Coupled in situ micro-X-ray computed tomography and volumetric digital image correlation (V-DIC) strain measurements of expanding plug tests revealed the three-dimensional, microstructure-dependent mechanisms behind strain localization, damage initiation and stress redistribution in braided SiC/SiC composites. Hoop strain varied significantly through the composite thickness and was highest at regions of tow crossover; at higher loads, tow fracture initiated at these locations, and sample rupture propagated axially by connecting points of tow overlap. Finally, strain measurements after the failure of a tow on the interior surface quantified the three-dimensional stress redistribution mechanisms and damage tolerance of the SiC/SiC composite.

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A new, in situ hermeticity testing apparatus has been developed to allow helium leak evaluation of ceramic tubes, including nuclear‐grade SiC/SiC fuel cladding ceramic matrix composites (CMC), during four‐point bending with simultaneous monitoring of local deformation and damage, using stereoscopic digital image correlation (DIC) and acoustic emissions. The capabilities of the experimental apparatus are demonstrated using alumina, borosilicate glass, and 4130 steel tubes with representative cladding dimensions and then applied to study the deformation‐hermeticity relationship of SiC/SiC CMCs. Results of three CMCs appear to indicate that matrix cracking occurs near the deviation from linearity strain at strains ranging from 0.04% to 0.06% and is shortly followed by an initial loss of gas tightness by 0.09% bending strain. Leaking increased in distinct steps over 0.1%‐0.2% bending strain, and within this range, results indicate that prior to fiber fracture, it is likely possible to regain gas tightness upon unloading. This technique and uncovered hermetic failure behavior are intended to progress the standardization of a test methodology for nuclear reactor components and to begin to resolve the mechanisms controlling distinct steps of ceramic matrix composite failure.

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