EFFECT OF ADHESION ON THE FRACTURE-TOUGHNESS OF CALCIUM CARBONATE-FILLED POLYPROPYLENE

The effect of adhesion on the strain energy release rate (G(c)) and Charpy notched impact strength (NIS) of calcium carbonate (CaCO3)-filled polypropylene (PP) at room temperature is investigated over a wide interval of particulate filler volume fractions. The concentration dependence of G(c) and NIS are discussed in terms of competition between the effects of increasing stiffness, decreasing effective matrix cross section, and the transition from a plane strain to a plane stress mode of failure. In all cases the plane stress and plane strain limits of the critical strain energy release rate for initiation of cracks were not affected by the presence of the filler and are the same as those for neat matrix. In the case of no adhesion between components, the size of the crack tip plastic zone increases with increasing filler volume fraction (upsilon(f)) because of the reduction of the material yield strength. In the region 0 < upsilon(f) < 0.12, there is a mixed mode of failure, and the measured value of G(c) for crack initiation increases steadily as the sample cross section approaches a fully plane stress state. The reduction in yield strength also results in the increase in G(c) for crack propagation as reflected by an increase in NIS. Above upsilon(f) = 0.12, the specimen cross section is in a fully plane stress state, and further increase in filler volume fraction (decrease in matrix effective cross section) has the net effect of reducing both G(c) and NIS. In the case of ''perfect'' adhesion, the yield strength increases only slightly with upsilon(f). In the region 0 < upsilon(f) < 0.05 there is also a mixed mode of failure, but the increase in G(c) is much less than that for the no-adhesion case since the size of the plastic zone in front of the crack is much smaller. Above upsilon(f) = 0.05, the combined effects of increasing stiffness, reduction of the size of the plastic zone, and decreasing matrix cross section dominate the behavior, causing a steady reduction in both G(c) and NIS. Good agreement was found between experimental data and calculations based on fracture mechanics principles.