On the relevance of a notch creep test for the comprehension and prediction of slow crack growth in PVDF

Modeling slow crack growth (SCG) in polyvinylidene fluoride (PVDF) needs shorter tests than the burst resistance tests usually performed on pipes. In this paper, creep experiments were performed on three-notched extruded and compression-molded specimens. Two distinct regimes were indicated from the kinetics of crack opening displacement (COD). At high temperatures and low stresses, the COD rate decreases until reaching a steady-state stage from which stress sensitivity and activation energy were determined. Investigations of the damage morphology by scanning electronic microscopy (SEM) revealed that the steady-state stage actually corresponded to the process zone thickening. Unlike in polyethylene, the process zone length could not be deduced from the COD measurement and the crack propagation kinetics could not be predicted. Nevertheless, this kind of test appears relevant to indicate the creep resistance of fibrils ahead of the crack tip, which remained a key mechanism in SCG in PVDF also, to predict more rapidly its stress sensitivity and to discriminate materials.