Inspection of notch depths in thin structures using transmission coefficients of laser-generated Lamb waves

• We characterized laser generated Lamb waves in thin plates.
• We developed a simple finite element model to simulate how a pure mode Lamb wave transmits across varying notch depths.
• We developed a quick method to determine transmission coefficients from a single signal based on continuous wavelet transform.
• Experiments are conducted to investigate how varying notch depths will affect the transmission coefficients of different laser-generated Lamb waves, and the results match well with finite element results.

The non-contact feature of the Laser/EMAT ultrasonic (LEU) technique is attractive for its NDT applications. However, it is challenging to apply it in thin structures because of the difficulties in the signal interpretations. In this work, the LEU technique is used to inspect the notch depths in thin steel plates. A Continuous Wavelet Transform (CWT)-based algorithm is developed to calculate the transmission coefficients of laser-generated Lamb waves. The effect of varying notch depths on Lamb waves’ transmission coefficients is investigated both numerically and experimentally. The transmission coefficients of laser-generated Lamb waves calculated using CWT have been used successfully to predict the notch depths in thin structures.