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Knowmats is an informal repository of information related to materials and simulation. The information helps simulation professionals perform best-in-class simulation with a better understanding of how materials are represented in FEA and simulation. read more...


Verification and Validation of a Three-Dimensional Generalized Composite Material Model

"A general purpose orthotropic elasto-plastic computational constitutive material model has been developed to accurately predict the response of composites subjected to high velocity impact. The three-dimensional orthotropic elasto-plastic composite material model is being implemented initially for solid elements in LS-DYNA® as MAT213. In order to accurately represent the response of a composite, experimental stress-strain curves are utilized as input, allowing for a more general material model that can be used on a variety of composite applications. The theoretical details are discussed in a companion paper. This paper documents the implementation, verification and validation of the material model using the T800-F3900 fiber/resin composite material."

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Mechanical Plasticity Yielding/Failure analysis Aerospace and Defense Automotive High Speed Testing LS-DYNA Composites Research Papers Validation


A Material Model for Transversely Anisotropic Crushable Foams in LS-DYNA

"Recently new materials were introduced to enhance different aspects of automotive safety while minimizing the weight added to the vehicle. Such foams are no longer isotropic but typically show a preferred strong direction due to their manufacturing process. Different stress/ strain curves are obtained from material testing in different directions. A new material model was added to the LS-DYNA code in order to allow a correct numerical simulation of such materials. Ease-of-use was a primary concern in the development of this user-subroutine: we required stress/ strain curves from material testing to be directly usable as input parameters for the numerical model without conversion. The user-subroutine is implemented as MAT_TRANSVERSELY_ANISOTROPIC_CRUSHABLE_FOAM, Mat law 142 in LS-DYNA Version 960-1106. In this paper we summarize the background of the material law and illustrate some applications in the field of interior head-impact. The obvious advantage of incorporating such detail in the simulation lies in the numerical assessment of impacts that are slightly offset with respect to the foam’s primary strength direction."

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Mechanical Foams Rate Dependency Automotive High Speed Testing LS-DYNA Research Papers


Nonlinear viscoelastic modeling for foams

Lightweight design is one of the major principles in automotive engineering and has made polymer materials to inherent parts of modern cars. In addition to their lightweight thermoplastics, elastomers, fabric and composites also incur important functions in passive safety. In the age of virtual prototyping, assuring these functions requires the accurate modeling of the mechanical behavior of each component. Due to their molecular structure, polymer materials often show viscoelastic characteristics such as creep, relaxation and recovery. However, considering the general state of the art in crash simulation, the viscoelastic characteristics are mainly neglected or replaced by viscoplastic or hyperelastic and strain rate dependent material models. This is either due to the available material models that are often restricted to linear viscoelasticity and thus cannot model the experimental data or due to the time consuming parameter identification. In this study, a nonlinear viscoelastic material model for foams is developed and implemented as a user material subroutine in LS-DYNA. The material response consists of an equilibrium and a non-equilibrium part. The first one is modeled with a hyperelastic formulation based on the work of Chang [8] and formerly implemented as *MAT_FU_CHANG_FOAM in LS-DYNA (*MAT_083). The second one includes the nonlinear viscoelastic behavior following the multiple integral theory by Green and Rivlin [9]. The polyurethane foam Confor CF-45 used as part of the legform impactor in pedestrian safety was chosen for its highly nonlinear viscoelastic properties to test the presented approach. The investigation shows the ability of the method to reliably simulate some important nonlinear viscoelastic phenomena such as saturation.

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Mechanical Foams Visco-elastic Automotive Nonlinear Material Models LS-DYNA Research Papers


Development of Material Input Data for Solid Elements under Crash Loads

"Heavy trucks have large masses and only small deformation zones. Because of this, they are loaded relatively severe in case of a crash. Under those conditions structural response is characterised not only by plastic deformation but also by failure in terms of cracks or fracture. Hence, failure prediction is essential for designing such parts. The following article describes the procedure of generating material models for failure prognosis of solid parts in the Commercial Vehicles Division at Daimler. Sheet metal parts are mostly discretised by shell elements. In this case the state of stress is characterized by hydrostatic pressure over von-Mises effective stress, the so-called triaxiality. For many real-life load cases which can be modeled by thin shells this ratio is between –2/3 and –2/3. Within this range the Gurson material model with the Tvergaard Needlemann addition leads to sufficiently accurate results. Furthermore, the Gurson material model allows considering the effect of element size, which amongst others is important for ductile materials. Most often however, in the case of solid parts the state of stress is more complex, which results in a triaxiality smaller than –1 or larger than 2/3. Gurson material models are usually validated based on shell meshes and tensile tests with flat bar specimen. If applied to solid parts, these models tend to underpredict failure . Thus, for solid parts the GURSON_JC material model is used. The Johnson Cook parameters are derived from an existing Gurson material model. Afterwards the material model is adapted to test results by modifying the load curve giving failure strain against triaxiality. This requires tensile tests"

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Mechanical Metals Rate Dependency Yielding/Failure analysis Automotive High Speed Testing LS-DYNA Research Papers Validation


Experimental and Numerical Investigation of Fracture in Aluminium

"To assess the problem of containment after a blade-off accident in an aero-engine by numerical simulation the FAA has instigated a research effort concerning failure prediction in a number of relevant materials. Aluminium kicked off the program which involved an intensive testing program providing failure data under different states of stress, different strain rates and different temperatures. In particular split Hopkinson bars were used to perform dynamic punch tests on plates of different thicknesses allowing to investigate the transition between different failure modes such as petaling and plugging. Ballistic impact tests were performed at NASA GRC for the purpose of validation. This paper focuses on the numerical simulation effort and a comparison with experimental data is done. The simulations were performed with LS-DYNA and a tabulated version of the Johnson-Cook material law was developed in order to increase the generality, flexibility and user-friendliness of the material model."

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Mechanical Metals Yielding/Failure analysis Aerospace and Defense High Speed Testing LS-DYNA Research Papers Validation


A Constitutive Formulation for Polymers Subjected to High Strain Rates

"Reliable prediction of the behavior of structures made from polymers is a topic under considerable investigation in engineering practice. Especially, if the structure is subjected to dynamic loading, constitutive models considering the mechanical behavior properly are still not available in commercial finite element codes yet. In our paper, we present a new constitutive law for polymers which recovers important phenomena like necking, crazing, strain rate dependency, unloading behavior and damage. In particular, different yield surfaces in compression and tension and strain rate dependent failure, the latter with damage induced erosion, is taken into account. All relevant parameters are given directly in the input as load curves, i.e. time consuming parameter identification is not necessary. Moreover, the models by von Mises and Drucker-Prager are included in the description as special cases. With the present formulation, standard verification test can be simulated successfully: tensile and compression test, shear test and three point bending tests."

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Mechanical Plastics Plasticity Rate Dependency Yielding/Failure analysis Automotive High Speed Testing LS-DYNA Research Papers


A Comparative Review of Damage and Failure Models and a Tabulated Generalization

"Reliable prediction of damage and failure in structural parts is a major challenge posed in engineering mechanics. Although solid material models predicting the deformation behaviour of a structure are increasingly available, reliable prediction of failure remains still open. With SAMP (a Semi-Analytical Model for Polymers), a general and flexible plasticity model is available in LS-DYNA since version 971. Although originally developed for plastics, the plasticity formulation in SAMP is generally applicable to materials that exhibit permanent deformation, such as thermoplastics, crushable foam, soil and metals. In this paper, we present a generalized damage and failure procedure that has been implemented in SAMP and will be available in LS-DYNA soon. In particular, important effects such as triaxiality, strain rate dependency, regularization and non-proportional loading are considered in SAMP. All required physical material parameters are provided in a user-friendly tabulated way. It is shown that our formalism includes many different damage and failure models as special cases, such as the well-known formulations by Johnson-Cook, Chaboche, Lemaitre and Gurson among others. "

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Mechanical Plastics Plasticity Rate Dependency Yielding/Failure analysis Automotive High Speed Testing LS-DYNA Research Papers


Accuracy Issues in the Simulation of Quasi-Static Experiments for the Purpose of Mesh Regularization

Generating a LS-DYNA material model from cupon-level quasi-static experimental data, developing appropriate failure characteristics, and scaling these characteristics to mesh sizes appropriate for a variety of simulation models requires a regularization procedure. During an Investigation of an anisotropic material model for extruded aluminum, numerical accuracy issues led to unrealistic mesh regularization curves and non-physical simulation behavior. Sensitivity problems due to constitutive material behavior, small mesh sizes, single precision simulations, and simulated test velocity all contributed to these accuracy issues. Detailed analysis into the sources of innaccuracy led to the conclusion that in certain cases, double precision simulations are necesscary for accurate material characterization and mesh regularization.

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Mechanical Metals Yielding/Failure analysis Aerospace and Defense Automotive Extrusion Nonlinear Material Models LS-DYNA Research Papers


A Simplified Approach for Strain-Rate Dependent Hyperelastic Materials with Damage

"Simulation of rubber-like materials is usually based on hyperelasticity. If strain-rate dependency has to be considered viscous dampers are added to the rheological model. A disadvantage of such a description is timeconsuming parameter identification associated with the damping constants. In this paper, a tabulated formulation is presented which allows fast generation of input data based on uniaxial static and dynamic tensile tests at different strain rates. Unloading, i.e. forming of a hysteresis, can also be modeled easily based on a damage formulation. We show the theoretical background and algorithmic setup of our model which has been implemented in the explicit solver LS-DYNA [1]-[3]. Apart from purely numerical examples, the validation of a soft and a hard rubber under loading and subsequent unloading at different strain rates is shown."

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Mechanical Rubbers Hyperelastic Rate Dependency Yielding/Failure analysis Automotive High Speed Testing LS-DYNA Research Papers


A semi-analytical model for polymers subjected to high strain rates

"Reliable prediction of the behaviour of structures made from polymers is a topic under considerable investigation in engineering practice. Especially, if the structure is subjected to dynamic loading, constitutive models considering the mechanical behaviour properly are still not available in commercial finite element codes. First, we give an overview of material laws for thermoplastics and show how the behaviour can be characterized and approximated by using visco-elasticity and metal plasticity, respectively. Experimental work is presented to point out important phenomena like necking, strain rate dependency, unloading behaviour and damage. A constitutive model including the experimental findings is derived. In particular, different yield surfaces in compression and tension and strain rate dependent failure, the latter with damage induced erosion, need to be taken into account. With the present formulation, standard verification tests can be simulated successfully. Also, an elastic damage model is used to approximate the unloading behaviour of thermoplastics adequately."

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Mechanical Rate Dependency Yielding/Failure analysis Automotive High Speed Testing LS-DYNA Research Papers