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Structural health monitoring using Lamb waves is based on guided waves introduced to a structure at one point and sensed at a different location. Actuation and sensing can be accomplished using various types of transducer. The paper demonstrates a non-contact method for low-frequency Lamb wave sensing. The technique utilizes a laser Doppler velocimeter. Lamb wave responses are enhanced using data smoothing and filtering procedures. The results are validated using classical piezoceramic-based sensing and numerical simulations. The study shows the potential of laser vibrometry for Lamb wave sensing.

Vibro-acoustic nonlinear wave modulations are investigated experimentally in a cracked aluminum plate. The focus is on the effect of low-frequency vibration excitation on modulation intensity and associated nonlinear wave interaction mechanisms. The study reveals that energy dissipation -not opening-closing crack action -is the major mechanism behind nonlinear modulations. The consequence is that relatively weak strain fields can be used for crack detection in metallic structures. A clear link between modulations and thermo-elastic coupling is also demonstrated, providing experimental evidence for the recently proposed non-classical, nonlinear vibro-acoustic wave interaction mechanism.