Research On Axial Stress Measurement Technology Of High-strength Bolt Based On Acoustic Elasticity Theory

As a common connector,high strength bolts are widely used in the field of mechanical equipment.In order to ensure the stability and safety of the structure,not only the tightening quality of the bolt should be strictly controlled during installation,but the tightening of the bolt should be regularly detected during service.Conventional detection methods can’t meet the requirements of field detection due to the problems of detection accuracy or detection efficiency.A method for fast and accurate measurement of bolt fastening state is needed.The thesis first studies the method of ultrasonic nondestructive measurement of bolt axial stress.Based on Hooke’s law and acoustic elasticity theory,the relationship between transverse and longitudinal wave and bolt axial stress is deduced.Eliminates the measurement error caused by the friction and uneven stress distribution between the bolt head and the test piece.Next,build a bolt axial stress measurement platform and design a special measurement software.Using ultrasonic probe to realize bolt axial ultrasonic propagation and end-face echo collection.Next,study the factors that affect the bolt axial stress measurement results and analyze the reasons that affect the test results.Control the measurement error within an acceptable range by changing the test method and test conditions.According to the needs of the experiment,a special bolt fixture tooling is designed,and the structure and principle of the small-size transverse and longitudinal wave integrated ultrasonic transducer are introduced.Finally,the use of hydraulic tensioning machine,bolt clamping tool and vertical and horizontal wave ultrasonic stress measurement system,real-time measurement of the changes in the sound velocity of the bolt tensioning process,the measurement parameter calibration.The calibration data is curve-fitted,and the functional relationship between the bolt axial stress and the ultrasonic sound is derived.The loading value of the stretching machine is compared with the measured value.The average deviation of the range is 5%,which proves the accuracy and reliability of the measurement system.After that,industrial field measurements were carried out for diesel engine cylinder block bolts and bearing bolts.The measured value is compared with the bolt torsion loading result,and the axial stress detection value is within the loading stress range,which verifies the effectiveness of the ultrasonic method of measuring the bolt axial stress.The measurement results prove that the technology can be applied to actual field testing.