Development of a Composite Material Shell-Element Model for Impact Applications



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To achieve lower weight of vehicles and higher specific energy absorption in crush load cases, automotive companies are moving towards the utilization of composite materials. While the response to loading of traditional engineering materials, such as plastics and steel, is well understood and can be simulated accurately, designers of composite structures still rely heavily on physical testing of components to ensure the requirements of load bearing capabilities are met. The majority of composite material models that have been developed rely on non-physical material parameters that have to be calibrated in extensive simulations. A predictive model, based on physically meaningful input, is currently not available. The here presented material model is a step towards the goal of a truly predictive material model for composite materials. The developed orthotropic material model includes the ability to define tabulated hardening curves for different loading directions with strain-rate and temperature dependency. Strain-rate dependency was achieved by coupling the theories of viscoelasticity and viscoplasticity to allow for rate dependency in both the elastic and plastic regions of the material deformation. In crush and impact load cases the material is loaded beyond its capabilities, and therefore, accurate modeling of damage accumulation and failure is essential. A damage model was implemented, where a reduction of stiffness and stress degradation in the individual material directions can be tracked precisely. Modeling of failure and Finite Element erosion was achieved by implementing a new strain-based generalized tabulated failure criterion, where failure strains can be precisely defined for specific states of stresses. Most components in the automotive industry are thin in comparison to their dimensions, shell (plane stress) elements are usually the Finite Elements of choice in these applications. Composite materials are generally used in a layup of plies with different fiber directions. These individual plies are very thin which leads to impractically small mesh sizes when modeled with three dimensional solid elements. The developed material model is, therefore, made available for shell elements. Composite materials show a large variation in their response to loading; the material model incorporates the ability to define a statistical distribution for certain material parameters that were found to be of influence on a component level. The tabulated nature of the input to the material model allows for the simulation of a large variety of composite materials ranging from fiber reinforced polymers to metal matrix composites. In extensive verification and validation simulations during and after the development process, accuracy and reliability of the model in numerically challenging situations and realistic loading scenarios was ensured. The presented material model can be implemented into most available Finite Element software. As part of this research it was implemented into the commercial Finite Element Solver LS-DYNA as *MAT_COMPOSITE_TABULATED_PLASTICITY_DAMAGE (*MAT_213) for shell elements.