Laser ultrasonic inspection of subsurface defects at high temperature using mode-converted surface wave

Subsurface defects are detrimental to the safety and integrity of critical components, particularly for those used in high-temperature environments. A reliable non-destructive evaluation method for in-situ inspection of subsurface defects in high-temperature components is highly desired. Laser ultrasonic techniques favorably accommodate the demands by virtue of non-contact generation and detection of surface waves. In this work, laser ultrasonic inspection of subsurface defects in high-temperature components is proposed, exploiting the mode-converted surface wave from skimming longitudinal waves as a distinctive signature for the rapid recognition of subsurface defects. Wave transition is numerically revealed with finite element simulation, from which the phenomenon of wave mode conversion is readily identified. Meanwhile, the dependence of surface wave velocity on temperature is established to adaptively compensate the velocity variation at high temperatures. Thereafter, delay-and-sum method is used to constructively enhance the signal-to-noise ratio of the mode-converted waves. A scanning laser ultrasonic setup is built to experimentally verify the feasibility and effectiveness of proposed method on the inspection of subsurface defects at different temperatures. Collectively, this work offers a viable route for in-situ inspection of subsurface defects in high-temperature components, where conventional ultrasonic methods fail. This approach has potential applications in broad fields, such as coating quality evaluation during fabrication, bearing assessment under load, and in-situ monitoring for additive manufacturing.