Optical detection of ultrasonic leaky waves from metallic structures in air

摘要

A recently developed optical heterodyne interferometer is proved to be sensitive to detect ultrasonic waves leaking out from metals (and from non-metals as well) at centimeters away from the metal surface in air. This measurement is based on the detection of the optical index variation in air due to the leaky wave. Experiments were carried out using both ultrasonic leaky Rayleigh and leaky Lamb waves in the low megahertz region. Optical measurements of these leaky waves velocities in semi-infinite materials and plates, which are related to elastic constants of the materials, showed good correlation to predicted values. Optical measurements of the amplitude of the leaky waves were used to obtain attenuation coefficients, which correlates to structural integrity of the materials. Surface and near surface defects were also detected. This optical method can be used for metals with low reflectivity surfaces. Thus it can be used for non-polished surfaces and can be extended for noncontact nondestructive evaluation applications.