Correction of coupling error in contact-type ultrasonic evaluation of bolt axial stress

• Coupling error within backwall TOF (Time of Flight) utilized in ultrasonic bolt axial stress measurement is investigated. • An analytical model of coupling error compensation using the scattering of bolt bearing surface is established. • Finite element simulation based on multi-physics strategy is carried out to verified the proposed method. • The synchrosqueezed wavelet transform is introduced to extract the scattering component from the pulse echo signal. The stress monitoring of bolted connections is crucial in the evaluation of mechanical systems’ structural health, and the ultrasonic method using TOF (Time of Flight) is considered promising in this application. However, the discrete nature of couplant layer characteristics can deteriorate the robustness of the ultrasonic method significantly. Here, for the purpose of deducting the coupling error, a compensating method of TOF is proposed. In this method, the components of the waves backscattering from bolt bearing surface within the temporal signal are filtered and utilized. The analytical coupling error correction model is established and the influences of the installation eccentricity upon the model are analyzed. Meanwhile, the proposed method is further verified by a finite element simulation and the results are consistent with the analytical derivations. Ultimately, the proposed method is experimentally validated and the scattering components are extracted from the pulse echo signal by using the synchrosqueezed wavelet transform. The results imply that, after being compensated by the proposed method, the stress measurement error decreased from 5 % to 1 %. This research provides a novel perspective in reducing the coupling error in a concise manner and is potential in improving the practicality of the ultrasonic bolt axial stress measurement methods.