Measurement of Axial Force of Bolted Structures Based on Ultrasonic Testing and Metal Magnetic Memory Testing

Bolted structures are widely utilized in flange sealing of the petrochemical high-pressure facilities, measurement of axial force of bolted structures can contribute to monitor flange sealing status. In this paper, static tensile tests are conducted in laboratory. The stress distribution of bolt with different axial tensile load is analyzed by using finite element (FE) method. Two non-destructive testing (NDT) methods including ultrasonic testing and metal magnetic memory testing are used to measure axial force of two bolt materials (low-carbon steel 4.8 and medium-carbon steel 8.8 bolts) during the whole tensile process. For the ultrasonic testing, the test results show that there is a stable linear correlation between the axial force of the bolt and the ultrasonic time of flight (TOF), and the TOF measurement is upgraded by using cross-correlation analysis technique. For the metal magnetic memory testing, magnetic signals increase with the increase of axial force because of the effect of stress-magnetization. Furthermore, magnetic memory characteristic parameters have a good linear relationship with axial force within the scope of a certain stress. Therefore, metal magnetic memory testing can be used as a novel method to measure axial force of bolted structures. Finally, the advantages and drawbacks of these two NDT methods are analyzed, which can provide a reference for the engineering application of effective NDT methods and technologies in bolt axial force measurement.