Gunther Blankenhorn

Development of an Orthotropic Elasto-Plastic Generalized Composite Material Model Suitable for Impact Problems

The need for accurate material models to simulate the deformation, damage, and failure of polymer matrix composites under impact conditions is becoming critical as these materials are gaining increased usage in the aerospace and automotive industries. There are a variety of material models currently available within commercial transient dynamic finite-element codes to analyze the response of composite materials under impact conditions. However, there are several features that are lacking in the currently available models that could improve the predictive capability of the impact simulations. To address these needs, a combined elasto-plastic model with damage suitable for implementation within transient dynamic finite-element codes has been developed. A key feature of the improved material model is the use of tabulated stress-strain data in a variety of coordinate directions to fully define the stress-strain response of the material. Currently, the model development efforts have focused on creating the plasticity portion of the model. A commonly used composite failure model has been generalized and extended to a strain-hardening-based orthotropic yield function with a non-associative flow rule. The coefficients of the yield function are computed based on the input stress-strain curves using the effective plastic strain as the tracking variable. The coefficients of the flow rule are determined in a systematic manner based on the available stress-strain data for the material. The evolution of the yield surface is examined, in detail, for a sample composite. A numerical algorithm based on the classic radial return method is employed to compute the evolution of the effective plastic strain. A specific laminated composite is examined to demonstrate the process of characterizing and analyzing the response of a composite using the developed model. The developed material model is suitable for use within commercial transient dynamic finite-element codes for use in analyzing the nonlinear response of polymer composites.

Using Virtual Tests to Complete the Description of a Three-Dimensional Orthotropic Material

AbstractGeneralized constitutive models, which can be used in explicit finite-element analysis, are being developed to accurately model composite systems under impact conditions. These models requi...

Experimental characterization of composites to support an orthotropic plasticity material model

Test procedures for characterizing the orthotropic behavior of a unidirectional composite at room temperature and quasi-static loading conditions are developed and discussed. The resulting data consisting of 12 stress–strain curves and associated material parameters are used in a newly developed material model—an orthotropic elasto-plastic constitutive model that is driven by tabulated stress–strain curves and other material properties that allow for the elastic and inelastic deformation model to be combined with damage and failure models. A unidirectional composite—T800/F3900, commonly used in the aerospace industry, is used to illustrate how the experimental procedures are developed and used. The generated data are then used to model a dynamic impact test. Results show that the developed framework implemented into a special version of LS-DYNA yields reasonably accurate predictions of the structural behavior.

Implementation of a tabulated failure model into a generalized composite material model

The need for accurate material models to simulate the deformation, damage, and failure of polymer matrix composites under impact conditions is becoming critical as these materials are gaining increased use in the aerospace and automotive communities. To attempt to improve the predictive capability of composite impact simulations, a next generation material model is being developed for incorporation within the commercial transient dynamic finite element code LS-DYNA. The material model, which incorporates plasticity, damage, and failure, utilizes experimentally based tabulated input to define the evolution of plasticity and damage and the initiation of failure as opposed to specifying discrete input parameters such as modulus and strength. The plasticity portion of the composite constitutive model is based on an extension of the Tsai-Wu composite failure model into a generalized yield function. For the damage model, a strain equivalent formulation is used to allow for the uncoupling of the deformation and damage analyses. For the failure model, a tabulated approach is utilized in which a stress- or strain-based invariant is defined as a function of the location of the current stress state in stress space to define the initiation of failure. Failure surfaces can be defined with any arbitrary shape, unlike traditional failure models where the mathematical functions used to define the failure surface impose a specific shape on the failure surface. In the current paper, the complete development of the failure model is described and the generation of a tabulated failure surface for a representative composite material is discussed.

Damage characterization of composites to support an orthotropic plasticity material model

The focus of this paper is the development of test procedures to characterize the damage-related behavior of a unidirectional composite at room temperature and quasi-static loading conditions and use the resulting data in the damage sub-model of a newly developed material model for orthotropic composites. This material model has three distinct sub-models to handle elastic and inelastic deformations, damage, and failure. A unidirectional composite—T800/F3900 that was the focus of our previous work, is used to illustrate how the deformation and damage-related experimental procedures are developed and used. The implementation of the damage sub-model into LS-DYNA is verified using single-element tests and validated using impact tests. Results show that the implementation yields reasonably accurate predictions of impact behavior involving deformation and damage in structural composites.

Analysis and Characterization of Damage Utilizing an Orthotropic Generalized Composite Material Model Suitable for Use in Impact Problems

The need for accurate material models to simulate the deformation, damage and failure of polymer matrix composites under impact conditions is becoming critical as these materials are gaining increased usage in the aerospace and automotive communities. In order to address a series of issues identified by the aerospace community as being desirable to include in a next generation composite impact model, an orthotropic, macroscopic constitutive model incorporating both plasticity and damage suitable for implementation within the commercial LS-DYNA computer code is being developed. The plasticity model is based on extending the Tsai-Wu composite failure model into a strain hardening-based orthotropic plasticity model with a non-associative flow rule. The evolution of the yield surface is determined based on tabulated stress-strain curves in the various normal and shear directions and is tracked using the effective plastic strain. To compute the evolution of damage, a strain equivalent semi-coupled formulation is used in which a load in one direction results in a stiffness reduction in multiple material coordinate directions. A detailed analysis is carried out to ensure that the strain equivalence assumption is appropriate for the derived plasticity and damage formulations that are employed in the current model. Procedures to develop the appropriate input curves for the damage model are presented and the process required to develop an appropriate characterization test matrix is discussed.