Abstract
Abstract : The fatigue crack growth properties of various high strength steels, including the 250 and 300 grades of the 18Ni-Co-Mo maraging steel, H11 steel, and D6A steel, have been studied using a center-cracked specimen. Experiments were conducted by cycling either at a constant stress or at a constant value of the stress intensity parameter, K. Electron microfractographs taken along the path of fatigue crack extension in the 250 grade maraging steel showed the growth ring pattern characteristic of fatigue. Measurement of the growth ring spacings and knowledge of the crack length permit calculation of the rate of crack extension as a function of K. The rates of crack extension, as determined by fractography and actual measurement on the specimen, are in good agreement. A technique for calculating the cyclic life of structure involving a numerical integration of K as a function of crack length and stress was programmed for computer operation. This calculation allows for the change in slope of the K vs da/dN at higher values of cyclic K. The results of these calculations are compared with the life prediction made by simple mathematical integration of the fourth power relationship for an infinite plate.
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