The effect of bolt clamping force on the fracture strength and the stress intensity factor of a plate containing a fastener hole with edge cracks

Research has been completed to establish the effect which the clamping force, resulting from torque tightening a nut and bolt, has on the fracture strength and the stress intensity geometry factor of a fastener hole containing a symmetrical pair of edge cracks. The work has involved the carrying out of a programme of experimental tests and also the conducting of a numerical study. The tests were carried out using specimens made from aluminum alloy, grade AL7075-T6 rectangular plate containing a central hole with fatigue propagated edge cracks. Three batches of specimens were produced, one without a bolt inserted (for the purpose as a benchmark) and two with nut and bolts fitted but with different tightening torques applied. The joint fracture strengths were obtained using a tensile testing machine. In the numerical investigation, a finite element package was used to model the three test specimen variants used and thereby establish their stress intensity geometry factors. The numerical analyses considered the effect of parameters such as the coefficient of friction between contacting surfaces and the magnitude of the remotely applied axial load. The results show that the bolt tightening torque, and hence the plate clamping force, has a significant effect on reducing the stress intensity factor, and thus the joint fracture strength compared to bolt-less specimens.