Effects of Loading History on Mechanical Performance of Semi-crystalline Polymers and Their Composites

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The proposed program is concerned about the influence of loading history on the change in mechanical properties for semi-crystalline polymers (SCP) and their glass-filled composites. Motivation for the program comes from the discovery of changes in mechanical properties owing to the loading conditions that are actually acceptable by plastic pipe industry for repair or regular maintenance. Three projects are proposed, of which the overall objective is to develop knowledge and technology to assess the possibility of crack initiation and growth in SCP and SCP composites that are intended for long-term, load-carrying applications.***    *Two SCPs and their glass-filled composites, based on polyethylene (PE) and poly(ether-ether-ketone) (PEEK), will be selected for the study. The research will be carried out using two approaches. One is through experimental testing to evaluate the influence of loading histories on the mechanical properties, and the other through finite element (FE) simulation to establish the constitutive equations and critical stress and strain states for crack initiation and growth. Concepts from the damage mechanics will be used to identify parameters that can be used to represent the variation in the stress and strain state, such as disintegration of the ordered structures in SCP or presence of micro-cracks at the glass-matrix interface, and to establish the relationship between damage evolution and changes in mechanical properties.***    *Two of the three projects are about the pure SCPs and the other their glass composites. The projects on pure SCPs are to explore the possibility of using accumulation of deformation, instead of change in temperature and strain rate, to generate the unusual ductile-brittle (uDB) transition that is known to lead to slow crack growth (SCG) in plastic pipes. These two projects are to understand the role of loading history on the change in mechanical properties, and to develop a non-destructive evaluation (NDE) technique for monitoring the stress and strain state in order to prevent catastrophic failure due to the SCG development. All possible loading modes, i.e. tension, compression, shear and their combinations, will be investigated, to identify the critical loading conditions to initiate the uDB transition. The project on SCP composites, on the other hand, is to search for the possibility of characterizing the crack coalescence and growth from micro-cracks at the fibre-matrix interface, and to establish the influence of the presence of micro-cracks on the overall mechanical properties. ***    *The three projects will provide training to 3 doctoral students in the next 5 years. When opportunities arise, undergraduate (UG) students will be recruited during the summer, to conduct small-scaled projects that are complementary to the proposed study and are suitable for the UG students' background.