Effect of Cavities on Ultrasonic Attenuation and Velocity
Cavitation, voids, or porosity in materials can initiate several types of failure mechanisms. One type is creep, which begins with cavitation and eventually leads to microcracking, cracking, crack coalescing, and ultimate failure. Failure from creep can be avoided if cavitation is detected early.
A theoretical investigation of nondestructive ultrasonic methods for detection of creep cavitation in steels was performed. Ultrasonic scattering theories available in the literature were converted into computer software and used to predict the effect of cavities on ultrasonic attenuation and velocity. From the results, we theoretically concluded that measurements of (1) absolute velocity and (2) velocity and attenuation variation with frequency have a strong potential to detect cavitation.
The study also looked at the effects of scattering from grain boundaries and from graphite and carbide inclusions in steels. Grain scattering affects attenuation and velocity, and methods were formulated to compensate for such effects. The scattering from inclusions is expected to be less than that from cavities, as cavities produce a higher impedance mismatch than inclusions. The study concludes that ultrasonic methods will be able to detect creep cavitation in carbon steels used for high-temperature applications.
KeywordsUltrasonic Wave Ultrasonic Velocity Cavity Size Ultrasonic Attenuation Impedance Mismatch
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