Lifetime of HDPE under stress relaxation at large strains

Slow Crack Growth (SCG) in HDPE under stress relaxation was studied by measuring the time to fracture of notched pipe samples subjected to constant hoop strains. Tests were made in a 2 % Arkopal N-100 tenside solution at various temperatures. The constant deformation was accomplished by pressing the pipe samples onto oversized steel cylinders with diameters giving strain levels ranging from 5 to 50 %. In addition, efforts were made to measure the stress relaxation by applying strain gauges to the steel cylinder wall. Two grades of HDPE were studied—one PE63 grade and one PE100 grade. The two materials were found to behave very differently. Somewhat surprisingly, the PE100 grade generally exhibited shorter fracture times. Furthermore, the time to fracture showed significant temperature dependence but was more or less independent of strain level above the yield point. For the PE63 material, on the other hand, the temperature effect was much less prominent, but a slight strain level dependence was seen. It is likely that the reason for this is differences in the stress relaxation behaviour. As the stress level decreases over time, so does the crack driving potential, and an elevated temperature may not necessarily lead to an overall acceleration of the crack propagation. A frequently used empirical model for the calculation of fracture times was expanded to account for a time dependent stress level. The results indicate that SCG under stress relaxation may be analytically treated in analogy with the more common creep case