Axial Stress Evaluation of Service-Exposed Double-Headed Bolts Using EMAT Techniques

An electromagnetic ultrasonic method was proposed for quantifying the axial stress of service-exposed double-headed bolts in rocket launch platform steel structures. A shear-wave excitation scheme was adopted, and an acoustoelastic time-of-flight model was developed for axial stress inspection. A specialized EMAT for double-headed bolts was designed and optimized to improve excitation efficiency and signal sensitivity. The effect of clamping length, an essential geometric parameter, on inspection coefficient was analyzed, and a corresponding correction strategy was introduced. Laboratory and on-bolt tests confirmed the model's linearity, the robustness of the correction method, and the capability to accurately measure axial stress after compensating for clamping-length differences. The proposed technique provides a non-contact, rapid, and broadly applicable approach for assessing the integrity of steel structural connections.