Optical techniques for examining mechanical materials
Advances in sensors-actuators technology and signal processing are revolutionizing Structural-Health Monitoring and Non-destructive Testing with the integration of sensing-actuating capabilities into the structure. We demonstrate ultrasonic guided-wave detection capabilities of four optic fibre sensors (OFS) and experimentally address their damage detection and location potential, based on their sensing high directivity. Various signal processing methods that look into perturbations caused over guided-waves propagation characteristics are discussed for damage detection and location applications. We model the modulation that the acoustic wave pressure field induces over the sensing property of an integrating OFS. The basic trends predicted, for varying sensor length, distant and orientation to the ultrasonic source, are experimentally confirmed. The model characterizes the directivity pattern of these sensors with obvious implications in damage location. We also prove that a polarimetric sensor exhibits similar integration behaviour than an interferometric system. The unparallel remote inspection capabilities and broadband (spatial and temporal) ultrasonic generation and detection features attainable by combination of laser generation and interferometric detection are demonstrated. An all-optical remote inspection tool for materials is constructed and experimentally applied to aluminium samples. The processing of the detected ultrasonic data by 2D-Fourier transform and reassigned-spectrogram provides high quality and high resolution information of the structurally localized and global dispersion characteristics. This is utilized to demonstrate sensitivity to temperature changes and to illustrate hole-damage detection. Finally an inversion technique is applied to the broadband dispersion information allowing accurate and repeatable estimation of elastic and geometrical properties of the structure. The technique provides values of Young’s modulus (E) 71.0GPa, Poisson’s ratio (ν) 0.352, and plate thickness (d) 1.16 mm, experimentally validated within an error of 1% for E, and 2% for ν and d. Monitoring the deviation of these values with respect to an undamaged signature can be used as indicator of structural damage or deterioration. The technique provides values of Young’s modulus (E) 71.0GPa, Poisson’s ratio (ν) 0.352, and plate thickness (d) 1.16 mm, experimentally validated within an error of 1% for E, and 2% for ν and d. Monitoring the deviation of these values with respect to an undamaged signature can be used as indicator of structural damage or deterioration.