Effect of different morphologies on slow crack growth of high-density polyethylene

Abstract

With the wide application of polymer pipes in the world, people pay more attention to the longevity of the pipes. Slow crack growth is one of the most important factors that influence the longevity of materials, especially PE pipes. Thus, the study of slow crack growth has become deep and concrete. In this paper, slow crack growth in specimens with different morphologies was studied. A self-designed oscillatory shear injection molding (OSIM) device was utilized to prepare pure HDPE 5000s specimens with different morphologies. The process of slow crack growth in OSIM specimens was compared with that in specimens prepared by conventional injection molding (CIM); so it was possible to study the influence of the different morphologies in specimens on the process of slow crack growth under the same external conditions. Different morphologies have great influences on the ability of materials to resist slow crack growth. The time taken for the slow crack growth to fracture in the OSIM specimens is considerably longer than that in the conventional ones. Under the same conditions of 80 °C and 9.0 MPa, the time of complete fracture is 650 min in OSIM specimens, in contrast to 60 min in conventional specimens. Moreover, in the OSIM specimens, the initial stress of the brittle–ductile transition is increased. At 80 °C, the brittle–ductile transition takes place at 4.7 MPa for conventional specimens, whereas for OSIM specimens, it occurs at 5.6 MPa. Furthermore, the mechanisms of their slow crack growth processes are different. The SCG process in conventional specimens is that stress concentration initiates craze, which grows rapidly and becomes a crack. However, the process in OSIM specimens is a cyclic process, in which stress concentration initiates craze, the crack grows, a new craze appears, and the new crack grows until the specimens fracture.