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Digimat Material Model for Short Fiber Reinforced Plastics at Volvo Car Corporation

A pilot study has been done at Volvo Cars Corporation (VCC) to explore the potential of Digimat as material model for car components made of short fiber reinforced thermoplastics. It has been a joint project between Safety and Durability departments. It thus spans several types of analyses using both LS-DYNA, Abaqus and Nastran as finite element solvers. Additionaly, nCode DesignLife is used for evaluation of fatigue. The aim has been to find out if Digimat is a suitable tool for VCC with respect to predictability, material testing and process data requirements, ease of use and computational effort. One component was chosen for the study: a front end carrier (FEC) made of Lanxess Durethan BKV 30, PA6 with 30% glassfiber, and overmolded steel consoles. Four types of component tests have been done: two quasistatic bending loadcases, two dynamic bending loadcases, one thermal creep loadcase and one fatigue loadcase. In the component simulations, the predictability of the model in terms of stiffness, initial failure and progressive failure was examined and compared to alternative models. Furthermore, two fullscale loadcases were run including the component with the Digimat model. These were a pedestrian safety loadcase and a strength loadcase. The objective was then to evaluate the difference in results and simulation time compared to the alternative models. Fiber reinforced plastics are anisotropic due to their microstructure of glass fibers embedded in plastic matrix. Moreover, the macroscopic (anisotropic) material properties in each material point are inherent from the fiber orientations in each point. As the orientation is dependent on the melt flow in the injection process, it needs to be considered somehow to determine the properties in each point. The common way to do this is to simulate the injection process with a specific tool like Moldflow or Moldex. As input to the structural analysis, the orientations of the integration points in the injection mesh are mapped to the integration points in the structural mesh. The Digimat material model is micromechanical and the material properties are defined for the separate phases, fiber and matrix, and the macroscopic response is determined by mean field homogenization. However, in most cases, as a step before structural simulation, the micromechanical parameters are translated to parameters for a macromechanical model with its response dependent on the fiber orientation in each point. For the different analysis types mentioned above, different material models for the polymer matrix are used. For strength, a linear elastic fiber, elastoplastic matrix and a Tsai-Hill 3D transversely isotropic failure indicator, applied at the so-called pseudo grain level, is used. For crash, the matrix is elastoviscoplastic.