Strain evolution around corrosion pits under fatigue loading

The effect of pit depth on the crack initiation life of the API-5L X65 steel was investigated by varying the depth of corrosion pits generated on fatigue specimens using a micro-electrochemical cell. Fatigue specimens were subjected to fatigue loading in air, during which time images of the surface around the pits were captured in-situ. Digital image correlation was employed to make full-field strain measurements at the mouth of corrosion pits from the images captured during testing. From these measurements, an average threshold strain value for crack initiation of 0.24 ± 0.06% was established. Whilst crack initiation life was observed to decrease with increasing pit depth, the threshold value was observed to be unaffected by pit depth. Images captured during the fatigue cycling were also used to calculate crack growth data. Crack growth rate was found to be influenced by pit depth for the first 150 μm of crack length, where greater pit depth facilitated higher crack growth rates due to an increased strain concentration around the deeper pits.