Application of the stress interaction failure criterion in platelet composite compression molded parts

Abstract

The stress interaction failure criterion was applied to three compression‐molded sheet molding compound (SMC) materials. Two are epoxy‐based compounds with randomly oriented carbon fiber tows, and the third material consists of glass fiber bundles randomly oriented in a vinyl ester resin. The failure surface was built using tensile test results at different orientations with respect to the flow direction, resulting in combined loads in the principal stress coordinate system. Three unknown variables arise from using only tensile strength values. Solving a system of three equations derived from the criterion function and using the failure values obtained from destructively testing at three different orientations, the unknown variables as well as the interaction strength components were determined. Results show good agreement for all three materials. Overprediction was found in some instances, attributed to the effect of fiber orientation distribution in the part, and to the complex interactions of the filled structures. To allow a more direct comparison between predicted and experimental data, the resulting failure surface was translated to the global coordinate system (σxx) and compared to the data derived from destructive tensile tests. Good agreement for all materials was observed, with underprediction noted for some orientations, attributed in part to the equation solving method and the nature of the mesostructures of the composites, where interactions between the tows, resin, mold walls, and fibers are present. The results of this work show that the methodology described permits the development of a failure surface for SMC materials when limited data is available.