Experimental study on effects of clearance on single bolt, single shear, composite bolted joints

An experimental study is presented on the effects of bolt-hole clearance in composite bolted joints, in single shear, single bolt configurations. Variable clearances were obtained using specially manufactured reamers, ranging from neat fit to 240 μm (which is somewhat larger than the largest allowed clearances in the aircraft industry). The specimens were manufactured in accordance with the appropriate ASTM standard from graphite-epoxy HTA/6376, with quasi-isotropic and zero dominated layups. Both protruding head and countersunk bolts were used, with two different applied torque levels. Purpose built jigs were designed for drilling high quality holes and centring the bolts within the holes (to remove bolt position as a variable). Some tests were performed to failure with several repeats of each clearance, and some to percentages of failure for later analysis with x-ray and microscopy. Joint stiffness, 2% bearing offset strength, ultimate bearing strength, and ultimate bearing strain were obtained. The main effects of clearance were on stiffness and ultimate strain with less effect on strength; a delay in load take-up also occurred with the higher clearance joints, which has implications for load distributions in multibolt joints.