Active Damage Detection by Scattered Plate Waves
Dr. Ching-Chung Yin
|關鍵字:||散射;主動偵測;壓電材料;scattering;active damage detection;piezoceramics|
This thesis experimentally investigates feasibility of detecting the locations and sizes of damages such as a circular hole and its edge crack in a plate using an active damage detection technology based on scattered plate waves. The experiments were carried out by an array of piezoelectric ceramic PZT discs surface mounted on both sides of the specimens as actuators and sensors to launch and detect the fundamental anti-symmetric plate waves. Damage positions are determined through time-of-flight of the envelop of wave group, which and the scattered spectra of plate waves from damages are characterized by time-frequency analysis. The time-frequency analysis on signals of plate wave was processed by continuous wavelet transform with a mother wavelet of Gaussian cosine pulse. Besides scattered plate waves from the damages, the signals for the damaged plate also include the signals of plate wave propagating direct from actuators to sensors. Under only ideal conditions, the signals of scattered plate waves can be evaluated from the difference between those signals for damaged plates and undamaged plates. The location of damage area is determined using the simplex algorithm, in which the objective function is a summation of the square of time-of-flight differences among scattered waves and those waves direct propagating between actuators and sensors. It guarantees seeking the exact location of damage due to existence of only one global minimum for the specimens having a single damage. A very good agreement between experimental results and predictions was achieved. The characteristics of scattered spectra are found to be dependent on damage types, sizes, paths of wave propagation, and angle of incidence. It results in a critical need to develop a numerical analysis of scattered spectra for various damages as references to compare them with experimental results in the near future. The present method can detect damages such as circular holes in a large plate. However, it is difficult to detect edge cracks extending from circular holes since the diffracted plate waves decay fast from the crack tips, which are far away from the actuators and sensors in these specimens.