DEVELOPMENT OF LOW FREQUENCY ACOUSTIC MICROSCOPE

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The purpose of this project is to develop a low frequency acoustic microscope which measures precisely the velocities and attenuation of the leaky surface waves and visualize small defects within solids, with lens-less focused ultrasonic transducers operated at frequency less than 5OMHz and digital signal processing. The main results are summarized in the following.1. A digital measurement system has established for precise velocity measurement of the leaky creeping, surface SY and Rayleigh waves as well as attenuation measurement of the leaky Rayleigh wave.2. The mass density within a surface layer has estimated with the measured velocities and 'attenuation mentioned above and equation of leaky surface waves. The measured density of aluminum alloy and fused quartz agrees with bulk density within 10%.3. With dynamic finite element method, the wave propagation in water and solid has analyzed for a model of line focused transducer and aluminum plate. The velocities of the leaky creeping and Rayleigh waves thus calculated agree with the measured within 2%.4. The change in Young's modulus in a functional gradient Al2O3/Ni ceramics has been estimated with the measured velocities of the leaky Rayleigh and longitudinal waves.5. A sizing method of minute defects far less than the focused beam diameter is proposed, where the reflection amplitude from defects is reduced by that of the wide plane at the same depth. The spherical voids of 50 mu m in ceramics is estimated within 20%.6. The growth or healing of spall damage within a target plate under repeated high velocity impacts is well visualized with a lens-less point focused PVDF transducer, from which we can monitor nondestructively the evolution process of internal voids or cracks.