MICROSTRUCTURAL PROCESSES OF FRACTURE OF RUBBER-MODIFIED POLYAMIDES

The microstructural processes of fracture in rubber-modified polyamide s were studied in blends of polyamide 66/ethylene-propylene-diene rubb er (PA66/EPDR) in Izod bending impact experiments. Izod tests were per formed at various temperatures to determine the ductile-brittle transi tion as a function of temperature, rubber weight fraction and particle size. Subsequent analysis of the fracture surfaces by scanning electr on microscopy revealed unique morphologies for various regions of toug hness: in the brittle region, the fracture surface is patchy; in the t ransition region, there are occasional striations present on the fract ure surface, along with the brittle fracture morphology; in the tough region, the fracture surface is fully covered by striations which pene trate only similar to 2 mu m beneath the surface in the form of shallo w cracks perpendicular to the surface. The striations lie parallel to the main crack front and are a signature of the effective toughening o f the polyamide through the incorporation of rubber particles. This dr astic change in the fracture surface morphology and formation of stria tions at the ductile-brittle transition point is the result of elastic -plastic buckling of previously highly stretched material layers on th e crack flank as they undergo a constrained accommodation. Morphologic al studies of the process zone, mainly below the crack flanks, have co nfirmed this hypothesis of buckling of surface layers. The spacing bet ween the striations observed on the fracture surface of tough specimen s varied with rubber content, particle size, temperature and strain ra te. These effects are discussed in terms of local adiabatic heating an d variations in the plastic resistance of the matrix material. A model for striation formation based on plastic buckling of a thin surface l ayer on a porous elastic foundation is provided that serves as a scali ng relation accounting well for the temperature-dependent changes in s triation spacing.