Fatigue characterization of drilled components in polymer matrix composites

Composite materials have been initially used in the aerospace, defence and aircraft industries, but nowadays they are finding applications in consumer oriented applications. For some requirements of connection in components, it is necessary to drill workpiece materials. Drilling of fibre reinforced composites can produce typical problems such as delamination of material and fibre pullout, which decrease the fatigue strength of components. The prediction of fatigue life in composites components is not easy because of the several number of possible failure mechanisms which can occur when these materials are cyclically stressed. In this paper an experimental investigation on the behaviour concerning fatigue strength and life of drilled components in composite materials subjected to fatigue loading is presented. The cutting force components measured during drilling operation have been related to the cutting parameters. The influence of cutting speed and feed rate on the fatigue strength of machined holes has been taken into consideration. Results obtained from repeated flexural fatigue tests on glass and carbon fibres reinforced epoxy matrix have been used to compare the behaviour of different composite materials. Both thrust force and torque measured during drilling operation show a decreasing trend as the ratio between cutting speed and feed rate increases. Fatigue tests have been carried out both on non drilled and drilled specimens. Tests conducted on non drilled materials have allowed to define a threshold typical level of maximum force of fatigue loading. Failure cycles number has been taken into consideration comparing the threshold value with the traces of fatigue behaviour acquired during drilled components characterisation. Failure cycles number determined for glass fibres reinforced composites, is not dependent on speed ratio. The extension of damaged area at the exit side of the hole does not significantly affect the fatigue strength of drilled components.