Porosity in configured structures : effect of ply drops and caul sheets in the processing of composite parts

Fiber reinforced polymer composites offer a variety of processing advantages in the manufacturing of high performance structures. However, due to a variety of potential quality defects, there is a great deal of risk associated with composite processing. Amongst these numerous defects, porosity has always been one of critical concern. Although there exists a great deal of literature on the subject, the bulk of existing research to date is restricted to the processing of flat uniform parts. As such, there exists a discrepancy between the current academic understanding and the practical knowledge needed in current practice. The objective of this thesis is to advance our knowledge of porosity to processing scenarios commonly seen in current practice. This has been done by conducting a comprehensive examination of the relationship between the mechanisms driving void evolution and the use of two commonly used structural configurations. These are ply drops and caul sheets. In this study, a series of configured composite parts were manufactured to parametrically assess the effect of ply drops and caul sheets both separately and in combination. The porosity content and final thickness profile of the parts were evaluated through optical microscopy and thickness measurements. The results from this parametric study show that resin pressure shielding due to lack of compliance between the laminate and the caul sheet can be a primary cause of porosity. It has also been found that lack of compliance caused resin migration which carries with it adverse effects on final part quality. The resin pressure distribution of ply-drop laminates processed with and without caul sheets was tracked in-situ through the use of instrumented tooling. The results from these experiments support the findings of the parametric study and provide a comprehensive understanding of the dominant mechanisms. These mechanisms where simulated with state of the art finite element software. These simulations demonstrate that commercially available software packages can be used to enhance our understanding of void evolution in complex processing scenarios. As such, the findings presented in this thesis are of great engineering value to current practices since they can be applied to a wide variety of applications.Applied Science, Faculty ofMaterials Engineering, Department ofGraduat