Load history effect on FCGR behaviour of 304LN stainless steel

FCGR behaviour of 304LN stainless steel was studied employing (i) constant R and (ii) constant Kmax cyclic loading envelopes. The closure free crack growth rates obtained by the constant Kmax method were always lower than that of the constant R procedure. Martensitic transformation at the crack tip was observed in both the cases. Qualitatively, the constant Kmax method produced more martensite than the constant R procedure. The martensitic transformation is thought to decrease the applied stress amplitude due to volumetric expansion and strain associated with phase transformation. The dissimilarity in the martensitic transformation and the resultant reduction in the applied stress cycle produced by the two methods were thought to be the reasons for the observed difference in the crack growth rates.