Observation of crack-closure behavior after single overload cycles in 7075-T6 single-edge-notched specimens

Electrical potential and strain gage techniques were used to determine changes in crack-closure during the delay period after single overload cycles in 7075-T6 SEN specimens. For the specimen geometry (SEN), alloy (7075-T6), and instrumentation used, no significant changes in crack-closure were measured during propagation of the crack through the delay region after the overload cycle, even though some crack-closure was taking place during constant-amplitude cycling and changes in crack-closure loads were observed with changes in R values. The small change in crack-closure after the overload cycle was not enough to explain the delay. However, after relaxation for 16 hr, as well as after a high overload ratio, significant crack-closure was observed by the potential method. This crack-closure disappeared within 200 cycles. Thus, these results showed that the observed changes in crack-closure could not completely account for the observed number of delay cycles. These observations, which appear contrary to the crack-closure hypothesis, are in agreement with some of the other reported results. These results are applicable to the alloy, specimen geometry, thickness, and sensitivity of the instrumentation used in this investigation.