Selective Guided Waves Generation in a Bi-dimensional Waveguide for Damage Detection

Abstract

SHM systems aim to detect defects in the structure they are attached to. Carefully selected pure guided waves are to be sent in the structure that acts as a waveguide in order to properly discriminate defects. For plates, this process has been developed in the past decades, e.g. using Lamb and SH waves. However, for a waveguide of finite crosssection, unconventional modes exist and particular attention must be paid to select the generated modes in the waveguide. For a same frequency-thickness product, the number of propagating modes in a bar is much larger than in an infinite plate and they may exhibit several displacement nodes over the bar width. In this paper, guided waves in a rectangular cross-section aluminum bar are studied, as representative of guided wave propagation along joint and extrusions. The two-dimensional Semi-Analytical Finite Element (SAFE 2D) method is used to obtain dispersion curves of all possible propagating modes, as well as mode shapes. Bar modes are generated experimentally in an aluminum bar using an ultrasonic multi-element probe, and detected with a Laser Doppler Vibrometer (LDV). Displacements and amplitudes obtained after processing of the measured data are in agreement with the predicted dispersion curves. Amplitude and phase measurements of modes generated by each excited element of the probe provide a transfer matrix, the inversion of which allows finding the excitation signals to simultaneously send to all elements in order to generate a desired pure mode. Finally, experiments are performed to confirm the selective generation, and to demonstrate a damage detection.