Digital image correlation for continuous mapping of fatigue crack initiation sites on corroded surface from offshore mooring chain

Effect of cyclic softening and mean stress relaxation on fatigue crack initiation in a hemispherical notch

“…44 and the current crack detection criterion, which signals a crack when the inter-node displacement across the crack plane reaches 2 μm. 19 The solution for the shallowest crack of Raju et al (still a relatively deep crack with a depth-to-plate-thickness ratio of a=t ¼ 0:2), when assuming a=c ¼ 0:8 and a bending surface stress equal to the cyclic yield stress, yields a crack of 2c ¼ 0:53 mm for a crack mouth opening displacement of 2 μm.…”

“…The applied mesh in Figure 10B was used to track movement of surface points and detect crack initiation according to the detection criterion in Coffin. 19 Figure 10C shows a color plot of the displacement field in z direction for a crack that has grown past the mouth of a notch. Cracks at the initiation stage did not show up as distinct features in color plots but could be identified by tracking the inter-node displacements in MATLAB.…”

“…The tests by Zarandi and Skallerud were also conducted on material from a link from the same chain. The current fatigue testing was performed in conjunction with a study of fatigue initiation from natural corrosion pits 19 and thus involved a chain that had been in service and submerged in sea water. The chain links had been in service on a floating production storage and offloading (FPSO) vessel for 20 years.…”

“…The current work aims to improve the understanding of MSR and its influence on fatigue initiation. Since corrosion pits vary in size and shape, 19 machined, hemispherical notches are considered here in order to develop a generic approach for MSR in R4‐grade steel chains. This simplification is consistent with a previous study of local strains around corrosion pits 1 .…”

Suitability of DIC and ESPI optical methods for monitoring fatigue damage development in X10CrMoVNb9-1 power engineering steel

“…Additionally, DIC could be successfully used to identify cracks on irregular surfaces. Qvale et al [42] used full-field digital image correlation technique to identify crack initiation and early crack growth on corroded surfaces from offshore mooring chain. The methodology of DIC fatigue damage development monitoring may be used together with the procedure presented in the authors' previous work [43], where the power exponent approximation of the fatigue damage measure ϕ and fatigue damage parameter D were successfully used to determine the fatigue life of P91 steel.…”