Bolt-hole clearance effects and strength criteria in single-bolt, single-lap, composite bolted joints

Effects of bolt-hole clearance on the stiffness and strength of composite bolted joints were investigated. The configuration studied was single-lap, single-bolt. Four different clearances were obtained using variable size reamers, ranging from neat-fit to 240 μm. The specimens were manufactured in accordance with ASTM standard D5961/D5961 M-96, from graphite/epoxy HTA/6376, with quasi-isotropic and zero-dominated lay-ups. Both protruding head and countersunk bolts were used, with two different applied torque levels. Specimen dimensions were chosen to obtain bearing as the primary mode of failure, with ultimate failure being mostly through bolt failure. Joint stiffness, 2% offset bearing strength, ultimate bearing strength and ultimate bearing strain were obtained according to the Standard. In addition, an alternative definition of strength was derived, which has some advantages over the offset method, and the results were evaluated according to this definition. Increasing clearance was found to result in reduced joint stiffness and increased ultimate strain in all tested configurations. Finger-tight joints with protruding head bolts showed a link between clearance and strength, but countersunk and torqued joints did not. A delay in load take-up also occurred with the higher clearance joints, which has implications for load distributions in multi-bolt joints.