ASME Conference Presenter Attendance Policy and Archival Proceedings

2014;():V009T00A001. doi:10.1115/IMECE2014-NS9.

This online compilation of papers from the ASME 2014 International Mechanical Engineering Congress and Exposition (IMECE2014) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Computational Engineering and Simulation

2014;():V009T12A001. doi:10.1115/IMECE2014-36371.

This paper provides a combined computational and analytical study to investigate the lateral impact behavior of pressurized pipelines and inspect all the parameters such as the outside diameter and internal pressure affects such behavior. In this study, quartic polynomial functions are applied to formulate the maximum crushing force (F), maximum permanent displacement (W), and absorbed energy (E) of the pressurized pipelines during the impact problem. The effects of the diameter and pressure on F, W, and E are therefore illustrated through analyzing those functions. Response surfaces are also plotted based on the generated quartic polynomial functions and the quality (accuracy) of those functions are verified through several techniques.

Commentary by Dr. Valentin Fuster
2014;():V009T12A002. doi:10.1115/IMECE2014-36489.

Modeling progressive failure of composite materials can be a challenging task. It is further complicated by mesh dependency of finite element solvers when implementing the Hashin criterion. As one continues to refine the mesh, the finite element analysis (FEA) continues to yield varying solutions; hence there is no converged solution for continuous mesh refinement. This is due to mesh dependency when modeling strain softening. The FEA package Abaqus attempts to mitigate this but the issue is not eliminated.

Methods that address mesh dependency include experimental validation and numerical analysis. Experimental validation tailors the mesh to match a specific result. This mesh would then be applicable to other configurations with the same geometry and loading; however, experimental validation with increasingly complex parts for FEA is costly and time consuming. Numerical approaches to mesh selection do not require experimental validation; however, these methods may be computationally expensive depending on the analysis. A mesh selection strategy that does not require experimental validation, while computationally efficient, should be implemented for design purposes.

This study investigates a mesh selection strategy based on a converged elastic solution; the coarsest mesh that converges to a solution in the linear-elastic portion of the material response is chosen for analysis. Previous studies using an implicit solver yielded good results for out-of-plane loading conditions; however this procedure has not been implemented for explicit solvers. In this study, an investigation was conducted to determine the appropriate mesh to model progressive damage for notched, carbon fiber composite panels in out-of-plane shear (mode III) using an explicit solver in Abaqus/Explicit. This study analyzed 20 ply thick panels and considered three stacking sequences: 10%, 30%, and 50% zero degree plies. The procedure initially disabled damage and identified the coarsest mesh that approached a converged elastic solution. Using this mesh, damage was enabled and the models were run with loading proceeding through damage initiation until failure. The panels’ material response were extracted from the finite element (FE) model and filtered in order to determine their maximum load-carrying ability. The FE predicted maximum loads were then compared to corresponding experimental data in order to validate the mesh selection procedure. This process is not limited to out-of-plane shear; the potential for this mesh selection method would allow for progressive failure simulation to be more applicable in the design process of composite structures instead of post-damage analysis.

Commentary by Dr. Valentin Fuster
2014;():V009T12A003. doi:10.1115/IMECE2014-36929.

Vibration of a thin, rectangular-cross-section beam submerged in a viscous, quiescent fluid undergoing small amplitude oscillations is studied using a Boundary Element (BE) approach in which the free-surface is modeled through a stress-free boundary condition. The Stokes approximation is used where nonlinear convective terms are negligible and the problem is formulated in Fourier and Laplace transform space when appropriate. Results are expressed in terms of non-dimensional hydrodynamic force and its components, namely added mass and damping coefficients. Several parametric studies are conducted to evaluate the effects of depth of submergence, frequency and the amplitude of oscillations on the hydrodynamic functions. The results are compared with the classical solution for a vibrating lamina in an infinite fluid as the limit case and with a recent study using Smoothed Particle Hydrodynamics (SPH) analysis in the presence of a free-surface.

Commentary by Dr. Valentin Fuster
2014;():V009T12A004. doi:10.1115/IMECE2014-37254.

When a simply supported composite plate is subjected to a lateral load, the presence of the twist coupling stiffnesses in the governing differential equations of equilibrium does not allow the determination of an exact solution for the deflection and numerical methods must be used. This paper describes a comparison of computed approximations to the deflection of composite laminates subjected to transverse loading obtained using the Ritz method and the finite element method. The Ritz method is implemented with the symbolic manipulation program Maple and ANSYS is used to perform the finite element calculations. Reliable results are obtained using both methods.

Commentary by Dr. Valentin Fuster
2014;():V009T12A005. doi:10.1115/IMECE2014-37396.

The current work is focused on numeric investigation of aerodynamic load developed on a wind turbine blade and its effects on aeroelastic characteristics of a wind turbine blade. In order to do that proper turbulent model along with appropriate assumptions need to be determined. Geometry is modeled with actual blade data for both twist and tapper. The blade tip is not considered during the modeling. Validation is done by NREL phase VI wind turbine blade data as well as other published data. Finally the aerodynamic load obtained from the CFD simulation is transferred to perform the structural analysis. It has been found that the load distribution along the blade span is not linear. It varies with the span length and it also varies along the chord of the blade airfoil. Due to this varying load the stresses developed in the blade are dissimilar which dictates the skin thickness of the blade and the shape of the spur inside the blade. It has also been observed that the aerodynamic characteristics such as lift coefficient (CL) and pressure coefficient (CP) changes with the deflection of the blade which affects the power output of the wind turbine. Finally a pre-bent blade model has been analyzed and the effect due to the bent on the performance of the wind turbine has been observed and presented. It has been found that the pre-bent blade has better Cp distribution than deflected blade and the deviation of Cp from the actual straight blade reduce significantly in pre-bent blade compare to deflected blade. The pressure distribution along the chord of the blade airfoil at different locations have been observed and presented.

Commentary by Dr. Valentin Fuster
2014;():V009T12A006. doi:10.1115/IMECE2014-37443.

A thermo-mechanical coupling contact model between a fractal rough body and a flat body is established. In the model, the heat flux coupling between the sliding surfaces and the effect of elasto-plastic deformation of the rough body are considered. To obtain the transient microcontact process between the rough body and the flat body during rotating sliding friction, the thermo-mechanical problem under this three-dimensional model is solved by the nonlinear finite element multi-physical methods. The comparisons of the real contact area are analyzed under two different working modes, including loading processes with and without frictional rotating. During the loading and rotating process, the shear stress and the total frictional force on the frictional rough interface, and the equivalent plastic strain of the contact asperity are larger. All these including the thermal expansion make the real contact area increase with the applied normal load much faster under the working mode of loading and rotating than it does under the only loading mode.

Commentary by Dr. Valentin Fuster
2014;():V009T12A007. doi:10.1115/IMECE2014-38542.

This paper investigates the influence of multiple inclusions on the Cauchy stress of a spherical particle-reinforced metal matrix composite (MMC) under uniaxial tensile loading condition. The approach of three-dimensional cubic multi-particle unit cell is used to investigate the 15 non-overlapping identical spherical particles which are randomly distributed in the unit cell. The coordinates of the center of each particle are calculated by using the Random Sequential Adsorption algorithm (RSA) to ensure its periodicity. The models with reinforcement volume fractions of 10%, 15%, 20% and 25% are evaluated by using the finite element method. The behaviour of Cauchy stress for each model is analyzed at a far-field strain of 5%. For each reinforcement volume fraction, four models with different particle spatial distributions are evaluated and averaged to achieve a more accurate result. At the same time, single-particle unit cell and analytical model were developed. The stress-strain curves of multi-particle unit cells are compared with single-particle unit cells and the tangent homogenization model coupled with the Mori-Tanaka method. Only little scatters were found between unit cells with the same particle volume fractions. Multi-particle unit cells predict higher response than single particle unit cells. As the volume fraction of reinforcements increases, the Cauchy stress of MMCs increases.

Commentary by Dr. Valentin Fuster
2014;():V009T12A008. doi:10.1115/IMECE2014-38669.

A new method for implementing contact/impact in an implicit finite element formulation has recently been developed. The method uses the idea of buoyancy and fluid drag forces to enforce the contact constraint as well as provide a method of dissipating energy due to the contact. Additionally, the method provides a simplified means for accounting for contact friction. The method has potential usefulness in a certain class of problems such as contact with bodies having soft and viscous foundations as well as problems with sliding. In this work, we introduce the method and illustrate it with an example that shows the benefits and utility of the method.

Commentary by Dr. Valentin Fuster
2014;():V009T12A009. doi:10.1115/IMECE2014-38717.

Air pollution has been proven as a significant risk factor for multiple health conditions. A major portion of urban air pollution is attributed to vehicle emissions. In this study, a high school which is close to an interstate highway is numerical simulated to estimate the impact of traffic emissions on air quality. Two probability density functions, Weibull distribution and Rayleigh distribution, were used in wind data statistical analysis. A numerical method was used to estimate the wind speed at study site based on the wind data in meteorology stations. Both indoor and outdoor environment were simulated using computational fluid dynamics (CFD). The airflow and the dispersion of particulate air pollutants emitted from the highways surrounding the high school building were analyzed. The wind flow was simulated using Reynolds-Averaged Navier Stokes (RANS) model. The particulate matters are tracked using Lagrangian model. For the indoor simulation, the standard k-ε model is employed to model the air-phase turbulence. Discrete phase model (DPM) and Eulerian multiphase model were utilized for the particle phase, respectively. The comparison shows that the Lagrangian approach has better agreement since the dispersed-phase volume fractions are less than 10%.

Commentary by Dr. Valentin Fuster
2014;():V009T12A010. doi:10.1115/IMECE2014-38974.

The U* index has been used for load transfer analysis to show its capability in giving general awareness regarding performance of structure. Although U* index and stress values have been proven to be useful indexes as structure design criteria, a thorough comparison between conventional stress analysis and loads transfer analysis (based on U* index) is lacking. In this study, we evaluate load transfer behaviors of a parcel rack of multiple passenger vehicles under different loading conditions using the U* index. Then by demonstrating the unique capabilities of U* as an index for stiffness, it is shown that the load path concept can be combined with the stress analysis results to provide comprehensive information about the structure responses to loading. In addition to the agreement between stress analysis and the U* analysis, it is shown that U* can provide additional information about the structure response that stress analysis fails. Such information includes: interpreting high and complicated stress distributions in structure and detection of questionable stiffness in certain parts of structure. More importantly, the load path index U* can detect the area where significant changes in the structure stiffness occurs. Such information can be used as a guideline for structure design with the goal to reduce the weight while still keeping the structure integrity.

Commentary by Dr. Valentin Fuster
2014;():V009T12A011. doi:10.1115/IMECE2014-39230.

In recent years, there has been an increasing demand for tractor usage for agricultural activities in the world. Tractors are an integral part of mechanization and have a crucial role to play to enhance agricultural productivity. They are used for many kinds of farm work, under various soil and field conditions. It provides agricultural activities in challenging conditions by using several farming equipment. During the operations, tractors have to efficiently transfer power from the engine to the drive wheels and PTO through a transmission. Tractor clutch is the essential element in this system. During the torque transmission, loads which occur on the clutch components cause damages. In many cases, especially PTO clutch finger mechanism is fractured under the torque transmission.

In this study, finger mechanism, which used in tractor clutch PTO disc, is investigated. Finite element analyses were performed for two different thicknesses (3.5 and 4 mm) of the finger mechanism. Stress and deformation values which occur during the transfer of power in a safe manner are investigated for these thicknesses. The finger mechanism CAD models were created using CATIA V5 and then imported into ANSYS for static finite element analyses. As a result of the analyses, approximately 13% stress decreasing was observed with the increment of the 0.5 mm for the finger thicknesses. Results from the analyses provide an accurate prediction of the material yielding and load path distribution on the PTO clutch finger. To verify the analyses results prototype PTO finger mechanism was manufactured and was conducted bench tests. Consequently, a good correlation was achieved between finite element model and test results.

Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Damage and Failure of Composites

2014;():V009T12A012. doi:10.1115/IMECE2014-36376.

The ever increasing use of composite materials in today’s society has created a drastic demand for better modeling of their behavior. The difficulty arises in that many modern composite structures are unique in shape and are exposed to a variety of loading situations. More specifically, loading scenarios which cause out-of-plane shear (Mode III) or mixed mode (Mode I + Mode II + Mode III) failure are of greatest challenge to model. This study investigates the capabilities of Simulation Composite Analysis (SCA), a composites software by Autodesk, in modeling failure in notched carbon-fiber composite panels loaded in Mode III. SCA was used with the finite element modeling software Abaqus/Standard (Dassault Systèmes) to model six different laminate stacking sequences. Three of the layups featured 40 plies through the thickness and the other three had 20 plies, with each containing either 10, 30, or 50 percent zero degree plies. The modeled panels were displaced as to create for a Mode III loading condition and the resulting maximum loads, load-displacement plots, and damage propagation outputs were compared to experimental results. It was found that SCA can determine the maximum failure load of the panels with an average of 11.6 percent deviation from experimental values. For one laminate stacking sequence in particular, the software determined maximum loads that deviated less than 1 percent from the experimental data. The load-displacement plots showed good correlations with experimental data in the linear region; however, the load-displacement behavior after damage was well modeled for only certain layups. The damage propagation paths for all the panel models were similar to the experimental panels in general, though self-similar damage propagation was not captured by the FEA models. Overall, Mode III failure in the notched carbon fiber panels was satisfactorily modeled for maximum load, but continued development is needed for predicting damage propagation paths. Modeling Mode III failure in composites is a difficult task; therefore, determining accurate methods in which to model such failure will be a substantial benefit to the composites engineering community. If low cost computer models can be established which accurately capture material damage and failure, the need for expensive and time-intensive experiments may be greatly reduced.

Commentary by Dr. Valentin Fuster
2014;():V009T12A013. doi:10.1115/IMECE2014-36580.

Damage initiation and progression in uni-directional continuous polymer composites at the microscale has been investigated by considering a 3D Repeating Unit Cell (RUC) with square distribution of fibers. Three damage modes under static loading have been looked at, viz., matrix damage, fiber failure and fiber-matrix debonding. The matrix has been modeled as an isotropic elastic-plastic material via a user material subroutine (UMAT) within the framework of the finite element software Abaqus. In addition, fiber-matrix debonding has been simulated using a traction-separation criterion via Cohesive Zone Modeling (CZM) approach. Finally, a user defined field (USDFLD) has been used to simulate the fiber breakage. The combined effect of matrix-damage, fiber failure and interfacial debonding has then been studied using homogenization principles. Preliminary results from the current modeling approach have been found to be encouraging and this approach paves way for more complex multi-scale damage simulations in heterogeneous materials.

Commentary by Dr. Valentin Fuster
2014;():V009T12A014. doi:10.1115/IMECE2014-38018.

The development of a composite cryogenic fuel tank is desirable for the creation of a reusable single-stage launch vehicle. The cyclic loading and temperature changes experienced during launch and re-entry conditions result in the microcracking of conventional composites. To increase the fracture strength of this composite, a property often limited by the matrix, the nanoplatelet known as graphene or exfoliated graphite, has been introduced. Three nanocomposites were produced using graphene and Phenylethynyl Terminated Imide oligomer (PETI-5). The nanocomposites were machined in to flexure samples and tested at room temperature. Results from these tests indicate that the ideal concentration of graphene in our PETI-5 nanocomposite is 0.08%.

Commentary by Dr. Valentin Fuster
2014;():V009T12A015. doi:10.1115/IMECE2014-38040.

Microcapsules containing epoxy resin EPON 862, phenyl acetate solvent, and multi-walled carbon nanotubes were fabricated by in-situ polymerization for the development of a self-healing composite. The microcapsules were embedded in an epoxy matrix in single-edged notch bend shapes for fracture testing on a three-point bend test fixture. Healing efficiency was calculated for specimens with various microcapsule loading ratios. Fracture tests confirmed healing ability with the microcapsules. Preliminary results indicated higher healing efficiencies with increased microcapsule loading.

Commentary by Dr. Valentin Fuster
2014;():V009T12A016. doi:10.1115/IMECE2014-38109.

A major benefit of advanced fiber-reinforced polymer composites is that they can be tailored and optimized to suit a particular structural application by orienting the reinforcing fibers along multiple directions. For practical load-bearing structural components manufactured from multidirectional laminates, predicting their mechanical behaviour is quite complex. This is specifically the case for progressive failure analysis of these materials when subjected to quasi-static or fatigue loading since local cracks will initiate and evolve in multiple directions simultaneously. The difficulty of the problem increases further when these laminates are subjected to complex multiaxial stress states. This is due to the fact that the multidirectional crack state will be subjected to additional crack driving stress components, which will ultimately alter the crack evolution characteristics. A synergistic damage mechanics (SDM) methodology has recently been developed to address these issues in progressive damage analyses of composite laminates containing multiple damage modes and subjected to uniaxial loading [1]. By combining micromechanics and continuum damage mechanics, the SDM methodology provides a rigorous and practical tool for accurate prediction of progressive damage behaviour in composite structures. This is essential for accurately predicting the integrity and durability of practical structures, which will lead to safer and more efficient designs.

Commentary by Dr. Valentin Fuster
2014;():V009T12A017. doi:10.1115/IMECE2014-38112.

Recycled paper and plastics are widely used in current society. Recycled paper is very helpful to reduce waste emissions and energy consumption, saving resource and cost and environment protection. Recycled paper tube is one of the most successful cases in application of recycled paper. Plastics and Recycled paper are excellent choices for packing materials. They are also used to construct temporary structures for both exhibition spaces or for rapid-recovery shelters in emergency operations. As paper tubes are laminated composite materials composited of paperboard which is inherent anisotropy materials, the research on mechanical property and fracture behavior becomes more complicated. In order to analyze deformation and fracture behavior of paper tube and paper tube laminated combined with polymer under lateral compressive load, both the paper tube and hybrid paper tube which was dipped in to the thermosets resin were tested and compared. The energy analysis was also conducted here.

Topics: Plastics
Commentary by Dr. Valentin Fuster
2014;():V009T12A018. doi:10.1115/IMECE2014-38825.

The effect of different hygrothermal aging conditions on the glass transition temperature of a six-ply quartz-fiber-reinforced bismaleimide composite is investigated via dynamic mechanical analysis. Bismaleimide is a polymer matrix material suitable for high-temperature structural and electrical applications such as the radar-protecting structure on supersonic aircraft. This particular material is usually subjected to water absorption due to humid air, precipitation, condensation, and accumulation of water in the interior of its constituent structure. In the fully dry, un-aged condition, the glass transition temperature of the laminate is approximately 380°C. Hygrothermal aging conditions are simulated by full-immersion of laminate specimens in distilled water ranging from 30 to 200 days at 25, 40, 60, 75, and 90°C. Specimens immersed at 40°C for 200 days showed the largest depression in glass transition temperature, to approximately 365°C. A subtle, secondary transition marked by a depression and recovery in storage modulus is consistently present in specimens exposed to 75 and 90°C conditions, independent of total immersion time, near 260°C. However, the specimens aged at 75 and 90°C did not exhibit a significant decrease in glass transition temperature as expected. Results indicate variations in glass transition temperature as a result of water absorption in BMI/quartz laminates are dependent on hygrothermal aging history, rather than solely a function of moisture content.

Commentary by Dr. Valentin Fuster
2014;():V009T12A019. doi:10.1115/IMECE2014-39677.

In this paper we compare two manufacturing techniques namely vacuum infusion and compression molding, used in manufacturing S2 glass fabric/epoxy composites for high-speed impact applications. Even though compression molding and vacuum infusion are two widely used manufacturing techniques, the resulting product may be very different. Compression molding has the advantage of achieving a much higher fiber density for the same thickness. With a higher fiber density, the composites made by compression molding have better mechanical properties than a composite made by vacuum infusion. However, vacuum infusion is faster and more economical. The mechanical performance of the composites manufactured by these two processes are compared by performing tensile tests and high speed impact tests for the determination of the limit speed V50. For the same number of plies, preliminary results for compression molded specimens indicate a 50% increase in stiffness and a 40% increase in strength. Also, for panels of the same thickness, the V50 was higher for compression molding specimens.

Topics: Manufacturing
Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Effects of Defects, Damage Tolerance and Repair of Composites

2014;():V009T12A020. doi:10.1115/IMECE2014-36483.

Traditionally, corroded pipelines with metal loss defects have been repaired by replacing the defective areas with metallic pipe segments or by welding on metallic sleeves. Considering the technical and economic advantages of composite materials, literature shows that these defects can be repaired or reinforced with a composite sleeve system. In these systems, epoxy filler is used to fill the corrosion defect followed by wrapping the piping segment with concentric coils of composite material. However several challenges need to be met to obtain the desired repair quality. The major considerations are surface preparation, reduction in operating pressure for the repair, the duration required (e.g. for curing of the composite material) for the composite repair to share the load from the pipe, and the most important is the in-field quality assessment and to ensure consistency between several repairs. In this work, a numerical model is developed to investigate the performance of composite repair systems and compare with the traditional metallic welded sleeve. The effect of fiber orientation, repair sleeve thickness, material and installation pressure have been studied. Results show the potential of composite repair system in replacing the existing welded metallic non-pressure containing sleeve for metallic pipelines.

Commentary by Dr. Valentin Fuster
2014;():V009T12A021. doi:10.1115/IMECE2014-38407.

The focus of this paper was to investigate the effects of microvoid content in quartz/BMI laminates on both short and long-term moisture absorption dynamics. The moisture absorption characteristics for the laminates were experimentally obtained by water immersion tests at 25°C of three-ply quartz/BMI samples that contain voids, ranging from 8.6% to 13.7% by volume. The void levels were obtained by conditioning the prepreg at different moisture levels for 48 hours in an environmental chamber before curing in a hot press. The curing process was carried out at 69 kPa, which leads to a more uniform fiber volume fraction for the laminates. Having a constant fiber volume fraction ensures the same amount of fiber-matrix interface present in all the test samples, therefore eliminating the effect of fiber-matrix interface as an experimental variable. It is shown that the presence of microvoids leads to an increased non-Fickian absorption behavior. Hence, the anomalous, non-Fickian absorption parameters are obtained by using a one-dimensional absorption model that accounts for both bound and unbound free water within the laminate. It is shown that the microvoids act as storage sites for moisture which can be described by the one-dimensional, non-Fickian absorption model. Finally, possible relationships between the four absorption model parameters and the process-induced microvoid content are discussed.

Commentary by Dr. Valentin Fuster

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

2014;():V009T12A022. doi:10.1115/IMECE2014-36731.

Within the framework of European project STYLE (Structural integrity for lifetime management), fracture tests on two large scale pipes containing a through wall crack have been performed. Two Mock-ups have been tested: MU1 is a narrow gap Inconel Dissimilar Metals, provided and designed by AREVA France, and MU2 is a an austenitic steel butt-weld with a thermally aged weld repair austenitic weld, provided by EDF British Energy.

The four-points bending tests were carried out by the French Alternative Energies and Atomic Energy Commission (CEA), in order to study the mechanical properties and integrity of component such as welding pipes. A through wall crack was machined in the both pipes. After a fatigue pre-cracking step carried out at RT, the monotonic fracture test was performed (at 300°C on MU1). Optical camera and Electrical Potential Drop Method have allowed following the crack growth during fatigue and final fracture stages. The observations made post-mortem showed ductile tearing of a few millimeters in those pipes.

The first part of this paper is devoted to the four-points bending tests. The second part of this paper deals with first numerical analysis related to the Mock-up-1. Previous results concerning the mechanical characterizations of the constitutive materials are discussed. Fracture mechanics small scale specimens are interpreted using FE Analysis to obtain the fracture parameters used in global approaches. First computation is shown on the Mock-up-1 in order to predict the behavior of the large scale test mechanical and fracture behavior.

Commentary by Dr. Valentin Fuster
2014;():V009T12A023. doi:10.1115/IMECE2014-36865.

In order to clarify the characteristics of high-cycle fatigue of the modified 9Cr-1Mo steel, a high temperature rotary bending test was carried out. As a result, the fatigue strength of this alloy decreased monotonically at elevated temperatures. It decreased from 440 MPa at room temperature to about 350 MPa at 400°C. This decrease of the fatigue strength was attributed to the temperature dependence of the yielding strength of this alloy. The fatigue limit appeared near 107 cycles at 400°C, whereas it appeared around 106 cycles at room temperature. The most important result is that the fatigue limit disappeared up to 108 cycles at temperatures higher than 500°C. Thus, the number of cycles at which the fatigue limit appeared shifted to higher cycles with increasing the testing temperature. Clear striation was observed in the stable crack growth region on the fracture surface of all the specimen tested at room temperature, 400°C, 500°C, 550°C, and 600°C. Intergranular cracking, which have been observed in creep-fatigue tests, was not observed. Since the estimated operating temperature of FBR is 550°C, it is very important to consider this fatigue strength in the structural and reliability design of the modified 9Cr-1Mo steel.

In this study, the change of crystallinity of this alloy under fatigue loading was also analyzed by applying an EBSD method. The image quality (IQ) value obtained from the analysis was used for the quantitative evaluation of the crystallinity in the area where an electron beam of 20 nm in diameter was irradiated. The quality of the atomic alignment was found to degrade under the cyclic loading, and a crack started to occur on the surface of the alloy when the quality of the atomic alignment decreased to a certain critical value.

Commentary by Dr. Valentin Fuster
2014;():V009T12A024. doi:10.1115/IMECE2014-37340.

Fretting fatigue is one of the typical failure forms of engine block. The aim of this study is to investigate the fretting fatigue mechanism of the V type engine and guide engine design. An experiential system was developed to simulate fretting fatigue failure under typical engine working condition. And a submodel was used in the finite element calculation to analyze contact status and stress distribution of the structural model. Through the fretting fatigue experimental observations and finite element analysis, it can be concluded that the additional rotate torque caused by bearing load and the bolt pretension load are the two main factors which affect the fretting fatigue mechanism of the V type engine. Appropriate increasing of the bolt pretension load and using extended skirt block with cross-bolted main bearings design will restrain the oscillation of the main bearing cap can be beneficial to fretting fatigue lives of the engine block.

Topics: Fatigue , Engines
Commentary by Dr. Valentin Fuster
2014;():V009T12A025. doi:10.1115/IMECE2014-37391.

The presence of hydrostatic pressure in the range of 10 to 600 bar (typical of sea depths from 100 meters to 6 kilometers) and cyclic wave loading in off-shore structures and equipments operating in deep-water environment requires suitable criteria accounting for high hydrostatic load during design of such structural elements. The presence of corrosive species due to sea water and marine organisms also adds complexity to this problem. Plain fatigue data evaluated under ambient conditions or fatigue data evaluated in atmospheric conditions under soaked sea water environment may not be appropriate for the design of systems subjected to hydrostatic pressure. High hydrostatic pressure alters the yield criterion of materials. Thus, it becomes essential to generate the fatigue strength data of structural steel in pressurized simulated sea water (3.5% NaCl solution) conditions.

This paper presents the comparison of fatigue life data evaluated for a structural steel material under ambient conditions and under hyperbaric pressure conditions. A special hyperbaric chamber was designed for this purpose and 3.5% NaCl solution was pressurized into the chamber by a manual pump. Mechanical fatigue loading was applied on the specimen in the presence of two pressures, viz., 30 bar and 50 bar hydrostatic pressure. The results suggest that hydrostatic pressure has a significant influence on fatigue life data of steel. The fracture surfaces were examined using a scanning electron microscope which suggests change in fracture mode from being pure ductile to mixed mode of fracture. Work is in progress to evaluate the strength degradation of welded joints subjected to hyperbaric fatigue.

Commentary by Dr. Valentin Fuster
2014;():V009T12A026. doi:10.1115/IMECE2014-37598.

Pipeline is the common mode for transporting oil, gas, and various petroleum products. Structural integrity of oil and gas transmission pipelines is often threatened by external interferences such as concentrated lateral loads and as a result, a failure of the pipeline may occur due to “mechanical damages”. Sometime, this load may not cause immediate rupture of pipes; rather form a dent which can reduce the pressure capacity of the pipeline. A dent is a localized defect in the pipe wall in the form of a permanent inward plastic deformation. This kind of defect is a matter of serious concern for the pipeline operator since a rupture or a leak may occur. Accordingly, an extensive experimental study is currently underway at the Centre for Engineering Research in Pipelines (CERP), University of Windsor on 30 inch (762 mm) diameter and X70 grade pipes with D/t of 90. The aim of this research is to examine the influence of various parameters such as dent shape and service pressure on strain distributions of dented pipe. Also, three-dimensional finite element models were developed and validated for determining strains underneath the indenter. The load-deformation behavior of pipes subject to this type of lateral denting load obtained from experimental study and finite element analysis is discussed in this paper. In addition, distributions of important strains in and around the dent obtained from the study are also discussed.

Topics: Stress , Pipes
Commentary by Dr. Valentin Fuster
2014;():V009T12A027. doi:10.1115/IMECE2014-37660.

Elastic plastic fracture mechanics (EPFM) is the domain of fracture analysis which considers extensive plastic deformation at crack tip prior to fracture. J integral and crack tip opening displacement (CTOD) have been commonly used as parameters for EPFM analysis. The relationship between these parameters has been extensively studied by industry and academia. The plastic constraint factor can serve as a parameter to characterize constraint effects in fracture involving plastic deformation. Therefore, the characteristics of plastic constraint factor are important in EPFM analysis. In this study, the relationship between J Integral and CTOD was investigated by conducting fracture toughness tests using single edge notched bend (SENB) specimens. Also, plastic constraint factor was investigated by using finite element analysis. Numerical analysis was carried out using ABAQUS elastic-plastic analysis mode.

Commentary by Dr. Valentin Fuster
2014;():V009T12A028. doi:10.1115/IMECE2014-37703.

In this paper, the in-situ scanning electron microscopy (SEM) experiments are performed in the edge-cracked specimen under the single overload in order to investigate transient fatigue crack growth behavior. The specimen is made of Al7075-T6 and under the plane stress condition. During the testing, several loading cycles of interest are selected and divided into a certain number of steps. At each step, high resolution images around the crack tip region are taken under the SEM. Imaging analysis is used to quantify the crack tip opening displacement (CTOD) at each corresponding time instant in a loading cycle. In the current experimental work, the crack closure phenomenon is not only directly observed under constant amplitude loadings, but also under the variable amplitude loading. The experimental results provide the evidence that the crack closure may disappear or become inconsequential right after the single overload. And some observations imply that the crack closure is not the only parameter which controls fatigue crack growth rate, other factors need to be considered. A detailed discussion is given based on the current investigation.

Commentary by Dr. Valentin Fuster
2014;():V009T12A029. doi:10.1115/IMECE2014-37909.

This study deals with the reverse bending method as a residual stress modification technique for CTOD test of the thick weld. In the reverse bending method, the residual stress around the notch tip of the specimen is redistributed by compressive plastic deformation by reverse bending load. The reverse bending method requires a smaller level of applying load comparing to the local compression method. However, the excessive reverse load may introduce the excessive plastic deformation in way of notch tip which can affect the CTOD value of the weld. So, this study investigates how to establish the proper bending load for ultra-thick weld. In order to do it, stress intensity factor was determined by the plastic zone size around the notch tip by 3-dimensional finite element analysis. The validation of the reverse bending method was verified by examining the configuration of pre-crack at CTOD weld test specimen with various thickness and strength.

Commentary by Dr. Valentin Fuster
2014;():V009T12A030. doi:10.1115/IMECE2014-39430.

In order to make clear the mechanism of the directional coarsening (rafting) of γ′ phases in Ni-base superalloys under uni-axial tensile strain, molecular dynamics (MD) analysis was applied to investigate dominant factors of strain-induced anisotropic diffusion of Al atoms and nanotexture change of fine dispersed γ′ precipitates. In this study, a simple interface structure model corresponding to the γ/γ′ interface, which consisted of Ni as γ and Ni3Al as γ′ structure, was used to analyze the effect of alloying element on diffusion properties. The diffusion constants of Al atoms were changed drastically by the dopant elements and their contents. When the lattice constant of the γ phase was increased and its melting point was decreased by the addition of Cr or Al atoms, the strain-induced anisotropic diffusion of Al atoms in the γ′ phase was accelerated. On the other hand, the addition of Co decreased the diffusion significantly. Therefore, changes of lattice constant and melting point depending on the chemical composition of the γ/γ′ interface are the dominant factors controlling the strain-induced anisotropic diffusion of Al atoms in the Ni-base superalloy.

Commentary by Dr. Valentin Fuster
2014;():V009T12A031. doi:10.1115/IMECE2014-39513.

Fracture toughness and Fatigue Crack Growth (FCG) experimental data represent the basis for accurate designs and integrity assessments of components containing crack-like defects. Considering ductile and high toughness structural materials, crack growing curves (e.g. J-R curves) and FCG data (in terms of da/dN vs. ΔK or ΔJ) assumed paramount relevance since characterize, respectively, ductile fracture and cyclic crack growth conditions. In common, these two types of mechanical properties severely depend on real-time and precise crack size estimations during laboratory testing. Optical, electric potential drop or (most commonly) elastic unloading compliance (C) techniques can be employed. In the latter method, crack size estimation derives from C using a dimensionless parameter (μ) which incorporates specimen’s thickness (B), elasticity (E) and compliance itself. Plane stress and plane strain solutions for μ are available in several standards regarding C(T), SE(B) and M(T) specimens, among others. Current challenges include: i) real specimens are in neither plane stress nor plane strain - modulus vary between E (plane stress) and E/(1-ν2) (plane strain), revealing effects of thickness and 3-D configurations; ii) furthermore, side-grooves affect specimen’s stiffness, leading to an “effective thickness”. Previous results from current authors revealed deviations larger than 10% in crack size estimations following existing practices, especially for shallow cracks and side-grooved samples. In addition, compliance solutions for the emerging clamped SE(T) specimens are not yet standardized. As a step in this direction, this work investigates 3-D, thickness and side-groove effects on compliance solutions applicable to C(T), SE(B) and clamped SE(T) specimens. Refined 3-D elastic FE-models provide Load-CMOD evolutions. The analysis matrix includes crack depths between a/W=0.1 and a/W=0.7 and varying thicknesses (W/B = 4, W/B = 2 and W/B = 1). Side-grooves of 5%, 10% and 20% are also considered. The results include compliance solutions incorporating all aforementioned effects to provide accurate crack size estimation during laboratory fracture and FCG testing. All proposals revealed reduced deviations if compared to existing solutions.

Commentary by Dr. Valentin Fuster
2014;():V009T12A032. doi:10.1115/IMECE2014-39514.

Structural integrity assessments regarding Fatigue Crack Growth (FCG) and fracture phenomena are based on fracture mechanics theoretical background and rely upon the notion that a single parameter (usually K or J, respectively for linear elastic and elastic-plastic fracture mechanics) characterizes the crack-tip stress fields and controls local damage. However, the validity of K/J as crack-tip driving forces representative of local stress fields is only achieved if SSY (Small Scale Yielding) conditions prevail. It means that plasticity ahead of the crack must be small. Current standards (e.g.: ASTM E399, E1820, E647, ISO 12135) impose severe geometrical restrictions for the specimens (minimum thicknesses and crack depths) looking for plane strain (high constraint) conditions and therefore K and J-dominance. The main challenge is that thicknesses and/or planar dimensions of current real structures made of high toughness structural steels are in several cases not enough for the extraction of “valid” C(T), SE(B) or SE(T) specimens. In this context, subsized specimens are of great interest. As an example, Charpy geometries have been investigated during the last decades. This work is concerned about testing high structural steels and investigates the applicability of fatigue-precracked Charpy specimens for determining FCG (da/dN vs. ΔK) and J-R curves. The main issues are: i) verify the feasibility of the experiments in a servohydraulic machine in terms of scatter, control and repeatability; ii) quantify the validity limits of K and J for such reduced geometries. Samples had notches machined by EDM and were precracked reaching a/W=0.25 and a/W=0.45. FCG and J-R tests were successfully conducted with repeatability and refined 3D non-linear FE models were developed to provide compliance solutions and verify K and J dominance. Consequently, mechanical properties from subsized samples could be obtained and compared to data obtained from standardized C(T) specimens made of the same steel. The applicability of precracked Charpy geometry could be investigated, motivating further investigations in the field.

Topics: Fatigue cracks
Commentary by Dr. Valentin Fuster
2014;():V009T12A033. doi:10.1115/IMECE2014-40032.

The paper presents a correlation of fatigue crack growth rate (FCGR) behavior for different load ratios R (= min load/max load). The proposed method of correlation is explained in detail using comprehensive experimental FCGR data from 2524 aluminum alloy. The consistency of the proposed method has been verified using test data taken from literature for more than 10 different materials including aluminum, steel, titanium and other alloys. For all the materials studied, a transition load ratio Rt was found, which marks the transition between a dominant influence of ΔK or Kmax on FCGR behavior. For R>Rt the FCGR is dominated by ΔK. On the other hand, for R<Rt it is found that Kmax is to be the dominating parameter. Two equations, in terms of ΔK or Kmax, have been developed to represent FCGR curves for various load ratios. ΔKdriving is used to represent FCGR behavior for R>Rt and Kmax driving for R<Rt. The study reveals that the FCGR curves for different R-ratios can be collapsed into two narrow scatter bands, where each band is influence by either ΔK or Kmax. The final correlation was further simplified to a single equation, which represents “the master curve” corresponding to the transition load ratio, Rt. Thus by knowing FCGR data for one load ratio and using the proposed method, FCGR curves for any other load ratio may be predicted within a narrow scatter band.

Topics: Fatigue cracks
Commentary by Dr. Valentin Fuster
2014;():V009T12A034. doi:10.1115/IMECE2014-40203.

The reliable characterization of fatigue behavior and progressive damage of advanced alloys relies on the monitoring and quantification of parameters such as strain localizations as a result of both crystallographic deformation mechanisms and bulk response. To this aim, this article attempts to directly correlate microstructural strain at specific fatigue life to global strain as well as surface roughness in Magnesium alloys. Strain at the grain scale is calculated using Digital Image Correlation (DIC), while surface topography gradients are computed using roughness data at different stages of the fatigue life. The results are further correlated to Electron Back Scatter Diffraction (EBSD) measurements which reveal the profuse and spatially inhomogeneous nature of the crystallographic deformation mechanisms related to yielding and fatigue crack initiation. Emphasis is given on using multimodal NDE data to formulate first a description of the current state of the material subjected to fatigue loading and on identifying conditions that can probabilistically drive the affected by both local and global response, governing degradation process.

Commentary by Dr. Valentin Fuster
2014;():V009T12A035. doi:10.1115/IMECE2014-40208.

Fretting fatigue raises many challenges in modeling and predicting of a turbine engine blade disk attachment response. 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. Fretting causes a very high local stress near the edge of contact resulting in wear, nucleation of cracks, and their growth, which can result in significant reduction in the life of the material. Fretting depends on geometry, loading conditions, residual stresses, nonlinear response, and surface roughness, among other factors. These complexities make fretting a significant driver of fatigue damage and failure risk of disks. That is, fretting is often the root cause of the nucleation of cracks at attachments of structural components, and the cyclic plastic cumulative deformation and damage occur within depths of only several grains. Hence, resolving the deformation at the scale of individual grains, in order to understand the crystallographic orientation dependence of plasticity driven fretting fatigue and its relation to surface contact conditions, is important. In this study, a finite strain computational crystal plasticity constitutive law will be implemented to simulate and investigate time dependent response of turbine engine blade to disk attachment. The present work leverages the computational model of early efforts, which focused on modeling damage initiation and propagation due to fretting fatigue using micro-thermo-mechanical model, and further enhances the capabilities of capturing the micro-scale nature of the fretting small oscillatory relative displacement at grain level. These efforts provided a high fidelity approach to capture the life of the material at the blade to disk attachment and to simulate the realistic mechanism associated with fretting.

Topics: Gas turbines , Disks , Blades
Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Full-Field Experimental Techniques for Quantifying Fracture and Failure

2014;():V009T12A036. doi:10.1115/IMECE2014-38248.

Nowadays “eco-design” is becoming a philosophy to guide next generation of materials and products as global environmental issue produced by fossil fuels and resource overusing. With an industrial increasing interest in sustainable, eco-efficient and green material’s application, natural fiber in polymer composite is guided to develop rapidly. As well know that, natural fibers possess advantages over synthetic or manmade fibers due to its abundance, biodegradability, CO2 neutrality, excellent price/performance ratio and comparable specific strength properties. However, outdoor applications of natural fiber composite are still constrained and raising concerns in terms of their durability, including UV resistance, moisture resistance and extreme temperature withstand and dimensional stability. Continuing with previous research on kenaf non-woven reinforced unsaturated polyester composites three months degradation performance, in order to get a good knowledge of its degradation process/cycle in complicated outdoor environments, longer degradation periods up to 6 months and 12 months in this paper were added for further investigation and comparison. Initially, three sets of kenaf fiber mat composite samples were located in extreme cold temperature (Harbin), mild sea climate Kyoto (Japan), subtropical marine monsoon climate Shanghai (China) and tropical monsoon climate Zaria (Nigeria) respectively from the same starting time until predetermined ageing periods, afterwards weight change and mechanical behavior in terms of tensile, flexural, impact and fracture toughness were measured instrumentally for ageing effect discussion and comparison. As expected, the aged specimens in those different positions all showed the dropped mechanical properties with increasing ageing periods. Furthermore, the trend of degradation in various mechanical parameters was established, which demonstrated weight loss made more serious effect on aged sample’s mechanical properties’ reduction than water absorption behavior. In a word, dropped mechanical properties of the degraded composites accompanied with weight change behavior were clarified, in which degradation phenomenon of embrittled the matrix polymer, deteriorated reinforced fiber and interfacial properties were detected.

Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: General

2014;():V009T12A037. doi:10.1115/IMECE2014-36241.

The disturbances on the surface of a moving liquid sheet in a moving gaseous medium are studied to analyze the dynamics and breakup of the liquid sheet with co-flowing gas. The problem, composed of the Navier-Stokes systems associated with surface tension forces, is solved by the Volume of Fluid (VOF) technique with a Continuum Surface Force (CSF) manner artificially smoothing the discontinuity present at the interface. The investigation provides the insights into the dynamics and breakup processes. The inlet velocities of liquid and gas are determined by liquid and gas Weber number, respectively. It is found that the disturbances occurred by the gas Weber number controls the instability process for the liquid sheet breakup. The results show that there is a range of gas Weber number for the occurrence of droplet. In this range, the gas Weber number causes an aerodynamic normal force at the tip of the liquid sheet which is able to form a droplet from the tip of the liquid sheet. Below that range of gas Weber number, the aerodynamic normal force at the tip of the sheet is too low to produce a droplet and above the range, the aerodynamic normal force stretches the liquid sheet too much and no droplet occurs.

Commentary by Dr. Valentin Fuster
2014;():V009T12A038. doi:10.1115/IMECE2014-36285.

Conventional design and manufacturing techniques of fibre-reinforced laminated materials keep the fibre orientation angle constant within each ply. However, with the development of advanced tow-placement technology it is now feasible to produce composites with curved fibres. This offers more flexibility to tailor the mechanical properties and improve the performance of laminated structures. In this paper, fibre path optimisation of a laminated cylindrical shell is studied. Curvilinear variations for the fibre orientations are adopted in the circumferential and longitudinal directions of the shell. In order to reduce the computational cost a surrogate-based optimisation strategy is proposed to pursue the optimum design. The laminated shell is subjected to bending and torsion loads and the maximum displacement magnitude is minimised while a constraint on the buckling load is imposed. Numerical studies are presented for two cases. First, only circumferential variation in the fibre orientations is considered. Then, circumferential and longitudinal variations are assumed.

Commentary by Dr. Valentin Fuster
2014;():V009T12A039. doi:10.1115/IMECE2014-36712.

A vast majority of objects around us arise from some growth processes. Many natural phenomena such as growth of biological tissues, glaciers, blocks of sedimentary and volcanic rocks, and space objects may serve as examples. Similar processes determine specific features of many industrial processes which include crystal growth, laser deposition, melt solidification, electrolytic formation, pyrolytic deposition, polymerization and concreting technologies. Recent researches indicates that growing solids exhibit properties dramatically different from those of conventional solids, and the classical solid mechanics cannot be used to model their behavior. The old approaches should be replaced by new ideas and methods of modern mechanics, mathematics, physics, and engineering sciences. Thus, there is a new track in solid mechanic that deals with the construction of adequate models for solid growth processes. The fundamentals of the mathematical theory of growing solids are under consideration. We focus on the surface growth when deposition of a new material occurs at the boundary of a growing solid. Two approaches are discussed. The first one deals with the direct formulation of the mathematical theory of continuous growth in the case of small deformations. The second one is designed for the solution of nonlinear problems in the case of finite deformations. It is based on the ideas of the theory of inhomogeneous solids and regards continuous growth as the limit case of discrete growth. The constitutive equations and boundary conditions for growing solids are presented. Non-classical boundary value problems are formulated. Methods for solving these problems are proposed.

Commentary by Dr. Valentin Fuster
2014;():V009T12A040. doi:10.1115/IMECE2014-38132.

The analysis and design of fluid-filled structures is a common problem in mechanical engineering. The fluid will add mass, stiffness, and damping properties to the structure that are not present in the dry configuration, and that alter the dynamic response of the structure. Understanding the fluid-structure dynamics can be an essential part of designing a fluid container, particularly if large structural deflections or dynamic motion are expected. For many structures, it is important to appreciate the modal harmonics of the structure and how the fluid will influence them. For instance, the modal frequencies of the tank change with the fluid level, requiring a study of frequency response for different fluid levels. Actual testing to confirm these effects can be expensive and impractical so it is useful to have an accurate model to supplement or replace physical data. Finite element analysis (FEA) codes are well suited to modeling the hydroelastic coupling between the fluid and container. FEA is commonly used to model the structural dynamics of complex systems and to generate the modal frequencies and shapes. One such application is the design of the fuel tank on a flight vehicle, where pressure fluctuations in the fuel line can lead to oscillations in thrust at the engine and high vibrations in the structure. If the frequency of these vibrations matches a natural frequency of the fluid-tank structure, a feedback loop forms coupling the structural response to the fluid oscillations through the engine thrust. This phenomenon is often referred to as “pogo” instability in flight vehicles. The resulting vibrations can cause the vehicle to break apart. The fuel tank must be designed to avoid natural frequencies in the range likely to be excited vehicle vibrations. This report describes the use and verification of hydroelastic modeling capabilities using FEA software to model a fluid-filled fuel tank. Test data from previous experimental work is used to validate the natural frequencies and mode shapes of the tank at different fluid fill levels. The FEA model compares very well to experimental data and theoretical calculations, providing good validation for the modeling technique. The FEA modeling method is thus authenticated as a valuable tool for predicting the dynamics of fluid-tank systems.

Commentary by Dr. Valentin Fuster
2014;():V009T12A041. doi:10.1115/IMECE2014-38701.

Steel tubes are widely used in industries as machine components and are most common in heavily loaded mechanisms subjected to high dynamic torsional and compressive stress. Hence, they should have higher strength than that of the conventional mechanisms to resist failure. Quenching, an industrial heat treatment process, can improve the microstructure, hardness, toughness, and corrosion and wear resistance of materials. Steel tubes, if quenched, would have desired properties to serve the purposes. However, besides improving material properties, quenching generates some residual stress and deformation in the material due to rapid temperature drop and phase transformation. Therefore, to estimate the temperature distribution, residual stress, and deformation computationally; a three-dimensional fluid-structure interaction model is developed for the steel tube with different quenchants. The quenching characteristics by water, brine, and propylene glycol are estimated and compared with each other. The time-varying nodal temperature distributions in the tube are observed and the critical regions are identified having maximum residual stress and deformation. The time-varying residual stress and deformation at a particular point and along the axial and radial directions of the tube are studied. The convergence of the model is checked and validation of the model is done.

Commentary by Dr. Valentin Fuster
2014;():V009T12A042. doi:10.1115/IMECE2014-39216.

The micropolar elasticity theory provides a useful material model for dealing with fibrous, coarse granular, and large molecule materials. Though being a well-known and well-developed elasticity model, the linear theory of micropolar elasticity is not without controversy. Specially simplification of the microppolar elasticity theory to the couple-stress and classical elasticity theories and the required conditions on the material elastic constants for this simplification have not been discussed consistently. In this paper the linear theory of micropolar elasticity is reviewed first. Then the correct approach for a consistent and step-by-step simplification of the micropolar elasticity model with six elastic constants to the couple-stress elasticity model with four elastic constants and the classical elasticity model with two elastic constants is presented. It is shown that the classical elasticity is a special case of the couple-stress theory which itself is a special case of the micropolar elasticity theory.

Topics: Elasticity , Stress
Commentary by Dr. Valentin Fuster
2014;():V009T12A043. doi:10.1115/IMECE2014-39335.

Recently, separation of fine liquid droplets from gaseous flow medium has become an important subject for process and aerospace industry. The aim of this work is to study collection efficiency of high concentration droplets from high velocity air stream using electrostatic separators. To improve the separation efficiency, different designs of separator were studied experimentally including wire-to-plate and wire-to-cylinder designs. The results demonstrated that two-stage planar and cylindrical separators, if designed properly, could provide high collection efficiencies for both small and large particle sizes at high concentration conditions.

Commentary by Dr. Valentin Fuster

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

2014;():V009T12A044. doi:10.1115/IMECE2014-36567.

Cellular materials have two important properties: structures and mechanisms. These properties have important applications in materials design; in particular, they’re used to determine the modulus and yield strain. The objective of this study is to gain a better understanding of these two properties and to explore the synthesis of three-dimensional (3D) compliant cellular materials (CCMs) with compliant porous structures (CPSes) generated from modified hexagonal honeycombs. An orthotropic constitutive CCM model in the Cartesian coordinate system is constructed using the strain energy method, which uses the deformation of hinges around holes and the rotation of links. A finite element (FE) based simulation is conducted to validate the analytical model. The moduli and yield strains of the 3D CCMs with an aluminum alloy are about 1.2GPa and 0.4% in the longitudinal direction and about 0.08MPa and 30% in the lateral direction. The CCMs have extremely high positive and negative Poisson’s ratios Display Formulaνxy*±30 due to the large rotation of the link member in the transverse direction caused by an input displacement in the longitudinal direction. This paper demonstrates that compliant me so structures can be used for next generation materials design in tailoring mechanical properties such as moduli, strength, strain, and Poisson’s ratios.

Topics: Design
Commentary by Dr. Valentin Fuster
2014;():V009T12A045. doi:10.1115/IMECE2014-36620.

Aluminium honeycombs is a lightweight cellular material and a good energy absorber. In different engineering applications, it is usually used as structural components. Comprehensive study has been conducted to analyse the compressive behaviour of aluminium honeycombs. However, research related to mechanical response of aluminium honeycombs material subjected to different type of loadings, such as indentation, is still limited. In this paper, experimental and numerical studies were conducted to investigate the deformation mechanism and energy dissipation of a HEXCELL® aluminium honeycomb subjected to dynamic indentation. A high speed INSTRON machine was used to conduct dynamic tests at velocities of 0.5 m/s and 5 m/s. Numerical analysis was conducted using ANSYS LS-DYNA at velocities of 5 m/s, 15 m/s and 25 m/s. The simulation results were in good agreement with the experimental results in terms of stress-strain curve profile and deformation mode. In the experiment, it was found that with the increase of velocity (strain rate) the average plateau stress of indentation also increases which was validated in the numerical analysis. The deformation of aluminium honeycombs under indentation showed that the compression of hexagonal honeycomb cells under the indenter and also tearing of honeycomb cell walls along the four edges of the indenter. The dissipation of energy in compression and tearing was calculated and discussed. The effect of loading velocity on the plateau stress and energy absorption was also analyzed.

Commentary by Dr. Valentin Fuster
2014;():V009T12A046. doi:10.1115/IMECE2014-37580.

The work presented here is a continuation of the study performed in exploring the energy absorption characteristics of non-Newtonian fluid-filled regular hexagonal aluminum honeycomb structures. In the previous study, energy absorbing properties were investigated by using an air powered pneumatic ram, dynamic load cell, and a high speed camera. This study was conducted using a pneumatic ram which was designed to exploit only its kinetic energy during the impact. Experimental samples included an empty honeycomb sample and a filled sample as the filled samples showed the largest difference in energy absorption with respect to the empty samples in the previous study. Therefore, the filled samples were further investigated in this study by measuring the impact forces at the distal end as well as the damage on the impact end. Upon impact, the filled samples were able to reduce the damage area on impact end and were able to lower average and peak forces by 71.9% and 77.4% at the distal end as compared to the empty sample.

Commentary by Dr. Valentin Fuster
2014;():V009T12A047. doi:10.1115/IMECE2014-37581.

This study was conducted to investigate the effects of cross-sectional geometry on thin wall axial crushing members for the purpose of improved energy absorption. A total of five geometrically equivalent shapes (same wall thickness area, material, and length) were analyzed namely, triangle, rectangle, square, pentagon, and circle. The deformation modes and energy absorption of the members were studied under compressive loads and compared using ABAQUS/Explicit module, finite element analysis software. The simulations revealed that for the five geometrically equivalent cross sections under equal loading conditions, the pentagon shaped member absorbed the highest amount of energy. As compared to baseline rectangle member, the pentagon member absorbed approximately 25–28% more energy.

Topics: Absorption
Commentary by Dr. Valentin Fuster
2014;():V009T12A048. doi:10.1115/IMECE2014-37608.

An interpenetrating phase composite is made by injection molding thermoplastic polymers into the voids of open-cell aluminum foam. Two types of polypropylene and an acetyl were mechanically introduced into the open cells of a Duocel® aluminum foam. Prior experimental work revealed that the combination of the polymer and the metal foam yields a hybrid that is stiffer than the polymer alone but has a reduced tensile strength. A finite element model using a tetrakaidecahedral unit cell is used to model the metal foam ligaments with the polymer occupying the remaining space. The geometric model as well as the interface between the two materials were validated against the experimental results. The resulting conclusions are that the aluminum ligaments oriented along the load direction cause an increase in stiffness but ligaments oriented laterally cause stress concentration that yield lower strength. The finite element model is used to give both qualitative and quantitative explanations of the physics of the interrelations between the metal foam and the polymer.

Commentary by Dr. Valentin Fuster
2014;():V009T12A049. doi:10.1115/IMECE2014-37802.

Natural fiber composite materials are expected as capable materials which may replace the conventional and synthetic materials for the practical applications where manufacture requires less weight and energy conservation. In this study, three kinds of cellulosic-fiber mats including kenaf, bamboo and jute mats were used to fabricate composites by hand lay-up and compression molding methods. As the basic investigation, low cycle fatigue tests were carried out to analyze the material’s fatigue properties by using different bending or tensile loads. Moreover, the scanning electron microscope observation (SEM) on the fracture surfaces has been carried out respectively to investigate the degradation under cycle loads and discuss the possibility of kenaf/bamboo/jute composites achieving hypothetical outstanding mechanical properties in engineering uses.

Commentary by Dr. Valentin Fuster
2014;():V009T12A050. doi:10.1115/IMECE2014-38099.

Paper recycling is an effective way in reducing deforestation and energy consumption. Therefore recycling paper and paper products has been widely applied in many areas, such as packaging industry, furniture decoration, temporary structures in building and so on. Paper products are made from plant fibers and they are laminated materials. So it is of possible to generate interlaminar fracture in the use of paper products, especially in the construction made of paper such as paper tubes which have been used widely. In order to improve the interlaminar performance of paper products and then improve the construction performance of paper products, delamination behavior of laminated paper has been studied in this paper. By a series of peel tests, comparative analysis about different paperboard were carried out. The cause of delamination behavior of laminated paper was analysis based on the detailed observation using a scanning electron microscope (SEM).

Topics: Delamination
Commentary by Dr. Valentin Fuster
2014;():V009T12A051. doi:10.1115/IMECE2014-38925.

Cellular materials, often called lattice materials, are increasingly receiving attention for their ultralight structures with high specific strength, excellent impact absorption, acoustic insulation, heat dissipation media and compact heat exchangers. In alignment with emerging additive manufacturing (AM) technology, realization of the structural applications of the lattice materials appears to be becoming faster. Considering the direction dependent material properties of the products with AM, by directionally dependent printing resolution, effective moduli of lattice structures appear to be directionally dependent. In this paper, we develop a constitutive model of a lattice structure, which is an octet-truss with a base material having an orthotropic material property considering AM. One case study is conducted with an orthotropic property of a base material in 3D Printing. A polyjet based 3D printing material having an orthotropic property with a 9% difference in the principal direction provides difference in the axial and shear moduli in the octet-truss by 2.3 and 4.6%.

Commentary by Dr. Valentin Fuster
2014;():V009T12A052. doi:10.1115/IMECE2014-39005.

Many natural and modern man-made materials are porous materials. In many cases, for practical applications and for the proper design of products it is necessary to understand the effective mechanical properties of the porous structure. Even though some general applicable theories exist to estimate the mechanical properties in function of the porosity fraction, or apparent density, the relation between the pore structure and size distribution is not clear. In particular, in case of structured porosity the mechanical properties demonstrate anisotropic behavior. For specialized applications there is an interest to be able to design materials with specified anisotropic properties or with properties varying in space.

In order to design and optimize porous structures with specific anisotropic properties, a flexible simulation model is developed and tested to predict and understand the mechanical anisotropic properties of a large variety of porous structures. In order to be able to change the geometric pore structure without recreating a new model, the pore structure is implemented into a space dependent smoothed binary function.

A comparison is made between results obtained with FEM models with a fully defined geometry and the FEM model with the porosity function to describe the pore structure in order to evaluate and quantify the error that is introduced by the latter implementation and investigate the possibility to mitigate the error.

The model results are also compared with experimental en numerical results reported in literature.

Commentary by Dr. Valentin Fuster
2014;():V009T12A053. doi:10.1115/IMECE2014-39090.

Polymeric cellular materials were primarily developed as means to reduce density of solid polymers and thus saving cost for applications where mechanical strength is not required like the packaging industry. Nevertheless, functionally graded cellular materials showed an attractive mechanical behavior in experimental studies. The fatigue life of porous polycarbonate (PC) with above 90% relative density is reported to be as much as four times that of a solid PC, and greater impact strength with relative density over 60%. The focus of this paper is on fabricating bio-polymeric-based functionally graded porous material with polylactic acid (PLA) and analyze the buckling behavior of their plate-like structures. The analysis includes numerical modeling supported with experimental findings. The modeling is carried out with a higher order shear deformation plate theory (HSDT) accounting for extension in the transverse direction. The proposed HSDT satisfies the constraint on the consistency of transverse shear strain energy a priori in addition to the traction conditions on plate surfaces. Few theories in the area satisfy both conditions. Finite element is used to implement the HSDT with C1 continuity using conforming elements. The through-thickness varying properties are homogenized at planar-level with the generalized self-consistent scheme. The graded cellular structure is manufactured to vary through the plate thickness while being uniform on-average for the other two planar dimensions. Constrained foaming process is adopted to control the pores’ size and structure through the plate thickness.

Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Mechanics in Biology and Medicine

2014;():V009T12A054. doi:10.1115/IMECE2014-39676.

The stiffness of plant tissue largely influences the overall mechanical response of plant organs, such as stems, branches and leaf petioles. This work examines the structural hierarchy of the plant tissue; in particular of the collenchyma tissue of the Rheum rhabarbarum. The goal of the paper is to develop a multiscale model capturing features of two orders of its structural hierarchy: cell wall and tissue architecture. The former is considered as a fiber reinforced composite, where the cellulose microfibril (CMF) is the main load bearing component. The longitudinal stiffness of the middle (S2) layer of the secondary cell wall is affected by the microfibril angle (MFA) up to 45° to a greater extent, which in turn plays a role in the overall wall stiffness. The latter, i.e. tissue architecture, influences the tissue stiffness through its random distribution of cells. Finite-edge Centroidal Voronoi Tessellation (FECVT) is used to model the non-periodic microstructure of the rhubarb collenchyma, whose effective elastic properties are obtained through finite element analysis. The results from the FECVT model show that the effective stiffness in the longitudinal direction is 15 to 25% higher than that in the transverse direction for relative density between 5 and 30%. The variation reflects the stiffening effect of the shape and size of the cells in the collenchyma tissue, as well as its aperiodic cellular distribution.

Commentary by Dr. Valentin Fuster

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

2014;():V009T12A055. doi:10.1115/IMECE2014-37917.

In this paper, the sliding contact of a rigid sinusoid over a viscoelastic halfplane is studied by means of an analytical procedure that reduced the original viscoelastic system to an elastic equivalent one, which has been already solved in [1]. In such a way, the solution of the original viscoelastic contact problem requires just to numerically solve a set of two integral equations. Results show the viscoelasticity influence on the solution by means of a detailed analysis of contact area, pressure and displacement distribution. A particular attention is paid to the transition from full contact to partial contact conditions.

Commentary by Dr. Valentin Fuster
2014;():V009T12A056. doi:10.1115/IMECE2014-38345.

The static and dynamic friction characteristics of a steel pin on injection molded polyoxymethylene homopolymer disk system were studied under both lubricated and dry contact conditions. Samples were tested at externally-applied normal loads ranging from 20 to 160 N. Under dry test conditions, friction coefficients displayed two distinct regions with very low friction coefficients at low loads and rising to approximately 0.7 (static) and 0.6 (dynamic) at loads above 60N. This phenomenon is attributed to an enhanced contribution of the ploughing friction mechanism at higher loads. As load increases, the pin penetrates through the injection mold-induced skin layer and into the core. At loads lower than 60 N, however, the pin does not significantly penetrate the disk during the test and the adhesive mechanism dominates the tribological properties. Additional tests were performed in order to determine the effects of a lithium soap thickened, low viscosity, synthetic hydrocarbon grease. The average static and dynamic friction coefficients for the lubricated interface were found to be 0.031 ± 0.01 and 0.027 ± 0.01, respectively. The friction coefficients exhibited a linear dependence on the load. This result indicates a shift from the more optimal elastohydrodynamic lubrication regime at lower loads to a mixed lubrication regime and a behavior closer to dry contact at higher loads. Results are interpreted in light of the principal static and dynamic friction mechanisms.

Commentary by Dr. Valentin Fuster
2014;():V009T12A057. doi:10.1115/IMECE2014-38353.

Adhesion and cohesion of materials are of importance in many different areas of science and engineering, such as friction between objects, flow of liquids on solids or other liquid surfaces, and phase change heat transfer. One contribution to adhesive energy, irrespective of the type of material(s), is from van der Waals interactions, which arise from alteration of the quantum and thermal fluctuations of the electrodynamic field due to the presence of interfaces. Despite its importance, the theory of van der Waals interactions between macroscopic bodies, which is mainly due to Lifshitz, Dzyaloshinskii, and Pitaevskii, remains shrouded in relatively complicated language of quantum statistical physics. In this paper, we will present an alternate derivation which skirts quantum statistical physics (for most part) and relies primarily on a combination of classical electrodynamics and energy conservation.

Topics: Adhesion
Commentary by Dr. Valentin Fuster
2014;():V009T12A058. doi:10.1115/IMECE2014-38446.

In this paper, we discuss the mechanism of detachment of thin pre-stressed films from a flat smooth rigid substrate. Indeed, we develop an analytical solution in closed form which shows how the critical value of the pull-off force strongly depends on the press-stress P0. In detail, the critical pull-off force needed to detachment is shown to be higher for pre-stressed tapes. Furthermore, we notice that, when a high pre-stress is present, tapes may behave in different manner and spontaneously detach from the rigid substrate.

Topics: Adhesion , Stress
Commentary by Dr. Valentin Fuster
2014;():V009T12A059. doi:10.1115/IMECE2014-38848.

Surface topography parameters commonly used do not give information on directional structures on a technical surface, that originate e.g. from manufacturing and finishing processes. On the other hand these sturctures might show large influence on the friction behaviour of these surfaces. A numerical study is performed with an unlubricated dynamic sealing contact, which is e.g. utilized in vacuum applications: These results are compared to directional evaluation of surface parameters. It is shown, that surface roughness parameters commonly used are neither appropriate to predict the level of friction on technical surfaces nor can they give information on directional properties of a counter surface.

Commentary by Dr. Valentin Fuster
2014;():V009T12A060. doi:10.1115/IMECE2014-39845.

The aim of the present work is to investigate the adhesive properties of a biomimetic micro-structured surface with strongly direction-dependent adhesion properties. The system is constituted by parallel elastic wall-like structures topped with a thin film. The micro-walls are assumed in perfect contact with a rigid substrate and the adhesive interaction is modeled by considering full adhesion. In a previous work of the authors, it is shown that this geometry, when loaded with an external moment acting perpendicularly to the walls direction, enhances the adhesive properties with respect to a simple flat surface as a result of its crack trapping behavior.

In the present paper we investigate what happens when the crack propagates with a generic angle with respect to the walls, in order to determine the variation of the critical conditions for detachment with the direction of the applied moment. Results show that the crack trapping can occur only when the crack propagates perpendicularly to the walls. In all the other cases, the system compliance linearly increases with the crack length. As a result, the energy release rate at the crack tip is constant during the crack propagation and the crack trapping phenomenon cannot occur.

Topics: Adhesion
Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Modeling Materials With Morphological Complexities and Evolving Microstructures

2014;():V009T12A061. doi:10.1115/IMECE2014-38728.

Craniosynostosis is a condition defined by premature closure of cranial vault sutures, which is associated with abnormalities of the brain and skull. Many causal relationships between discovered mutations and premature suture closure have been proposed but an understanding of the precise mechanisms remains elusive. This article describes a computational framework of biological processes underlying cranial growth that will enable a hypothesis driven investigation of craniosynostosis phenotypes using reaction-diffusion-advection methods and the finite element method. Primary centers of ossification in cranial vault are identified using an activator-substrate model that represents the behavior of key molecules for bone formation. Biomechanical effects due to the interaction between growing bone and soft tissue is investigated to elucidate the mechanism of growth of cranial vault.

Commentary by Dr. Valentin Fuster
2014;():V009T12A062. doi:10.1115/IMECE2014-39619.

Based on the braiding process and force analysis of yarn, a mesoscopic numerical modeling approach was established, which divided the modeling process as follows: establishing the control points according to the braiding process, establishing the fixed points during jamming, adjusting the control points after jamming, changing the position of fiber bundle due to the fiber bundle intertwined each other and establishing the fiber bundle trajectory according to the minimum strain energy. In the process of adjusting the intertwined fiber bundle trajectories, the fiber bundle trajectory was scattered. Using extrapolation adjustment method, discrete points of fiber bundle trajectory intertwined were adjusted in turn from the control points to the fixed points. Adjusted discrete points were equivalent at the corresponding location points of the corresponding trajectory, and at the same time, there was non-interference between the fiber bundle trajectories. Using this method, fiber bundle trajectory and cross section of the models of 2-D woven and 3-D four-directional braided composite materials were established, compared with the experiment result, which were consistent with the electronic microscope scan images and calculated woven structure size was in agreement with the measured data. The maximum relative calculation error of braiding bitch of 3-D four-directional braided structure was about 5%, especially braiding angle was 21° or so, the relative calculation error was below 2%. The maximum relative calculation error of surface braiding angle of 3-D four-directional braided structure was about 4%, especially braiding angle was 21° or so, the relative calculation error was below 2.4%. This modeling approach was fundamental for further analysis of the micromechanical strength and life of braided composites, which was applied to aero-engine hot section.

Commentary by Dr. Valentin Fuster

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

2014;():V009T12A063. doi:10.1115/IMECE2014-36887.

IM7/PEEK composite materials may experience large mechanical and thermal stresses during service, and their failure mechanisms can be complex and have various modes. In this study, IM7/PEEK specimens for interlaminar shear strength testing were evaluated by both nondestructive ultrasound method and standard destructive test. First, IM7/PEEK specimens were fabricated under various processing parameters including roller speed, torch temperature, roller temperature, compaction level, N2 flow rate, and material tension level. Then ultrasound longitudinal wave velocity that is normal to fiber direction was evaluated and the influence of measuring locations and ultrasound frequencies from 0.5 MHz to 10.0 MHz were studied. Experimental results showed that 5.0 MHz is the most sensitive frequency to fabrication conditions and that ultrasound velocity can be related to some processing parameters such as materials tension, N2 flow rate and temperature. Finally, interlaminar shear strength experiments were carried out by standard short beam bending destructive tests. The relationship between ultrasound velocity and standard destructive tests were analyzed and the potential of ultrasound velocity for interlaminar shear strength evaluation was discussed.

Commentary by Dr. Valentin Fuster
2014;():V009T12A064. doi:10.1115/IMECE2014-37083.

The advantages of using graphite as reinforcement in aluminum composites have been addressed by several authors, e.g. Warner et al. [1]. The addition of graphite has been demonstrated to improve the relation strength to density ratio because of the low weight of graphite. Also a significant hardening caused by the difference in the thermal expansion coefficient between graphite and Al matrix have been observed [2]. Furthermore, the presence of graphite has been demonstrated to reduce the wear and abrasion of tools. The structural response of metal matrix composites (MMC) has been studied recently to evaluate the elastic and plastic performance by using Finite Element Method (FEM) [3, 4].

In this paper an analysis of the structural response of Al\graphite composites is presented. The analysis is based on the Finite Element Method (FEM) in the elastic and plastic regions. Several Al\graphite composites with different graphite contents were considered. A FEM model based on a unit cell was proposed and two different Al/graphite interface conditions were assumed: 1) strongly bonded interface, and 2) sliding-friction interface. Also, both regular and irregular arrays of graphite particles in the aluminum were considered in the model. The results in the elastic region are evaluated in terms of the effective elastic modulus and compared with some theoretical models in the literature [5]. From these results it has been observed that when a friction-sliding interface is considered, a slightly smaller elastic modulus is obtained, compared with the strongly-bonded interface condition. Regarding the numerical simulation in the plastic region, the results were compared with theoretical models [6] and experimental data obtained from compression tests.

Commentary by Dr. Valentin Fuster
2014;():V009T12A065. doi:10.1115/IMECE2014-38400.

In the present work, the three-dimensional analysis for the deflection and stress distributions of functionally graded ceramic–metal sandwich plates is developed based on the method of sampling surfaces (SaS). In accordance with this method, into each layer of the plate, reference surfaces that are not equally spaced and are parallel to the mid-surface of the plate are introduced, and the displacement vectors of these surfaces are chosen as unknown functions. Such a choice allows the representation of the governing equations of the proposed higher order layer-wise plate theory in a very compact form and also permits the derivation of strain–displacement relationships correctly describing all motions including the rigid-body motions of the functionally graded plate. Hence the 3D elasticity problem of the thick plate is efficiently solved. The material properties of sandwich plate’s face layer are assumed to be that of a two-constituent material that vary continuously through the thickness of the face sheet according to a power law distribution of the volume fraction of the constituents. The core layer is homogeneous and made of an isotropic ceramic material. The effects of the volume fraction of the material constituents and their distribution on the deflections and, in particular, the 3-D stress distributions as well as the effects of the length-to-width and length-to-thickness ratios of the plate are investigated. Comparison of the results of the present work with the results available in existing literature is carried out for a benchmark problem. It is shown that considering large number of SaS, which are located at interfaces and Chebyshev polynomial nodes, the accuracy of the solutions can be improved significantly wherein the error will approach zero value as the total number of surfaces in each layer become very large.

Commentary by Dr. Valentin Fuster
2014;():V009T12A066. doi:10.1115/IMECE2014-39688.

One of the common structures of smart compliant systems is in the form of slender beams, which have the ability of undergoing large deformations due to non-mechanical stimuli. In this study, large deformations of cantilever beams having piezoelectric patches are investigated. The nonlinear kinematics relation of finite beams reported by Reissner (1972) is adopted. The electric field inputs are applied through the thickness of the piezoelectric patches in order to induce curvature changes. Analytical solutions of the governing equations of the deformations of the electro-active beams under actuation of arbitrary number of patches are presented.

Commentary by Dr. Valentin Fuster
2014;():V009T12A067. doi:10.1115/IMECE2014-40059.

The volume ratio between piezoelectric and magnetostrictive phases is an important parameter for magneto-electric (ME) composites. The ME voltage coefficients can be enhanced greatly at optimum volume ratio. However, no previous report has focused on the study of the volume ratio effect on ferromagnetic-ferroelectric-substrate multilayer composites. We consider an arbitrary laminated structure of length 2L and N layers. In this case, there is no middle plane of the bar that can serve as a plane of symmetry. For simplicity we assume that the multilayer structure is two dimensional (i.e. bar structure), and the field functions depending only on the space coordinates X1, X2. In the Cartesian system of coordinates the X1 axis is directed along the bar length, the X2 -across the width, and X3 is orthogonal to X1 and X2. It is assumed that piezoelectric layers are poled in the X1 direction (L-L mode). It should be mentioned that the proposed theory can be successfully applied to multilayer structures when the polarization direction of the piezoelectric layers is along the X2 direction, or when some of them are along the X1, and others along X2, or X3 directions. An averaging method is used for deriving effective material parameters of composites. We consider only (symmetric) extensional deformation in this model and at first ignore any (asymmetric) flexural deformations of the layers that would lead to a position dependent elastic constants and the need for other methods to be applied. Using the continuity conditions for magnetic and electric fields, as well as the open and closed circuit conditions, one obtains the analytic expression for longitudinal ME voltage coefficient, which depends on electro-mechanical material properties and thicknesses of the layers. Analytical expression for ME coefficient allows us to find the optimal volume ratio of layers, for which the ME coefficient approaches to its maximum value.

Commentary by Dr. Valentin Fuster

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

2014;():V009T12A068. doi:10.1115/IMECE2014-36772.

This work uses a compressible Eulerian multi-material solver with three modeling approaches to examine shock and pressure wave propagation in a bubbly medium. These approaches represent different levels of complexity from fully resolving the dispersed bubbles to treating the bubbly medium as a homogeneous mixture. An intermediate approach is based on treating bubbles as discrete singularities. Propagation of the pressure wave through the bubbly medium is compared between the simplified approaches and the fully resolved bubble simulation. Different scenarios demonstrating the effect of pressure amplitude, void fraction, and bubble size distribution are presented to further understand wave propagation in bubbly media.

Commentary by Dr. Valentin Fuster

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

2014;():V009T12A069. doi:10.1115/IMECE2014-37014.

Advanced high strength steels cover a vast range of applications more specifically in aerospace and oil industry where large deformation of a material is desired in order to attain a specified shape and geometry of the product. The main reason behind their successful implementation is having an optimum combination of strength and formability. Austenite based twinning induced plasticity steel lies in the second generation and has excellent strength-cum-formability combination among the group of advanced high strength steels. The stress assisted phase transformation from austenite to martensite, which is known as twinning, found to be principal reason behind an enhancement of these properties. This work is aimed to investigate an elastic-plastic behavior of an austenite dominated steel, which undergoes slip and mechanical twinning modes of deformation. Initially, a micromechanical model of twining induced plasticity phenomenon is developed using crystal plasticity theory. Then, the developed model is numerically implemented into finite element software ABAQUS through a user-defined material sub-routine. Finally, finite element simulations are done for single and poly-crystal austenite subjected to combined load. This replicates the complex loading condition which exists in material forming processes like pipe expansion, extrusion, rolling. The variation in stress-strain response, magnitude of shear strain, and volume fraction of twinned martensite are plotted and analyzed.

Commentary by Dr. Valentin Fuster
2014;():V009T12A070. doi:10.1115/IMECE2014-37358.

We use atomistic simulations to study mechanical properties of monolayer molybdenum disulfide MoS2. Using molecular dynamic (MD) simulations, we investigate the nano-fracture properties of monolayer MoS2 under mixed mode I and II loadings. The MD simulations are used to obtain the critical stress intensity factors of both armchair and zigzag cracks as a function of applied loading phase angle. Our atomistic simulations predict that armchair cracks are tougher than zigzag cracks, and both armchair and zigzag cracks tend to propagate along a zigzag path. Furthermore, we use density functional theory (DFT) to investigate how point defects influence the mechanical properties of nanoribbons. Our DFT simulations show that missing one S atom does not significantly affect the mechanical strength of monolayer MoS2, whereas missing one Mo atom can reduce the maximum strength of single layer MoS2 sheet by about 10%.

Commentary by Dr. Valentin Fuster
2014;():V009T12A071. doi:10.1115/IMECE2014-39395.

A facile electrochemical anodization method was used for producing hierarchically textured surfaces based on TiO2 nanotubes in two different configurations. It was found that perfluoro-functionalized TiO2 nanotubes exhibit high static contact angles for a variety of liquids such as apolar, polar aprotic and polar protic solvents. Wenzel and Cassie-Baxter theories were applied for theoretical contact angle calculations for the present study. By using Cassie theories, it is shown that a drop of polar liquid was in a fakir or Cassie-Baxter (CB) state on perfluoro-functionalized nanotube surfaces. The fakir state prevents spreading of the liquid on the surface. On the other hand, the wetting of non-polar liquids such as hexane is characterized by either Wenzel states or transition states characterized by partial imbibition that lie in between the CB and Wenzel states.

Topics: Wetting , Nanotubes
Commentary by Dr. Valentin Fuster
2014;():V009T12A072. doi:10.1115/IMECE2014-40262.

The effectiveness of helmets in preventing internal damage due to blast waves requires understanding of not just the strength of the helmet material but also its energy absorption characteristics. To understand and develop ballistic helmets with improved protection, it is necessary to develop computational procedures that will enable the accurate modeling of traumatic head injuries as well as the precise measurement of the mechanical properties of composite materials used in helmets. In this study, a multiscale simulation strategy is used to estimate the mechanical characteristics of advanced composite structures with embedded nanostructures in a ballistic material. In most of the previous theoretical works, an analysis dedicated to improving the design of the helmet using composite nano-structures was not included due to a lack of understanding of the interactions of the nano-structures with the matrix materials. In this work, the role of the helmet on the over pressurization and impulse experienced by the head during blast wave is studied. The properties of the nano-composite materials are estimated using molecular dynamics (MD) simulations and the properties are scaled to the macroscopic level using continuum mechanics formulations. Finally, the analysis is also carried out on an unprotected head to compare the results to those obtained when protected by a helmet containing carbon nanotubes. The developed multiscale model can be used to improve the composition of helmets and the understanding of the traumatic effect of blast shock wave, thereby leading to the mitigation and prevention of traumatic head injuries.

Commentary by Dr. Valentin Fuster

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

2014;():V009T12A073. doi:10.1115/IMECE2014-39658.

Growing demand for clean, affordable energy has driven the power industry towards generation plants with higher thermal efficiency and higher operating temperatures. ASTM Grade 91 is a high chromium (9Cr-1Mo) creep resistant steel commonly used in high temperature pressure vessel and piping applications. These service conditions often involve a combination of stationary and cyclic loads at elevated temperatures. Lifecycle assessments of components under such conditions require modeling of both creep and fatigue behaviors. This paper develops two approaches to modeling mixed creep and fatigue crack growth for lifetime assessment of high service temperature components. Both approaches model fatigue crack growth using the Paris law integrated over the number of lifetime cyclic reversals to obtain crack extension. A strip yield model is used to characterize the crack tip stress-strain fields.

The first approach employed an explicit method to approximate creep crack growth using C* as a crack tip parameter characterizing creep crack extension. The Norton power law was explicitly solved to model the primary and secondary stages of creep.

The second approach used an implicit method to solve a set of constitutive equations based on properties of the material microstructure to model all creep stages. Constitutive equations were fit to experimental data collected at stresses 10–60% of yield and temperatures 550–650°C. These methods were compared to published experimental data under purely stationary loads, purely cyclic loads and mixed loading. Both models showed good agreement with experimental data in the stress and temperature conditions considered.

Topics: Creep , Fatigue , Steel , Stress , Strips
Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Multiscale Modeling of Textile Composites

2014;():V009T12A074. doi:10.1115/IMECE2014-39538.

The bottle-neck issues to resolve for numerical simulation of real scale ballistic tests of fabric body armors are computer capacity limitation and prohibitive computational cost. It is not realistic to use micro-level computer simulations for an open end design process. Most numerical simulations are only applicable for small scale parametric analyses, which could facilitate apprehension of fabric failure mechanisms during ballistic impact, but not applicable for the design process.

In this paper, a sub-yarn model, the digital element approach, is applied to simulate real scale ballistic tests for soft body armors. In this approach, a yarn is discretized into multiple digital fibers and each fiber is discretized into many digital elements. In order to improve efficiency, two hybrid element mesh concepts are investigated: area based hybrid mesh and yarn based hybrid mesh.

The area based hybrid mesh procedure is similar to one utilized in the conventional finite element approach. A fine element mesh is adopted in the area near the impact center; a course element mesh in the area far away. However, numerical simulation results show that the stress wave travels along the principal yarns at the speed of sound immediately after ballistic impact. High yarn stress develops quickly from the impact center to a distance along the principal yarn. As such, the area based hybrid mesh approach fails to obtain improved computer efficiency without loss of accuracy.

Because the high stress only develops within principal yarns after a ballistic impact, a yarn based hybrid element mesh procedure is adopted. In this procedure, only principal yarns and yarns near principal yarns are discretized into fine digital fibers; other yarns are discretized into coarse digital fibers. Because only a few principal yarns resist load in a typical ballistic impact, the yarn based hybrid technique could improve simulation efficiency up to 90–95% without sacrificing accuracy.

A numerical tool is then developed to generate fabric with a yarn based hybrid mesh. Accuracy of the approach is analyzed. The hybrid mesh technique is applied to simulate real scale ballistic tests of ballistic armors made of 4 to 20 piles of 2-D plain woven fabrics. Numerical results are compared to real scale standard ballistic results.

Topics: Textiles , Simulation
Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Polymer Nanocomposites and Nanostructured Materials: Simulations and Experiments

2014;():V009T12A075. doi:10.1115/IMECE2014-37517.

Recently nanoparticle-reinforced polymer nanocomposite materials, comprised of the inclusion, (non-bulk polymer) interphase and the bulk polymer matrix, have received considerable interest. Because of interaction between the nanoinclusion and surrounding polymer matrix, the non-bulk polymer in the vicinity of the nanoinclusion has different properties than the bulk polymer. With tremendous amount of surface area, the interphase may have a large influence on the overall nanocomposite properties and complicate micromechanical predictions of effective properties. Although several micromechanical approaches can provide approximations of the effective elastic modulus, they require one to calculate the dilute concentration tensor using the well-known Eshelby tensor that treat interphase as separate, physically distinct inclusions. However, their elegant solutions are no longer available when the real geometry of the annular interphase must be considered. This work analytically determined the components of the dilute strain concentration tensors for both the inclusion and the interphase by addressing four auxiliary loading cases, which can be directly implemented within standard micromechanical approaches, such as the Mori-Tanaka model, to predict the effective properties of polymer nanocomposites with cylindrical/fibrous nanoinclusions. Comparison of the predictions of the proposed model with predictions based on the traditional Multiphase Mori-Tanaka approach show that differences between the models are largest when the annular interphase region is softer than the matrix material, attributed to the ability of the proposed model to capture the “stress-shielding effect” in the case of the softer annular interphase. In addition, we have examined several sets of experimental data from the literature for both stiff and soft interphase systems to shed further insight on the utility of the proposed model. The model proposed here would provide an important guideline to evaluate the impact of chemical functionalization techniques and other strategies that seek to tailor the properties of the interphase region in nanocomposite materials.

Commentary by Dr. Valentin Fuster
2014;():V009T12A076. doi:10.1115/IMECE2014-39507.

A 10.3 m wind turbine (WT) blade has been designed to improve structural efficiency of rotor blades. The blade was featured with glass/polyester composites, flatback in the inboard region, thick airfoils in the mid-span region and transversely stepped spar cap thickness. This paper provided an overview of static bending test performed on the blade. Deflections, strains, load-carrying capacity, and failure behavior of the blade were investigated. Finite element (FE) analysis was carried out to complement test and to provide more insights into structural performance of the blade. The blade exhibited linear behavior in spar caps and aft panels at the maximum chord, and it continued to withstand applied loads well beyond the occurrences of local buckling of the shear web and the flatback at the maximum chord. The inboard region showed exceptional load-carrying capacity with failure loads larger than 420% test loads. Through this study, potential structural advantages by applying proposed structural features to large composite blades for multi-megawatt (MW) wind turbines were addressed.

Commentary by Dr. Valentin Fuster
2014;():V009T12A077. doi:10.1115/IMECE2014-40216.

The mixing of fluids using AC Electrokinetic is presented in this paper. Both AC electrothermal (ACET) and AC electroosmosis (ACEO) techniques are investigated for mixing operation. AC electrokinetic mixing utilizes the characteristics of short diffusion distance and large specific interface area, and the characteristics of laminar flow and multiphase flow in a microchannel. The proposed mixer will have advantages of easy implementation and compatibility with microchip fabrication. Furthermore low and high conductive fluid has been experimented for mixing operation. In this research, the ACET and ACEO mixing will be optimized by surface modification using a biocompatible hydrophobic nanocomposite monolayer. This coating will modify the mixer surface to a hydrophobic surface and improve the friction losses at the interface, and eventually increase the mixing rate. Both ACEO and ACET flow is a promising technique in microfluidic mixing toward laboratory automation applications, such as clinical diagnostics and high-throughput drug screening. But the mixing efficiency and type of AC electrokinetic usage depends on the conductivity range of the fluids. These mixers can be integrated with the lab-on-a-chip and can provide inexpensive disposable devices.

Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Polymer Nanocomposites: Simulations and Experiments

2014;():V009T12A078. doi:10.1115/IMECE2014-38596.

This work investigated and characterized the electrical conductivity of carbon nanotubes (CNT)/polymer composites. Surface modification has been applied to improve the homogeneous dispersion of MWCNTs in epoxy. After treatment, MWCNTs were mixed into low viscosity epoxy matrix at room temperature. Dispersion and structural integrity of MWCNTs before and after surface modification were examined by SEM images. The dispensability of treated MWCNTs and electrical conductivity of nanocomposites are evaluated and also compared with MWCNTs/polymer composites in literature prepared using the same commercial MWCNTs. The electrical conductivity of MWCNTs and MWCNTs/epoxy composites were evaluated by the four-point probe method. The results of electrical property will lay a foundation for establishing the relationship between electrical resistance and strain of MWCNTs/epoxy composites. The results also confirm that reducing CNT agglomerate size can greatly improve the electrical conductivity of composite and decrease the percolation threshold.

Commentary by Dr. Valentin Fuster
2014;():V009T12A079. doi:10.1115/IMECE2014-38815.

The relative permittivity and loss tangent at 10 GHz of a nanoclay-reinforced epoxy is investigated as a function of nanoclay loading percentage and moisture content. The energy dissipation associated with frictional and inertial losses during the reorientation of absorbed dipole water molecules exposed to an oscillating electromagnetic field has a significant impact on the relative permittivity and loss tangent of moisture-contaminated polymer materials. This can damage the performance of polymer-based radar-protecting structures (radomes) designed to protect sensitive radar equipment. Thus, prevention or minimization of water absorption in these materials is critical to mitigating this effect. The moisture barrier properties of nanoclay reinforcement are well known, and are targeted in this study as a potential method to reduce the moisture absorption rate and therefore improve the performance of polymer-based radomes exposed to precipitation and humid air. The ability of a water molecule to rotate freely in the presence of an EM field is dependent on its physical and chemical state; whether it be bound and unable to rotate, or unbound and able to dissipate energy through unrestricted rotation. Therefore, any potential dielectric property changes associated with the physical and chemical interaction of water and nanoclay must be quantified prior to exploiting prospective moisture-barrier benefits. In this study, the relative permittivity and loss tangent of an epoxy system reinforced with nanoclay up to 5% content by weight are assessed using a resonant cavity technique at 10 GHz during moisture uptake due to immersion in distilled water at 25°C. Variations in moisture diffusion behavior are observed due to the nanoclay loading percentage. Although deviations in the dielectric properties due solely to nanoclay loading percentage are minimal, effects due to moisture absorption are much more prominent. In the most extreme case, a nearly 15% increase in relative permittivity is observed at 5% moisture content by weight, with a direct correlation between diffusion behavior and degradation of relative permittivity observed for all samples. Likewise, an increase in the loss tangent of approximately 220% is observed at 5% moisture content by weight.

Commentary by Dr. Valentin Fuster
2014;():V009T12A080. doi:10.1115/IMECE2014-39972.

Self-sensing via piezoresistivity of multiwall carbon nanotube-epoxy resin composite was studied in order to assess its feasibility in strain and damage detection. Self-sensing is an economical and durable component of Structural Health Monitoring of manufactured composite, in which the material is employed as sensor. Objective of the study centered on strain and damage sensing and particularly focused on how applied tension and /or inflicted damage affected electrical conductivity. The MWCNT-epoxy composite was manufactured with non-treated multiwall carbon-nanotubes. They were dispersed in the epoxy employing ultrasonic dispersion and mechanical mixing, prior to addition of curing agent. The samples were rectangular specimen of dimensions 150 mm × 25 mm × 5 mm. Surface electrodes were created with sliver paint, to which copper wire leads were wrapped and affixed with more silver paint or carbon based electrically conductive glue. Kelvin in-line resistivity measurement technique was adopted to assess and monitor composite resistivity. The technique minimizes contact resistance between electrodes and the composite, which could be order of magnitude larger than the material resistance. For strain sensing, the specimen was subjected to increasing tensile strain until failure. In damage sensing, the specimens received different depth surface cuts across their width. During the tensile strain testing the specimens failed at a typical strain of about 4%. The corresponding change in resistance was in range of 2–3%. In damage sensing mode, changes of up to 40% in resistance were recorded for 75% thickness deep damage. For lower depth damages, the change in resistivity were about 4%–10%. The data obtained so far indicate that with proper technique, self-sensing of CNT-epoxy resin composites could be a viable method for monitoring applied strain and structural integrity.

Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Posters

2014;():V009T12A081. doi:10.1115/IMECE2014-36106.

The prediction of the plastic collapse load of cylindrical pressure vessels is very often made by using expensive Finite Element Computations. The calculation of the collapse load requires an elastic-plastic material model and the consideration of non-linear geometry effects. The plastic collapse load causes overalls structural instability and cannot be determined directly from a finite element analysis. The ASME (2007) code recommends that the collapse load should be the load for which the numerical solution does not converge. This load can be only determined approximately if a expensive nonlinear analysis consisting of a very large number of sub steps is done. The last load sub step leading to a convergent solution will be taken as the critical load for the structure. In the instability regime no standard finite element solution can be found because of the lack of convergence of the numerical procedure. Other methods for the calculation of the allowable pressure proposed by the ASME code are the elastic stress analysis and the limit load analysis. In the present paper the plastic collapse load for a cylindrical pressure vessel is determined by an analytical method based on a linear elastic perfectly plastic material model. When plasticity occurs the material is considered as incompressible and the tensor of plastic strains is parallel to the stress deviator tensor. In that case the finite stress-strain relationships of Henkel can be used for calculating the pressure for which plastic flow occurs at the inside of the vessel wall or in the case of full plasticity in the wall. The analytical results are fully confirmed by finite element predictions both for axisymmetric and high costs three dimensional models. The analytical model can be used for fast predictions of the allowable load for the design of a large variety of pressure vessels under safety considerations. The accuracy of the predicted collapse load largely depends on the quality of the temperature dependent wall material data used both in the analytical and numerical calculations.

Commentary by Dr. Valentin Fuster
2014;():V009T12A082. doi:10.1115/IMECE2014-36150.

Precise estimation of wall stress distribution within an abdominal aortic aneurysm (AAA) is clinically useful for prediction of its rupture. In this paper a computational fluid dynamic model incorporating two-way coupled fluid-structure interaction is employed to investigate the role of laminar-turbulent flow transition and wall thickness in altering the distribution and magnitude of wall stress in an AAA. Blood flow in axially symmetric aneurysm models governed by a compliant wall mechanics was simulated. Menter’s hybrid k-epsilon/k-omega shear stress transport (SST) model with a correlation-based transition model was used to capture laminar-turbulent transition in the blood flow. Realistic physiological transient boundary conditions were prescribed. The numerical model was validated against experimental data available from the literature. Fluid flow analysis showed the formation of recirculating vortices at the proximal end of the aneurysm after the peak systole which then, moved towards the distal end of the aneurysm along with the bulk flow and were dissipated eventually due to viscous effects. These vortices interacted with the aortic wall and led to local pressure rise. Von Mises stress distribution on the aneurysm wall and location of its peak value were computed and compared with those of a separate numerical simulation performed using a laminar viscous flow model. The predicted peak wall stress was found to be significantly higher for the SST model as compared to the laminar flow model. The location of maximum stress shifted more towards the posterior end of the aneurysm when laminar-turbulent flow transition was considered. In addition, a small reduction of 0.4 mm in wall thickness resulted in the elevation of peak wall stress by a factor of 1.4. The present study showed that capturing flow transition in an AAA is essential to accurate prediction of its rupture. The proposed numerical model provides a robust computational framework to gain more insight into AAA biomechanics and to accurately estimate wall stresses in realistic aneurysm configurations.

Commentary by Dr. Valentin Fuster
2014;():V009T12A083. doi:10.1115/IMECE2014-36334.

Limit load analysis is a well known method to calculate the allowable design pressure of container components. A limit load of a pressurized container is achieved, when the stress of a wall and the flow stress are equal. In the following paper the transferability of limit load analysis from small scale tank containers up to large scale containers (railway tank) are investigated. Finite element calculations are carried out and compared with experimental results. It can be concluded that the limit load analysis works very well. Furthermore, the yield strength of the material should be used as flow stress.

Commentary by Dr. Valentin Fuster
2014;():V009T12A084. doi:10.1115/IMECE2014-37549.

Analyses involving both modeling and experimental measurements for the residual stresses evaluation in Ni thin films on silicon wafers have been studied with focus on determining the optimum condition that leads to spallation. Different thicknesses of stressor thin films (Ni in our case) have been electroplated on Si(100) and Si(111) wafers to predict the critical residual stress in the Ni/Si system that are required for steady-state crack depth. The Ni film thicknesses and the total critical stresses in the Ni films required for steady state spalling processes were specified for different crack depth in Si(100) and Si(111) wafers.

Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Processing and Performance of Nanocomposites

2014;():V009T12A085. doi:10.1115/IMECE2014-37182.

Cellulosic nanofibers have been electrospun with an antitumor agent Cisplatin. Cellulose acetate (CA) and Cisplatin were co-electrospun using a coaxial electrospinning system. For the outer sheath, a solution of 7.5wt% CA in Acetone and DMAc (2:1) was used. The inner core consisted of Cisplatin dissolved in DMF at a concentration of 5mg/ml. Drug-loaded nanofibers from Cellulose pulp (2wt%) dissolved in NMMO. H2O were also produced. The solutions were electrospun in a high voltage electric field of 25–30 kV. Characterization of neat and drug-loaded nanofibers was performed using Scanning electron microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDS). The characterization studies have shown the formation of nanofibers having both sporadic beads with internal agglomeration and conjugation of Cisplatin on the nanofiber surfaces.

Commentary by Dr. Valentin Fuster
2014;():V009T12A086. doi:10.1115/IMECE2014-37183.

Ultrahigh molecular weight polyethylene (UHMWPE) fiber blends with Nylon-6 and reinforced with single-walled carbon nanotubes (SWCNT) were produced using a solution spinning process. Polyethylene-graft-Maleic Anhydride (PE-g-MAH) was used as a compatibilizer to enhance the interfacial bonding between the polymer phases. The loading of Nylon-6, MAH, and SWCNTs with respect to UHMWPE was 20 wt.%, 10 wt.% and 2 wt.% respectively. The development of morphological characteristics due to the inclusion of a compatibilizer in an immiscible hybrid polymer nanocomposite fiber is hereby discussed. Characterization studies of the hybrid fibers were performed using scanning electron microscopy (SEM), Energy-dispersive X-Ray Spectroscopy (EDS) and Fourier-transform infrared spectroscopy (FTIR).

Commentary by Dr. Valentin Fuster
2014;():V009T12A087. doi:10.1115/IMECE2014-38123.

Single-walled Carbon nanotubes (SWCNTs) have been shown to have excellent conductive properties. SWCNTs were dispersed in a SiC nanoparticle matrix to form a homogeneous mixture that is both mechanically durable and conductive. The SWCNT amount has been varied. SiC/SWCNT mixtures were then doped with various N- and P-type agents, and the resulting samples were analyzed by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). Raman spectra of the samples were also measured for evidence of structural changes. Seebeck coefficients were measured for the doped samples demonstrating the change in thermoelectric properties. Shifts in the G peak (1580.6 cm-1) of the Raman spectra of the samples provides evidence of an increase in charge carrier concentration in the doped samples, correlating well with the Seebeck coefficient results.

Commentary by Dr. Valentin Fuster
2014;():V009T12A088. doi:10.1115/IMECE2014-38408.

The objective of this study is to examine lattice thermal conductivity (κ) of Fe-Cr alloys containing different 〈001〉 tilt grain boundaries (GBs). The effects of Cr concentration (2 and 10%) and three different 〈001〉 tilt boundaries (Σ5{310}, Σ13{510}, and Σ17{530}) have been examined at 70K using the reverse non-equilibrium molecular dynamics (rNEMD) simulation technique. The results exhibit higher κ for Fe or Fe-Cr models with Σ5[310] GB. The values are 2–4% and 12–16% more than those of models with Σ13[510] and Σ17[530] GBs, respectively. Pure Fe single crystal models exhibit higher conductivities than Fe/Fe-Cr models with various Σ tilt boundaries. κ decreases 7–9% as GBs are introduced into the pure Fe single crystal models. On the other hand, the conductivities of Fe-Cr models are affected more by the Cr concentration than the presence of a particular GB. As 10% Cr is added into the system the conductivity decreases by 7.6–9.4% compared to the pure Fe models.

Commentary by Dr. Valentin Fuster

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

2014;():V009T12A089. doi:10.1115/IMECE2014-36917.

Since the introduction of assistive technologies for enhancing human mobility, there has been a high demand for compact, lightweight, powerful and energy efficient actuator. The Pneumatic Artificial Muscle (PAM) is a distinctive compliant pneumatic actuator, which has properties similar to the biological skeletal muscle, making it a great candidate for applications in human mobility assistive devices. Whereas the PAMs can be used actively or passively, until now, it has been mostly used in active applications to power various mechanisms, such as robotic arms, most notably the Shadow Robot Company Dextrous hand. For those applications, static and dynamic models of PAM have been developed by researchers to fairly accurately predict the muscle-force carrying capabilities and muscle contraction distance behavior, respectively. However, limited passive models have been developed, with results varying from acceptable to poor in terms of accuracy. Recognizing the significance of characterizing the PAM passive behavior, especially for legged locomotion application, this paper proposes a PAM stiffness model that is based on Newtonian mechanics and considers geometric, mechanical and material properties of the muscle. The proposed stiffness model is experimentally validated for a wide range of operating conditions.

Topics: Modeling , Muscle , Stiffness
Commentary by Dr. Valentin Fuster
2014;():V009T12A090. doi:10.1115/IMECE2014-39006.

During fission yeast cytokinesis, actin filaments nucleated by cortical formin Cdc12 are captured by myosin motors bound to a band of cortical nodes. The myosin motors exert forces that pull nodes together into a contractile ring. Cross-linking interactions help align actin filaments and nodes into a single bundle. Mutations in the myosin motor domain and changes in the concentration of cross-linkers alpha-actinin and fimbrin alter the morphology of the condensing network, leading to clumps, rings or extended meshworks. How the contractile tension developing during ring formation depends on the interplay between network morphology, myosin motor activity, cross-linking and actin filament turnover remains to be elucidated. We addressed this question using a 3D computational model in which semiflexible actin filaments (represented as beads connected by springs) grow from formins, can be captured by myosin in neighboring nodes, and get cross-linked with one another through an attractive interaction. We identify regimes of tension generation between connected nodes under a wide set of conditions regarding myosin dynamics and strength of cross-linking between actin filaments. We find conditions that maximize circumferential tension, correlate them with network morphology and propose experiments to test these predictions. This work addresses “Morphogenesis of soft and living matter” using computational modeling to simulate cytokinetic ring assembly from the key molecular mechanisms of viscoelastic cross-linked actin networks that include active molecular motors.

Commentary by Dr. Valentin Fuster
2014;():V009T12A091. doi:10.1115/IMECE2014-39525.

Unique among animal flyers, bats have highly flexible and stretchable thin wing membranes. The connection between the structural constituents of bat wing skin, its material behavior, and flight abilities is not yet known. In this work we propose a structurally motivated constitutive model for the wing skin. Within a continuum mechanics framework, the proposed strain energy function for the wing skin is the sum of contributions due to the matrix and two mesoscopic fiber families, one oriented primarily spanwise consisting of elastin fiber bundles and the other family oriented chordwise consisting of muscle fibers. While the fibers are flat and straight when the wing is somewhat open, the matrix exhibits corrugations due to compressive loading from the pre-stretched spanwise fibers. This mismatch in the natural configurations of components is accounted for in the model by a decomposition of the deformation gradient of the spanwise fibers. The material parameters are fit with a procedure motivated by the underlying deformation mechanisms of the tissue corresponding to the regions of the j-shaped constitutive curves. The proposed model is fit to the first set of biaxial experimental stress-strain data for bat wing skin and captures the general features of the tissue response well.

Commentary by Dr. Valentin Fuster
2014;():V009T12A092. doi:10.1115/IMECE2014-40430.

The large strain behaviour of a short fibre-reinforced composite is studied through numerical simulations. The reinforcing fibres yield the macroscopic response transversely isotropic which is indeed the case of many reinforcements currently used in composites: short carbon fibres, cellulose whiskers, carbon nanotubes. As a result of the analysis, it is shown that the reorientation of the fibres that takes place at large strain has a significant effect on the overall material response by changing the axis of isotropy. This behaviour can be adequately described by using a transversely isotropic model whose strain energy function depends on three invariants: two isotropic and one representing the stretch along the direction of the fibres. To assess its capabilities, the model is compared to the results of experiments carried out by the authors on nickel-coated chopped carbon fibres in a vulcanised natural rubber matrix for which the fibre orientation is achieved by controlling an external magnetic field prior to curing. Possible applications include micro-sized propulsion devices and actuators.

Topics: Fibers , Elastomers
Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Symposium on Multiphysics Simulations and Experiments for Solids

2014;():V009T12A093. doi:10.1115/IMECE2014-37606.

The exhaust manifold is an essential component of an engine, which has become increasingly important because of innovations in the industry. Thus, the efficiency of an exhaust manifold is a key factor in overall engine efficiency. In operating conditions, there are many factors that may influence the performance of an exhaust manifold, such as temperature, pressure, wall thickness, coolant velocity, etc.

A manufacturer of diesel engine’s exhaust manifolds was interested in investigating the performance of its manifolds. This paper describes the method of analysis and results obtained by Fluent and ANSYS software. The purpose of the project is to analyze the stress distribution and locate the areas most prone to failure.

Commentary by Dr. Valentin Fuster
2014;():V009T12A094. doi:10.1115/IMECE2014-38619.

Assessment of residual stresses in railroad rails without destructing the material plays a vital role in rail road safety. Ultrasonic testing is a commonly used nondestructive technique to determine the stresses in any structure. Ultrasonic stress evaluation technique is based on acoustoelastic effect which refers to the changes in the speed of the elastic wave propagation in a structure undergoing static elastic deformations. Critically refracted longitudinal (LCR) waves can be used as the propagating waves because it is a bulk wave and can reflect the surface and subsurface characteristics by the wave property linked to material elasticity. In this paper, a COMSOL Multiphysics module-based Finite Element Method (FEM) model is developed and numerical simulations are carried out for critically refracted longitudinal wave propagation in a railroad rail head for residual stresses. The time travel data results from this FEM Model are validated with reported experimental results.

Commentary by Dr. Valentin Fuster
2014;():V009T12A095. doi:10.1115/IMECE2014-39053.

Recently, the importance of lead support structure design has been growing. This is due to very large electromagnetic forces generated by high current flow in short-circuit failure of a transformer. In this paper, a coupled electromagnetic-structural analysis based on numerical method was conducted in order to calculate electromagnetic force which is difficult to calculate analytically. Then, an analysis on change of stress generated from lead support structure was carried out to evaluate structural safety in fault current conditions. A Structural analysis was performed on two types of domains (between leads and bolts, between leads and supports) considering nonlinear contact conditions because these two domains initially have gaps or can be separated. As a result, the arrangement of lead support structures considering their allowable stress was derived to secure the structural safety against the short-circuit force. The proposed arrangement of lead support structure was applied to the structures was applied to the initial designs and this led to improvement of lead support structures designs. Through this process, the evaluation technology suggested in this paper was validated.

Topics: Safety , Circuits , Failure
Commentary by Dr. Valentin Fuster

Mechanics of Solids, Structures and Fluids: Time-Dependent Materials and Their Composites: Experimental, Theoretical and Numerical Studies

2014;():V009T12A096. doi:10.1115/IMECE2014-36831.

In this paper we present a new constitutive framework, the Parallel Rheological Framework (PRF), for modeling polymers that has been recently developed by the authors and implemented in the commercial finite element software Abaqus [1]. The framework is based on parallel finite-strain viscoelastic and elastoplastic networks. For each viscoelastic network a multiplicative split of the deformation gradient into elastic and viscous components is assumed. The evolution of the viscous component of the deformation gradient is governed by a flow rule obtained assuming the existence of a creep potential. The flow rule is expressed as a function of stress invariants and internal variables, and different evolution laws for the internal variables are allowed within the framework of the model. Similar to the viscoelastic networks, the deformation gradient in the elastoplastic network is decomposed into elastic and plastic components. The yield surface is defined assuming combined isotropic/kinematic hardening. The yield surface is a function of a scalar internal variable that describes isotropic hardening, and a tensorial internal variable (backstress) that describes the shift of the yield surface in the stress space. The evolution of the scalar variable is governed by associated flow rule, while the evolution of backstresses is determined by the Armstrong-Frederick law [2], which is extended to finite-strain deformations. Finally, stress softening is introduced into an elastoplastic network using a modified version of Ogden and Roxbourgh’s pseudo-elasticity model [3].

This paper presents an outline of the framework, including two recent enhancements: a new creep model (the power law model) and combined isotropic/kinematic hardening plasticity model. The framework is then applied to analyze numerically the uniaxial loading/unloading behaviors of filled natural rubber and an EPDM polymer. The results obtained using finite element simulations show very good correlation with experimental data.

Topics: Modeling , Polymers
Commentary by Dr. Valentin Fuster
2014;():V009T12A097. doi:10.1115/IMECE2014-36985.

Sheet metals usually exhibit a certain degree of plastic anisotropy because of the rolling effect. To characterize the anisotropic behavior in simulations related to large deformation, strain-rate independent phenomenological models are frequently used in quasi-static conditions. Two functions are generally included in such a model, i.e. the yield function and the plastic potential. The former limits the stress state within the yield surface while the latter determines the direction of the plastic strain increment.

Traditional plasticity models mostly assume associated flow rule, in which the two functions mentioned above are identical. With the enhanced demand of accuracy, the forms of the associated models become too complex with more and more parameters to achieve an easy calibration procedure. Alternatively, in the past decade the non-associated models were increasingly used for sheet metals. Separate functions for the two aspects of plasticity lead to efficient characterization and convenient calibration.

In numerical study of dynamic loading cases, how to characterize strain-rate dependence of plasticity is an important issue. Some visco-plastic models were developed to take the rate effect into account, e.g. Johnson-Cook and Cowper-Symonds models, where the isotropic J2 flow theory was commonly used. However, when the material is severely anisotropic, this approach is very likely to be insufficient, and a model including both anisotropy and rate dependence would be needed. Extending a non-associated anisotropic model to be rate-dependent is a promising approach which has not been published in open literature to the best knowledge of the authors.

Objective of the present study is to develop an applicable model for characterizing dynamic mechanical behavior of a typical high-strength steel sheet. Two steps are performed. The material is investigated under quasi-static loading firstly. Tensile test results show an obvious anisotropy which cannot be described by traditional associated models. So the non-associated Hill48 model is chosen and calibrated. Accuracy of the model is verified by a quasi-static punching test. Thereafter the dynamic material properties are obtained by conducting tensile tests at quite a few strain-rate levels covering 0.0004–1200s−1. To characterize the positive strain-rate effect in strength, the non-associated Hill48 model is extended to be visco-plastic after checking two rate-dependence formulations in existing isotropic models. With implementing the extended model into a user subroutine of ABAQUS/explicit, simulations of the dynamic tension tests are run and compared to the real experiments. A good agreement between the simulated and the experimental result is achieved using the VUMAT.

Commentary by Dr. Valentin Fuster
2014;():V009T12A098. doi:10.1115/IMECE2014-39237.

Active contraction of smooth muscle results in the myogenic response and vasomotion of arteries, which adjusts the blood flow and nutrient supply of the organism. It is a multiphysic process coupled electrical and chemical kinetics with mechanical behavior of the smooth muscle. This paper presents a new constitutive model for the media layer of the artery wall to describe the myogenic response of artery wall for different transmural pressures. The model includes two major components: electrobiochemical, and chemomechanical parts. The electrochemical model is a lumped Hodgkin-Huxley-type cell membrane model for the nanoscopic ionic currents: calcium, sodium, and potassium. The calculated calcium concentration serves as input for the chemomechanical portion of the model; its molecular binding and the reactions with other enzyme cause the relative sliding of thin and thick filaments of the contractile unit. In the chemomechanical model, a new nonlinear viscoelastic model is proposed using a continuum mechanics approach to describe the time varying behavior of the smooth muscle. Specifically, this model captures the filament overlap effect, active stress evolution, initial velocity, and elastic recoil in the media layer. The artery wall is considered as a thin-walled cylindrical tube. Using the proposed constitutive model and the thin-walled equilibrium equation, the myogenic response is calculated for different transmural pressures. The integrated model is able to capture the pressure-diameter transient and steady-state relationship.

Commentary by Dr. Valentin Fuster

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