Fracture of notched polypropylene specimens under creep conditions

The kinetics of slow crack growth of isotactic polypropylene (Vestolen PP P 9022) was observed in single-edge-notched specimens under a constant tensile load and at a temperature of 95 degrees C. The notch opening under different stress levels was measured as a function of time. Two types of specimens (perpendicular and parallel to the extrusion direction) were examined. The development of the processing zone and the crack advance was observed after unloading at surfaces introduced along and across the crack plane. Optical and scanning electron microscopy was used for these observations. Morphologies of the processing zone as a function of loading time and stress level were assessed. The relationships between (i) stress and time to rupture for ductile and brittle region and (ii)minimal notch opening rate (delta) over dot(o) and stress intensity K-Io were expressed quantitatively. A pronounced effect of the orientation on the crack propagation was established. The time to break of specimens taken along the extrusion direction was about 20 % higher than that of the specimens taken in the cross direction.