Biaxial yielding of polypropylene/elastomeric polyolefin blends: effect of elastomer content and thermal annealing

The yield behavior of commercial homopolymer poly- propylene modified by elastomeric metallocene-cata- lyzed polyolefin blends was investigated by carrying out uniaxial tension, uniaxial compression, plane strain compression, and simple shear mechanical tests. Investigation was performed using specimens machined from isotropic compression molded plates. The onset of yielding was determined by means of the residual strain method. Experimental data was fitted according to the two most popular yield criteria in the polymer field—modified Tresca and modified Von Mises criteria. Both criteria provided reasonable predictions of the yield onset locus despite the tendency of polymers to develop crazes under positive hydrostatic pressure. A generalized yield locus based on the modified Von Mises criterion and the Lazzeri and Bucknall relation- ship was constructed for PP/POE blends. In addition, for one blend composition the effect of the polypropyl- ene matrix crystalline morphology—altered by thermal annealing—was investigated.