Process Development and Characterization of Novel VARTM Fiber Reinforced Thermoplastic Composites

Fiber reinforced plastic (FRP) composite materials offer numerous advantages compared to traditional materials, like very low density, very high strength-to-weight ratio, higher fatigue resistance and better corrosion resistance. Nevertheless, their utilization in the automotive sector has been limited mainly in the design of race, high performance or luxury cars. In fact, despite their great advantages, employing FRP materials in vehicles components is more expensive than utilizing common materials like steels and it is not economically feasible for mass productions. Therefore, a fundamental aspect for increasing the utilization of FRP in the automotive sector is the reduction of the cost of raw materials and manufacturing processes. Another important aspect in the modern engineering is the possibility of recycling the materials. Nowadays, most of the FRP composites produced make use of thermosetting resins matrices that cannot be recycled. The objective of the thesis was investigating the use of an innovative thermoplastic resin for manufacturing glass fiber reinforced plastic (GFRP) panels through the vacuum assisted resin transfer molding (VARTM) process. The novel thermoplastic resin, called Elium®, has very low viscosity and can be manufactured at room temperature with the same processes adopted for thermosetting materials. Being thermoplastic, the resin is recyclable and it has higher toughness and fatigue resistance than the thermosetting resins commonly employed. The VARTM process allows to produce composite materials at low cost, exploiting the vacuum inside the mold for injecting the resin in the preform. The polymerization of the Elium® resin can be carried out inside the mold at room temperature, allowing to decrease further the production costs. The initial part of the work was dedicated in setting up the VARTM process. During the panels production, the main difficulties encountered were analyzed and possible improvements were applied to the process. The injection system was optimized in order to allow a better control, avoiding the introduction of air inside the mold and minimizing the waste of resin. The influence on the resin flow given by the position of the inlet and outlet hoses and the direction of the fibers of the preform was studied experimentally. The second phase consisted in testing the quality of the panels produced in order to validate the manufacturing process. The acid digestion of the matrix was employed for discovering the fiber volume fraction of the specimens, the microstructure of the material was investigated using optical microscopy and the material glass transition temperature was found with the dynamic mechanical analysis (DMA). The results showed that the overall quality of the panels produced was good. The thermoplastic resin was able to impregnate completely the preform, obtaining full fiber wetting and low void content. The polymerization of the resin was completed correctly. The thesis includes also a computational work involving the characterization of the growth of microscopic cracks in GFRP tubular test specimen subjected to combined compressive and shear loads using a computational model. The objectives of this work were modelling the fracture behavior observed during the experimental tests performed on the tubular specimens in a previous study, and computing the strain energy release rate (SERR) related to the crack initiation and propagation using fracture mechanics-based theory.