Abstract
The pulse-echo technique for ultrasonic velocity measurement was used for non-destructive study of creep damage in an advanced silicon nitride, SN 88, during interrupted tensile creep tests at 1400 °C under stress of 150 MPa. Linear relationships were found between strain and the longitudinal and shear wave velocities, Poisson's ratio, Young's, shear and bulk moduli. Density measurement indicated a coefficient of proportionality of ~ 0.7 between the volume fraction of cavities and tensile strain. Cavitation was therefore concluded to be the main creep mechanism and the reason for ultrasonic velocity and elastic moduli degradation in the studied silicon nitride. The ultrasonic velocity technique was demonstrated to be simple, sensitive and reliable nondestructive method for monitoring of creep damage during intermittent creep in silicon nitride.

Key words
tensile creep, cavitation, ultrasonic velocity, silicon nitride, elastic moduli

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