13342 Ultrasonics

Creep damage detection has also been under study by ultrasonic methods using precise ultrasonic velocity measurements [37]. Since creep changes the elastic property of the material, it also changes the acoustic velocity. Figure 13.18 shows typical profiles of ultrasonic longitudinal wave velocity in an Si3N4 crept at 1300°C for 200 h under various creep stress conditions. The profiles show the velocity of two parts of the creep specimens: the gage part and the grip part. Compared to the virgin specimen, the velocities in the gage part are shown to be lower than those in the grip parts. The velocity decreases with increasing creep stress. Another specimen crept under 200-MPa stress shows much larger reduction of the velocity in the gage part, which corresponds to larger creep strain and perhaps development of creep cavities. To obtain net change of velocities

caused by creep cavities, each difference at the grip part was subtracted from the original total wave velocity. It was assumed that this provided the net effect of creep cavities.

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