During the last few decades, the acoustic microscope has become a common instrument for the investigation of the internal structure of materials and for industrial non-destructive evaluation. Such microscopes are typically built on a reflection scanning scheme, where the pictures obtained present two-dimensional distributions of the reflective capability at some depth below the specimen surface. The sharp changes in the acoustical impedance at the boundaries of structural components cause significant reflection and produce a quality scan picture. But some specific objects cannot be resolved without certain difficulties: thin cracks (less then wavelength), thin glue layers or incusions with the similar values of impedance. The acoustic response from such artifacts is usually insufficient to form any acceptable image or quantitative data. In these cases, additional information can be obtained by using the phenomenon of acoustical nonlinearity with its accompanying effect of higher harmonic generation. The idea of taking advantage of nonlinear effects in order to improve acoustic images has been discussed in the literature from 1976 to the present. The principal applicability and advantage of nonlinear methods were demonstrated, and a significant increase of resolution was especially noted. But wide practical application is restrained by the fact that, in some cases, the nonlinear contribution is quite uniform, and, in others, the distribution of nonlinearity in the specimen is coincident with the distribution of acoustical impedance. Therefore nonlinear images often contain very few additional details compared to linear ones.
展开▼
