Phase-resolved fatigue damage evolution in cement mortar: from interfacial initiation to matrix-assisted crack connectivity

Fatigue damage in mortar is greatly affected by the aggregate–paste ITZ, but the shift from crack initiation to connectivity under cyclic compression is not well understood. This study combines time-lapse X-ray computed tomography (XCT), digital volume correlation (DVC), deep-learning-based phase segmentation, and dissipation-based mechanical descriptors to track three-dimensional fatigue damage evolution in mortar. In a single mortar cylinder (∅7 mm × 14 mm) subjected to cyclic compression up to 0.8 F c and interrogated by time-lapse XCT at six loading states over 1000 cycles, with each XCT scan requiring approximately 57 min, crack voxels at early cycles accumulate preferentially in near-interfacial neighborhoods, consistent with the mechanically vulnerable nature of the ITZ. With continued cycling, the rate of new interfacial crack formation decreases, while the cement matrix increasingly participates in linking previously formed cracks into a connected damage network. DVC analysis shows that strain peaks occur before XCT cracks, indicating strain localization as an early damage signal. Dissipation metrics indicate a transition from early irreversible deformation to enhanced energy dissipation associated with crack interaction and connectivity. Independent XCT data confirm the same temporal ordering: strain localization, cracking, and connectivity. In the specimen examined here, fatigue damage is consistent with a staged sequence comprising sub-resolution strain localization, ITZ-neighborhood initiation, and matrix-assisted coalescence, and the same temporal ordering is recovered in a qualitative re-analysis of an independent dataset. These results are presented as a mechanistic case study; therefore, statistical generalization to population-level mortar behavior requires replication across specimens, mix designs, and loading protocols