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

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Abstract
The conditions governing growth or dormancy of fatigue cracks in mild steel subjected to the general tensile loading cycle σm± σa, where σm ≥ σa, 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 σa3 l; if σa3 l ≥ Cm a crack would grow, whereas if σa3 l < Cm it would remain dormant, where Cm depended on the value of the ratio σm/σa. Its value at σm/σa = 1 corresponded to that derived from the zero mean load value by assuming that the loading cycles ± σa and 0 to σa were equivalent. The value of Cm then decreased linearly as the ratio σm/σa increased to 3, after which it remained constant as σm/σa 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.

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