Answer Happy

6) Estimate the number of cycles an aluminium alloy wing skin can tolerate to failure if its crack growth rate is dominated by a maximum stress range of 250 MPa, using the following information. (10 marks) Fitted data for the Paris law for fatigue crack growth in the alloy, where the stress was cycled from 0 to positive tensile, gives: da = CAK" mm per cycle dN C = 2 x 10-13 and m = 4 = -1/2 The alloy's fracture toughness, is 40 MPa mi where Kic Ocrit Vita. Assume an edge crack length, a, grows to a critical length before fast fracture, from an initial defect size of a = 0.5 mm, and the maximum stress is 250 MPa. HINT: First show with m = 4 this gives - Nf 1 Cπ2 Δσ4 racrit a-2 da (Note: the constant C is defined in MPa so that it is consistent with stress in MPa and, to be compatible, the crack length is in meters)