Scanning laser source Lamb wave enhancements for defect characterisation

• We identify enhancements of Lamb waves as a laser source is passed over a defect.
• Time–frequency analysis used to track variations in individual Lamb modes.
• Several contributing mechanisms to this enhancement are identified.
• Defect position and severity can be found from the observed enhancement.

Surface-breaking defects in thin storage structures can cause costly component failure if left undetected. Here, a method for detecting and characterising surface breaking defects using near-field Lamb wave enhancements is presented for measurements in which a laser generation source passes over the defect. A dual laser scanning system is used to generate and detect Lamb waves in aluminium plates containing v-shaped laser micro-machined slots of different depths. Time–frequency analysis techniques are used to identify and track the magnitudes of individual wave-modes across a scan at different frequencies, and large enhancements in signal magnitude at certain frequencies are observed for the fundamental A0 and S0 wave modes when the laser source passes over the defect lip. The mechanisms responsible for this enhancement are identified and examined, and a characterisation process for identifying the position and severity of the defect is presented.