Ultrafast phased-array imaging with pump excitation for closed-crack imaging

Nondestructive evaluation of closed cracks is one of the most challenging problems in ultrasonic testing. Here, we propose a novel closed-crack imaging technique combining ultrafast phased-array with pump excitation. The pump excitation with kHz frequency can effectively induce the crack opening/closing behaviors since it can generate a large displacement of 1000 nm order. At the same time, ultrafast phased-array imaging, i.e. plane wave imaging, with MHz frequency stroboscopically captures the high-speed crack dynamics induced by pump excitation. We successfully proved the concept in a closed fatigue crack specimen.