An Investigation Of The Influence Of Applied Stress Ratio On Fatigue Crack Propagation In Structural Steels

TWI Industrial Member Report Summary 72/1978

By S J Maddox, T R Gurney, A M Mummery and G S Booth

Background

Fatigue tests have been carried out under several stress ratios, ranging from R = -4 to R = 0.67, on centre cracked sheet specimens of four structural steels. Reasonable correlation of the results could be achieved by basing the stress intensity factor range, Delta K, on the tensile part of the stress cycle although the results for R = 0 gave the minimum rate of crack propagation indicating that for R<0 the compressive part of the cycle contributes some damage and for R>0 da/dN increases with R. The effect of R could not be taken into account satisfactorily using the Forman method of correlation. Limited experiments have been carried out to determine that part of the applied stress cycle for which the crack tip was open, U Delta sigma, and these indicated that for negative values of R the effect of R on rate of crack propagation could be explained in terms of U Delta K. It seems possible that U can be expressed as a simple function of R, but further experimental results are required, particularly under fully tensile loading, before that suggestion can be confirmed.