Fatigue-Crack Propagation of High-Density Polyethylene Homopolymers: Influence of Molecular Weight Distribution and Temperature

Abstract

The present study focuses on the influence of the molecular weight distribution (MWD) on the crack-growth kinetics of fatigue-crack propagation in high-density polyethylene (HDPE) homopolymers. Compact-tension specimens of HDPE homopolymer grades, with polydispersities ranging from 2 to 45 and weight-averaged molar mass ranging from 49 to 450 kg/mol, are tested in cyclic loading at temperatures ranging between 23 and 92 °C. Through a variation of sample thickness, linear elastic-fracture mechanics is shown to apply for the chosen geometry (compact tension). It was found that the crack-propagation kinetics obey the Paris–Erdogan law, for which the Paris–Erdogan exponent m is (highly) similar for all grades tested (m = 3.9), implying that the Paris–Erdogan prefactor A is the governing parameter for the crack-growth kinetics. Relatively poor correlations are observed when the prefactor A is plotted as a function of both the tie-molecule fraction derived from the theoretical model by Huang and Brown, J. Mater. Sci. 1988, 23, 3648, and the average number of effective physical cross-links per chain as derived by Tervoort et al., Macromolecules 2002, 35, 8467. A far better correlation is observed between prefactor A and the weight-average molecular weight (Mw), which improved further when Mw is corrected for the width of the MWD, taking into account the z-average molecular weight Mz, through the ratio Mz/Mw. A power-law correlation of prefactor A with Mw and the width-corrected Mw reveals slopes of −3.4 and −3.3, respectively. Because a molecular slip within the fibrils would require chain transport through the crystalline blocks, the temperature dependence of the fatigue-crack-growth kinetics is investigated to identify the underlying molecular processes. This investigation reveals the existence of a high-temperature and a low-temperature deformation process, both of which can be related to chain-slip mechanisms through their respective activation energies (125 and 50 kJ/mole), as their activation energies are considerably lower than that required for chain scission (430 kJ/mol). This, combined with the power-law exponent of −3.4, would suggest a possible connection between the underlying failure mechanisms of craze fibrils and reptation-like dynamics. Furthermore, experiments at elevated temperatures on a selection of homopolymer grades suggest that the MWD has no influence on the temperature dependence of fatigue-crack propagation for HDPE homopolymers.