Influence of microstructural voids on the mechanical and impact properties in commingled E-glass/polypropylene thermoplastic composites

In recent years, the compression moulding of E-glass/polypropylene commingled composites has been thoroughly investigated. In particular, in the University of Nottingham, a number of studies have been carried out, trying to correlate moulding parameters with mechanical properties and microstructural void content. However, some aspects of commingled composites have received less coverage so far and are therefore dealt with in this paper. These concern the effect of the processing conditions of these materials on interlaminar shear strength and impact properties and the influence of the synergy between processing, microstructure and properties on the impact performance of commingled composite structures. With this aim, flat plaques of E-glass/polypropylene commingled composites with a different fibre architecture (two-and three-dimensional) were non-isothermally compression moulded under various moulding conditions and then tested. The test programme included falling weight impact tests with a staircase procedure, Charpy impact tests and interlaminar shear strength (ILSS) tests. To evaluate the consolidation of the laminates, void content measurement using optical microscopy was related to ILSS and impact test results. In particular, the specific issues arising in moulding laminates with added three-dimensional fibres were studied. These include correct placement of the tow, sufficient preheating of thick laminates and nesting of the layers during moulding. The results of these tests are discussed in the light of the moulding conditions and quality, and conclusions are drawn regarding optimum moulding conditions for impact performance. Finally, indications on the reliability and possible improvement of the moulding procedure to yield a sufficient moulding quality, even with large thickness, are also provided. The knowledge acquired on material consolidation properties is applied in the manufacture of an automotive side intrusion beam: problems due to the scale effect are also discussed.