SKM Ting, JG Williams and A Ivankovic
Fracture - RCP # 2004 Milan
Recent experimental data (Ting et al (1-2) and Ting (3)) has shown that the onset and subsequent evolution of the damage (craze) was found to be sensitive to constraint effects, in addition to rates. In particular, the effect of notch depth was found to exert a greater influence on the material response than through the thickness effects. The results showed that the deeper the notch (crack), the greater was the notch tip (plastic) constraint to promote constrained brittle failure where cavitation was preferred. As the notch tip constraint was relieved, the crack tip ductility was increased to invoke the blunting process, which in turn increased the separation (or displacement) and energy values. This dependency (on constraint) therefore can lead to contrasting damage-failure behaviours between different grades of polyethylene. Different macroscopic deformation mechanisms such as, voiding, cavitation, lamellar fibrillation, large-scale fibrillation, multiple crazing and shear deformation, were observed leading to distinct failure behaviour. The pipe grade polyethylene exhibited a craze-shear transition at low constraint and rate conditions. Clearly, there is a competition between craze and shear formation at this condition. On the other hand, the damage once initiated in a high-density polyethylene often led to the growth of radial cracks. Consequently, the nature and the form of the damage affect the overall energy dissipation, Gc. The implication on the fracture toughness and hence the long-term fracture properties is significant. This paper attempts to establish the linkage between the structural and mechanical properties. Examination of the influence of the microstructural properties and their interrelated nature on the crack growth resistance forms a basis of discussion. The study performed here should provide pipe manufacturers a valuable understanding of the crack growth behaviour of polyethylene.
