Application of Ultrasonic Stress Measurement in the Evaluation of Bolt Fastening Force

The acoustic elasticity effect means that when the material is subjected to mechanical load, the stress changes the velocity of sound wave propagation. This property can be used for the surface of the material itself or for the overall stress measurement of the material. The ultrasonic method is much more accurate than using torque wrench to control the pretightening force of fastening bolt. The use of this mechanical-based measuring device can lead to uncertainty caused by random torque caused by mechanical manufacturing tolerances. The measurement of ultrasonic pretightening force gives the tensile force information inside the bolt and eliminates the uncertainty of mechanical method. Fastening stress of bolts is measured by generating compression wave and shear wave (double wave method) from electromagnetic coupling transducer (EMAT) in situ. Ultrasonic method does not need to remove bolts and do not need to measure bolt length when bolt preload has been tightened. The uncertainty in this method of measurement is about 5 percent.