A New Dual-Mode Electromagnetic Acoustic Transducer for Preload Measurement of Superalloy Bolts

Electromagnetic ultrasonic measurement of preload in highly attenuating superalloy bolts faces two critical challenges: the low ultrasonic signal amplitude and impracticality of measurement without prior knowledge of the bolt’s initial condition. To address these issues, this study proposes a dual-mode electromagnetic acoustic transducer (EMAT) featuring a new permanent magnet configuration. For superalloy bolt specimens, the proposed configuration not only resolves the challenge of insufficient Longitudinal Wave (LW) excitation inherent to traditional EMAT but also achieves a 280.78% enhancement in Shear Wave (SW) signal amplitude. The experimental results on bolt preload measurement demonstrate that the regression model established from measurement data obtained by the new permanent magnet-configured EMAT exhibits a 0.0672 higher R² coefficient compared to that of the traditional EMAT. In the comparative analysis of bolt preload measurement accuracy between mono-wave and bi-wave methods, the relative errors for SW mono-wave, LW mono-wave, and bi-wave methods are 0.45%, 0.18%, and 0.51%, respectively. The new dual-mode EMAT proposed in this study provides a robust methodology and critical data references for aerospace engine bolt preload monitoring.