Strain evolution around corrosion pits under fatigue loading

Abstract

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.

The influence of corrosion pit–pit spacing on the pit‐to‐crack transition and fatigue lifetime

“…This consequently resulted in greater strain levels being present in the component compared to when interaction is minimum. In a digital image correlation study of the same material, the average “threshold” principal strain required for crack initiation was determined to be 0.24% 48 . Because the nucleation of cracks is generally associated with accumulated localized plasticity, the greater strain generated per fatigue cycle, when pits are closer, will enable the threshold principal strain required for crack initiation in the material to be attained in fewer cycles compared to when the pits are farther apart 46 .…”

“…In a digital image correlation study of the same material, the average "threshold" principal strain required for crack initiation was determined to be 0.24%. 48 Because the nucleation of cracks is generally associated with accumulated localized plasticity, the greater strain generated per fatigue cycle, pits are closer, will enable the threshold principal strain required for crack initiation in the material to be attained in fewer cycles compared to when the pits are farther apart. 46 This behavior has recently been further investigated using probabilistic cellular automaton and extended finite element modeling and is the subject of a paper currently being prepared for publication.…”

“…The replica method has recently been used for similar purpose 47 . Limitations of the measurement methods could not allow identification of the cycle number at which a crack initiates hence a fatigue crack was assumed to initiate at an N th cycle, given by 48 : where and are, respectively, the number of cycles when a crack was first observed and the number of cycles at the last interval, that is, the previous measurement where no crack was observed. Tests were interrupted every 2000 cycles to observe whether or not a crack has initiated.…”

“…The replica method has recently been used for similar purpose. 47 Limitations of the measurement methods could not allow identification of the cycle number at which a crack initiates hence a fatigue crack was assumed to initiate at an Nth cycle, N i given by 48 :…”

Stress-corrosion-cracking assessment of X52 pipeline steel exposed to mexican soils

“…When an SSRT is being performed, the corrosive environment generates pits and these, in turn, increase the local stresses with a plastic deformation, producing plastic zones in the boundary corrosion pits. 24,25 The pits observed at the high-stress regime may cause incubation and nucleation of cracks.…”