Unveiling hermetic failure of ceramic tubes by digital image correlation and acoustic emission

Abstract

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.

Analysis of SiC/SiC composites for energy applications at ambient conditions

“…Few measurements of tubular bending properties exist in literature, 29,30 so expected values must be derived from both relevant bending literature sources when available and tubular tension test results. Past results for similar tubular SiC/SiC CMCs under identical four‐point bending have been reported by the authors in Bumgardner, CH, Heim, FM, et al 17 There is a slight increase in the reported Young's modulus compared to past results and expected values for similar tubular specimens; this increase may be attributed to a thicker, more consistent outer CVI matrix coating compared to past claddings. Importantly, expected mechanical performance of tubular, bending specimens generally trails that of planar, tensile specimens 24 .…”

“…The current failure analysis for both tow structures indicates that matrix cracking occurs preferentially along tow‐to‐tow contact points. For SiC/SiC CMC nuclear fuel cladding tubes which operate with internally pressurized helium gas, delays in significant matrix or coating cracking are desirable to increase the stress at which hermetic seal is lost due to through thickness cracks, which has been found to occur well before fracture of the tow structure 17 . Here, it is seen that in four‐point bending cracks appear to be spaced accordingly to the biased tow spacing; therefore, we can make a few recommendations to CMC manufacturers.…”

“…For SiC/SiC CMC nuclear fuel cladding tubes which operate with internally pressurized helium gas, delays in significant matrix or coating cracking are desirable to increase the stress at which hermetic seal is lost due to through thickness cracks, which has been found to occur well before fracture of the tow structure. 17 Here, it is seen that in four-point bending cracks appear to be spaced accordingly to the biased tow spacing; therefore, we can make a few recommendations to CMC manufacturers. While it is possible the diamond biaxial braid ( Figure 9A) works to widely distribute cracks and consume energy, the many highly discontinuous crossover regions would seem to provide a more concentrated fracture zone than for a regular biaxial braid ( Figure 9B).…”

“…Tension specimens were epoxied (3M Scotch‐Weld 2214) on both faces to steel tab adapters, which were pinned into a custom‐designed, double universal joint fixture with a free‐floating base mount (Figure 2A). Four‐point bend specimens were loaded in a custom‐designed fixture (Figure 2B), previously detailed and validated by Bumgardner, CH, Heim, FM, et al 17 The bend fixture was equipped with pivoting cradles manufactured specific to the outer diameter of the CMC tubes and sized as specified in ASTM C1684‐18. A support span of l U / l L = 34/100 mm/mm was used, with DIC cameras angled upwards to capture the location of maximum bending strain.…”

“…Unlike the tensile specimens, the outer SiC coating prevented directly relating microstructural features to crack propagation, but the fracture mechanisms remained consistent with those previously reported. 17 The tension under bending initiates cracking along the bending zone within the outer coating matrix, marking the DFL point in the stress-strain curve. These cracks progress, leading to circumferential cracking across the tubular specimen, and eventually, fiber tow fracture leads to final failure of the specimen.…”