Experimental Study of the Role of Gap Size on Bolt Failure in Clevis and Lug Joints

Abstract

Abstract A common standard for conservatively evaluating the required bolt strength in a clevis–lug joint is to assume that the bolt must survive under an interaction equation that combines loading caused by bending, shear, and axial preload–induced stresses. In practice, this leads to unnecessarily large bolts because the true failure mode is typically closer to simple shear with some bending stresses. In this experimental study, a clevis and lug joint with a variable gap between the clevis arms was used to determine the force necessary to break high strength bolts with various gaps between the clevis and the lug, ranging from pure shear (no gap) to a gap 2.8 times the diameter of the bolt. The testing results clearly show that failure occurs at forces that are often significantly higher than would be predicted by classic, closed-form analysis of the combined loading. In particular, the bolt failed at the force predicted by simple double shear failure theory for gap sizes up to half of the bolt diameter, and for larger gaps, the failure force was consistently higher than would be predicted by classic interaction equations. Thus, although assuming failure that is due to combined bending, shear, and axial stresses is safely conservative, the necessary bolt size is actually significantly smaller than this method predicts.