Mechanism-informed 3D analysis of fracture surfaces in hydrogen-charged line pipe steels

• Introduces a mechanism-informed 3D fracture surface analysis framework using optical profilometry to assess hydrogen embrittlement in pipeline steels. • Defines and applies three geometric metrics—total flat surface area, mean flat patch area, and maximum flat patch area – to quantify fracture morphology and correlate with HELP and HEDE mechanisms. • Demonstrates a reproducible, multi-scale approach that links microscopic fracture features with macroscopic embrittlement severity across varying hydrogen charging durations. Hydrogen embrittlement (HE) poses a critical threat to the structural integrity of pipeline steels in hydrogen-rich environments. This study presents a mechanism-informed, quantitative analysis of fracture surfaces in hydrogen-charged API 5L X65 steel using three-dimensional (3D) optical profilometry for quantitatively assessing and apprehending the susceptibility of steels to HE under different conditions. Steel specimens charged for 0–8 h were evaluated via slow strain rate tensile (SSRT) testing, SEM, and high-resolution 3D surface scanning. Three geometric parameters, total flat surface area, mean flat patch area, and maximum flat patch area, were introduced to quantify fracture morphology. A steady increase in total flat area and a sharp rise in mean and maximum patch areas at 8 h marked a transition from ductile to quasi-cleavage fracture. These metrics reliably captured HELP (Hydrogen-Enhanced Localized Plasticity) and HEDE (Hydrogen-Enhanced Decohesion) associated transitions, offering a reproducible and mechanism-sensitive framework for assessing hydrogen-induced damage in steels.