Optimal excitation frequency of lamb waves for delamination detection in CFRP laminates

To improve the reliability of delamination detection techniques based on Lamb waves for carbon fibre reinforced plastics (CFRP) composite laminates, the influence of excitation frequency of Lamb waves on the intensity of reflected waves from a delamination in cross-ply CFRP delaminated beams was investigated. The A₀ Lamb wave mode was chosen although the conclusion in this work is also applicable to S₀ mode. A Chebyshev pseudospectral Mindlin plate element proposed by authors was employed to simulate the A₀ mode propagation and the interaction of the A₀ mode with the delamination. Numerical results show that the sensitivity of A₀ mode to the delamination at different frequencies is remarkably different. Each delamination with different lengths or different locations along the through-thickness direction of the laminates generally has at least one or more optimal excitation frequencies which can generate the strongest reflected wave signals from the delamination. The reason for this phenomenon was also investigated by analyzing the natural vibration of the local upper-and lower-delaminated regions. The optimal excitation frequencies were identified to be close to the resonant frequencies of the local delaminated regions with pure flexural vibration modes similar to the deformation pattern of A₀ mode.