Crack Gages for Detecting Fatigue Crack Initiation

Abstract

The use of fatigue crack gages is examined, in order to provide a clear indication of fatigue crack initiation and early subcritical crack growth in structural steel details. The crack gage consists of a thin steel coupon that can be bonded to a structural member in the vicinity of a known stress raiser, such as at the end of a cover plate or near a stiffener. The crack gage concept has traditionally been used to correlate fatigue crack growth in the gage to crack growth in the structure. The present emphasis, however, is to use the crack gage to detect the onset of fatigue crack growth. The geometry of the crack gage is designed such that either initiation or significant crack propagation in the gage provides a direct indication that the structural element has developed a detectable fatigue crack.