Pretightening stress measurement method of aero-engine fastening components based on acoustic time difference separation of the coupling layers

Accurate measurement of the pretightening stress of the fastening components in the rotor system is crucial for ensuring assembly accuracy and operational safety in aero-engines. The acoustic time difference (ATD, ultrasonic propagation time difference) of the coupling layer reduces the pretightening stress measurement accuracy of the fastening components. This paper proposes a method for measuring the pretightening stress of the fastening components of the aero-engine based on the separation of the ATD of the coupling layer. Firstly, the echo signal characteristics of the bottom surface of the fastening components are analyzed by simulation, and the influence of the coupling layer ATD on the stress measurement error is studied. Then, based on the ultrasonic multi-layer medium propagation model, a scheme of adding a transition layer to construct the first echo is proposed. The ATD of the coupling layer is fed back by the first echo propagation time difference, and the ATD caused by the coupling layer and the stress is solved by the second echo. On this basis, the ATD separation method of the coupling layer is proposed and the pretightening stress measurement model of the fastening components based on the ATD separation of the coupling layer is established. The experimental results show that after the ATD of the coupling layer is separated, the maximum measurement error of the pretightening stress of the fastening component is reduced from 13.016 MPa to 4.738 MPa, which effectively improves the measurement accuracy.