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Crack closure of physical short and long cracks of LY12CZ aluminum alloy during fatigue process was investigated using acoustic emission (AE) technique. Results showed that the effective fatigue crack growth curve (da/dN vs. Delta K(eff)) of physical short and long cracks obtained by the AE technique was consistent with the effective fatigue crack growth Curve at high stress ratio (R = 0.8), which implied that the AE technique Could measure the crack closure level, especially for physical short crack. The growth rate of physical short crack was Much higher than that of long crack at the same Delta K, and the lower crack Closure level of short crack was the main reason. (C) 2008 Elsevier Ltd. All rights reserved.