Measurement of Load Distribution in Multi-bolt, Composite Joints, in the Presence of Varying Clearance

An experimental study is presented on the effects of bolt-hole clearance on the load distribution in multi-bolt composite joints. Single-shear, three-bolt joints were studied. The specimens were manufactured from graphite/epoxy HTA/6376, with quasi-isotropic lay-ups. Protruding head bolts of 8 mm diameter and torqued to finger-tight conditions were used. Different combinations of clearance were obtained by using four different reamers and a purpose-designed high-precision drilling jig. Instrumented bolts with strain gauges attached for measurement of shear strain at the shear plane, were used to measure bolt load distribution. Prior to use in the multi-bolt joints, the bolts were calibrated in single-bolt joints. For assembly of the multi-bolt joints, a special jig had to be designed to allow simultaneous aligning of the bolt along the axis of the joint, centring of each bolt in its hole, and torquing of the bolts to a prescribed level. The results of the study show that relatively small amounts of clearance can have substantial effects on load distribution. As an example, the middle bolt in a joint of this type is normally assumed to carry less load than the outer bolts, and is therefore considered not to be under threat of failure. But in the presence of clearance the load can shift to the middle bolt, potentially causing an unexpected failure mode.