Microstructural processes of fracture of rubber-modified polyamides

Abstract

The microstructural processes of fracture in rubber-modified polyamides were studied in blends of polyamide 66/ethylene—propylene—diene rubber (PA66/EPDR) in Izod bending impact experiments. Izod tests were performed at various temperatures to determine the ductile—brittle transition as a function of temperature, rubber weight fraction and particle size. Subsequent analysis of the fracture surfaces by scanning electron microscopy revealed unique morphologies for various regions of toughness: in the brittle region, the fracture surface is patchy; in the transition region, there are occasional striations present on the fracture surface, along with the brittle fracture morphology; in the tough region, the fracture surface is fully covered by striations which penetrate only ≈2 μm beneath the surface in the form of shallow cracks perpendicular to the surface. The striations lie parallel to the main crack front and are a signature of the effective toughening of the polyamide through the incorporation of rubber particles. This drastic change in the fracture surface morphology and formation of striations at the ductile—brittle transition point is the result of elastic—plastic buckling of previously highly stretched material layers on the crack flank as they undergo a constrained accommodation. Morphological studies of the process zone, mainly below the crack flanks, have confirmed this hypothesis of buckling of surface layers. The spacing between the striations observed on the fracture surface of tough specimens varied with rubber content, particle size, temperature and strain rate. These effects are discussed in terms of local adiabatic heating and variations in the plastic resistance of the matrix material. A model for striation formation based on plastic buckling of a thin surface layer on a porous elastic foundation is provided that serves as a scaling relation accounting well for the temperature-dependent changes in striation spacing.