CHANGES IN CRACK-OPENING STRESS AFTER UNDERLOADS AND OVERLOADS IN 2024-T351 ALUMINUM-ALLOY

A variable-amplitude block-loading history consisting of high, near-yield-stress, underloads or compression-tension overloads followed by constant-amplitude small cycles was used to examine underload and compression-tension overload induced reductions in crack closure and the subsequent build-up of crack-opening stress to its steady-state level in a 2024-T351 aluminium alloy. Special attention was given to the near-threshold region, where the crack growth rate was less than 10(-9) m cycle(-1). The crack-opening stress level and the crack growth rate were measured for four different R-ratios of the small cycles following underloads and compression-tension overloads using a 900 power short focal length optical microscope. The crack-opening stress levels were measured at frequent intervals after the underload and compression-tension overload applications until the crack opening stress returned to a steady-state level. The crack-opening stress build-up was then described by an empirical formula in terms of the ratio of the difference between the instantaneous crack-opening stress of the small cycles (S-op) and the post-underload and compression-tension overload crack-opening stress levels, (S-opol), and the difference between the steady-state crack-opening stress of the small cycles (S-opss) and the post-underload and compression-tension overload crack-opening stress levels, (S-op-S-opol)/(S-opss-S-opol). Effective stresses calculated using this formula were used to opol predict crack growth rates. The formula's predictions showed good agreement with experimentally measured crack growth rates. For simplicity, both underloads and compression-tension overloads will be referred to as overloads in the remainder of this paper.