Propagation of Fatigue Cracks in Various Sheet Materials

Abstract

Fatigue tests have been carried out on sheet specimens approximately 10 in. wide by 0·1 in. thick containing a small central slit, and measurements of the growth of the fatigue cracks initiated by the slit were made as the test proceeded. Nine common engineering sheet materials were tested, the applied nominal cyclic stresses being of a relatively low order, that is, between about ±1/2 and ±5 ton/in²; in each test a mean tensile stress was applied so that the loading was never compressive. The results were analysed in accordance with the geometrical similarity hypothesis suggested by Frost and Dugdale (1)†, that is, dl/dN = Kl, where l is the crack length and N is the number of cycles. It was found that over the region of stable continuous fatigue crack growth the coefficient K was proportional to the cube of the alternating stress for all materials tested and the growth law could be represented by dl/dN = σ³l/N_s where N_s depended on nominal mean stress and material, and σ is the alternating stress.