Self-reinforced polymer composites for energy absorbing applications
Self-reinforced polymer composites (SRC), with a highly oriented thermoplastic reinforcement and a matrix with less degree of crystallinity–from the same or similar polymer family - have been introduced to mitigate some of the drawbacks of widely-used conventional composites, such as recyclability and ease of manufacture. SRCs are fully recyclable due to the same chemical structure of its constituents, which also ensures a strong interfacial bond between the fibre and the matrix. Self-reinforced polypropylene is one of the most common SRCs, available in the market. Self-reinforced polypropylene (SRPP) materials are used for energy-absorbing applications due to their high ductility and impact resistance. The main aim of the current work was to investigate self-reinforced polypropylene materials and their response under impact events. Understanding the processing and testing parameters that affect the properties, improve the performance of SRPP and their reparability were the main objectives of the work. Manufacturing parameters affecting the mechanical performance of self-reinforced polypropylene composites were investigated. Processing temperature and pressure were the primary variable which affected the properties of SRPP. The cooling rate and holding time were also the secondary processing variable. The properties of the consolidated laminates did not change significantly within the processing window (125°C-150°C). It was also found that minimum pressure of 50 bars is required to prevent unwanted shrinkage with the highest value at 70 bar pressure. Impact resistance of the laminates was found to decrease by increasing the consolidation temperature, which is linked to a higher degree of interlayer delaminating at lower consolidation temperatures. SRPP materials, even at the optimised processing conditions, had low tensile Young’s modulus (~ 4GPa) compared to many conventional composites, which constraint their application. Glass fibre was hybridised with SRPP for stiffness enhancement without curtailing the ductility and exhibiting progressive failure and resulted in novel hybrid composites with higher modulus and ductility than SRPP. Glass fibre volume fraction, thickness and the layup were the crucial design parameters to reach progressive failure. Pseudo-ductility was achieved for glass fibre volume fraction below 6.5%. Furthermore, the dispersed layups (were the glass fibre layers were dispersed between the SRPP fabrics) worked in favour of increasing the maximum glass fibre to 10% and still having the progressive failure. Energy-absorbing capability of the hybrid laminates were increased for some as the interlayer delamination of the interfaces worked in favour of the energy absorbing mechanisms and energy dissipation. Repairability of SRPP and hybrid laminates were investigated by re-thermoforming the impacted laminates. Through the repair procedure, some layups retained their pristine stiffness and strength completely while others could be repaired up to 50%. Finally, a road map was set for optimising self-reinforced polypropylene composites and potential benefits of using hybrids were discussed. The reparability of SRPP and their hybrids was attempted for the first time and was very promising due to the possibility of repairing a damaged part in service.
Self-reinforced composites, thermoplastic composites, Hybrid composites, Failure mechanisms of composites, impact resistance polymer composites
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