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[References: 9 tit.]

The paper describes near-tip experimental strain measurements under simple variable-amplitude loading. Local strain was estimated by means of Digital Image Correlation (DIC) technique using optical microscope equipped with digital camera. Fatigue crack growth tests were performed on single edge crack tension specimens made of Al-Cu alloy under specially designed load sequences, which included baseline cycles, single overload or underload with measurement cycles for DIC. The main idea of the study is to compare strain response and shapes of hysteresis loops at the crack tip and crack wake. The obtained results suggest that COD values measured at the crack tip are significantly affected by both underload and overload, but the influence is different for each case. The observed behavior is attributed to abrupt changes in Kop and ΔKth and their subsequent recovery to the basic level.