Nonlinear acoustic nondestructive testing of cracks.

This paper is a review of new nonlinear acoustic methods. Acoustic imaging and NDT methods are widely used in various tecnical fields. Conventional methods are based on the principles of linear acoustics: Nonlinear distortions are ignored. The strength of the distortion can highly increase due to the compliant features of the cracks. Such distortion can be observed by using different nonlinear effects such as high-harmonic generation, modulation of sound by vibrations, and subharmonic generation. Nonlinear acoustic methods are based on these effects and examples of its usage are varied. First, a similar method was developed which tested the glue quality of thermoprotective covers of the Russian Buran space shuttle. Using a nonlinear effect of high-frequency sound modulation by vibrations, an enormous increase of the acoustic nonlinear parameter in steel that has been fatigued has been experimentally demonstrated. The single microcracks in metal construction were detected as well. The method based on second-harmonic generation due to cracks was tested in firm ‘‘PECHINEY’’ in France. The sensitivity of nonlinear new methods to the appearance of fatigue damage to the material is much larger than that of conventional methods of NDT. [Work was supported by Russian Foundation of Fundamental Research Grant N0-93-05-8074.]