Microstructural fracture processes accompanying growing cracks in tough rubber-modified polyamides

Abstract

The process of massive cavitational deformation in an ethylene—propylene—diene grafted maleic-anhydride modified polyamide 66, initiated from cavitating rubber particles and associated with stable crack propagation in compact tension and Izod impact experiments, has been studied in the region ahead of the crack front by scanning electron microscopy. The very tough stable ductile tearing behaviour of the material was manifested by steady-state crack opening angles of 60° and large stress-whitened zones. The process zone ahead of the advancing crack comprised two regions with distinct morphologies: (a) within a radius of 30 μm in an ‘intense’ deformation zone in front of the crack, the cavities are elongated to large draw ratios (≈ 10); and (b) in the remaining ‘weak’ zone cavities were found to be substantially equiaxed. The material with the elongated cavities of the crack front is parted by the propagating crack and immediately sheared by ≈ 60° and left lying parallel to the crack flank surfaces. The very tough behaviour of the material has been analysed by the well known mechanics of fracture of tough structural materials, and the calculated tearing moduli were found to be quite comparable with those of the toughest structural steels.