Choice Of Material Model Research Articles

Physics-based and phenomenological plasticity models for thermomechanical simulation in laser powder bed fusion additive manufacturing: A comprehensive numerical comparison

The present study investigated the sensitivity of material constitutive models on thermomechanical responses in laser powder bed fusion additive manufacturing of Ti-6Al-4V. Uniform scan strategies with scan lengths of 0.5, 1, and 2 mm were applied so that wide ranges of thermal histories could be generated. The Johnson-Cook (JC) and Mechanical Threshold Stress (MTS) material plasticity models were chosen to capture the influence of strain, strain rate, and temperature. The JC model is a phenomenological model which is known for its easy implementation and excellent agreement with material testing results. On the other hand, the MTS model is a more complex physics-based internal state variable plasticity model that is expected to provide more accurate estimation, particularly for cases involving changes in strain rate and temperature. Numerical results revealed that both JC and MTS models provided a similar stress evolution, however, the plastic strain evolution was more realistic using the MTS model. Moreover, it was found that the maximum strain and the strain rate in the LPBF process are high compared to typical quasi-static testing, i.e., ~ 2% and ~ 4 s−1, respectively. Accordingly, the material models should be calibrated with data obtained under similar deformation conditions. The choice of scan length also strongly affects in-plane stress anisotropy. Ultimately, we show both qualitatively and quantitatively the dependency of mechanical behavior prediction in LPBF on the choice of material models.

Simulations of laser-driven metal ejecta

Results reveal how the choice of material model affects microjets simulations, and that jet formation can be classified into three regimes depending on the shock strength.

Interpretation of the effect of hydrostatic test in components working under pressure

After construction and installation, the natural gas and oil transport structures must undergo hydrostatic tests before their installation and periodically during commissioning. These hydrostatic tests can be classified into two categories: resistance test and leak proof test. The application of an overload is a spectacular phenomenon and can be used as an example while testing components working under pressure. This is a destructive control method. If a critical fault, with respect to the proof test, exists in the pipeline, the latter is destroyed. If not, (i.e., not broken), the lower than critical defects will see their subsequent propagation by fatigue slowed down or even eliminated due to the effect of overload. Tow-dimensional finite element method is utilized to analyze the plasticity-induced crack closure phenomenon in a cracked pipe under variable amplitude loading conditions. To accurately capture this process the choice of material model employed is of significant importance, therefore this paper considers the Lemaitre-Chaboche model, with isotropic hardening and nonlinear kinematic hardening. The study shows significant results obtained while employing this model.

Inverse finite element characterization of the human thigh soft tissue in the seated position.

Pressure ulcers are localized damage to the skin and underlying tissues caused by sitting or lying in one position for a long time. Stresses within the soft tissue of the thigh and buttocks area play a crucial role in the initiating mechanism of these wounds. Therefore, it is crucial to develop reliable finite element models to evaluate the stresses caused by physiological loadings. In this study, we compared how the choice of material model and modeling area dimension affect prediction accuracy of a model of the thigh. We showed that the first-order Ogden and Fung orthotropic material models could approximate the mechanical behavior of soft tissue significantly better than neo-Hookean and Mooney-Rivlin. We also showed that, significant error results from using a semi-3D model versus a 3D model. We then developed full 3D models for 20 participants employing Ogden and Fung material models and compared the estimated material parameters between different sexes and locations along the thigh. We showed that males tissues are less deformable overall when compared to females and the material parameters are highly dependent on location, with tissues getting softer moving distally for both men and women.

Femoral fracture load and fracture pattern is accurately predicted using a gradient-enhanced quasi-brittle finite element model

Nonlinear finite element (FE) modeling can be a powerful tool for studying femoral fracture. However, there remains little consensus in the literature regarding the choice of material model and failure criterion. Quasi-brittle models recently have been used with some success, but spurious mesh sensitivity remains a concern. The purpose of this study was to implement and validate a new model using a custom finite element designed to mitigate mesh sensitivity problems. Six specimen-specific FE models of the proximal femur were generated from quantitative tomographic (qCT) scans of cadaveric specimens. Material properties were assigned a-priori based on average qCT intensities at element locations. Specimens were experimentally tested to failure in a stumbling load configuration, and the results were compared to FE model predictions. There was a strong linear relationship between FE predicted and experimentally measured fracture load (R2 = 0.79), and error was less than 14% over all cases. In all six specimens, surface damage was observed at sites predicted by the FE model. Comparison of qCT scans before and after experimental failure showed damage to underlying trabecular bone, also consistent with FE predictions. In summary, the model accurately predicted fracture load and pattern, and may be a powerful tool in future studies.

An empirical measure of nonlinear strain for soft tissue indentation.

Indentation is a primary tool in the investigation of the mechanical properties of very soft tissue such as the brain. However, the usual material characterization protocols are not applicable because the resulting deformation is inhomogeneous, with even the identification of the amount of strain ambiguous and uncertain. Focusing on spherical indentation only, a standard is needed to quantify the amount of strain in terms of the probe radius and displacement so that different indentation experiments can be compared and contrasted. It is shown here that the minimum axial value of the Eulerian logarithmic strain tensor has many desirable properties of such a standard, such as invariance under the choice of material model, and experimental conditions for a given probe displacement. The disadvantage of this measure is that sophisticated finite element techniques need to be used in its determination. An empirical relation is obtained between this strain and the probe radius and displacement to circumvent this problem, and it is shown that this relationship is an excellent predictor of the strain measure. Two essential features of this empirical measure for nonlinear strains are that the exact strain measure for the linear theory is recovered on restriction to infinitesimal deformations and that the simulations use models based on reliable and accurate indentation data obtained from freshly harvested murine brains using a bespoke micro-indentation device.

On the Correlation of FEM and Experiments for Hyperelastic Elastomers

Correlation of modern finite element methods (FEM) with advanced experimental techniques for elastomers, biomedical materials, and living organs requires study and modification of the behavior of these materials. In this study, the mechanical behavior of a commonly-used elastomer, silicone rubber, which provides excellent biocompatibility, was examined under different applied loading configurations, and large deformations were investigated through both experiment and simulation. The stress-strain behaviors of silicone rubber were tested, using multiple homogeneous experiments, including uniaxial extension and equibiaxial tension, the load-apex displacement response, and digitized deformed shapes of two of the most-used structures for nonlinear hyperelasticity—the inflation of a clamped circular membrane, and indentation of the membrane by a spherical indenter. Uniaxial and equibiaxial data were evaluated simultaneously, characterized by various constitutive models for implementation in the FE simulation. These constitutive models examined the prediction of the FE simulations for the inflation and indentation tests in comparison to the results of experiments at various load-apex displacement levels. The results showed that the constitutive models calibrated with the uniaxial and equibiaxial tests, predicted nearly the same results as the actual experimental results, particularly for the applied loads that generated moderate strain. However, when the FE simulations based on the constitutive models were adjusted, employing only uniaxial or equibiaxial tests, they predicted different results, where the degree of their correlations with experimental results was incomplete or in some states simply poor. The simulations suggested that the inverse FE procedure should not be restricted to the choice of material models, while more attention should be given to the choice of ranges of deformation.

Microstructural residual stress in particle-filled dental composite

The main goal of this study is to develop a micromechanical model of a particle-filled dental composite focused on the residual stress (RS) field developed during the curing process in its microstructure. A finite element model of a representative volume element of filler and resin was developed, and volumetric shrinkage was simulated during the curing process. Four material models (von Mises plasticity model, Drucker–Prager plasticity model, von Mises plasticity model with stress relaxation and Drucker–Prager plasticity with stress relaxation) of the polymer resin were built to assess the influence of the material model on the resulting internal stress. The relationship between the curing process and the magnitude of the stress components will be described, and an analysis of the post-curing state of the material in particular microstructure locations will be conducted in this study. Obtained RS is comparable to the stresses developed in the material under the external load. The substantial dependence on the choice of material model for resin is to be observed, and the suitability of particular models is discussed.

Viscoelastic behavior of polymer tape in a wound roll

AbstractA viscoelastic computational model is developed that uses experimentally determined viscoelastic material properties as input and can be used to predict the behavior of a tape material in a wound roll as stresses relax over time. Experimental creep test results are used to find best‐fit creep‐compliance parameters to describe two high density data storage tape media. The two tapes used in the analysis are a developmental tape with a poly(ethylenenaphthalate) (PEN) substrate and metal particle (MP) front coat similar to linear tape open (LTO4) (referred to in this work as “Tape C”), and LTO3, a commercially available tape with a PEN substrate and MP front coat. Sets of best‐fit creep‐compliance parameters are determined for both tapes. The differences between the predicted behavior using three‐, five‐, and seven‐parameter Kelvin–Voigt models are evaluated, both for a benchmark case and in a viscoelastic wound roll model. The choice of material model is found to significantly influence the predictions of the wound roll model. The differences between different material models for the same material are on the order of the differences found between the two different materials. A material model with a higher number of creep‐compliance parameters, although more computationally expensive, produces better results, particularly over long spans of time. The relative differences between the three‐, five‐, and seven‐parameter models are shown to be qualitatively consistent for several variations in the computational model setup, allowing predictions to be made based on simple benchmarks. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Deformation Histories Relevant to Multipass Girth Welds: Temperature, Stress and Plastic Strain Histories

Residual stresses predicted from welding simulations are known to be sensitive to the choice of material model. Recent work has shown large differences in the predicted residual stress profile when different types of hardening model are used (isotropic, kinematic or mixed models). More information is required regarding the exact deformation conditions that exist during welding. We consider finite element models of two multi-pass, stainless steel girth welds. Temperature, stress and plastic strain histories are recorded in the weld material and in the heat affected region (HAZ). Material in the weld and HAZ is observed to undergo three to five cycles of active plastic deformation followed by thermal cycling that is purely elastic. The stress state varies from biaxial compression to triaxial tension. These deformation histories are used as the basis for a discussion of the formulation of a suitable material testing schedule and subsequent constitutive modelling.

3D modelling of plasticity induced fatigue crack closure-effect of material constitutive relations

Abstract 3D finite element method is utilized to analyze the plasticity-induced crack closure (PICC) phenomenon in a cracked plate under constant-amplitude cyclic loading. To accurately capture the PICC process the choice of material model employed is of significant importance. This paper considers a relatively new model, the Ellyin–Xia elastic–plastic constitutive relations, and the more widely used kinematic hardening model. The study shows considerable difference in the results obtained while employing the two material models. Experimental results support the predictions by the Ellyin–Xia material model.

Low-cycle deformation of the structures in a nuclear power plant and methods for calculating them

It is proposed that the fatigue strength of structures in nuclear power plants with a quite long service life be analyzed in two stages: first, the type and location of the process and the effect of basic factors are determined by method of direct analysis of stable cycles and then a more accurate stepped analysis of the deformation kinetics is performed. The real properties of a material are systematized, taking account of creep, in ways that depend on the different types of processes. The methods for calculating the conditions and mechanisms of the progressing shape change and alternating-sign flow are refined. An example of the instability of the processes of cyclic deformation, which limits the possibility of operating structures beyond the limits of elastic adaptability, is examined. The possibilities and conditions for algorithmitization of the choice of material models and ensuring computational accuracy in calculations of deformation kinetics are discussed.

A Simplified Treatise of the Scott Bond Testing Method

The Scott bond test method has been used extensively in the paper industry over the years as a means to assess the bond strength of paper. The method has been a subject of some controversy lately since it does not always correlate to the sensitivity of the material to fracture by delamination. To gain some further insight into which parameters govern the fracture process in a Scott bond test a simplified approach has been chosen in order to formulate an analytical mathematical/mechanical model of the test. The model is dynamic in the sense that inertia effects are included. The material model utilised is a simple cohesive theory that assumes a linear behaviour between stress and crack opening when the material has started to degrade. This choice of material model makes the mathematical model very nonlinear. In fact, a system of three coupled nonlinear second order partial differential equations have to be solved and adjusted to the correct initial conditions. The material parameters needed for the model are the elastic modulus in the thickness direction, the transverse shear (elastic) modulus, the tensile strength (in the thickness direction) and the fracture work (per unit area) for a delamination crack. To investigate the ability of the model, a Scott bond testing apparatus have been equipped with a piezoelectric load sensor. The load cell was mounted on the apparatus’ pendulum so that the load acting on the sample holder could be recorded during the whole impact stage. This was done for a number of different initial velocities of the pendulum and it is found that the model gives a fair prediction of the contact load.

Predicting Knee Replacement Damage in a Simulator Machine Using a Computational Model With a Consistent Wear Factor

Wear of ultrahigh molecular weight polyethylene remains a primary factor limiting the longevity of total knee replacements (TKRs). However, wear testing on a simulator machine is time consuming and expensive, making it impractical for iterative design purposes. The objectives of this paper were first, to evaluate whether a computational model using a wear factor consistent with the TKR material pair can predict accurate TKR damage measured in a simulator machine, and second, to investigate how choice of surface evolution method (fixed or variable step) and material model (linear or nonlinear) affect the prediction. An iterative computational damage model was constructed for a commercial knee implant in an AMTI simulator machine. The damage model combined a dynamic contact model with a surface evolution model to predict how wear plus creep progressively alter tibial insert geometry over multiple simulations. The computational framework was validated by predicting wear in a cylinder-on-plate system for which an analytical solution was derived. The implant damage model was evaluated for 5 million cycles of simulated gait using damage measurements made on the same implant in an AMTI machine. Using a pin-on-plate wear factor for the same material pair as the implant, the model predicted tibial insert wear volume to within 2% error and damage depths and areas to within 18% and 10% error, respectively. Choice of material model had little influence, while inclusion of surface evolution affected damage depth and area but not wear volume predictions. Surface evolution method was important only during the initial cycles, where variable step was needed to capture rapid geometry changes due to the creep. Overall, our results indicate that accurate TKR damage predictions can be made with a computational model using a constant wear factor obtained from pin-on-plate tests for the same material pair, and furthermore, that surface evolution method matters only during the initial "break in" period of the simulation.

The effect of material model in describing mechanism of plasticity-induced crack closure under variable cyclic loading

Three-dimensional finite element method is utilized to analyze the plasticity-induced crack closure (PICC) phenomenon in a cracked plate under variable amplitude loading conditions: the effect of a single spike tensile overload, and also the case of a single compressive overload (underload). To accurately capture the PICC process the choice of material model employed is of significant importance, therefore this paper considers a relatively new model, the Ellyin-Xia model, and the more widely employed kinematic hardening model. The study shows significant difference in the results obtained while employing the two models.

3-D modelling of cyclically loaded composite patch repair of a cracked plate

A three-dimensional finite element model is utilized to capture the effects of plasticity-induced crack closure (PICC) on an adhesively bonded patch repair of a cracked plate. To accurately capture the PICC process the choice of material model employed is of significant importance, therefore this paper considers a relatively new model, the Ellyin–Xia model. The paper shows that the PICC phenomenon is beneficial to the repair.

Ballistic limit prediction using a numerical model with progressive damage capability

The ultimate objective of this study is to provide further understanding of the behaviour of laminated composites of varying lamina orientations and stacking sequences, when under high-velocity impact. Emphasis is placed on the determination of ballistic limits of these composites. To this end, an experimental program is carried out and a computational model, with progressive damage modeling capabilities, is developed using LS-DYNA. Experiments are performed whereby striking velocities are measured, via high-speed photography, to determine the ballistic limits of carbon fiber-reinforced polymer (CFRP) laminates of various stacking sequences. The results are reproduced closely by a numerical simulation, indicating that the numerical analysis conducted, including the choice of material model and contact definition, is an accurate means for modeling the high-speed impact characteristics of CFRP laminates. It is found that the use of static elastic and strength properties to describe the material is reasonable, since strain rate effects are found to be negligible. The kinetic energy of the projectile, plotted over the simulated impact duration, is used as the prime parameter to compare the experimental and numerical results. The numerical results accurately predict the experimental ballistic limit for six of the seven tested laminate stacking sequences. Failure due to delamination is found to play a vital role with respect to the energy absorbing ability and lamina stacking sequence of CFRP laminates.

Concrete Subjected to Triaxial Stress States: Application to Pull-Out Analyses

This paper presents the application of three-dimensional (3D) -constitutive models for concrete formulated in the framework of plasticity theory to structural analyses of anchor devices. For this purpose, two commonly employed concrete material models are considered. The first model, the extended Leon model, is based on one yield surface for the description of compressive and tensile failure of concrete. The second material model is a multisurface plasticity model consisting of three Rankine yield surfaces and a Drucker-Prager yield surface. The predictive capability of the models is demonstrated by means of anchor devices, commonly employed in structural engineering for the connection of steel and concrete members. Such devices induce strongly nonuniform triaxial stress states in the surrounding concrete, ranging from tensile, overcompressive, to confined compressive stress states. In the vicinity of the anchor head, even nearly hydrostatic stress states may occur. The numerical simulations on the basis of the employed 3D material models for concrete give insight into the load-carrying behavior of the investigated anchor devices. Two headed studs characterized by different shapes of the anchor head and an undercut anchor are considered. Comparison of the peak loads and failure modes of the respective anchor device predicted by the numerical models with experimental data highlight the strength and weakness of the employed material models. It is shown that some load cases may lead to rather large differences in peak load depending on the choice of material model. These differences are based on the individual properties of the constitutive models for concrete and, hence, detailed knowledge of the model under consideration is essential for giving accurate estimates of the peak load of the anchor device and the failure mode of concrete.

Comparison of elastic-plastic and rigid-plastic implicit FEM simulations in sheet forming applications

A general formulation for numerical finite element simulation of complex sheet forming processes with arbitrary tools is briefly introduced. An incremental displacement (“solid”) approach is assumed. Special attention is drawn to the choice of material models. The performance of the reviewed elastic-viscoplastic and rigid-viscoplastic algorithms is documented and the numerical results are compared to the experimental measurements. Test problems as well as deep drawings with complex irregular die geometries are simulated. Direct comparisons of the results obtained for identical tool-sheet sets from elastic-plastic and rigid-plastic models are presented and discussed. Provided that the overall numerical model is properly derived, it is shown that both approaches are accurate for many typical applications in sheet forming. Consequently, additional criteria to aid in the choice of material models are proposed and discussed. These include robustness, efficiency, and type of resulting information sought.