DUCTILE-TO-BRITTLE TRANSITION OF RUBBER-MODIFIED POLYPROPYLENE .2. FAILURE MECHANICS

The deformation behavior of blends of polypropylene (PP) with ethylene-propylene rubber was studied as a function of temperature and composition under tension. The damage ahead of the deliberately introduced defect was traced quantitatively as a function of external load with the aid of intensity analysis. In unmodified PP or in rubber-modified blends, no stable crack growth was obtained up to 99 percent of the maximum stress. A hierarchy of failure events was observed. First, there was a gradual occurrence of a fan-shaped damage zone. Then an intense damage zone initiated. Finally, a slow tearing mode of crack growth occurred at the maximum stress and the sample failed. The size and shape of the damage zone were influenced by temperature, composition and the artificially introduced stress-raiser. Voids were domainant in the fan zone with some crazes close to the intense damage zone. In the intense damage zone, the crazes coalesced to form a network of deformation bands. The description of the fan zone was achieved by a non-linear or elastoplastic failure approach, analogous to the Hilton-Hutchinson formalism, and the analysis of the intense zone by a critical stress instability criterion.