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.
This paper investigates the nonlinear vibro-acoustic modulation technique for damage detection in metallic structures. Surface-bonded, low-profile piezoceramic actuators are used to introduce a high-frequency ultrasonic wave and low-frequency modal vibration into an aluminium specimen. The response of the vibro-acoustic interaction is monitored by a third low-profile piezoceramic transducer. In contrast to previous applications analysing the response in the frequency domain, current investigations focus on the instantaneous characteristics of the response using the Hilbert-Huang transform. The study shows that both modulations, i.e. amplitude and frequency, are present in the acoustical responses when the aluminium plate is cracked. The intensity of amplitude modulation correlates far better with crack lengths than the intensity of frequency modulations.
Matching layers of acoustic impedance are intensively studied in ultrasonic transducers for the efficiency of wave transmission. Large impedance mismatch between the active element of piezo and parent material in long range ultrasonic is also expected to have the similar affects on the ratio of the transmitted and reflected waves which can cause high reflection at the interface that result acoustic wave ringing and indicate low transmitted energy for inspection over large areas. This simulation study present analysis of Lamb wave propagation through a single matching layer from a piezoelectric transducers. It explains transmitted waves into aluminum plate using different materials of matching plates at thickness of quarter wavelength. Four matching plates with close to the computed value of acoustic impedance had been used in FEM simulations to study effect of the matching layers on the transmitted Lamb wave in aluminum plate. The results indicated slightly different phenomenon of multiple wave reflections from the transmitted S0 and A0 modes at boundary of the matching layer.
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