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.