Polymer infiltration and pyrolysis (PIP) is a widely used route for fabricating ceramic matrix composites (CMCs), but matrix shrinkage during pyrolysis often induces defects. In this study, a sequential multiscale framework is developed to investigate defect evolution in PIP-processed CMCs. At the macroscale, the temperature, mass conversion, pyrolysis degree, and gas concentration are simulated during pyrolysis. The local temperature and mass conversion histories are then transferred to meso- and microscale models, where pyrolysis-induced stress and damage evolution are analyzed using a phase-field fracture approach. The results show that the mass conversion degree remains nearly uniform during pyrolysis, while the gas-induced expansion strain is negligible compared with the pyrolysis-induced shrinkage strain. At the local scales, distinct intra-tow crack modes, including perimural, longitudinal, and semi-perimural crack modes, are identified, while inter-tow cracks segment the matrix into block-like regions and promote delamination between yarn and matrix. The predicted crack features are consistent with previously reported experimental observations and numerical results.
