Effects of Compressive Overloads on Fatigue Crack Growth

Abstract

A phenomenological examination of acceleration in the rate of fatigue crack growth on H-6AL-4V beta annealed and 7050-T73 plate materials produced by compress!ve overloads was conducted. The effects of various magnitudes of compressive overloads and the sequence of tension-compression overload cycle with applied load ratio R= -1.0 on fatigue crack growth were investigated. The effects of truncation of compressive loads in a realistic flight-bv-flight spectrum of the C-141 aircraft were also examined. Specimens used in the test had center cracks and the through-the-thickness cracks emanating from one side of a A in. hole. When the overload cycle was changed from R = 0 to R = -1.0, with the constant amplitude loads at R = 0 and peak values of overload held constant, fatigue crack growth rate was increased by 5 to 40%. The rate increases with the increase of the overload ratio. The influence of sequence of tension-compression overload cycle with R = -1.0 on crack growth was less than 10%. The fatigue crack propagation times were shortened less than 15% when the compressive overloads were applied in blocked constant amplitude fatigue tests. Increasing the magnitude of compressive overload did not change the crack growth rate significantly. If the compressive loads in a realistic flight-by-flight spectrum of the C-141 aircraft are truncated, the predicted fatigue crack propagation time will be unconservative by as much as 50%.