Microstructural aspects of tensile deformation of high density polyethylene

Abstract

The paper describes tensile flow behaviour and related microstructural aspects of injection moulded polyethylene. The flow stress increased with increase in strain rate indicating that the tensile flow behaviour is sensitive to strain rate. Also, the yield stress exhibited a linear relationship with strain rate and followed the thermal activation concept with the activation volume consistent with the thickness of the lamellar crystallite. The behaviour of mechanically induced surface damage on polyethylene was studied by SEM. At low strain rates, the deformation process was characterised by features that looked like deformation bands. With increase in strain rate and strain, the deformation bands developed into a distinct array of crazes and grew inwards, followed by tearing. Also, with increasing strain rate the crazes multiplied and secondary crazes were generated that were at an angle to the tensile axis. The examination of the morphology of the fracture surface of polyethylene at various strain rates provided an insight into the mode of fracture process. At low strain rates polyethylene exhibited a ductile type of fracture with extreme fibrillation and, at intermediate strain rates, crazing - tearing was the predominant mode of fracture at the edges, while fibrillar failure occurred in the mid-thickness of the fractured surface. But, at higher strain rates the percentage of fibrillation was relatively small in comparison to lower strain rates. The different modes of deformation processes can be represented in the form of mechanical deformation - strain diagrams, which provide a broad perspective of the deformation processes operating in the different regimes.