Trepanning on unidirectional composites: delamination studies

Drilling of fibre reinforced plastics with conventional tools often results in defects like delamination, debonding, fibre pull-out, etc. Thrust is a major factor responsible for delamination and it mainly depends on tool geometry and feed rate. Trepanning tools, which were used in this study, were found to give reduced thrust while making holes on thin laminated composites. In this work the peculiarities of trepanning over drilling of unidirectional composites has been emphasised. The models for prediction of critical thrust and critical feed rate at the onset of delamination during trepanning of unidirectional composites based on fracture mechanics and plate theory also have been presented. Mathematical models correlating thrust and torque with tool diameter and feed rate have been developed through statistically designed experiments and effect of various parameters on them have been discussed. The critical feed rate is a function of strain energy release rate, elastic properties, sub-laminate thickness and diameter of the tool. It is observed that sub-laminate thickness is the most decisive parameter from the viewpoint of critical feed rates.