Effect of Mean Stress on the Rate of Growth of Fatigue Cracks in Sheet Materials

Abstract

Fatigue tests have been carried out on sheet specimens, 10 in. wide by about 0·1 in. thick, containing a small central slit, and measurements made as the test proceeded of the lengths of fatigue cracks initiated by the slit. Seven common engineering materials were tested at various tensile mean and alternating stresses. Similar tests have been reported previously (Frost and Dugdale (1)† and Frost (2)) in which the tensile mean stresses were relatively low; in the present paper the effect of higher mean stresses on the rate of crack growth was investigated. It was concluded that, for values of mean stresses greater than 2 ton/in2, the rate of growth could be represented by: where l is the crack length, σalt the nominal semi-range of alternating stress, σmean the nominal tensile mean stress and σmean ≥ σalt (i.e. stress cycle wholly tensile), N the number of cycles, and P and Q are material constants. For mild steel and copper the rate of crack growth was independent of tensile mean stress,