1344 Impact Damage Detection

Impact damage or foreign object damage (FOD) can occur on any component. To date, there has been a very limited effort to develop NDE methods to assess extent of FOD. Recently, detection of FOD has been approached by two NDE methods: thermal imaging and air-coupled ultrasound. In one recent set of experiments [47], five 76 x 152-mm 8-ply (3 mm thick) SYLRAMIC S200 CMC panels were studied for impact damage. Impact damage was introduced by using a 12.5-mm-diameter steel rod with a hemispherical head. Visual inspection revealed limited surface damage but could not assess the depth or extent of the damage. NDE data were used to determine extent of damage. In addition, since

FIGURE 13.36 Detection of porosity in melt-infiltrated (MI) SiC/SiC: (a) air-coupled ultrasonic through-transmission data, (b) thermal diffusivity image data, and (c) destructive verification of porosity variation.

FIGURE 13.36 Detection of porosity in melt-infiltrated (MI) SiC/SiC: (a) air-coupled ultrasonic through-transmission data, (b) thermal diffusivity image data, and (c) destructive verification of porosity variation.

repair of CMC components is an area of interest, NDE/C methods are necessary to assess quality of the repair and assess condition of repaired regions at inspection intervals. As part of the effort on the impact-damaged samples, to assess repair, the samples were repaired and reexamined with NDE.

Conventional, through-transmission X-ray radiographs revealed few small, dark (low-density) lines near the impact, and it is well known that conventional through-transmission X-ray imaging does not detect planar delams. However, through-thickness air-coupled ultrasonics and through-thickness thermal imaging did reveal the impact damage. Figures 13.37 and 13.38 show through-thickness air-coupled ultrasonic and thermal diffusivity images, respectively, before and after repair processing. The correlation of the size and shape of the damaged region between diffusivity and ultrasound images is very good.

What is to be noted in the image data are the sizes, shapes, and the gray-scale levels. Recall, see Fig. 13.31, that as density increases, the through-transmitted air-coupled ultrasound signal decreases but the thermal diffusivity increases. Looking at Figs. 13.37 and 13.38, there is a significant increase in the density of the panel not impacted after repair by additional PIP cycles. Further, especially on panel A, the impact-damaged zone density has also increased. Observation of the repaired region increased suggesting an increase in density of the impact region after repair.

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