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 has already been extensively studied but mainly in the high-cycle fatigue domain. However, nuclear components are generally loaded under low-cycle fatigue and large-scale plasticity conditions.

The present work aims at assessing the impact of a surface irregularity on the fatigue life of typical nuclear materials, and to investigate the influence of characteristic parameters under such conditions. In addition a special attention is paid here in the quantification of the characteristic stages of fatigue life, namely crack initiation, micro-propagation and propagation.

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. Strain amplitudes ranging from Δεt=±0.2% to Δεt=±0.6% were investigated. Surface irregularities have been introduced on polished cylindrical samples (diameter of 9 millimeters) by two methods: electro discharge machining and with a special device, developed by Pprime Institute, consisting in a progressive removal of material by abrasion by means of a ceramic disc and lubricating with diamond powder. Surface irregularities depth varies between 100 and 350 micrometers which corresponds to possible industrial imperfections.

The direct current potential drop method has been used to monitor the crack propagation and thereafter to derive crack growth rate data. A Finite Element numerical simulation has been implemented and experimental markings of the crack front for different cycle numbers have been conducted to calibrate the potential variation as a function of crack depth. A relatively limited discrepancy appeared between experimental and numerical calibration curves.

Fatigue test results demonstrated a significant impact of the surface irregularity on the 9 millimeters diameter specimen fatigue lifetime, depending on both irregularity depth and applied strain amplitude. The analysis of crack initiation from imperfections has been made possible thanks to the identification of a potential variation threshold associated with a crack depth close to 100 micrometers. Besides the strain intensity factor range ΔKε has been shown to represent an efficient crack driving force parameter under large-scale yielding for various experimental conditions. Crack growth data analysis using this parameter were useful to provide crack growth rate expressions and efficient to predict experimental fatigue lifetimes.