1345 Through Thickness Crack Detection

Through-thickness crack detection using NDE/C technology is important to CMC materials used as hot-gas confinement, such as a gas turbine combustor, because

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FIGURE 13.37 Through-transmission air-coupled ultrasonic C-scan image data of impact damaged PIP SiC/SiC 8-ply lay-up panels: (a) test panel E and (b) test panel A.

FIGURE 13.37 Through-transmission air-coupled ultrasonic C-scan image data of impact damaged PIP SiC/SiC 8-ply lay-up panels: (a) test panel E and (b) test panel A.

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FIGURE 13.38 Through-thickness thermal diffusivity image data of impact-damaged PIP SiC/SiC, 8-ply lay-up: (a) test panel E and (b) test panel A.

cracks can provide a leakage path for hot gases to supporting structures. One recent example occurred in a large 75-cm-diameter CVI SiC/SiC combustor liner for a gas turbine. The NDE methods used for data acquisition were thermal imaging and air-coupled ultrasonics. All NDE data suggested an axial separation of unknown depth in the as-processed condition; see Fig. 13.39. The liner was installed and run for 5016 h and subsequently was removed for analysis. What the destructive analysis suggested (see Fig. 13.39a) is that at the time the liner was fabricated, the plys at the seam had been distorted such that the ply layers no longer were concentric but rather had an orientation toward the outer surface. Thus destructive analysis confirmed the early NDE data in that there was a seam with a weakness.

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FIGURE 13.39 Detection of through-thickness crack in 3-mm-thick 8-ply, CVI SiC/SiC 75-cm-diameter combustor liner: (a) crack detected by thermal diffusivity image, (b) progression of crack as monitored by thermal diffusivity imaging. (a) Optical photo micrograph of cross section through liner wall after 5016 h, (b) initial thermal image, (c) thermal image after 2250 h, and (d) thermal image after 5016 h.

FIGURE 13.39 Detection of through-thickness crack in 3-mm-thick 8-ply, CVI SiC/SiC 75-cm-diameter combustor liner: (a) crack detected by thermal diffusivity image, (b) progression of crack as monitored by thermal diffusivity imaging. (a) Optical photo micrograph of cross section through liner wall after 5016 h, (b) initial thermal image, (c) thermal image after 2250 h, and (d) thermal image after 5016 h.

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