b'and to apply it to assess the structural reliability of SiC/SiC composite clad-dings. Toward this goal, the objectives of this project are: 1) design a new testing system, which is capable of producing different multi-axial stress states. 2) Use acoustic emission and X-ray tomography analyses to examine the damage status of specimens. 3) Develop a finite weakest-link statistical model for SiC/SiC composites under multi-axial loading. The model will be calibrated by the multi-axial tests.In this research, a robust multi-axial testing systems was developed for generating various load combinations of axial force and internal pressure, as depicted in Figure 1. To provide(Figure 3). The convex shape of theFigure 1. Multiaxial test apparatusthe effective isolation (jacketing)failure surface indicated an enhance-from the internal fluid pressure,ment in material resistance under viton membranes of a special shapebiaxial tension.were manufactured to seal both theIn parallel with the experimental internal surface of specimen andinvestigation, a mechanistic probabi-interface between the specimenlistic model was developed to predict and apparatus. High strength epoxythe lifetime distribution of SiC/SiC was used in-between the contractedcladdings corresponding to the PLS surface of specimen and apparatus limit. The model takes into account to transmit axial loading. With the mechanism of subcritical damage this design, the system is able to growth. The direct consequence load the specimen up to ultimateis that the failure probability of failure. Figure 2 shows the failurethe cladding at any given time is pattern of a specimen in the multi- governed not only by the current axial experiment. loading state, but also by the previous During the test, strain gage readingsloading history. The analysis also and acoustic emission signal ratesshowed that a long cladding would were recorded and subsequently usedexperience a considerably higher to determine the PLS. The currentfailure risk as compared to a short experiments involved five loadone (Figure 4). Such a size effect is paths: pure axial tension, pure hoopexpected to play a crucial role in tension, and 1:1, 2:1 and 1:2 ratiosdesign extrapolation from laboratory between the hoop stress and axialtest results to full-length cladding.stress. These test results (both the PLS and the ultimate limit states, ULS) were presented in terms of failure surfaces with statistical variability 2022|AFC ACCOMPLISHMENTS 67'