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The accurate representation of fatigue crack threshold, the region defining crack growth as either very slow or nonexistent, is extremely important. If the experimentally measured threshold is unconservatively high, a structural component designed with this data may fail long before the fatigue analysis predicts. The fatigue crack growth threshold is experimentally defined using ASTM standard E 647, which has been shown to exhibit anomalies. Alternate test methods have been proposed, such as the constant Kmax test procedure, to define the threshold regime without ambiguity. However, only the current test method defined by ASTM is designed to produce the range of fatigue crack thresholds (e.g., low and high R) needed to characterize an aerospace loading environment. It is the scope of this paper to determine the fatigue crack growth threshold of a well characterized aerospace alloy, 7075-T7351 aluminum, using different methods, to compare the results, and to draw conclusions.
fatigue, threshold, crack growth, test methods, plasticity induced crack closure
Research Engineer, NASA Langley Research Center, Hampton, VA
Professor, Mississippi State University, Mississippi State, MS
Senior Scientist, NASA Johnson Space Center, Houston, TX