Fracture mode transition mechanism in shield tunnel linings: Insights from automated recognition of random multi-crack evolution

Shield tunnels face significant structural health challenges attributed to the presence of random cracks. Due to the invisibility of underground structures, accurately quantifying crack morphology, particularly in depth, remains a formidable challenge. To address this, a cohesive-based multiscale modeling approach incorporating concrete macroscale and mesoscale information is employed to simulate the evolution of random cracks. Concrete is modeled as a three-phase composite of aggregates, mortar, and the interfacial transition zone (ITZ), with polyhedral aggregates generated according to Fuller’s grading curve and cohesive elements governed by a bilinear traction-separation law embedded in the mortar and along aggregate-mortar interfaces to capture mixed-mode cracking behavior. Red–green–blue (RGB) crack images obtained from numerical simulations are subsequently processed using a quantitative recognition frame-work based on the A* algorithm to measure crack depth. Results reveal that energy dissipation increases nonlinearly with the total crack length in depth, reflecting a progressive transition of fracture modes from pure tensile to tensile-shear, and ultimately to compression-shear. The framework enables precise identification of complex multi-crack patterns and quantification of crack depth, providing insights into fracture mode transitions and associated energy dissipation, and supporting reliable assessment of tunnel lining structural health.