An experimental study of bolt-hole clearance effects in single-lap, multibolt composite joints

An experimental study on the effects of variable bolt-hole clearance in single-lap, multibolt composite joints is presented. To the authors' knowledge, this is the first time that clearance effects in multibolt joints have been quantified experimentally. Joints with different clearances in each hole have been tested and the effects on load distribution, quasi-static strength, fatigue life, and failure modes are reported. Instrumented bolts have been used for measuring load distribution and specialized jigs have been used for positioning the bolts in the holes and drilling the joints. The clearances examined ranged from neat-fit to clearances slightly larger than those allowed in the aircraft industry. Earlier finite element studies with linear elastic material properties have indicated a significant effect on load distribution, and have postulated significant effects on strength due to reduced contact areas between bolts and holes, with correspondingly increased bearing stresses. The present experimental results confirm that clearance has major effects on the load distribution; the measured load distribution effects agree well with a three-dimensional finite element analysis with a nonlinear contact analysis. However, no significant effect on the ultimate quasi-static strength has been found, which is in line with earlier experimental studies on single-bolt joints. On the other hand, clearance has been found to have quite significant effects on fatigue life, which represents the first published data on this issue to the authors' knowledge. For both quasi-static and fatigue loadings, clearance had a stronger influence on failure initiation loads than on ultimate failure loads, indicating that clearance is of concern when designing for no damage (e.g., limit load design of aircraft).