A Highly Sensitive Laser-Magnetic Focusing EMAR Method for Thin Metal Wall Thinning Measurements at Large Lift-Offs

In this article, we propose a laser-magnetic focusing electromagnetic acoustic resonance (EMAR) (laser-MFEMAR) method for flaw detection at large lift-off. This method effectively addresses the problem that thinning defects of thin metal walls cannot be easily and quickly detected by electromagnetic acoustic transducer (EMAT) at large lift-off, under high temperature and vibration conditions. Being different from traditional EMAR, the laser-electromagnetic acoustic resonance (laser-EMAR) does not require swept-frequency excitation and can excite resonance signals in a wider frequency bandwidth. The modified laser-MFEMAR can receive detection signals at a lift-off as much as 15 mm. The signal-to-noise ratio (SNR) and spatial resolution for large lift-off detection of thin-walled metal parts can be further improved by employing a power spectrum estimation method. Finally, we performed EMAR and laser-MFEMAR detection comparison experiments on thin-walled superalloy standard specimens thinned in a 0.1-mm gradient. The results showed that laser-MFEMAR is capable of performing high-precision thickness detection of thin-walled metal parts at greater lift-off than EMAR, and the relative errors of detected wall thickness did not exceed 3.89%.