13331 Surface Breaking Cracks

Surface-breaking cracks have been shown to be detected by two NDE methods: fluorescent dye penetrants and elastic optical scatter based on a modified reflectometry setup. There are two penetrant methods: visible and fluorescent penetrants. For surface-breaking cracks, conventional visible penetrants have been shown to be unsatisfactory because the surface tension does not allow penetration into the very tight cracks associated with modern structural ceramics [30, 31]. Several investigators [30, 31] have demonstrated detection of tight surface-breaking cracks in Si3N4 structural ceramics using fluorescent methods. These results suggest that observation/detection of the cracks can be observed by using a 40-60X microscope equipped with an ultraviolet light source. It has been suggested that for best detection one should use an ultraviolet light intensity of about 1500 ^W/cm2 [31]. Crack detection sensitivity has been verified using Vickers indents with loads from 5 to 20 kg in several SiC and Si3N4 materials [30]. Fluorescent penetrant inspection was used to determine the length of the cracks across the Vickers indents. Figure 13.9 shows a comparison between what is seen in ordinary light compared to that seen with fluorescent for a typical 10-kg indent on a Si3N4 material. Examples of applying this penetrant method to a Si3N4 turbine blade are shown in Fig. 13.10.

FIGURE 13.9 Example of use of fluorescent penetrant to detect cracks in Si3N4 from 10-kg Knoop indent: (a) no penetrant and (b) with penetrant.

FIGURE 13.9 Example of use of fluorescent penetrant to detect cracks in Si3N4 from 10-kg Knoop indent: (a) no penetrant and (b) with penetrant.

FIGURE 13.10 Example of fluorescent penetrant to crack detection on HIPped Si3N4: (a) photograph of turbine blade and (b) penetrant image showing crack.

FIGURE 13.10 Example of fluorescent penetrant to crack detection on HIPped Si3N4: (a) photograph of turbine blade and (b) penetrant image showing crack.

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