Fatigue Crack Initiation and Growth in A517 SAW Weldments Under Variable Amplitude Loading
Defence Research Establishment Atlantic Dartmouth, Nova Scotia Canada
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A comparison between experimental and analytical predicted fatigue lives is made for a A517 SAW Weldment subjected to variable amplitude loading. Three, 10 inch wide butt-welded specimens were tested under a variable amplitude load spectrum consisting of 3 yearly storms. The magnitude of the loads in the storm increased and then decreased in a linear manner with the winter storm representing the most severe loading condition. Fatigue crack monitoring was achieved through the use of 20 localized potential drop probes affixed along the length of the weld. Multiple and independent fatigue crack initiation sites were found along the length of the weld. These eventually coalesced and formed a dominant fatigue crack, which led to the eventual failure of the specimens. Fatigue life estimates using local notch strain and fracture mechanics approaches were obtained. Fatigue crack initiation life estimates using the strain life approach were found to be un-conservative while the estimates of the fatigue crack propagation life were conservative. Accurate knowledge of the structural and weld geometries was found to be critical in the estimation of the fatigue lives.