Damage Assessment in Metal Plates by Using Laser Vibrometer Measurements

Abstract

Lamb waves, the ultrasonic guided waves in plates, have been increasingly used for long range damage detection in plate-like structures. They have been considered as the new generation ultrasonic structural health monitoring (SHM) and nondestructive evaluation (NDE) approaches that are able to provide efficient and effective sensing for critical structural components. In this paper, the laser Doppler vibrometer measurements are used for the damage detection. The laser Doppler vibrometer provides a non-contact means to acquire the ultrasonic wavefield and visualize the wave propagation. The ultrasonic wavefield as a function of time and space contains a wealth of information regarding the wave propagation in the structure and wave interaction with the damage. The frequency-wavenumber analysis and phased array beamforming are adopted to process the ultrasonic wavefield and visualize the damage. The proposed methodology was verified through an experimental test on an aluminum plate with a bonded quartz rod as a simulated damage. Keywords Lamb waves • Laser vibrometer • Damage detection • Phased arrays • Frequency-wavenumber analysis IntroductionLamb wave based SHM and NDE are growing rapidly due to their attractive features including the capability of travelling long distance with less energy loss comparing to bulk waves and sensitivity to small defects in the structure [1]. Over the last two decades, the advances of Lamb wave technologies have demonstrated the feasibility of damage detection in plate-like structures . Various Lamb wave based NDE and SHM methods have been developed . Giurgiutiu and Bao developed an embedded-ultrasonic structural radar by using phased arrays of piezoelectric wafer active sensors (PWAS) for in situ NDE and SHM of thin-wall structures [2]. Wilcox developed omni-directional guided wave phased arrays with electromagnetic acoustic transducers for the rapid inspection of large areas of plate structures [6]. Michaels developed an in situ array of spatially distributed ultrasonic sensors for the purpose of damage detection, localization and characterization [12]. Leonard and Hinders developed a Lamb wave based tomography technique to visualize the damage in the structure [18]. Park et al. investigated a time reversal process of the Lamb signals for the purpose of damage detection. Although recent advances in Lamb wave technologies have demonstrated the feasibility of damage detection and localization, there still remain many challenging problems for real-world applications. The obstacles that have confronted researchers in this field are the multimodal and dispersive natures of Lamb waves [1]. There are at least two Lamb modes, A 0 and S 0 , existing simultaneously. When the product of the wave frequency and structure half-thickness is large, several wave modes (such as A 0 , A 1 , S 0 and S 1 ) can coexist. The dispersive nature shows the Lamb wave speed depends on the wave frequency [24]. Various wave modes that are dispersive will make the interpretation of Lamb wave signals very...