NONCONTACT MEASUREMENT OF ULTRASONIC-ATTENUATION DURING ROTATING FATIGUE TEST OF STEEL

Acoustic resonance technique has been applied to monitor the fatigue d amage process of steel pipes exposed to rotating bending fatigue. The technique incorporates a superheterodyne spectrometer and an electroma gnetic acoustic transducer (EMAT). The EMAT was newly developed for th is purpose, and uses the magnetostrictive mechanism of ferromagnetic m etals and excites and detects axial shear waves traveling around the s ample pipe with axial polarization. Noncontact ultrasonic spectroscopy permits the accurate determination of the resonant frequency and the attenuation coefficient throughout the fatigue life. The attenuation c oefficient shows a sharp peak around 80%-90% of the life. The evolutio n is interpreted as reflecting dislocation multiplication, depinning, and formation of cell structures, which is supported by transmission e lectron microscopy observations. (C) 1997 American Institute of Physic s.