CRACK GAGES FOR PREDICTING FATIGUE DAMAGE IN STEEL STRUCTURES
Abstract
The onset and growth of fatigue cracks in metal structures can lead to catastrophic consequences if left unchecked. A frequent nondestructive inspection and evaluation program may avert an impending disaster and considerably extend the useful service life of a structure. However, due to the time and expense involved in thorough inspection programs, a simple and inexpensive method that provides a visual warning of potential fatigue damage would be quite useful. The study reported herein examines the use of notched crack gages to provide a clear indication of the onset of fatigue crack growth in structural steel details. The crack gage consists of a thin steel coupon which 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. In this study the structural element is a transverse welded butt joint member containing a full-length lack of penetration (LOP) discontinuity. The effect of the LOP discontinuity on the fatigue behavior of transverse welded butt joints is further investigated, with emphasis on the fraction of the cyclic life spent in fatigue crack initiation. The crack gage fatigue test results are compared with analytical predictions of the fraction of cyclic life spent in crack initiation and crack propagation. The results of the LOP and the Control fatigue tests from this study are compared with those from other studies. Finally, the suitability of the crack gage for actual use is evaluated with practical examples, and recommendations for design are made.
Degree
Ph.D.
Subject Area
Civil engineering
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