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The results of the Second Round-Robin on Opening-Load Measurement established the basis for a recent addition to ASTM E 647—“Recommended Practice for Determination of Fatigue Crack Opening Load from Compliance.” The technique involves characterizing the deviation in linearity of a load-displacement curve and reporting, as a minimum, the opening load corresponding to a 2% slope offset. The opening load and associated ΔKeff values reported showed significant scatter although this scatter was reduced when the data were subjected to a rigorous accept/reject criterion. Refinements in the method of handling data with high “noise” have further reduced scatter compared with the original analysis. Since each participant provided digitized load-displacement curves, the data from 17 test samples (10 participants) were reanalyzed using the “adjusted compliance ratio” (ACR) technique to evaluate ΔKeff. A comparison between the two methods shows that the ACR technique gives a higher mean value of ΔKeff than does the ASTM procedure. The ACR technique also shows a stronger correlation with crack growth rate data than does the ASTM procedure, with a slope comparable to that of a typical fatigue crack growth rate test. However, the mean value of ΔKeff based on the ASTM procedure shows better agreement with high stress ratio “closure free” data than does the ACR technique. This seemingly contradictory result can be partially explained in terms of second-order effects not normally considered significant.
fatigue, fatigue threshold, crack propagation, effective stress intensity, crack closure
Director, Fracture Technology Associates, Bethlehem, PA
Research engineer, NASA Langley Research Center, Hampton, VA