Development of a User-Defined Material Model for Sheet Molding Compounds

The compression molding of Sheet Molding Compounds (SMCs) is typically thought of as a fluid mechanics problem. The simulation of such materials is at present based on the background of compression or injection molded short fiber reinforced materials. The usage of CF-SMC consisting of high fiber volume content (over 50%) and long fiber reinforcement structures (up to 50 mm) challenges the feasibility of this point of view. The goal of this work is the development of a user-defined material model based on a solid mechanics formulation for SMC materials in LS-DYNA®. To allow for large deformations in the simulation an Arbitrary Lagrangian-Eulerian (ALE) approach is used. As a first step, a material characterization is carried out in a so-called press rheometer test where the mechanical behavior of the SMC material is analyzed during the compression molding process. The resulting stress response of the material then serves as input information for the material model. The material model itself is based on a modular building-block approach. The individual modules describe certain aspects of the material behavior (e.g. compaction, plastic flow behavior or fiber orientation) and interact with one another through the passing of parameters between the respective modules. This procedure allows for the flexible development of the mathematical description for each part of the material behavior. Initially, a simple mathematical model describes every module. In the further development of the model, each module is expanded by more complex mathematical descriptions. As the overall goal is a work in progress, this paper shows the current implementation of several of these modules including the characterized compression and flow behavior as well as a description for the fiber orientation based on the Folgar-Tucker equation. By simulating the press rheometer test itself using the developed user defined material model, a comparison between simulation and experiments is performed to check the accuracy of the various mathematical models used. The stress response and the flow front development provide the basis for the comparison and provide clues on how to proceed with the further development.