DEFORMATION AND FRACTURE OF STYRENE-ACRYLONITRIL COPOLYMER-RUBBER BLENDS - MICROSCOPY STUDIES OF DEFORMATION ZONES

A styrene-acrylonitril copolymer (SAN) was toughened by SAN-grafted po lybutadiene core-shell rubber particles. Notched tensile specimens wer e fractured with a tensile speed ranging from 10(-4) to 10 m s(-1). Th e deformation processes close to the fracture surface were studied by means of transmission electron microscopy. A marked difference in the structure of the deformation zone was observed between low speed (10(- 3) m s(-1)) and high speed (greater than or equal to 1 m s(-1)) deform ed samples. At low tensile speed the structure of the deformation zone correlated closely with fracture mechanics theory. When the tensile s peed was increased the deformation zone had a layered structure. In th e zone 400-1.5 mu m below the fracture surface the deformation structu re was similar to that at low speed. In the layer 1.5-0.5 mu m from th e fracture surface the rubber particles were strongly deformed, but no cavities or crazes could be observed: Directly next to the fracture s urface the high speed deformation zone showed a small layer (0.5 mu m) where all the deformation had vanished. it is suggested that due to h igh strain-rate plasticity at the crack tip a temperature rise occurs which is high enough to cause complete relaxation of the deformation i n this layer. Therefore, locally the glass transition temperature of t he matrix material was reached. The interaction between thermal effect s and deformation processes at the crack tip is discussed.