A periodic structure such as a diffraction grating is disposed on the surface of an object from which acoustic emissions are to be detected. A plane monochromatic light beam is incident on the periodic structure and forms a number of bragg diffraction orders at predetermined angles. A detector positioned to intercept one of the diffraction orders receives light whose frequency has been doppler shifted by surface motion on the object. A reference beam, also incident on the detector heterodynes with the doppler-shifted beam so that the doppler component can be recovered. In one embodiment, the reference beam is also incident on the surface and it produces bragg diffraction orders. The geometry produces up doppler in the bragg diffraction orders from one beam and down doppler from the bragg diffraction orders from the other beam and selected bragg diffraction orders from both beams are heterodyned in the detector.The acoustic emission analyzer is related to the detection of internal structural changes in an object and, more particularly, internal structural changes which may result in failure of the object. More particularly, acoustic emission analyzer is concerned with detecting acoustic emissions produced by fracture, grain boundary shift and the like, in an object particularly when under stress. When a stationary or moving object is stressed, internal irreversible changes can occur in the grain structure or in the gross structure of the body. In a shaft conveying a torque, for example, a sudden rearrangement of material domains can occur randomly and irreversibly from the stress. These occurrences give rise to acoustic emissions which are similar to tiny earthquakes typically originating within the body of the material and being transmitted to the surface. At the surface, longitudinal and transverse waves interact to produce surface waves. Knowledge of the acoustic emissions detectable from the surface waves can give insight into the response of the internal structures to stress. In particular, the frequency of occurrence of acoustic emissions can give advance warning of impending failure or the need for preventive maintenance. Acoustic emissions contain frequencies from the audible (below 10 khz) to the hundreds of mhz. Most of the emissions range in frequency up to about 5 mhz and involve motion over dimensions on the order of up to about 10 angstroms (1 angstrom equals 10-10 meters) by comparison, the wavelength of visible light ranges from 4,000 to about 7,500 angstroms or, roughly, 400 to 750 times greater than the surface displacement due to acoustic emissions.
Acoustic emissions are detected by piezoelectric transducers firmly clamped to the surface. The necessity for clamping the transducers to the surface, of course, disturbs the waves being measured and distorts the signals. In addition, such piezoelectric transducers are high q devices which add their own resonances to the signal.
