Lamb waves detection in a bovine cortical tibia using scanning laser vibrometry

Most of the techniques for generating and detecting ultrasonic Lamb waves (e.g. angle-beam piezoelectric transducers, micro-electro mechanical systems (MEMS), comb and interdigital transducers, phased array transducers, and piezoceramic transducers) require a firm physical contact with the measured objects. For objects with highly irregular surfaces such as bones, it will be very difficult to produce a good contact. Thus, a non-contact Lamb wave measurement technique, the scanning laser vibrometry, is proposed in this paper to examine a bovine cortical tibia in vitro. The ultrasonic Lamb waves used had the center frequency of 84KHz. The waves were generated using a planar transducer which was coupled with a cone-shaped resonant vibrator. Only the fundamental modes of a₀ and s₀ were expected to occur. 2-Dimensional images of the Lamb waves traveling in the bone were recorded. The scan results represent out-of-plane vibration of the surface of the bone. Lamb wave modes were verified with further post-processing analyses. In time-domain, time-history prediction of the modes is fitted onto the original detected signal as to confirm their common rising time for each mode. A frequency-domain method, i.e. wavelet analysis, is also employed to define the traveling modes and their group velocity. The expected modes can be clearly defined at the center frequency. Additionally, what seemed to be a new mode, a₁, was generated and detected at the higher frequency of the responses.