Technical Brief: Avoiding Bolt Failures

Bolts fail due to fatigue which involves three stages of damage including crack initiation at a thread root, radius or material defect, progressive cyclic fatigue growth and final sudden failure of the remaining cross section of the bolt. There are situations where torqued bolts may fail due to fatigue. This include when the initial preload torque value for the bolt is too low, the preload torque is above the yield stress of the bolt, the yield stress of the bolt material is too low, exposure to elevated temperatures which cause relaxation of the bolt, vibrations in equipment and stress amplitude above the endurance limit. There are various considerations that can be implemented to reduce the probability of a bolt fatigue failure. One is to check that engineering drawing torque value is appropriate for the specific bolt alloy, heat treatment and diameter. The selection of the proper strength and toughness material for the bolt is also crucial as well as using the proper wrenches for the given application.