The toughening mechanisms of blends of poly(acrylonitrile-butadiene-styrene) copolymer (ABS) and bisphenol-A polycarbonate (PC) have been studied by three point bending of round notched bars under plane strain state. The toughness of PC was decreased by adding small amounts of ABS particles, because the voids around the particles of ABS accelerate the development of the internal crazes. For further increase in ABS content, however, the toughness in the polymer blends was improved by the relaxation of stress concentration which increases the stability of craze to the catastrophic crack propagation. Above a certain critical content, which can be estimated from the critical stress for internal craze nucleation and shear yield stress, the shear plastic deformation occurred only in the matrix materials without the nucleation of crazes. It is concluded that the relaxation of stress concentration due to the formation of numerous voids is basic mechanism for the toughening of polymer blends. The energy dissipation by both crazing and shear deformation results from this relaxation of stress. © 1990.
