Low frequency acoustic microscopy for material characterization.

In recent years, acoustic microscopy has been found to be very useful for characterizing engineering as well as biological materials. With the present state of knowledge on acoustic microscopy, one can obtain the surface wave velocity of a homogeneous specimen or coating thickness of a coated material and can produce images of near surface internal defects and inhomogeneities in a specimen. Low frequency (≤ 1 MHz) acoustic microscopy has been found to be very effective for detecting cracks at a relatively greater depth. An unconventional low frequency acoustic microscope has been fabricated where the microscope lens has been replaced by two ultrasonic transducers with cylindrical concave front faces; one works as a transmitter and the other one as a receiver. Using this arrangement, it has been found that it is possible to detect internal cracks in a material and identify anisotropy in fiber reinforced composite plates. These experimental results are presented in this dissertation with appropriate theoretical formulations. A theoretical model to calculate AMS (acoustic material signature) of orthotropic materials is also developed and presented here. The theoretical model is verified by experimental results