The Fatigue Crack Propagation Delay Behavior In 2024 T3 Aluminum Alloy Due To Single Overload Underload Sequences PDF Download

Tests, represented in a three-dimensional matrix, were performed to investigate the effects of a single overload/underload sequence on the fatigue crack-growth rate in 2024-T3 aluminum alloy. Three different stress intensity ratios were studied to investigate the delay phenomenon: (1) overload level to maximum cyclic level, Kol/Kmax; (2) underload level to overload level, Kul/Kol; and (3) minimum cyclic level to overload level, Kmin/Kol. These ratios ranged from 1.6 to 3.0, -1.0 to +0.3, and 0.11 to 0.3, respectively. All tests had the same overload stress intensity level. Sufficient data were recorded to accurately determine the crack-growth rate through the overload affected zone. The crack length versus number of cycles data for each test were represented by a spline function which was then analytically differentiated to obtain the growth rate. The number of delay cycles was found to be directly related to the minimum growth rate following the overload/underload sequence. The ratios Kol/Kmax and Kul/Kol were shown to be of particular significance to the delay while the ratio Kmin/Kol was shown to be of less significance. The results were correlated with an extended crack closure concept. From this the maximum value of the opening stress intensity level following the overload/underload sequence can be determined and used to predict the number of delay cycles.