonaligned cracks are very source dependent, and those criteria and standards are often derived from on-site service experience without rigorous and systematic verification. Based on this observation, the authors previously reported on the influence of an embedded crack on an edge crack in 2-D scenarios and, more recently, in 3-D scenarios of the influence of a surface crack on a quarter-circle corner crack. However, realistic crack configurations detected using non-destructive methods are generally 3-D in nature and their influences are mutual. Thus the SIF distribution characteristics along the surface crack is equally important as the SIF distribution of the corner crack when Fitness-for-Service rules are to be applied. Therefore, non-aligned flaws with different configurations and shapes and the SIFs along their crack fronts are deemed necessary in order to obtain more practical guidance in the usage of rules speculated in Fitness-for-Service codes. In this study, the characteristics of the SIF distribution along a semi-elliptic non-aligned surface crack is examined under the influence of a quarter-circle corner crack of various geometries in an infinitely large plate. For any given geometry of a quarter-circle corner crack, a pair of horizontal (H) and vertical (S) separation distances between the two cracks is chosen followed by a detailed analysis of the effect of the quarter-circle corner crack on the 3D SIFs of the surface crack at different ellipticities. The analysis is repeated for various combinations of separation distances S and H. The results from this study are collectively significant to the understanding of the correlation between the criteria and standards in Fitness-for-Service community and the consequence of their usage in engineering practice.

Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Multi-Field Studies in Heterogeneous Materials: Experimental, Theoretical and Numerical Approaches

2017;():V009T12A051. doi:10.1115/IMECE2017-71151.

To clarify the deformation induced crystal texture evolution of rolled and drawn magnesium alloy sheets with strong basal texture, we developed a multi-scale finite element (FE) analysis code based on the homogenization theory, which combines the microscopic poly-crystal structure and the macroscopic continuum.

In our crystal plasticity constitutive equation of magnesium alloys, the plastic work induced temperature rise and twinning in the crystal slip systems was implemented into our multi-scale FE analysis code. To validate our numerical code to correctly predict macro and micro deformations including the crystal texture evolution, the tension and compression along normal direction (ND) and rolling direction (RD) at the room temperature 300K and the high temperature 673K were numerically investigated.

It is confirmed that numerical results showed the similar tendency to experimentally obtained results including the strengthening the basal texuture in compression along ND, the twinning, the polarity of twinning and the temperature-dependency that twinning is hardly appear at high temperature. Finaly, we concluded that our numerical code can predict the plastic strain induced texture evolution of magnesium alloys.

Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Multi-Physics Simulations and Experiments for Solids

2017;():V009T12A052. doi:10.1115/IMECE2017-71180.

A leaf spring is a simple form of spring commonly used for suspension system of vehicles which is originally called laminated or carriage spring. They perform isolation task in transferring vibration due to road irregularities to driver’s body. To improve the suspension system, many modifications have taken place overtime but recent innovations imply parabolic leaf spring and application of composite materials for these springs. The conventional flat profile of the leaf spring has been transformed into parabolic leaf spring which facilitates lighter, cheaper, better fatigue life and isolating more noise. This project basically includes designing a leaf spring with a conventional flat profile design following the standard dimension (SAE Manual) with acceptable tolerance and regard it as the base model for the project. To obtain the deformation, stress and fatigue life of the base model; a Computer Aided Simulation has been carried out in ANSYS Workbench considering the Structural Steel as the base material. Afterwards, the conventional flat profile design has been changed to parabolic shape consisting 1 Master leaf and 3 graduated leaves. In this case, initially the structural steel has been selected as the base material and later on SAE 5160 steel has been implemented to carry out the simulation. As only spring steel is the material widely used for parabolic leaf spring and many research has been carried out with spring steel, therefore different materials with combination of different spring design has been carried out in this project to get a better life cycle compared to the widely used one. After first modification, number of leaves has been increased to 5 but analysis has been carried out with the same two materials considered for initial simulation. Due to time constraint, the final optimized design has been selected among the analysis finished with the combination of leaves and materials which incorporate the better fatigue life, reduced deformation, reduced weight of the spring and increased factor of safety and later on following the final design (analyzed from CAE results) the parabolic leaf spring has been built with the assistance of a spring shop.

Commentary by Dr. Valentin Fuster
2017;():V009T12A053. doi:10.1115/IMECE2017-72342.

In fluid-structure interactions (FSI), one or more structures interact with incompressible or compressible fluids. These problems are non-linear multi-physics phenomena. Most FSI problems require a numerical solution rather than an analytical solution (Gene, 2012). This is particularly true for problems involving large deformations of elastomers and for problems related to plugging of flow. The classical two way coupling of fluid and structural solver does not work well and at times fail to reach a solution. Using a monolithic solver and non-conforming mesh techniques like Coupled Eulerian Lagrange (CEL) or Arbitrary Lagrange Eulerian (ALE) formulations we show solutions for such large deformation and plugging type FSI problems.

The technique has been applied to help develop an industrial flow regulator. The regulator uses a single piece elastomeric material that undergoes deformation due to pressure drop in the system. The deformation in turn restricts flow in the system and potentially developing flow related noise in the system. Rigorous FSI analyses techniques were used to design and develop the concepts minimizing prototype development time. The FSI analysis not only helped in designing the mechanical function of the regulator but also the helped evaluate the acoustic noise level in the system. The resulting design performance was validated by controlled performance tests. The design was shown to work reliably and meet performance requirements.

Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Multi-Scale Computations in Fluids, Structures, and Materials

2017;():V009T12A054. doi:10.1115/IMECE2017-70777.

One considers a linear elastic random structure composite material (CM) with a homogeneous matrix. The idea of the effective field hypothesis (EFH, H1) dates back to Faraday, Poisson, Mossotti, Clausius, and Maxwell (1830–1870, see for references and details [1], [2]) who pioneered the introduction of the effective field concept as a local homogeneous field acting on the inclusions and differing from the applied macroscopic one. It is proved that a concept of the EFH (even if this term is not mentioned) is a (first) background of all four groups of analytical methods in physics and mechanics of heterogeneous media (model methods, perturbation methods, self-consistent methods, and variational ones, see for refs. [1]). New GIEs essentially define the new (second) background (which does not use the EFH) of multiscale analysis offering the opportunities for a fundamental jump in multiscale research of random heterogeneous media with drastically improved accuracy of local field estimations (with possible change of sign of predicted local fields).

Estimates of the Hashin-Shtrikman (H-S) type are developed by extremizing of the classical variational functional involving either a classical GIE [1] or a new one. In the classical approach by Willis (1977), the H-S functional is extremized in the class of trial functions with a piece-wise constant polarisation tensors while in the current work we consider more general class of trial functions with a piece-wise constant effective fields. One demonstrates a better quality of proposed bounds, that is assessed from the difference between the upper and lower bounds for the concrete numerical examples.

Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Multifunctional and Micro-/Nano-Structured Materials: Modeling and Characterization

2017;():V009T12A055. doi:10.1115/IMECE2017-70151.

The selection of coating or surface treatments is a crucial step in the design of oil and gas equipment to protect against the deterioration caused by wear, corrosion, galling, fatigue, etc. Quench polish quench (QPQ) nitriding is a superior candidate to increase surface hardness for abrasion and galling resistance in carbon or stainless steels. The increased surface hardness improves the wear and corrosion resistance but reduces the surface material ductility. It is generally not recommended for application to V-shaped threads or sharp notches subjected to high stress.

During well perforation in cased-hole completion, the detonation of the gun string along with the induced pressure wave in fluids generates a large-magnitude dynamic motion in the gun string. The peak load of a perforating event, from detonation to fluid-structure interaction, happens in the range of microseconds to milliseconds. The coupled wellbore hydrodynamic and structural dynamic shock load may cause an overstress failure in the millisecond scale but is usually overlooked in engineering practice.

In this work, we investigated the behavior of QPQ coating under transient dynamic loads, employing both physical test and finite element analysis. We designed a combination of drop test fixture and specimens to simulate a notched specimen subjected to dynamic tensile loads. Two types of specimens were prepared in this study, QPQ-coated specimens and bare metal specimens without coating. The specimens without coating were tested to serve as a baseline for comparison. The methodology in this study provides a generic guideline for design of equipment potentially subjected to transient mechanical shock loads.

Commentary by Dr. Valentin Fuster
2017;():V009T12A056. doi:10.1115/IMECE2017-70903.

Ballistic fabrics made from high performance fibers such as para-aramid (synthetic) and basalt (natural) fibers, and composites utilizing these fabrics, are among the leading materials for soft body armor systems. Modern military and other law enforcement operations are technology-driven with weapons and ammunition that demand a flexible, damage- and moisture-resistant, and lightweight ballistic fabrics with superior energy absorbing capacity. Basalt fibers, which are extracted from igneous volcanic rocks, are natural fibers with mechanical and thermo-physical properties that are generally comparable or superior to glass and other synthetic fibers at a lower cost. This gives basalt-based composites an edge over contemporary materials for potential application as anti-ballistic body armor. The aim of the present study is to experimentally determine the V50 performance and penetration resistance of unidirectional woven basalt fiber laminated composites with three different combinations of ply orientations [0°, 45° and 90°] at both CW and CCW directions and number of layers namely, B1 (7 layers), B2 (12 layers) and B3 (16 layers). The V50 performance test was conducted in accordance to the MIL-STD-662F standard using the Universal Test Gun (Prototypa UZ-2002). The V50 ballistic velocity is computed based on a minimum of six shots including three complete penetrations (CP) and three partial penetrations. The MIL-STD-662F standard stipulates that the value of ballistic limit protection should be less than 38 m/s for the purpose of computing V50. The V50 value for sample B1, that yielded to CP at 192 m/s and 172 m/s clean through to the witness plate, could not be determined. The V50 for samples B2 and B3 are 117 m/s and 153 m/s, respectively.

The optimum number of layers of the basalt fabric to sustain the reference penetration velocity of 367 m/s corresponding to threat Level II of the NIJ Standard-0101.04 is calculated for each specimen for further study. The tensile properties and Shore-D hardness levels of the three samples are also measured and reported.

Commentary by Dr. Valentin Fuster
2017;():V009T12A057. doi:10.1115/IMECE2017-71138.

A microstructure based model is developed to predict the effective electrical conductivity of open-cell metallic foams. A tetrakaidecahedral cell is adopted as the repeating unit, which comprises of 32 ligaments and 24 joints. Each ligament contains a straight section with a uniform cross section and two identical joint sections. Each joint is formed with four identical joint sections. The geometries of the ligaments are constructed based on the minimum surface energy reached during the foaming process. The electrical conductivity of a joint section is first numerically computed. It is then used, along with that of the straight section of the ligament, to give rise to the conductivity of the ligament for a given relative density. The effective electrical conductivity of the foam is then analytically determined along four typical orientations of the unit cell with the resistor network method and Kirchhoff’s current and voltage laws. The numerical results indicate that the electrical conductivity of 3-D open-cell foams exhibits some degree of anisotropy. The predictions of the foam conductivity along the four identified orientations well encompass the scattering of the experimental measurements reported in the existing studies.

Commentary by Dr. Valentin Fuster
2017;():V009T12A058. doi:10.1115/IMECE2017-72475.

The variations in thermal conductivity of nanocomposites are found to depend not only the intrinsic properties of the fiber and matrix phases but also on the interfacial resistance of the reinforcing phase. As we go down the length scales, the interfacial thermal resistance due to size of the nanoparticle becomes significant. In order to address the effect of size (length and diameter) of nanotube on the thermal transport property of nanotube composites, thermal conductivity of different nanotube samples varying in length and diameter will be estimated first using molecular dynamic (MD) simulations with AIREBO potentials. This will be carried out using the ‘Heat-Bath’ method - non-equilibrium molecular dynamics (NEMD) approach. In the heat bath method, constant amount of heat is added to and removed from the hot and cold regions and the resulting temperature gradient is measured and the thermal conductivity is calculated using the Fourier Law. This will be followed by the study of interfacial thermal resistance of these nanostructures. These intrinsic properties are then used with continuum based mathematical formulations to study the effect of size of the nanoparticle on the overall thermal conductivity of the nanocomposite.

Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Multiscale Models and Experimental Techniques for Composite Materials and Structures

2017;():V009T12A059. doi:10.1115/IMECE2017-70684.

It had been claimed in an earlier article [1], using statistical theory of failure, that a triangular lattice composite structure is stiffer (i.e. has a higher overall Young’s modulus) than a square lattice composite structure for the same negligible volume fraction for a given statistical strength. In the present work, this claim has been experimentally validated. Copper/epoxy lattice composite structure is manufactured by hand-weaving copper wires of fixed size to create macroscopically isotropic structures of triangular and square shapes. The unit cell area in both cases is kept the same to ensure same statistical strength. Compressive linear response (not to fracture) of triangular and square composite structures has been recorded to measure the overall stiffness.

Commentary by Dr. Valentin Fuster
2017;():V009T12A060. doi:10.1115/IMECE2017-71311.

In the present research, an advanced methodology for the multi-scale analysis of composite structures is proposed. It is based on the Carrera Unified Formulation (CUF), according to which any theory of structures, either 2D plate/shell or 1D beam, can be expressed as a degenerate case of Elasticity by using generalized expansions of the fundamental unknown fields. By using an extensive index notation, CUF allows the governing equations of the problem under consideration, and eventually the related finite element arrays, to be stated in terms of fundamental nuclei, which are invariant of the theory approximation order and the analysis scale. In this manner, micro-, meso-, and macro-scale models of composite structures can be formulated with ease and in a unified way, without the need of changing the model paradigms from one scale to the other. The capability of the proposed methodology based on CUF is assessed and the results demonstrate the validity of the approach, whose mathematical formalism is scale-independent, but allows for the simultaneous analysis of composites from global to very local scales in an accurate manner.

Commentary by Dr. Valentin Fuster
2017;():V009T12A061. doi:10.1115/IMECE2017-71500.

To accurately predict fracture patterns in quasi-brittle materials, it is necessary to accurately characterize heterogeneity in the properties of a material microstructure. This heterogeneity influences crack propagation at weaker points. Also, inherent randomness in localized material properties creates variability in crack propagation in a population of nominally identical material samples. In order to account for heterogeneity in the strength properties of a material at a small scale (or “microscale”), a mesoscale model is developed at an intermediate scale, smaller than the size of the overall structure. A central challenge of characterizing material behavior at a scale below the representative volume element (RVE), is that the stress/strain relationship is dependent upon boundary conditions imposed. To mitigate error associated with boundary condition effects, statistical volume elements (SVE) are characterized using a Voronoi tessellation based partitioning method. A moving window approach is used in which partitioned Voronoi SVE are analysed using finite element analysis (FEA) to determine a limiting stress criterion for each window. Results are obtained for hydrostatic, pure and simple shear uniform strain conditions. A method is developed to use superposition of results obtained to approximate SVE behavior under other loading conditions. These results are used to determine a set of strength parameters for mesoscale material property fields. These random fields are then used as a basis for input in to a fracture model to predict fracture patterns in quasi-brittle materials.

Commentary by Dr. Valentin Fuster
2017;():V009T12A062. doi:10.1115/IMECE2017-71615.

Fiber-matrix debonding is a common type of failure in composite materials. A computational model that illustrates this damage-induced separation between glass fiber and epoxy matrix in a fiberglass composite model is created from the ground-up. Material properties for the simulations are determined via in-house performed experiments. The tensile modulus of the epoxy matrix is determined to be 8.21 GPa measured using resin-only specimens according to ASTM D638-14. The glass fiber is a bundle of glass fibers extracted from a lattice-woven tape with the original purpose of fiberglass crack patching. These bundles contain approximately 1000 individual fibers per roving. A field-emissive scanning electron microscope study of the glass fiber reveals that the individual fiber diameters vary from 11.04μm to 13.52μm with an average of 11.97μm and a standard deviation of 0.604μm. Tensile tests are performed to the glass fiber bundles using custom grips designed in accordance to ASTM D7269. From these tests, it is found that the fiber Young’s Modulus ranged from 42 GPa to 61.7 GPa. Furthermore, in order to simulate a physical analogue to our computational simulations cylindrical composite specimens of a single bundle of fibers running through the central axis of the cylinder with 6.35mm diameter and 100mm length were prepared and tested. It is found that all of the test specimens exhibit some amount of debonding post-failure. The computational simulation is created in ABAQUS 6.14 to replicate the effects of the experimental composite and elucidate the debonding behavior on smaller time scales. The lower outer face is given a static pinned boundary condition while the upper outer face is given a displacement boundary condition to simulate the fixed lower grips and moving upper grips during the tensile test. The material properties obtained by the experimental tests are applied to the materials in the simulation. Two separate damage models are used in order to simulate breakage; the fiber and matrix utilize the Johnson-Cook failure model as plastic strain-to-failure and the fiber-matrix interface is governed by cohesive-zone elements. The computational model exhibits realistic stress distribution as well as calculates element deletion to simulate interfacial debonding. The simulations are able to show the matrix cracking at the notch and subsequently a separation between the fiber-matrix interface similar to the physical composite analogue.

Commentary by Dr. Valentin Fuster
2017;():V009T12A063. doi:10.1115/IMECE2017-71843.

COPVs are currently used at NASA to contain high-pressure fluids in propulsion, science experiments and life support applications. These COPVs have a significant weight advantage over all-metal vessels; but, as compared to all-metal vessels, COPVs require unique design, manufacturing, and test requirements. The most significant difference from metal pressure vessel designs is that COPVs involve a much more complex mechanical understanding due to the interplay between the composite overwrap and the inner liner.

Often only limited analysis is performed to obtain an initial design, and then the design is refined through number of “build and burst” iterations. However, the cost in material and resources to fabricate multiple test specimens is extremely prohibitive. To avoid these high cost and time for build and burst iterations, FEA is often employed in an attempt to reduce the number of iterations required. FEA process becomes more of a design confirmation effort rather than a design iteration effort. In this research, we aimed to establish a detailed design optimization of a complete COPV through Finite Element simulation.

Commentary by Dr. Valentin Fuster
2017;():V009T12A064. doi:10.1115/IMECE2017-72009.

The anisotropic plane elasticity solution of the stresses throughout a thin, rectangular, laminated composite plate subjected to self-equilibrating cubically distributed shear stress at the two ends of the plate is obtained. The plate is assumed to be a symmetric laminate and hygrothermal effects are not considered. The characteristic decay length under the shear loading is estimated for different cases of isotropic, orthotropic, and anisotropic plate. The work is intended to be presented as an example during a lecture or to be published in new textbook demonstrating the numerical method.

Commentary by Dr. Valentin Fuster
2017;():V009T12A065. doi:10.1115/IMECE2017-72241.

Single Cantilever Beam (SCB) testing was conducted on foam and honeycomb core sandwich specimens to predict the fracture toughness and bonding strength. Adhesion characteristics can also be identified between facesheet and core materials conducting SCB test on precracked specimen. Disbonding under combined tensile and shear loading may involve failure modes such as kinking and cell buckling. Modified Beam Theory (MBT) method specifies required parameters to generate a least square plot based on experimentally determined compliance and disbond length obtained during the SCB test. Interfacial fracture toughness Gc, enumerated by following recommended data reduction methodologies, required to predict minimum Gc to initiate disbond growth. Finally, experimental compliance was compared with elastic foundation stiffness compliance to verify the performance of the test. Good agreement between two compliance solutions validates the accuracy of the SCB test.

Commentary by Dr. Valentin Fuster
2017;():V009T12A066. doi:10.1115/IMECE2017-72406.

Alike other polymer material, PolyVinyl Chloride (PVC) shows a clear creep behavior, the rate of which is influenced by temperature, load and time. Polyvinyl chloride bolted flange joints undergo relaxation under compression for which the material creep properties are different than those under tension. Since the sealing capacity of a flanged gasketed joint is impacted by the amount of relaxation that takes place, it is important to properly address and predict the relaxation behavior due to flange creep under compression and reduce the chances of leakage failure of PVC flange joints.

The main objective is study the creep behavior of PVC flanges under the influence of normal operating conditions. This is achieved by developing a PVC creep model based on creep test data under various compressive load, temperature and time. A simulation of a PVC flange relaxation behavior bot numerically and experimentally is conducted on an NPS 3 class 150 bolted flange joint of dissimilar materials one made of PVC material and the other one by steel SA105. The study also provides a clear picture on how the compression creep data on Ring specimen may be utilized for predicating the flange performance under various operating temperatures with time.

Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Peridynamic Modeling of Material Behavior

2017;():V009T12A067. doi:10.1115/IMECE2017-71527.

In this paper, reformulation of classical bond-based peridynamic thermomechanical model for irregular domain decomposition and its MOOSE-based implicit formulation are presented. First, the irregular grid based peridynamic thermomechanical model is formulated and model parameters are derived. Following this, an implicit formulation for the solution of static or quasi-static problems is presented. Some aspects of the MOOSE-based implementation are given. After that, the formulation is verified against benchmark solutions for thermomechanic problems. Crack initiation and propagation in circular (2D) and cylindrical (3D) nuclear fuels at high temperature are studied using irregular grids.

Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Quantitative Visualization

2017;():V009T12A068. doi:10.1115/IMECE2017-70314.

The degradation process of Ni-base superalloy CM247LC was investigated experimentally under the creep loading at 900°C. The initial excellent high-temperature strength of this alloy is attributed to its micro texture, the fine binary phase such as cuboidal γ’ (Ni3Al) precipitates orderly dispersed in the γ matrix (Ni-rich matrix). However, it was observed that γ’ precipitates started to coarse perpendicular to the applied uniaxial load direction during high temperature creep loading. The disappearance of the strengthened micro texture caused the acceleration of the crack growth along the phase boundaries of the layered texture and seriously degrades the strength of this material. Therefore, not only the outlook of micro texture but also the changes of the atomic configuration and atomic concentration which were based on the atomic diffusion behavior was investigated for the further explication of rafting mechanism more in detail.

It was found that the distribution of Image Quality (IQ) value which was obtained from EBSD analysis monotonically shifted to lower values and the full width of half maximum became wider as the creep loading time increased. This degradation of the order of atomic alignment indicated that lattice defects density increased and ordered superlattice structure (Ll2 structure) became disordered. In addition, the initial periodic distributions of component elements which corresponded to the fine periodic alignment of the γ and γ’ phases also disappeared and the concentration of each element became uniform even though both the γ and γ’ phases still remained even after rafting. The observed creep damage of CM247LC was, therefore, dominated by the degradation of the order of atomic arrangement, and this degradation was attributed to the strain-induced atomic diffusion of component elements. It is very important, therefore, to suppress this strain-induced acceleration of atomic diffusion in this alloy by modifying the microstructure of this alloy.

Topics: Superalloys , Stress
Commentary by Dr. Valentin Fuster
2017;():V009T12A069. doi:10.1115/IMECE2017-70317.

Ni-base superalloys are widely used for various power plants and jet engines. Since the operating temperature of thermal plants and equipment has been increasing to improve their thermal efficiency for decreasing the emission of carbon-dioxide, the initially designed microstructure was found to change gradually during their operation. Since this change of microstructure should deteriorate the strength of the materials, sudden unexpected fracture should occur during the operation of the plants and equipment. Therefore, it is very important to clarify the dominant factor of the change of the microstructure and the relationship between the microstructure and its strength for assuring the stable and reliable operation of the plants and equipment. In this study, the change of the strength of a grain and a grain boundary of Ni-base superalloys caused by the change of their microstructure was measured by using a micro tensile test system in a scanning ion microscope. A creep test was applied to bulk alloys at elevated temperatures and a small test sample was cut from the bulk alloy with different microstructure caused by creep damage by using focused ion beams. The test sample was fixed to a silicon beam and a micro probe, respectively, by tungsten deposition. Finally, the test sample was thinned to 1μm and the sample was stretched to fracture at room temperature. The change of the order of atom arrangement of the sample was evaluated by applying electron back-scatter diffraction (EBSD) analysis quantitatively. In this study, the quality of grains in Ni-base superalloys was analyzed by using image quality (IQ) value calculated by using Hough transform of the observed Kikuchi pattern. It was found that the order of atom arrangement was deteriorated monotonically during the creep tests and this deterioration corresponded to the change of the microstructure clearly. Both the yield strength and the ultimate tensile strength of a grain in the alloys decreased drastically with the change of the microstructure, in other words, the IQ value of the grains. There was a clear relationship between the IQ value of a grain and its strength. Therefore, this IQ value is effective for evaluating the crystallinity of the alloys and the remained strength of the damaged alloys. The change of the microstructure was dominated by the strain-induced anisotropic accelerated diffusion of component elements of the alloys and the activation energy of the diffusion was determined quantitatively as a function of temperature and the applied stress.

Topics: Creep , Superalloys , Damage
Commentary by Dr. Valentin Fuster
2017;():V009T12A070. doi:10.1115/IMECE2017-70494.

The change of the lath martensitic structure in modified 9Cr-1Mo steel was observed in the specimens after the fatigue and creep tests using EBSD (Electron Back-Scatter Diffraction). The Kernel Average Misorientation (KAM) value obtained from the EBSD analysis were used for the quantitative evaluation of the change in the lath martensitic texture. It was found that the average KAM values of the fractured specimens decreased clearly after 107−108 cycles of the fatigue loading at temperatures higher than 500°C when the amplitude of the applied stress exceeded a critical value. This change corresponded to the disappearance of the lath martensitic structure. The critical value decreased monotonically with the increase of the test temperature. This microstructure change decreased the strength of the alloy drastically.

It was found that the change of the microstructure started at a certain time at each test temperature as a function of the amplitude of the applied stress. There was the critical stress at which the microstructure change started at each test temperature higher than 500°C, and the activation energy of the change was determined as a function of temperature and the amplitude of the applied stress. The dominant factor of the microstructure change was the stress-induced acceleration of the atomic diffusion of the component elements in the alloy. In order to improve the long-term reliability of the alloy, it is very important to increase the activation energy by modifying the microstructure of this alloy.

Commentary by Dr. Valentin Fuster
2017;():V009T12A071. doi:10.1115/IMECE2017-70526.

An indentation test can easily measure the deformation characteristics of a material, because it does not require a test specimen to be cut from the material being examined. The applicability of this test is usually restricted to evaluating the fundamental characteristics of deformation, such as elasticity and plasticity; however, it is also useful if the test can be applied for the fracture evaluation of materials. Therefore, in this study, the fracture behavior of materials is discussed by performing indentation tests. The evaluation procedure depends on the variation in the indentation force owing to the differences in the deformation behavior. The observed variation is analyzed via the fractography of the material. A simple formulation is derived from the results for the development of a material evaluation method. Finally, the importance of the choice of the indenter diameter is explained in terms of the accuracy of the plateau stress of the porous material.

Commentary by Dr. Valentin Fuster
2017;():V009T12A072. doi:10.1115/IMECE2017-72490.

Experimental estimation of acurate material properties are key elements to the design of machine components and structures. In general, the elastic properties were determined using uniaxial tensile tests regardless of the final shape of the product including pipes, which exhibit different elastic properties in the longitudinal and tangential directions due to their manufacturing process. Several attempts have been made to estimate the mechanical properties of pipes along their longitudinal directions including ASTM D2290 and exhibited inconsistent results; thereby, calling for further design and analysis. This paper, therefore, presents various design alternatives to the ring hoop tensile test adopted in the ASTM D2290 standard through finite element modeling. The design optimization consisted of varying the ring width, dogbone width, dogbone gauge length, and the spacing between the two D-blocks to obtain the optimum values of the varying parameters that provide uniform normal stress across the cross-sectional area of the dogbone and more representative mechanical response. Finite element results revealed that the proposed dogbone sample design has an optimum length to width ratio of 4 with an orientation angle between 75° to 105° with respect to the horizontal axis. The proposed model was compared to adopted test methods such as ASTM D2290 and resulted in comparable stress contours uniformity across the dogbone gauge length but different contact pressure values. it was also found that the contact pressure for the ASTM ring hoop tensile test is higher than that for the proposed model by 22%.

Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Young Medalist Symposium

2017;():V009T12A073. doi:10.1115/IMECE2017-71397.

Slender, lightweight structures are demanded to meet efficiency targets or to enhance vehicle system performance characteristics. Yet, when subjected to static stress for load-bearing purposes, the flexible structural members may buckle. Furthermore, additional dynamic excitations may activate adverse snap-through responses in such post-buckled components, which accelerates fatigue and failure. The severe nonlinearity associated with these phenomena challenges traditional forms of analysis and necessitates studious experimental methods for conclusive system characterization and model validation. This research builds upon state-of-the-art analytical and experimental strategies to examine the complex forced, dynamic behaviors of built-up structures that contain one or more post-buckled members. An analytical modeling and solution formulation is reviewed that is uniquely amenable to the study of multistable structures and permits experimentally-observable measures of impedance to be identified. Through theoretical and experimental studies, the efficacy of the impedance measures is evaluated towards their usefulness in identifying the onset of dynamic bifurcations in the multistable structural dynamics. For moderate amplitudes of input energy, the analysis is found to provide qualitatively accurate prediction of the drive point impedance changes observed prior to dynamic bifurcations from low to high amplitude of displacement.

Topics: Bifurcation
Commentary by Dr. Valentin Fuster

NDE, Structural Health Monitoring and Prognosis: Computational NDE and SHM

2017;():V009T17A001. doi:10.1115/IMECE2017-72284.

Floating Production, Storage and Offloading (FPSO) units are floating vessels used by the oil and gas industry offshore for the production and processing of hydrocarbons as well as storage of oil. It is equipped with the required facilities to produce, process, and store produced fluids. Among the equipment on board of the FPSO are pumps used to inject water or pump crude oil. This FPSO is fitted with seven positive displacement pumps powered by diesel engines; three for water injection and four for power fluid. At the exhaust of the driving engines, stainless steel bellows experienced frequent failures on a monthly basis; thereby, incurring a huge financial overburden on operating companies. Therefore, the aim of this paper is to explore the possibility of identifying some remedies to overcome this problem. The paper discusses the root causes of bellow failures attached to the exhaust system of a diesel engine on board of an FPSO. Effect of vibration, temperature, corrosion, and vessel rolling on the bellow structural response were investigated to identify the root causes of failure. A finite element modeling of the problem under study was conducted by taking into account the combined effect of thermo-mechanical loads. It was found that the thermal stress was well below the allowable stress. In addition, vibration analysis of the bellow-engine system revealed that the fundamental frequency of the bellow was way below the engine’s natural frequency. However, it was found that the vessel’s rolling generated an elevated stress that can cause failure of the bellow in a very short period time. The paper also presents some possible solutions to remedy or delay failure from occurring frequently as in this case.

Commentary by Dr. Valentin Fuster

NDE, Structural Health Monitoring and Prognosis: NDE Methods for Damage Detection, Quantification and Characterization

2017;():V009T17A002. doi:10.1115/IMECE2017-70140.

In this work, an impact experiment on a composite plate with unknown material properties (its group velocity profile is unknown) is implemented to localize the impact points. A pencil lead break is used to generate acoustic emission (AE) signals which are acquired by six piezoelectric wafer active sensors (PWAS). These sensors are distributed with a particular configuration in two clusters on the plate. The time of flight (TOF) of acquired signals is estimated at the starting points of these signals. The continuous wavelet transform (CWT) of received signals are calculated with AGU Vallen wavelet program to get the accurate values of the TOF of these signals. Two methods are used for determining the coordinates of impact points (localization the impact point). The first method is the new technique (method 1) by Kundu. This technique has two linear equations with two unknowns (the coordinate of AE source point). The second method is the nonlinear algorithm (method 2). This algorithm has a set of six nonlinear equations with five unknowns. Two MATLAB codes are implemented separately to solve the linear and nonlinear equations. The results show good indications for the location of impact points in both methods. The location errors of calculated impact points are divided by constant distance to get independent percentage errors with the site of the coordinate.

Commentary by Dr. Valentin Fuster
2017;():V009T17A003. doi:10.1115/IMECE2017-71045.

Cylindrical structures have been applied in various pressure vessels and weapon systems, which may be subjected to harsh environmental conditions such as large mechanical stresses and thermal stresses. As a result, non-destructive evaluation of such structures is critical in quality control. Among the various defects that may be generated during fabrication, transportation, operation/firing, and so on, surface crack is a critical one and needs to be quantitatively and accurately evaluated. In this study, both ultrasound phased array technique and eddy current technique are applied for the inspection of 120 mm steel test cylinder. In the cylinder, a total of nine sets of surface defects of various size, depth and orientation are fabricated and quantitatively evaluated.

In ultrasound phased array evaluation, simulations and experiments on standard aluminum block were carried out first to calibrate the system parameter setup. During this calibration, ultrasound propagation and its interaction with defects were simulated and studied. The dependence of ultrasound field on the ultrasound parameters and on the characteristics of defects was analyzed and optimized. Then simulations and experiments on steel test cylinder were carried out for the detection of the smallest detectable defects. Results showed that the optimization of the number of active elements can improve the distortion of defect images; the steering angles and the beam focusing laws may change the ultrasound beam intensity and uniformity, which has a significant influence on the sensitivity and resolution of the phased array technique; the geometry and material properties of cylindrical structures could distort the ultrasound beam, and as a result, calibration is necessary and important during practical inspections. Frequency is a key factor for phased array technique to improve its sensitivity.

In eddy current evaluation, a prototype for wireless eddy current system was designed, and an eddy current probe interface and a main unit interface were developed. The main advantages of such wireless probe are safety, economic benefits and maneuverability when compared to conventional wired probe. During testing, the signal at the probe interface was activated, measured, digitized and transmitted wirelessly to the main unit interface. Experimental results showed that the eddy current signals can be wirelessly communicated with main unit, and the results are comparable with the wired eddy current tester. Testing results also showed that the wireless signal is about 8 dB lower compared to wired signals and phase difference exists between the wired and wireless signals.

Commentary by Dr. Valentin Fuster
2017;():V009T17A004. doi:10.1115/IMECE2017-71203.

Electro-mechanical impedance diagnostic is one of key structural health monitoring approaches in aerospace structures. Considerable number of studies have demonstrated its efficiency in monitoring bolted joints. This investigation focuses on effect of a bolted boundary on the electro-mechanical impedance response of the space structure. Many space vehicles incorporate cylindrical payloads featuring multiple plates connected with threaded rods. Position of nuts on the threaded rods determine layered structure of the payload. Because of the cylindrical configuration of the payload, internal layers are formed by circular plates bolted to the connection rods. The number of connection rods determines the number of bolted boundary conditions around plate’s circumference. In this case, the boundary of the plate is essentially a mix of bolted and free segments and is not associated with a classical boundary condition. It is suggested that this case may be represented by an elastic boundary conditions with boundary stiffnesses depending on torque applied to each bolted joint. Vibrations of a circular plate with indicated complex boundary conditions were studied in this contribution theoretically and experimentally. As a result of numerical studies, a range of stiffnesses was suggested to model the bolted boundary. An analytical expression for the electro-mechanical impedance of a circular plate was presented and was utilized in the calculation of the response of a circular plate with the complex boundary. Structural damage was modeled as deviation of the stiffness associated with the bolted joint. Experimental studies were carried out to validate results of theoretical investigations. Electro-mechanical impedance signatures of the circular plate with an attached piezoelectric active sensor were collected for different sets of boundary conditions representing theoretical scenarios. Effect of the compromised bolted joint on the electro-mechanical impedance response of the whole circular plate was explored and the analysis of changes due to different conditions of the bolted boundary was provided.

Commentary by Dr. Valentin Fuster
2017;():V009T17A005. doi:10.1115/IMECE2017-71278.

Thick-walled welds are one of the most important components of large structures, which requires effective NDT and NDE techniques. UT technique is commonly used for welds, but does not perform well in thick-walled welds because of the coarse-grained microstructures. This paper introduces an efficient method to simulate ultrasonic signals in thick-walled welds, based on an anisotropic and inhomogeneous weld model in 2D. The method consists of three major steps, including ray-tracing, field calculation and signal simulation. The ray-tracing algorithm is developed based on Ogilvy’s RayTRAIM. The ultrasonic field is calculated along the traced ultrasonic rays using the multi-Gaussian beam model. The simulated defect echoes are generated based on a flaw scattering model, according to the calculated ultrasonic fields. All the algorithms and models have been improved and optimized for higher efficiency, based on the characteristics of the 2D weld model. UT experiments were performed to verify the simulation method. An austenitic weld with four SDHs was inspected using the phased array technique. The simulated defect echoes fit with the experimental signals with acceptable deviations, indicating the validity of the simulation method. Improvement of the method is required in future study, in order to predict the ultrasonic signals more precisely.

Commentary by Dr. Valentin Fuster
2017;():V009T17A006. doi:10.1115/IMECE2017-71374.

The state of stress in mechanical components can be evaluated non-destructively using ultrasonic waves. The relation between stress and the velocity of elastic waves in a solid is given by the acoustoelastic theory. Among different types of ultrasonic waves, the longitudinal critically refracted (Lcr) wave is more sensible to stress variations. However, not only stress, but also other factors can influence the velocity of ultrasonic waves, as for example, the microstructure of the material under test. Since the Young’s modulus is affected by variations of the microstructure, a relation between wave velocity and the modulus would allow corrections of the values of wave velocity obtained during stress measurements. The aim of this work is to find a relation between the Young’s modulus and the velocity of Lcr waves for specimens of API 5LX70 steel. The material chosen in this work is used in the manufacture of pressure vessels and oil pipelines. Assessing the state of stress of such components in field can guarantee operational safety and economic gains by avoiding premature replacements. The time-of-flight (TOF) of the Lcr wave was measured in four samples of API 5LX70 steel and the results were related to the Young’s modulus measured in a tensile test and through ultrasonic tests. Since the distance traveled by the wave in the specimens was kept constant, the TOF was used instead of the wave velocity. The results showed a linear relation between the TOF measured and the Young’s modulus obtained by ultrasound and tensile test in the samples.

Commentary by Dr. Valentin Fuster
2017;():V009T17A007. doi:10.1115/IMECE2017-71498.

The objective of this study is to capture and characterize the acoustic emissions of radial ball bearings in operation. A comparison is made between healthy bearings and defective ones. A precise 0.012” wide groove was cut into either a ball, the inner or outer race of test bearings. To reduce vibrational interference, the test shaft is isolated using a magnetic coupling and is supported by a thrust air bearing. The test bearing is acoustically insulated from its surrounding with mass loaded vinyl and acoustic reflections are dampened by wedge foam to improve the signal quality. The acoustic emissions are captured and digitized using a smartphone. Various signal processing techniques are used to characterize the signals and analyze defect modes. The results provide a comparison between the acoustic emissions of healthy and defective bearings operated at various speeds and provide methods for characterization.

Commentary by Dr. Valentin Fuster
2017;():V009T17A008. doi:10.1115/IMECE2017-71516.

Glass fabric epoxy resin based composite panels enhanced with carbon nanotubes were subjected to damage while changes in electrical resistance were obtained via embedded electrodes. The purpose of the study was to develop an alternative method to Electrical Impedance Tomography (EIT), which generates conductivity field, hence, indicating presence of various damages. The current method provides damage field by taking meticulous measurements of electrical resistance of panel. The method does not monitor conductivity as in the EIT but utilizes electrical resistance changes to detect damage. In the current form, it employs a network of 64 (8 × 8 grid) electrodes distributed evenly in a typical panel instead of the boundary electrodes used in EIT. Even though 64 electrodes were employed, fewer electrodes were sufficient to produce accurate indication of damage location and its size. In previous studies percolation threshold for carbon nanotube-epoxy mixture was determined, which enabled selection of optimal CNT concentration used in manufacturing of glass fiber reinforced panels. The glass fiber reinforced panels were manufactured by vacuum infusion method. The typical panel consisted of 10 glass fabric (S-2) plies. Copper electrodes were embedded beneath the top layer fabric ply. Electrical resistances measurements were obtained using four-probe technique. In the four-probe method, two outer electrodes are used to source a known current through the panel, while the two inner electrodes provide voltage drop needed to compute resistance. The technique minimizes contact resistance between electrodes and the composite, which could be order of magnitude larger than the material resistance being measured. Electrical resistance of cured glass fiber reinforced CNT-epoxy panels was first measured without any damage. Afterwards, damages in form of circular hole were inflicted to the panel starting with 1/8” diameter and enlarging it to 1/2” in steps of 1/8”. After the largest hole, 0.04” (∼1 mm) width cracks emanating from the hole were inflicted. During all measurements, electrical current passing through the source and sink electrodes was kept constant and changes in voltage from the inner probes were recorded. The thrust of the method is to incorporate a curve fit for quantifying the changes in resistance. The method can be applied to damage quantification in panels. The smaller spaced electrode distribution was more sensitive to smaller damages as expected, but the larger spaced electrodes network was sufficiently responsive to smallest damage. Experimental results were fairly good at predicting the damage and its magnitude. Results also indicated a very good agreement with the finite element simulations of the panels. Application of this technique can be a powerful tool for real time structural health monitoring of manufactured composites.

Commentary by Dr. Valentin Fuster
2017;():V009T17A009. doi:10.1115/IMECE2017-72063.

This paper discusses the development and testing of an automated robotic ultrasonic guided wave based inspection system developed to provide an efficient, accurate and reliable method for performing nondestructive evaluation and longer term structural health monitoring in advanced composite structures. The development process and challenges in the design of the automated robotic system are described. A number of tests were performed using the developed robotic ultrasonic inspection system on composite honeycomb core sandwich materials. Experiments showed that the developed automated ultrasonic guided wave inspection system was successful at locating disbonds between the core and the facesheets. Environmental sensitivity testing was also performed to characterize the effect of changing temperature and humidity on system performance. These tests indicate that approach was relatively insensitive to environmental changes, so that this approach could be used in service environment without a significant reduction in performance. Current system testing indicates that the described robotic ultrasonic inspection approach offers an accurate and robust method for inspection and long term tracking of advanced structural system health.

Commentary by Dr. Valentin Fuster

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