Fracture properties analysis of rotationally moulded plastics for their application in skin-foam-skin sandwich structure.

Abstract

Rotational moulding is a low pressure, high temperature manufacturing method and is considered to be the best for making large hollow shape plastic parts. Due to its long heating cycle, mould rotation during heating and slow cooling rate, it is completely different from injection or other moulding processes. The mechanical properties of rotationally moulded plastics are totally dependent on unique heating or cooling cycles. With the growing demand for rotationally moulded plastics in load bearing and other applications, a better understanding of their fracture properties is essential. In the rotational moulding process, multilayer plastic products such as skin-foam-skin three layered sandwich structures can be manufactured in a single manufacturing step without any joints. It exhibits relatively high stiffness, strength-to-weight ratios and is used increasingly in various applications such as automotive and marine. During the lifetime of the sandwich material, it may face multiple or repeated impact events. Therefore, the aim of this work is to develop a better understanding of the fracture behaviour of rotationally moulded plastics in order to use them in skin-foam-skin sandwich structure and reduce in-service failures due to impact. Here, rotationally moulded two different commercially available Polyethylene (PE) and Polypropylene (PP) plastics are tested. Microstructural details of the plastics are investigated here. Fracture properties, particularly fracture toughness properties are studied using J-integral elastic-plastic fracture mechanics approach to identify the fracture initiation point. Impact properties are also investigated at a wide range of temperatures. PE materials are found to have better fracture properties. It is observed that with the fracture toughness plastic’s microstructure particularly crystal and amorphous region thickness are related. The understanding from these works is followed by the manufacture of rotationally moulded skin-foam–skin sandwich structure and testing of low velocity impact properties of this structure from 20 J to 100 J energy level with a drop weight impact testing machine. PE is used for both in skin and core layer and sandwich samples are manufactured at four different skin-core thickness combinations. Impact force resistance and bending stiffness are found to be increased with an increase of both skin and core layer thickness. Low velocity repeated impact properties of the rotationally moulded sandwich samples are also investigated from 20 J to 50 J energy level at the end of this project to understand the effect of repeated impact on the sandwich structure. The samples are subjected to single impact event repeatedly up-to penetration at each energy level. Impact energy-impact number curve obtained from repeated impact test provides an equation for prediction of the number of repeated impacts for the penetration of the sandwich samples at each energy level.