Abstract
This thesis presents and discusses a multiple thermally assisted piercing (TAP) technique that can be employed for introducing holes into thermoplastic composites. This technique uses heat to melt the thermoplastic composite matrix, which allows the reinforcing fibres to be displaced around a hole during piercing, instead of cutting the fibres as in the drilling technique. This study was concerned with the effect of the multiple perforations (drilled and pierced) on a thermoplastic composite material and the local strain distributions around the holes.
The process was found to introduce resin-rich regions and voids in the vicinity of the pierced holes and displacement of the fibres for distances of up to 3-hole diameters from the hole. The digital image correlation results showed distinctive behaviours between the drilled and pierced specimens under tensile, compression, and shear loading. It was found that the change in fibre orientation close to the hole edge can significantly affect the local stiffness of the pierced composite. The mechanical test results of the open-hole tensile and open-hole compression results showed no consistent difference on the composite strengths in tension between the pierced and drilled specimens; this might be a consequence of the small diameter of the hole used (2 mm). However, the compression strengths of the specimens with pierced holes were found to be smaller than those with the drilled holes which have been attributed to the fibre displacements and void formation introduced during the piercing process.
In addition to the experimental work, a finite element model was established to simulate the TAP process. The results of the model showed the displacement of the fibre bundles seen in the experiments recreated successfully and were reasonably representative of the microscopy findings. The model successfully demonstrated the increase and decrease in volume fraction close to the piercing pin, however, an accurate measurement of these changes is required to validate the model.