Damage Quantification in Plates Using Lamb Waves

Damage detection using Lamb waves in industry often employs excitation of flawless specimen followed by introduction of a defect. The data acquired from these specimens are then compared and differences exposed in time series and spectral decompositions were highlighted and labelled as potential indicators for the presence of damage. Such analyses are done from adoption of conventional ultrasonic techniques. However, Lamb waves are more complicated, requiring a different approach. A new approach to damage quantification deals away with complex analysis. It indiscriminately analyses any combination of modes. Therefore constraints previously imposed on Lamb wave damage detection such as frequency, actuator and geometry choice are not needed. In fact low profile PZTs excited at their resonant frequency yield excellent results, encouraging their exploitation. The method is conceptually simple, computationally quick, robust and easy to use. Differences in waveforms can be quantified between damaged and undamaged specimens. It is hypothesised that the size of the damage can be related to the size of the difference in the Lamb wave response when compared to a flawless specimen. This knowledge is used to detect as well as quantify subsurface defects in plate specimens. Damage was introduced progressively larger while an automated laser vibrometry rig was utilised to sequentially collect data. Two geometries were considered, a flat plate, then a more challenging aircraft wing skin with typical aerospace geometry was attempted. Subsurface defects 15% deep were detected as well as quantified. However the transmitter and sensor location is critical to the results achieved.