Visualization of Lamb Wave Interaction with a 5 mm Fatigue Crack using 1D Ultra High Frequency Laser Doppler Vibrometry.

摘要

Elastic waves can travel across relatively large distances in thin metallic plates, and can be affected by discontinuities such as cracks. An important research area in guided wave SHM is the characterization of the lower limits of detectable damage and identification of measurement and signal processing techniques for reliable detection of damage at those limits. This paper presents the results of experimental measurements of Lamb waves collected using an ultra-high-frequency laser Doppler vibrometer (UHF-LDV). The two test articles were 3.175 mm thick aluminum plates with a small (1.59 mm) center hole added to facilitate growth of a fatigue crack. One plate was left undamaged while the second contained a 5 mm fatigue crack emanating from the hole. The UHF-LDV provides out-of-plane displacement measurements at a much greater temporal and spatial resolution than is possible with most other commercial LDV systems. Point spacing of approximately 0.5 mm was used in a 21 mm x 22 mm scan region centered on the hole and crack. PZT transducers were used to induce Lamb waves in the structure at frequencies from 400 to 600 kHz. The data show small but measureable differences in elastic wave propagation patterns between undamaged and damaged plates. Recommendations for additional analysis and future experiments are also provided.