Strength MPa

FIGURE 13.13 Correlation between laser back scatter data and four-point band strength for two Si3N4 materials.

FIGURE 13.14 Optical transmission characteristics of several SiC and Si3N4 materials for X = 0.6328 ^m. Insets show: (a) visible observation of laser light on backside of Si3N4 turbine blade, (b) schematic of step wedge used to obtain through-transmission data.

ultrasonics, the data are presented in the form of a gray-scale image of the scanned area of the specimen. The elastic optical scatter method relies on the fact that many monolithic structural ceramic materials, including Si3N4, SiC, and zirconia, are optically translucent at various optical wavelengths as noted in Fig. 13.14.

Laser Scatter Image Laser Scatter Image

Optical photomicrograph after removal of 34 |im Optical photomicrograph after removal of 221 |im

FIGURE 13.15 Comparison of optical photomicrographs and laser back scatter data on crept Si3N4.

Optical photomicrograph after removal of 34 |im Optical photomicrograph after removal of 221 |im

FIGURE 13.15 Comparison of optical photomicrographs and laser back scatter data on crept Si3N4.

To demonstrate polarized backscattered laser light detection of creep, an Si3N4 sample was crept at 1400°C under 41.5-MPa stress. Creep damage was verified by taking elastic optical back scatter data, polishing off 50-60 ^m of material, and repeating these sequential steps through the thickness. Optical photomicrographs were taken of the surface before and after removing each layer. This sequence was repeated until the laser data suggested no more creep damage. The optical photomicrograph data and the corresponding laser scatter data consistently had one-to-one correspondence. Figure 13.15 shows two of these correlations.

The detection sensitivity of the laser back scatter method has been further tested using an Si3N4 polished surface flat specimen with intentionally induced subsurface Hertzian cone cracks. The Hertzian cracks were generated by loading small-diameter ceramic balls with different known loads. Figure 13.16 shows a schematic diagram of the specimen and resulting surface and subsurface laser scans. Note that in the surface scan, only the known surface breaking crack is detected, whereas in the subsurface scan, only the subsurface Hertzian cone cracks are detected. Figure 13.17 shows an enlarged view of the detection of the C-crack. Note that the subsurface detection suggest the larger diameter as would be expected of the "C"-crack.

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