Ultrasonic Inspection for Metal Creep Based on Flight-Time-Attenuation

Abstract

Attenuation coefficient is frequently-used for ultrasonic creep inspection in metals. However, in practical inspecting, e.g. thick wall weldment in high temperature pipings, it is usually difficult to measure the accurate propagating distance of ultrasound, which results in an inaccuracy of attenuation coefficient calculation. Therefore, a modified method for attenuation coefficient calculating was proposed here, in which the flight-time of ultrasound was substituted for propagating distance. The new calculated attenuation coefficient was named as flight-time-attenuation parameter, and had a unit as dB/us. A uniaxial tension creep experiment with pure lead was conducted to verify the detecting ability of this new parameter. Two piezoelectric wafers were pasted on the pure lead specimen, and ultrasonic testing was carried out simultaneously without interrupting the tension procedure. Attenuation coefficient and flight-time-attenuation parameter were calculated with inspecting signal acquired from creep specimen, and compared with each other. The results showed that two ultrasonic parameters had the same variation tendency along with creep procedure, and flight-time-attenuation parameter had a better sensitivity. Accordingly, the flight-time-attenuation parameter could be used for ultrasonic creep inspection in metals, and worked as a replacement of attenuation coefficient calculated with propagating distance. Another creep inspecting experiment with P91 steel weldments was conducted to verify the practicability of the new parameter, in which the new flight-time-attenuation parameter showed a good distinguishing ability for different creep status.