Optical Detection of Acoustic Emission Signals

Piezoelectric transducers, long used in the generation and detection of ultrasonic waves, have more recently been the detector of choice for acoustic emission signals. Optical probing methods, however, have several important advantages for acoustic emission studies: (1) they have an inherent broad frequency response, free from mechanical resonances, (2) they do not interfere with the acoustic waves. (3) since the focused optical beam d1ameters are typically only a few hundredths of a millimeter, optical methods can probe very close to a crack or a tw1n, (4) they can probe internally in transparent media, and (5) they can be used over a very wide temperature range. In this paper we compare the response of an optical probe with that of a com~ercial acoustic emission transducer. Since the optical probe permits absolute calibration, we can not only measure dcuustlc emission a.plltudes, but also determine the quantitative response of the piezoelectric transducer to known acoustic disturbances of various kinds over a range of frequencies. We include measurements of real acoustic emission from twinning in two metals and stress corrosion cracking in steel.