Ultrasonic Sensing and Time-Frequency Analysis for Detecting Plastic Deformation in an Aluminum Plate

We investigate the use of low frequency (10-70 MHz) laser ultrasound for the detection of fatigue damage. While high frequency ultrasonics have been utilized in earlier work, unlike contacting transducers, laser-based techniques allow for simultaneous interrogation of the longitudinal and shear moduli of the fatigued material. The differential attenuation changes with the degree of damage, indicating the presence of plasticity. In this paper, we describe a structural damage identification approach based on ultrasonic sensing and time-frequency techniques. A parsimonious representation is first constructed for the ultrasonic signals using the modified matching pursuit decomposition (MMPD) method. This decomposition is then employed to compute projections onto the various damage classes, and classification is performed based on the magnitude of these projections. Results are presented for the detection of fatigue damage in Al-6061 and Al-2024 plates tested under 3-point bending