Effects of In-situ Stress State on the Plastic Deformation, Fracture, and Size Scaling of Thermoset Polymers and Related Fiber-reinforced Composites

Abstract

This work proposes an investigation on the effects of in-situ stress state on the plastic deformation, fracture, and size scaling of thermoset polymers and their fiberreinforced composites. Towards this end, a comprehensive set of size effect tests on pristine epoxy is presented along with a computational analysis within the framework of Drucker-Prager (DP) plasticity. This analysis includes the following experimental tests on geometrically-scaled specimens representing various loading conditions: (1) uni-axial tension; (2) three-point bending; (3) Mode I fracture; (4) Mode II fracture. The foregoing results clearly show that the mechanical behavior of thermoset polymers is strongly affected by the local stress state since the material at the notch tip can exhibit remarkable ductility with about 70% equivalent plastic strain in Mode II fracture due to deviatoric stresses whereas the pronounced brittleness characterizes the material at the crack tip with about maximum 1% counterpart under Mode I scenario due to hydrostatic stresses. In addition, thermoset polymers also exhibit distinct mechanical behaviors at various length scales since the material strength at the micro scale can be four to six times higher than the macro-scale value from conventional tests whereas the fracture energy at the micro-scale can be forty times lower. These interesting conclusions were further confirmed through the successful microscale damage modeling of crossply [90/0]s and [0/90]s laminates under uni-axial tension and pure shear by leveraging two-scale constitutive model. In fact, the undesirable micro-scale simulations by only considering the macro-scale material properties of thermoset polymers clearly convey the important information of tensorial stresses and multi-scale material properties in fiber-reinforced polymer composites.