Effect of a Tensile mean Stress on the Alternating Stress Required to Propagate An Edge Crack in Mild Steel

The conditions governing growth or dormancy of fatigue cracks in mild steel subjected to the general tensile loading cycle σ_m± σ_a, where σ_m ≥ σ_a, have been studied using plate specimens containing two small opposite co-planar edge cracks. It was found that, as in the case of zero mean load, these conditions were defined by the value of the parameter σ_a³l; if σ_a³l ≥ C_m a crack would grow, whereas if σ_a³l < C_m it would remain dormant, where C_m depended on the value of the ratio σ_m/σ_a. Its value at σ_m/σ_a = 1 corresponded to that derived from the zero mean load value by assuming that the loading cycles ± σ_a and 0 to σ_a were equivalent. The value of C_m then decreased linearly as the ratio σ_m/σ_a increased to 3, after which it remained constant as σ_m/σ_a increased further, provided that the applied loading cycle did not result in the bulk of the specimen deforming plastically. This lower limiting value was found to correspond to that predicted from the known growth rate of a crack in mild steel, assuming that a crack cannot grow at a rate less than one atomic spacing per stress cycle.