Effect of indentation defects on fatigue crack propagation behavior of weld and base material: An in-situ fatigue study

Surface indentation defects generated during coiled tubing (CT) transportation and operation critically influence fatigue failure mechanisms, yet their microstructural interaction dynamics remain poorly characterized. Through in-situ fatigue testing coupled with electron backscattering diffraction (EBSD) characterization, this work systematically examines how indentation-induced microstructural modifications govern crack propagation behaviors in base metal (BM) and weld zone (WZ) materials. The results show that indentation generates fundamentally distinct microstructural responses in BM and WZ, subsequently dictating their crack progression mechanisms. In the plastic influence zone of the indentation, the interlocking basket weave structure formed by irregular grains nested in each other during the pressing process of BM indentation was damaged, resulting in a large amount of stress concentration at the grain boundary, accelerating the main crack propagation. Conversely, WZ develops enhanced resistance to crack formation and propagation effectively due to indentation-induced homogenization of dislocation density and a certain increase in grain and grain boundary strength. In the non-affected zone of indentation, the microstructures of both the BM and the WZ restored their original properties. In the BM, ferrite with varying sizes and shapes, along with a small amount of pearlite, were intricately nested and interwoven. This structure exhibited superior coordination deformation capability, effectively delaying crack propagation. In contrast, within the WZ, ferrite grains were larger and more independent, leading to diminished intergranular deformation compatibility. This structural characteristic facilitated localized strain concentration through intensive slip band formation, which subsequently accelerated the propagation of main cracks.