Non-contact optical/laser measuring techniques are very attractive in many engineering applications. The paper demonstrates examples related to structural health monitoring. Various methods based on strain, vibration and ultrasound measurements are presented together with relevant references. Applications examples utilise in-plane and out-of-plane measurements taken by 1-D and 3-D laser Doppler vibrometers.

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The paper presents the application of the nonlinear acoustic technique for fatigue crack detection. The method uses frequency modulation of the high-frequency ultrasonic wave by the low-frequency modal excitation. Low-profile, surface-bonded piezoceramic transducers are used for acousto-vibration actuation and sensing. The paper investigates the application of the broad-band low-frequency modal excitation. The study demonstrates that small fatigue cracks can be detected in an aluminum plate by the increase of amplitude level of modulation sidebands in the ultrasonic spectra. However, the sidebands can be also observed when the crack is not present in the plate due intrinsic nonlinear effects. Further studies are recommended to investigate these findings.

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