bluuebunny/crossref_metadata_2025 huggingface dataset:
Probabilistic Mesomechanical Fatigue Crack Nucleation Model
Authors: Tryon R. G. , Cruse T. A.
Published: February 2008

A probabilistic mesomechanical crack nucleation model is proposed to link the microstructural material heterogeneities to the statistical scatter in the macro structural response. The macrostructure is modeled as an ensemble of microelements. Cracks nucleate within the microelements and grow from the microelements to final fracture. Variations of the microelement properties are defined using statistical parameters. A micromechanical slip band decohesion model along with a Paris law crack growth model are used with first order reliability methods and Monte Carlo simulation to determine the distribution of fatigue life for the macrostructure. The modeled response is compared to trends in experimental observations from the literature.
Thermoelastic Finite Element Analysis of Subsurface Cracking Due to Sliding Surface Traction
Authors: Cho S.-S. , Komvopoulos K.
Published: February 2008

A linear elastic fracture mechanics analysis of subsurface crack propagation in a half-space subjected to moving thermomechanical surface traction was performed using the finite element method. The effect of frictional heating at the sliding surface on the crack growth behavior is analyzed in terms of the coefficient of friction, crack length-to-depth ratio, and Peclet number. The crack propagation characteristics are interpreted in light of results for the directions and magnitudes of the maximum shear and tensile stress intensity factor ranges, respectively. It is shown that, while frictional heating exhibits a negligible effect on the crack propagation direction, it increases the in-plane crack growth rate and reduces the critical crack length at the onset of out-of-plane crack growth at the right tip due to the tensile mechanism (kink formation). The effect of frictional heating becomes more pronounced with increasing contact friction, crack length-to-depth ratio, and Peclet number. Crack mechanism maps showing the occurrence of opening, slip, and stick regions between the crack surfaces are presented for different values of crack length-to-depth ratio, coefficient of friction, and position of thermomechanical surface traction.
Constitutive Equations for Solid Propellants
Authors: O¨zu¨pek S¸ebnem , Becker Eric B.
Published: February 2008

Mechanical behavior of the Space Shuttle redesigned solid rocket motor (RSRM) propellant is studied from a phenomenological point of view. Motivated by the study of the experimental data three initially isotropic constitutive models have been developed. All models represent the effect of strain rate, superimposed hydrostatic pressure, and cyclic loading on the stress and dilatation response of the material. A particular emphasis is given to the prediction of volume dilatation. The model resulting in the best representation of the available data is calibrated using only a few tests. The predictions of the model are compared with experiments for several loading conditions not used in the calibration.
Grain Size Modeling and Optimization of Rotary Forged Alloy 718
Authors: Domblesky J. P. , Shivpuri Rajiv
Published: February 2008

The study presented describes the simulation procedure and methodology used to develop two models for predicting recrystallized grain size in Alloy 718 billet. To simulate multiple pass forging of billet, controlled, high temperature compression testing was used to apply alternate deformation and dwell cycles to Alloy 718 specimens. Grain size obtained by simulation was found to be in excellent agreement with grain size from forged billet when cooling rate was included. The study also revealed that strain per pass and forging temperature were the predominant factors in controlling the recrystallized grain size. Both models were found to accurately predict the recrystallized grain size obtained by compression tests performed at super-solvus temperatures.
A Constitutive Model and Its Identification for the Deformation Characterized by Dynamic Recovery
Authors: Zhou M. , Clode M. P.
Published: February 2008

A constitutive model is presented to characterize the deformation behavior of materials at elevated temperatures that undergo dynamic recovery. Hot torsion tests were utilized to validate the constitutive equations. The tests were conducted with the material of AA5083 over a range of twist rate from 0.013 to 13.70 rev/s and for each twist rate at an initial temperature range of 350°C to 550°C, respectively. The material constants determination methods are demonstrated with the proposed linear regression and the nonlinear iteration formats. A good agreement between the experimental data and the predicted results has been achieved
High Temperature Multiaxial Low Cycle Fatigue of CMSX-2 NI-Base Single Crystal Superalloy
Authors: Kanda Mitsuru , Sakane Masao , Ohnami Masateru , Hasebe Tadashi
Published: February 2008

This paper describes studies of the tension-torsion multiaxial low cycle fatigue of CMSX-2 Ni-base single crystal superalloy. Tension-torsion low cycle fatigue tests were carried out at 1173K using CMSX-2 hollow cylinder specimens aligned with the {001} axis. Several multiaxial strain and stress parameters were applied to the experimental data to examine the suitability of the parameters to life prediction. All the strain parameters proposed so far gave a large scatter of the data correlation. Discrepant data correlation with the strain parameters resulted from the anisotropic stress response due to the crystallographic texture. Larger Mises equivalent stress was applied in torsion tests than in tension tests at the same Mises strain. However, Mises stress and the equivalent stress based on crack opening displacement gave a satisfactory data correlation. This study developed a new equivalent strain, taking account of the anisotropy of the elastic constants, which correlates the multiaxial low cycle fatigue data with a small scatter.
Combined Bending and Torsional Fatigue of Woven Roving GRP
Authors: Wafa M. N. Aboul , Hamdy A. H. , El-Midany A. A.
Published: February 2008

A study of biaxial fatigue of woven roving glass reinforced polyester (GRP) subjected to in-phase and out-of-phase cyclic bending and torsional moments is presented. To evaluate failure theories for this material, tests were conducted on two fiber orientations [0, 90] and [45, −45] tubes. The results showed that for [0, 90] composites the S-N curves in pure bending and in pure torsion are sufficient to predict life. For [45, −45] tubes, the value of the normal stress interaction component of the strength tensor, H12, has to be obtained. If the ratio of the global flexural stress amplitude, A, to the accompanied global shear stress, B, is less than 2, the value of H12 may be taken as presented by Tsai-Hahn theory. But, if A/B ≥ 2, the value of H12 has to be obtained from [45, −45] pure bending S-N curve, since the failure mode is a combination of interfacial shear and matrix failure. The out-of-phase loading results showed that the life of the specimens at high stress levels is less than that for the in-phase loading with the same peak values A and B.
Experiments on the Ratchetting Behavior of AISI 316L(N) Austenitic Steel at Room Temperature
Authors: Haupt A. , Schinke B.
Published: February 2008

Uniaxial tension-compression ratchetting tests were performed on AISI 316 L(N) austenitic stainless steel at room temperature. The mean stresses were low in comparison to the stress amplitudes. Nevertheless, they have been found to be of major importance to strain accumulation. Also, the stress amplitude exerts a distinct but, under the given conditions, smaller influence. No plastic shakedown has been found in the tests with constant mean stress and stress amplitude. Tests with an increase in the maximum load after a number of cycles, due to an increase in mean stress or stress amplitude, show a behavior similar to that in tests performed with the higher load only. Tests with a decrease in maximum load exhibit immediate shakedown, although tests performed with only the lower load showed ratchetting.
Micromechanical Analysis of Dynamic Behavior of Conventional and Negative Poisson’s Ratio Foams
Authors: Chen C. P. , Lakes R. S.
Published: February 2008

Both conventional and negative Poisson ’s ratio foams exhibit dispersion of acoustic waves, as well as cut-off frequencies, at which the group velocity tends to zero. This macroscopic behavior is attributed to micro-vibration of the foam cell ribs. The purpose of this article is to develop a micromechanical model of the cut-off frequency. This model is based on the resonance of ribs which may be straight, curved, or convoluted. As foam cell ribs become more curved, the cut-off frequency decreases. Therefore foams with curved or convoluted ribs are expected to provide superior performance in applications involving the absorption of sound.
Influence of Strain Rate and Temperature on the Radial Compression Behavior of Wet Spruce
Authors: Uhmeier Andreas , Salme´n Lennart
Published: February 2008

In the mechanical pulping process, wood is treated in compression and in shear at high temperatures in the wet state, to separate the fibers and also to increase the bonding ability and flexibility of the fibers in order to obtain strong paper sheets. This is probably enhanced by permanent deformation and collapse of the fibers. In this study, the influences of moisture content, density, strain rate and temperature on the mechanical response of spruce compressed radially to 50 percent strain were investigated. Regression models were obtained for the plateau stress (≈collapse stress), energy absorption, plastic strain and reduction of plateau stress after the first compression. Temperature and strain rate had a great influence on the mechanical behavior of spruce. It was found that lumen (= cell void) water had a significant effect on the deformation process at high strain rates. The reduction in plateau stress after one compression was about 30–55 percent, which might increase the collapsibility of the wood fibers.
Interactive Growth of Multiple Fiber-Bridged Matrix Cracks in Unidirectional Composites
Authors: Huang Y. , Li N.-Y. , Zhang H. W. , Hwang K.-C.
Published: February 2008

A model is developed for monotonic and cyclic fiber sliding in a fiber-reinforced composite containing multiple cracks. The model is used to study the fatigue growth of multiple cracks in a matrix reinforced with aligned, continuous fibers, where cracks are bridged by frictionally constrained fibers. It is established that the crack tip stress intensity factor is significantly reduced in multiple cracking due to interactions among cracks and among slip zones. The fatigue crack does not grow as fast as that for a single bridged crack or for multiple nonbridged cracks, thus the approach to steady-state crack growth is significantly delayed.
Holographic Evaluation of Edge Delamination in Bars Bonded to a Rigid Support With Foam-Adhesive
Authors: Qin S. , Shang H. M. , Giam C. L. , Tee C. E.
Published: February 2008

When a bar having one end bonded to a rigid support with foam-adhesive is loaded, the high peeling stresses at the bonded edge foster edge delamination along the interface of bonding. Upon inspection by double exposure holography, with an incremental point load applied at the free end of the bar between exposures, the indistinct fringe perturbation and the unknown resilience of the foam-adhesive (quantified by its foundation modulus) impede unambiguous evaluation of the condition of adhesion. This paper describes a simple method for rapid detection and assessment of artificially created edge delamination in such bar structures. The theoretical analysis elucidates the suitability of modelling the bar as partially clamped, and having an equivalent slope at its support related to the foundation modulus. Two dimensionless parameters are defined, which have between them a linear relationship for a nondelaminated bonding but a nonlinear relationship for a delaminated one. Finally, a simple iterative procedure is described for estimating the delaminated length without having to predetermine the elasticity of the support.
Influence of Grinding Direction on Fracture Strength of Silicon Nitride
Authors: Strakna T. J. , Jahanmir S. , Allor R. L , Kumar K. V.
Published: February 2008

It is generally found that grinding transverse to the tensile stress direction in flexure bars subjected to four-point bending results in a lower strength compared to grinding in the longitudinal direction. In the present study, standard flexure specimens made from a reaction-bonded and a sintered reaction-bonded silicon nitride (RBSN and SRBSN) were surface ground under three different conditions in both longitudinal and transverse directions to assess the effect of grinding direction on strength. Four-point flexure tests were performed on the specimens and Weibull parameters were calculated. The results showed that while the strength was not affected by the grinding condition when grinding was performed in the longitudinal direction, the strength of the samples ground in the transverse direction was reduced as the material removal rate was increased by a factor of 30. This result was confirmed by fractography, which showed that almost all the fracture initiation sites in the longitudinally ground samples were associated with near-surface microstructural features, whereas in the transverse ground samples fracture initiated from damage introduced by grinding. The strength reduction by grinding in the transverse direction was found to be material dependent, and was larger for SRBSN than for RBSN.
Mixed-Mode Fatigue Crack Growth in Stainless Steels Under Biaxial Loading
Authors: Qian Cai-Fu , Wang Ming-O , Wu Bao-Juan , Dai Shu-Ho , Li J. C. M.
Published: February 2008

Mixed-Mode fatigue crack propagation tests were carried out in stainless-steel cruciform specimens in which a center initial crack oriented at 45 deg was loaded biaxially. When the loadings were in-phase, the crack was deflected with the initial crack growth angles less than 50 deg and when they were out-of-phase, it was branched with the initial crack growth angles larger than 50 deg. In all the cases, the deflected or branched cracks propagated in Mode I, namely ΔKII was almost always zero along the crack paths. The relationship between deflected or branched cracks during propagation was also analyzed.
Effect of Stress Ratio on Short Fatigue Crack Growth
Authors: Wang C. H.
Published: February 2008

A model of short fatigue crack growth is proposed, which is based on the blocked slip concept and the shear decohesion mechanism. The analysis is extended to the case of mean stress loading. A theoretical proof is presented for the transfer of slip bands across grain boundaries. The rate of growth is proportional to the shear strain range and the maximum plastic zone size. There are no adjustable parameters in the theory for the case of high strain level, when the plastic strain dominates the decohesion process. Otherwise only one constant is needed, which may be derived from long crack growth data. The model is shown to provide satisfactory predictions of experimental results under uniaxial loading with various stress amplitudes and mean stresses.
Life Prediction Techniques for Variable Amplitude Multiaxial Fatigue—Part 1: Theories
Authors: Wang C. H. , Brown M. W.
Published: February 2008

Fatigue life prediction under multiaxis random loading is an extremely complex and intractable topic; only a few methods have been proposed in the literature. In addition, experimental results under multiaxis random loading are also scarce. In part one of this two-part paper, a multiaxial non-proportional cycle counting method and fatigue damage calculation procedure are proposed, which is compared with one published damage-searching method. Both theories are based on critical plane concepts, one being an extension of the local strain approach for uniaxial variable amplitude loading and the other employing a new counting algorithm for multiaxis random loading. In principle, these two methods can be considered as bounding solutions for fatigue damage accumulation under multiaxis random loading.
Life Prediction Techniques for Variable Amplitude Multiaxial Fatigue—Part 2: Comparison With Experimental Results
Authors: Wang C. H. , Brown M. W.
Published: February 2008

An extensive multiaxial random fatigue test programme was conducted at room temperature using tubular specimens. Experiments were performed under combined tension/torsion and triaxial loading, covering proportional and nonproportional variable amplitude loading cases. The two proposed life prediction methods discussed in Part 1 are evaluated using the experimental results, demonstrating that these two methods provide satisfactory predictions.
Energy Criterion for Fatigue Strength of Wood Structural Members
Authors: Liu Jen Y. , Ross Robert J.
Published: February 2008

This report describes a mathematical model for fatigue strength of cellulosic materials under sinusoidal loading. The model is based on the Reiner-Weissenberg thermodynamic theory of strength in conjunction with a nonlinear Eyring’s three-element model. This theory states that failure depends on a maximum value of the intrinsic free energy that can be stored elastically in a volume element of the material. The three-element mechanical model, which consists of a linear spring in series with a parallel array of another linear spring and an Eyring dashpot, provides a good description of rheological material properties. The strength model system was able to predict rupture occurrence of polymers and wood structural members under constant and ramp loading with satisfactory results. For sinusoidal loading, the present study shows that the strength model system can predict time at fracture as a function of applied mean stress, amplitude of cyclic stress, and stress frequency. Numerical examples with model parameters evaluated for small Douglas-fir beams are presented.
A Probabilistic Treatment of Microstructural Effects on Fatigue Crack Growth of Large Cracks
Authors: Chan K. S. , Torng T. Y.
Published: February 2008

A probabilistic model has been developed for treating the effects of microstructural variation on the fatigue crack growth response of large cracks in structural alloys. The proposed methodology is based on a microstructure-based fatigue crack growth law that relates the crack growth rate, da/dN, to the dislocation barrier spacing, yield stress, fatigue ductility coefficient, Young’s modulus, and the dislocation cell size or crack jump distance. Probabilistic treatment of these microstructure-dependent variables has led to a fatigue crack growth law that includes explicitly the randomness of the yield stress, fatigue ductility coefficient, and the dislocation barrier spacing in the response equation. Applications of the probabilistic crack growth model to structural reliability analyses for steels and Ti-alloys are illustrated, and the probabilistic sensitivities of individual random variables are evaluated.
Generalization of the Coffin Equations With Respect to the Effect of Large Mean Plastic Strain
Authors: Skoczen B.
Published: February 2008

Introduction of a correction factor accounting for large mean plastic strain to the kinetic law for the rate of damage per cycle is proposed. Integration of the corrected kinetic damage law under the assumption of a stable hysteresis loop (constant strain amplitude, constant mean plastic strain) leads to a generalized Coffin-Manson formula. The formula is useful for calculation of the fatigue life of structures after transition of transient ratchetting to the plastic shakedown state. Integration of the damage law is performed also for the case of superimposed cyclic and monotonic strain. The results of calculations are compared with the experimental results reported by Coffin (1970) and described by the author using a quasilinear criterion. An improved fatigue failure criterion for this case is presented and a good convergence with the results of tests for Nickel A samples is shown.
An Inelastic Constitutive Equation of Fiber Reinforced Plastic Laminates
Authors: Kanagawa Y. , Murakami S. , Mizobe T.
Published: February 2008

A Constitutive model for describing the time dependent inelastic deformation of unidirectional and symmetric angle-ply CFRP (Carbon Fiber Reinforced Plastics) laminates is developed. The kinematic hardening creep law of Malinin and Khadjinsky and the evolution equation of Armstrong and Frederick are extended to describe the creep deformation of initially anisotropic materials. In particular, the evolution equations of the back stresses of the anisotropic material were formulated by introducing a transformed strain tensor, by which the expression of the equivalent strain rate of the an isotropic material has the identical form as that of the isotropic materials. The resulting model is applied to analyze the time dependent inelastic deformation of symmetric angle ply laminates. Comparison between the predictions and the experimental observations shows that the present model can describe well the time dependent inelastic behavior under different loadings.
Phase Transformation Temperatures of an Ultra-Low Carbon Steel
Authors: Hwu Y. J . , Lenard J. G.
Published: February 2008

An ultra-low carbon steel is studied and the temperatures at which the austenite to ferrite transformations begin and are complete are determined. The methods of measurements of the temperatures are discussed. The effects of the cooling rate, initial austenite grain size, prestrain, residual strains, static recrystallization, and the mode of deformation on phase transformation temperatures are determined and discussed.
Measuring Strain Distribution During Mesoscopic Domain Reorientation in a Ferroelectric Material
Authors: Park S. B. , Park S. S. , Carman G. P. , Hahn H. T.
Published: February 2008

In this paper, we present the results of an experimental study focused on understanding the strain concentrations arising due to nonlinear phenomena associated with polarization switching. A moire interferometry technique is used to measure the normal and shear strains of a PZT-5H piezoceramic undergoing 180 and 90 deg switching. These results include the strain concentrations measured between polarized regions oriented 180 and 90 deg apart. The results show that very large strain mismatches (e.g., as large as 4500 microstrains) occur along the boundary of dissimilar oriented domains, suggesting a source of microcrack initiation and fatigue degradation.
Evaluation of Tensile Strength of Multistrand Conductors—Part I: Theoretical Basis
Authors: Lanteigne J. , Akhtar A.
Published: February 2008

A simple model was developed to predict the plastic behavior of a stranded conductor under tensile loading to fracture. Two cases are considered; the torque transmitted in the case of conductors with fixed extremities and the rotation angle in the case of free rotating conductors. The permanent set caused by bending and torsion during the stranding process associated with conductor fabrication was taken into consideration. The model predicts maximum failing load and total elongation, parameters currently used for design and selection of conductors.
Evaluation of Tensile Strength of Multistrand Conductors—Part II: Experimental Results
Authors: Akhtar A. , Lanteigne J.
Published: February 2008

Tension-torsion measurements were made with applied strain on ACSR, AACSR,ACAR, aluminum and aluminum alloy conductors, as well as on groundwires. Results were compared with predictions of a model based on mechanical properties of constituent wires and the geometry of the multistrand cable. True tensile strength of each cable was predicted accurately. Deviations were found in the torque values of cables containing galvanized steel strands. Interstrand friction, not considered in the model, is thought to be responsible for the deviation. Frictionless interaction of strands was found to be a reasonable approximation for aluminum and aluminum alloy stands.
Serrated Flow and Surface Markings in Aluminum Alloys
Authors: Li Ming , Lege Daniel J.
Published: February 2008

Serrated flow and associated progressive surface markings severely restrict the application of some aluminum sheet alloys for automotive body exteriors. This paper attempts to approach the phenomenon from the localization theory of continuum mechanics as well as from the classical atomistic and dislocation considerations. Plane strain tension tests were conducted for a commercial Al-Mg alloy (5182-O) at different strain rates and temperatures, and the local temperature changes were measured by an infrared thermal imaging system. Continuum mechanics analysis provided the insight into the myth that band surface markings never appear under biaxial tension strain states. In addition, continuum mechanics analysis shed light on the observation that PLC bands were not seen on the surface of plane strain tension specimens even though the stress-strain curves exhibited serrations. Finally, it is emphasized that only by combining the efforts of continuum mechanics at the macroscale and materials science at the microscale, can a complete understanding of the phenomenon be reached.
Cyclic Deformation and Fatigue of a TiNi Shape-Memory Alloy Wire Subjected to Rotating Bending
Authors: Tobushi H. , Hachisuka T. , Hashimoto T. , Yamada S.
Published: February 2008

The cyclic deformation and fatigue of a TiNi shape-memory alloy wire were studied by carrying out rotating-bending fatigue tests at various strain amplitudes, temperatures, and rotational speeds in air and in water. The results were summarized as follows: (1) for strain amplitudes in the rhombohedral-phase transformation region, the fatigue life was longer than 107 cycles and the deformation properties did not change under cyclic deformation; (2) for the strain amplitudes in the martensitic transformation region, the fatigue life was shorter than 105 cycles and the higher the temperature, the shorter was the fatigue life; (3) the rotational speed did not affect the fatigue life in water but a higher rotational speed led to a shorter fatigue life in air.
Experimental Characterization of Crack Tip Deformation Fields in Alloy 718 at High Temperatures
Authors: Liu Jin , Lyons Jed , Sutton Michael , Reynolds Anthony
Published: February 2008

A series of fracture mechanics tests were conducted at temperatures of 650°C and 704°C in air, using Inconel® 718. A noncontacting measurement technique, based on computer vision and digital image correlation, was applied to directly measure surface displacements and strains prior to and during creep crack growth. For the first time, quantitative comparisons at elevated temperatures are presented between experimentally measured near-crack-tip deformation fields and theoretical linear elastic and viscoelastic fracture mechanics solutions. The results establish that linear elastic conditions dominate the near-crack-tip displacements and strains at 650°C during crack growth, and confirm that KI is a viable continuum-based fracture parameter for creep growth characterization. Postmortem fractographic analyses indicate that grain boundary embrittlement leads to crack extension before a significant amount of creep occurs at this temperature. At higher temperatures, however, no crack growth was observed due to crack tip blunting and concurrent stress reduction after load application.
Rate Data and Material Model Parameter Estimation
Authors: Fossum A. F.
Published: February 2008

Traditionally stress-strain data are used to estimate material parameters in viscoplastic material models. Since the models exist in rate form, they must be integrated in time for the duration of the laboratory tests to give response models for parameter estimation. In this paper the alternative is considered in which the rate form of the material models is fitted to time differentiated stress strain data. Material parameteres, their 95 percent confidence intervals, and their correlation coefficients are compared with their counterparts determined from the more traditional stress versus strain form of the data. It was found that the use of the stress rate form of the data gave well determined parameter estimates with equivalent or superior fitting statistics to the same data can be used to extract significant and distinct sensitivity coefficients to enchance the overall robustness of an optimization algorithm for a parameter estimation.
The Numbers of Elastic Properties and Failure Parameters for Fiber Composites
Authors: Christensen R. M.
Published: February 2008

It is shown that there is a coordination between the numbers of the elastic properties and the numbers of the failure criteria parameters for aligned fiber composites under certain realistic conditions. These conditions require a high degree of anisotropy appropriate to polymeric matrix composites such that failure decomposes into separate fiber dominated and matrix dominated modes. Failure criteria are given in both five parameter and four parameter forms. The five parameter failure form coordinates with the usual five elastic property form for transversely isotropic composites. The four parameter failure form coordinates with a reduced four elastic property form which is shown to be applicable as an approximation for the same typical fiber composites.
Plastic Energy Dissipation Model for Lifetime Prediction of Zirconium and Zircaloy-4 Fatigued at RT and 400°C
Authors: Lin Xiao , Haicheng Gu
Published: February 2008

Low cycle fatigue properties of zirconium and zircaloy-4 were investigated at RT and 400°C. The microscopic structure was determined using scanning electron microscopy and transmission electron microscopy techniques. On the basis of analyses of fatigue damage mechanism, it is believed that fatigue is an irreversible energy dissipation process. Thus, the plastic dissipation energy per cycle is selected as a fatigue damage variable. The accumulated plastic dissipation energy is calculated at the condition of considering cyclic hardening, saturation and softening characters of zirconium and zircaloy-4 during cycling. The testing results show that there present a power law between the plastic dissipation energy and fatigue lifetime.
A Three-Dimensional Network Model for a Low Density Fibrous Composite
Authors: Stahl D. C. , Cramer S. M.
Published: February 2008

A procedure is described that predicts the mechanical behavior of a fibrous material by generating and analyzing finite element models of its three-dimensional microstructure. The approach is applicable to a class of materials with microstructures consisting of fibers connected at well-defined points. The procedure allows one to predict the effect of important sources of heterogeneity in these materials. Analyses determine initial elastic properties, failure mode, and strength of the composite; the failure analysis consists of tracking a progression of micro failures. The procedure is validated by comparison of predictions to test results.
A Finite Element Analysis of the Effect of Hardening Rules on the Indentation Test
Authors: Huber N. , Tsakmakis Ch.
Published: February 2008

Using the Finite Element Method, an analysis is given of the indentation of an elasticplastic half-space by a rigid sphere. In particular, attention is focused on the effect of hardening rules on the material response. The materials considered are supposed to exhibit isotropic and kinematic hardening. Moreover, it is shown that the possibility of similar behavior due to effects of friction can be ruled out.
Inelastic Response of Off-Axis MMC Lamina
Authors: Newaz G. M. , Zhang Ke
Published: February 2008

Damage evolution and failure characteristics were investigated for a 10 deg off-axis SiC/It unidirectional metal matrix composite (MMC) lamina subjected to monotonic loading. A replica technique was used to monitor sequential damage evolution under monotonic loading for the 10 deg MMC lamina. The replicas were then examined under a Scanning Electron Microscope (SEM). Characteristic debonding along the fiber—a dominant damage mechanism, matrix plasticity in the form of slip bands and fiber cracks were observed at various strain levels and were rationalized based on the state of stress in the off-axis lamina. This investigation provides an insight into the off-axis material response for MMC lamina and associated damage mechanisms and can guide mechanism-based multiaxial constitutive model and failure criteria development.
A Simulation Procedure for Panel Age Forming
Authors: Narimetla S. P. , Peddieson J. , Buchanan G. R. , Foroudastan S.
Published: February 2008

A method of simulating the age forming of initially flat panels is proposed. It conceptually divides the process into three phases (loading, aging, and unloading) and simulates each phase separately. Some sample simulations are presented for 7075 aluminum alloy based on a simplified version of the general procedure (linear loading and unloading, nonlinear aging). The results of these simulations are used to illustrate several aspects of the predictions. An interesting finding is that the part shape and tool shape are geometrically similar in two important special cases.
Study on the Thermo-Elastic-Plastic Cutting Model for 3-D Tool with Chip Breaker
Authors: Lin Zone-Ching , Zheng Yan-Liang
Published: February 2008

This paper used large deformation finite element theory, updated Lagrangian formulation, finite difference method, and incremental theory to develop a three-dimensional thermo-elastic-plastic simulation model for a tool with chip breaker. Both the critical strain energy density theory and the tool feed geometrical location were introduced as the chip separation criterion for cutting. The algorithm of tool movement geometrical limitations was used to examine and correctly the node so as to conform to real cutting conditions. In this model, the tool moved step by step in the simulation, which ran from the initial contact between tool and workpiece to the formation of steady cutting force. Finally, the numerical simulation model proposed in this paper was used to analyze the changes in workpiece and chip shapes, stress, strain rate, residual stress, temperature and cutting force of mild steel workpiece under different chip breaker lengths. The results were also compared with those from tools without chip breaker. The findings indicate that the chip breaker length affects the shorter the chip breaker length, the better the effects of chip breaker, and the lower the values of the aforementioned physical properties.
Numerical Simulation of Residual Stresses Caused by Quenching Process in Aluminium Gas Cylinders
Authors: Ibrahim R. N. , Ischenko D. , Lam Y. C.
Published: February 2008

Residual stresses introduced during quenching process in aluminium gas cylinders contribute to the development of cracks. This may result in leakage or fracture of the cylinders. Finite element studies were conducted to evaluate the effect of the quenching process on through thickness inelastic strain and residual stress distributions in the neck area of the gas cylinder. The numerical modeling and experimental studies confirmed that high level of tensile residual stresses exists at the internal surface of the neck of the aluminium gas cylinders which is susceptible to cracking.
Strength Prediction of RTM Single-Lap Joints With Transverse Stitching
Authors: Tong Liyong
Published: February 2008

It was experimentally shown that transverse stitching can provide an over 20 percent improvement in the strength of composite single-lap joints manufactured using the Resin Transfer Moulding (RTM) process. In this study, an analytical model is presented to predict the strength of the RTM single-lap joints with and without transverse stitching. Simple formulas are used to calculate the energy release rates of mode I and II using longitudinal membrane force and bending moment in a continuous adherent at an overlap end. The strengths of single-lap joints are determined using an interactive mixed-mode based failure criterion. A good correlation between the measured and the predicted failure loads validates the present analytical model.
Mechanistic Prediction of Fracture Processes in Ferritic Steel Welds Within the Transition Temperature Regime
Authors: Busso E. P. , Lei Y. , O’Dowd N. P. , Webster G. A.
Published: February 2008

This work examines the fracture behavior of ferritic steel welds in the transition temperature regime, where failure can occur either by ductile tearing or cleavage fracture. A computational and probabilistic-based mechanistic approach to cleavage fracture and ductile crack growth is adopted to model the fracture processes. The softening effect of ductile damage close to the crack tip is described by a Gurson-type material model. A statistical approach linked to both the Weibull stress and the initial void volume fraction is employed to determine the probability of cleavage fracture and the coupling between both fracture mechanisms. Finite element results are relied upon to interpret experimental fracture toughness data for the welds and to examine the effects of near crack tip damage and crack growth on the cleavage failure probabilities and cleavage and ductile fracture toughness distributions. The scatter in the weld experimental fracture toughness data is well reproduced by the proposed cleavage and ductile tearing models.
Analysis of Interfacial Failure in Particle-Filled Elastomers
Authors: Zhong X. Allan , Knauss Wolfgang G.
Published: February 2008

The evolution of microdamage (interfacial dewetting) in highly filled elastomers under consideration of high deformation gradients is examined. The interface between hard (rigid, two-dimensional) inclusions embedded in an elastomer characterized by a three-term Ogden (rate insensitive) model, and the elastomer matrix is represented by a cohesive-zone type interfacial model to follow the whole process of interfacial dewetting and its effect on the global (multiphase) material response in a plane strain setting. The analysis is carried out through a mixed finite element formulation for hyperelasticity, incorporating interface elements. We consider the effects of particle geometry and loading conditions on the process of interfacial failure. The results indicate that the distributed failure process is highly unstable and depends heavily on the size, shape, orientation and interactions of inclusions as well as the global loading conditions. The overall material behavior of the model agrees qualitatively with experimental observation.
Multiaxial Nonlinear Viscoelastic Characterization and Modeling of a Structural Adhesive
Authors: Popelar C. F. , Liechti K. M.
Published: February 2008

Many polymeric materials, including structural adhesives, exhibit a nonlinear viscoelastic response. The nonlinear free volume approach is based on the Doolittle concept that the “free volume” controls the mobility of polymer molecules and, thus, the inherent time scale of the material. It then follows that factors such as temperature and moisture, which change the free volume, will influence the time scale. Furthermore, stress-induced dilatation will also affect the free volume and, hence, the time scale. However, during this investigation dilatational effects alone were found to be insufficient in describing the response of near pure shear tests performed on a bisphenol A epoxy with an amido amine hardener. Thus, the free volume approach presented here has been modified to include distortional effects in the inherent time scale of the material. In addition to predicting the global response under a variety of multiaxial stress states, the modified free volume theory also accurately predicts the local displacement fields, including those associated with a localized region, as determined from geometric moire´ measurements at various stages of deformation.
Axisymmetric Adhesion Tests of Pressure Sensitive Adhesives
Authors: Shull K. R. , Crosby A. J.
Published: February 2008

The fracture mechanics of adhesion tests based on the contact of a flat punch or a spherical indenter with a thin elastic layer are studied. For a flat punch, existing literature data are used to derive semi-empirical analytic expressions for the compliance of the thin film. For spherical indenters, the load-displacement relationship, in addition to values for the compliance, are obtained from finite element analysis. Preliminary experimental results on the adhesion of a spherical indenter to commercially available pressure sensitive adhesives are also described, and the fracture mechanics analysis is placed in the context of these results.
Influence of Inclusion Microgeometry on Some Thermomechanical Properties of Isotropic Polymer-Matrix Composites
Authors: Li J. , Weng G. J.
Published: February 2008

The influence of inclusion shape on some selected thermomechanical properties of isotropic viscoelastic composites is investigated by a micromechanical theory. These properties include: (i) the cyclic stress-strain behavior; (ii) cyclic creep; (iii) the master compliance curve; and (iv) the effective thermal expansion coefficient. It is found that these viscoelastic properties are all strongly dependent upon the inclusion shape. Specifically, under a strain-controlled cyclic loading the transient stress-strain curves of the composites all exhibit cyclic hardening behavior, but the level of flow stress reached is controlled by the inclusion shape. Except for the disk-reinforced case the per-cycle energy loss of the composite at 20 percent of inclusion concentration is found to be greater than the loss of the pure viscoelastic matrix. The complex shear modulus of the composite with various inclusion shapes is shown to lie on or within Milton and Berryman’s bounds (1997). Creep under cyclic stress tends to oscillate around the creep curve under a constant, mean stress for all inclusion shapes, with disks showing the greatest resistance. To uncover the influence of temperature, the creep compliance of the composite with a thermorheologically simple matrix is investigated and it is demonstrated that the compliance curves at various temperatures can all be plotted into a single master one on a reduced time scale. Finally, the effective thermal expansion coefficient of the composite is shown to be generally time-dependent, but the degree of time-dependence is low with spherical inclusions and very high with disks, others lying in-between.
A Presentation and Comparison of Two Large Deformation Viscoelasticity Models
Authors: Govindjee Sanjay , Reese Stefanie
Published: February 2008

In this paper we present a theory of finite deformation viscoelasticity. The presentation is not restricted to small perturbations from the elastic equilibrium in contrast to many viscoelasticity theories. The fundamental hypothesis of our model is the multiplicative viscoelastic decomposition of Sidoroff (1974). This hypothesis is combined with the assumption of a viscoelastic potential to give a model that is formally similar to finite associative elasto-plasticity. Examples are given to compare the present proposal to an alternative formulation in the literature for the cases of uniaxial plane strain relaxation and creep.
A Numerical Study of Cavitation and Yield in Amorphous Polymer-Rubber Blends
Authors: Steenbrink A. C. , Van der Giessen E.
Published: February 2008

A numerical study is presented of cavitation and yield in amorphous polymer-rubber blends in terms of a unit cell model involving an initially voided rubber particle. The particle is described by a non-Gaussian rubber elasticity model, while the glassy matrix is described by a material model featuring time-dependent yield, followed by intrinsic softening and subsequent strain hardening. Large strain, finite element analyses are reported which give a detailed view on the growth of the void in the rubber particle in concurrence with progressive plastic deformation in the matrix. The study focusses on the effect of the rubber particle properties on the growth of the initially cavitated particle. The results indicate that below a certain value of the rubber modulus, the particle behaves as a void, whereas above that value the rubber will tend to increasingly suppress plasticity and void growth.
Modeling Foam Damping Materials in Automotive Structures
Authors: Wagner David A. , Gur Yuksel , Ward Susan M. , Samus Marsha A.
Published: February 2008

Foam damping materials judiciously placed in automotive structures efficiently reduce the vibration amplitudes of large, relatively flat exterior body panels such as the hood, roof, deck lid (trunk) and door skin. These polymer foams (typically epoxy or vinyl) have mechanical properties that depend on the foam homogeneity, degree of expansion, temperature and frequency of excitation. Standard methods for determining true bulk mechanical properties, such as Young’s modulus, shear modulus and damping terms, are discussed along with methods for determining engineering estimates of the properties “as used” in automotive applications. Characterizing these foam damping materials in a component or full body finite element structural model as discrete springs and dashpots provides an accurate and economical means to include these features. Example analyses of the free vibrations and forced response of a hood are presented accompanied by test data that demonstrate the accuracy of the structural model. A parametric study investigates the effect of foam material stiffness and damping properties on hood vibration amplitudes under dynamic air loading. A methodology is discussed to reduce the hood vibration level under cross-wind conditions to an acceptable level with the use of foam materials.
Dynamic Response of Epon 828/T-403 Under Multiaxial Loading at Various Temperatures
Authors: Chen W. , Zhang X.
Published: February 2008

The mechanical response and failure behavior of Epon 828/T-403 epoxy were experimentally obtained over a strain rate range of 1.7 × 10−5 to 4.7 × 103 s−1 and a temperature range of −190 to 100°C. Compressive loads were applied in both uniaxial and multiaxial manners. A modified split Hopkinson pressure bar was employed to perform dynamic experiments, whereas an Materials Test System (MTS810) was used to conduct experiments under quasi-static loading conditions to establish the trends in strain-rate sensitivity. Multiaxial compression was achieved by installing snug-fit steel sleeves on the lateral surfaces of the cylindrical polymer specimens. The effects of temperature were studied by heating/cooling specimens to desired temperatures before mechanical loads were applied. The experimental results show that the yield strength of the polymer increases with increasing strain rate until adiabatic heating offsets the strain rate hardening. Lateral confinement significantly increases the axial yield strength. Increasing temperature reduces both yield strength and Young’s modulus.
Deformation Micromechanics of Regenerated Cellulose Fibers Using Raman Spectroscopy
Authors: Hamad W. Y. , Eichhorn S.
Published: February 2008

It is demonstrated that Raman spectroscopy can be used to study the molecular deformation of TENCELR fibers. It is shown that the peak positions of the 895 cm−1 and 1095 cm−1 Raman bands shift to lower frequency upon the application of tensile stress or strain owing to the macroscopic deformation leading to direct stretching of the cellulose chains. Significant frequency shifts of 1.98 cm−1 and 3.26 cm−1 are obtained for the two Raman-sensitive peak positions, respectively, using the 632.8 nm laser radiation. It is found that the change in position of the Raman band is more closely related to molecular stress than strain, and that the molecular deformation processes for tencel resemble those for other conventional polymers (PET fibers) and high-performance fibers such as aromatic polyamides (aramids).