In-SEM analysis of plastic deformation and micro-rotation in the vicinity of a crack tip in pipeline steels

https://doi.org/10.3221/IGF-ESIS.79.01

High-strength pipeline and oil-country tubular goods (OCTG) steels are widely used in environments where fatigue and hydrogen-assisted cracking limit structural integrity. Sulfide stress cracking (SSC) is strongly influenced by the hydrogen – material interactions that unfold within the local mechanical response environment at the crack tip that determines the stress and deformation conditions controlling subsequent crack propagation. The present study investigates the evolution of crack-tip deformation and crystallographic orientation changes in high-strength pipe steels during controlled mechanical loading. API Spec 5CT C110 steel specimens containing fatigue pre-cracks were subjected to incremental tensile loading inside a scanning electron microscope. Repeated electron backscatter diffraction (EBSD) maps were acquired from the same crack-tip region at successive loading stages, while SEM imaging was used to monitor the crack opening, as well as the larger-scale deformation evolution at the sample surface. EBSD datasets were spatially registered (aligned), and the inter-step crystallographic orientation changes were evaluated through misorientation-angle mapping and rotation-axis analysis. Complementary digital image correlation (DIC) observations revealed the development of localized deformation fields originating from the crack tip. EBSD-based analysis showed that the majority of the investigated region experienced relatively small crystallographic orientation changes during loading, with micro rotation around in-plane axes contributing to out-of-plane grain relief emerging under elastic-plastic deformation. Localized regions with increased misorientation were observed near the crack-tip deformation field. The obtained results demonstrate the capability of repeated in situ EBSD measurements to track the evolution of crack-tip crystallographically governed response during mechanical loading and provide insight into the deformation processes that precede and then accompany hydrogen-assisted crack propagation.