Generation and Detection of Single Mode Lmab Waves Using Laser-Based Ultrasound

Lamb waves have been successfully used for the detection of defects in and on the surfaces of plate-like structures for many years [1–4]. In these applications, the Lamb waves have generally been generated and detected using angled-beam piezoelectric transducers or EMAT transducers. In certain applications where only the lowest order symmetric or antisymmetric mode was needed, laser-based ultrasonic (LBU) techniques have been used for generation and/or detection. The use of an LBU technique is attractive because of the potential for rapid inspection of large areas and because it is noncontact with large stand-off distances. The large stand-off distance is particularly attractive for the inspection of structures with elements that protrude slightly from the surface that would interfere with the movement over the surface of an angled-beam transducer or an EMAT. However, a potential difficulty is that Lamb waves are a family of guided waves that exist in plate-like structures, and a large number of modes of vibration may co-exist in a given plate thickness. An essential element in establishing the feasibility of this technique is to demonstrate that LBU techniques can select and efficiently generate a single Lamb wave mode. The selected mode will ideally have an energy distribution within the plate wall that is optimized for detection of specific defect types. Further, the dispersion characteristics of the waves depend on the specific mode and the operating point on the frequency vs wavenumber dispersion curve. The slope of the curve at the operating point of the mode selected should be nearly constant over the operating bandwidth. This implies that the group velocity is nearly constant and a pulse will maintain its compact shape as it propagates, i.e., it is nondispersive. This is especially important for the inspection of large areas where the pulse might be required to propagate over long path lengths. To obtain efficient Lamb wave generation, the mode and operating point should be selected to have a low mechanical admittance for the combination of in-plane and out-of-plane displacements produced by the thermoelastic source. Further, for an optical detector that is sensitive only to out-of-plane displacements, the mode must have a significant out-of-plane displacement at the selected operating point