Effects of testing speed and heat treatment on the fracture of kaolin-reinforced high density polyethylene

Abstract

In the case of ductile thermoplastic polymers in either neat form or in a composite, the volumetric fracture work is large and may be dominant, and the ideas of post-yield fracture mechanics (PYFM) are appropriate. Both surface and volumetric work depend heavily on the polymer type, morphology including crystallinity and its directionality, and reinforcement. The plastic zone often extends fully across the specimen before crack propagation proceeds, both in application and in fracture tests. An analysis appropriate to this physical behavior has been performed for polymers and has been extended to composites [1–3]. This analysis is also the subject of a proposed standard [4]. The described behavior is that of materials where fracture begins after the specimen ligament has fully yielded; that is, after the peak load has been reached. Briefly summarized, the plastic zone size can often be assumed to be proportional to the ligament length L squared with a non-dimensional proportionality constant or shape factor β which depends on the specimen geometry and material. The surface work is proportional to the surface area. The total work of fracture Wf may be normalized by the ligament surface area (LB), where B is the specimen thickness, giving