Detection and Characterization of Hydrogen-Induced Stepwise Cracks Using Multi-Plane Wave Imaging, Coherence Factor Fusion and Physics-Constrained Temporal Pair Tracking

Stepwise hydrogen-induced cracking (HIC) in carbon-steel components rarely appears as a single resolvable crack, instead it produces compact, non-coplanar reflectors distributed through the wall thickness. This study presents an automated, physically interpretable ultrasonic framework that detects this indirect signature directly. Its core is a deterministic, physics-constrained temporal pair-tracking detector that forms candidate non-coplanar reflector pairs within each scan frame, links physically admissible pairs across neighbouring frames and accepts only tracks satisfying predefined score, persistence and geometric criteria. Unlike data-driven classifiers, every decision is traceable to explicit geometric constraints and is accompanied by an interpretable decision margin. As a front end, plane wave imaging data were reconstructed with the total focusing method and several coherence-factor combinations were evaluated. Although SCF×CCF gave the highest CNR and lowest API, VCF×CCF was selected for its compact reflector-pair morphology and best detector compatibility, while still increasing CNR by 10.9 dB and reducing API from 7.77 to 0.47 relative to conventional PWI/TFM. The targeted signature was grounded in metallographic observation of stepwise cracking and in controlled reflector geometries in a calibration block, where calibration-compatible indications were detected with an absolute vertical-separation error of 0.1 mm. Relative to the expert NDT reference standard, the detector identified 41 of 46 positive sequences (89.1% sensitivity) and produced a 6.5% false-positive rate across 418 strict-background sequences. Ablation and robustness analyses confirmed that performance arose from the combined score, persistence and geometry criteria rather than any single threshold. The proposed framework supports identification and prioritization of rare stepwise-cracking indications for further operator review.