Effect of fiber-bundle-missing in fiber performs on flexural properties of SiC_f/SiC composites

Fiber-bundle-missing defects of four severity levels (no missing bundle, 1/3 bundle missing, 2/3 bundle missing, and an entire bundle missing) are precisely introduced into 2.5D braided SiC fiber preforms by deliberate defect engineering, and the corresponding SiC_f/SiC composites are fabricated via a combined chemical vapor infiltration (CVI) and polymer infiltration and pyrolysis (PIP) process. The effects of fiber bundle missing defects on the flexural properties and failure mechanisms of the composites are investigated using XRD, SEM, and CT characterization together with three-point bending tests. The results show that the as-fabricated SiC_f/SiC composites possess a well-bonded BN interphase and a dense matrix microstructure. The severity of fiber bundle missing exhibits a significant negative correlation with the flexural performance of the material. The defect-free specimens exhibit the highest and most stable flexural properties, with an average flexural strength of 349.2 MPa and a coefficient of variation of 1.98%. As the severity of bundle missing increases, the flexural strength decreases progressively, reaching a minimum of 246.6 MPa for the specimens with an entire bundle missing. Specifically, the failure mode evolves from a stochastic, fiber-dominated failure in the defect-free condition, through an uncertainty-driven failure induced by local fiber absence, to a deterministic, defect-dominated failure when an entire bundle is missing. Microstructural analysis reveals that fiber bundle missing not only directly reduces the load-bearing cross-section, but also degrades the load-carrying capacity of the material by inducing local stress concentration and altering the crack propagation path.