Effect of torque tightening on the fatigue strength of bolted joints

The effect of tightening torques on the life of plates bolted using single and double lap joints was investigated. The effect of plate thickness using an aircraft grade aluminium alloy with double lap joints was also studied. Constant amplitude fatigue tests, under load control, were carried out, with a near zero stress ratio, on plain specimens (for bench mark purposes) and on both single and double lap joint specimens, for which several torque levels were applied on the bolted joints. The objective of the fatigue tests was to demonstrate failure trends for each joint type, material thickness and torque loading, rather than the generation of comprehensive S–N curves. Possible factors that affected the fatigue life of the bolted joints are discussed and conclusions are drawn with respect to the beneficial effects of tightening torques on the bolted single and double lap joints.