Initiation and propagation of fatigue cracks from surface imperfections on 304L austenitic stainless steel

Given the stringent requirements of high levels of safety in nuclear components, stakeholders of French nuclear industry must anticipate the presence of residual surface imperfections in these components. Such imperfections could be introduced during manufacturing or maintenance operations. The incidence of surface irregularities on the fatigue strength of metallic components must be considered. Meanwhile, nuclear components are generally loaded under low-cycle fatigue and large-scale plasticity conditions. The present work aims at assessing the impact of a controlled surface irregularity on the fatigue life of typical nuclear materials. The influence of characteristic parameters under low-cycle fatigue conditions is also investigated. A 304L austenitic stainless steel used in components of French nuclear power plants was studied. Fatigue tests were conducted under fully-reversed total axial strain control in air at ambient temperature with total strain amplitudes ranging from Δϵ_t/2=0.2% to Δϵ_t/2=0.6%. Surface irregularities, whose depth varies between 100 and 350 micrometers, have been introduced on polished cylindrical samples. The direct current potential drop method has been used to monitor the crack propagation, and thereafter, to derive crack growth rate data. Experimental markings of the crack front for different cycle numbers have been carried out to calibrate the variation of the potential as a function of crack depth. It appears that the fatigue life is strongly reduced in presence of a surface irregularity.