Development Of Ultrasonic Bolt Stress Automatic Calibration Test System

High strength bolt fasteners are widely used in the fastening connection of various equipment.In order to ensure the safety and reliability of the equipment,it is very important to measure the axial stress of the bolt by scientific and effective means.At present,among many methods for measuring the axial stress of bolts,the ultrasonic stress measurement method based on the principle of acoustic elasticity has been widely paid more attention because of its advantages of non-destructive,simple,fast and high accuracy.The ultrasonic stress measurement method indirectly measures the bolt axial stress by measuring the ultrasonic time of flight in the bolt.Therefore,to use the ultrasonic method to measure the bolt stress,it is necessary to calibrate bolts of corresponding specifications in advance,establishing the relationship between the stress and the ultrasonic time of flight.The calibration accuracy will directly affect the accuracy of the actual stress measurement.Aiming at the problems such as inconvenient bolt clamping,manual loading and inaccuracy of loading force for current calibration methods,this thesis will develop a set of automatic calibration equipment for bolt stress based on ultrasonic method,to realize automatic and continuous loading control for the calibrated bolt,and real-time and accurate measurement and data processing of stress and ultrasonic time of flight,and finally achieve the relationship between axial stress of bolt and ultrasonic time of flight.Firstly,based on the acoustic elastic effect,the principle of ultrasonic bolt stress measurement was analyzed,and the mathematical model reflecting the relationship between the difference of ultrasonic time of flight and the bolt axial stress was established.Accordingly,the requirements of the calibration test system were analyzed,the design goal was dertermined,and the whole scheme design of the calibration system was completed.The whole calibration system is mainly composed of four parts: mechanical structure,stress loading control system,multichannel data acquisition and transmission system and application software system.The mechanical structure of the calibration test system was designed.The horizontal double sliding rails’ structure was designed to ensure the convenience of bolt installation and disassembly,and the spring coupling mechanism was also designed to ensure a good coupling state of the ultrasonic probe with the tested bolt.What’s more,the stress loading control system based on servo motor and the multi-channel data acquisition and transmission system based on NI data acquisition card and ultrasonic detection module were established.The stress loading control system adopts Modbus protocol to transmit the control command by RS485 serial port to realize loading the bolt stably,continuously and accurately.The multi-channel data acquisition and transmission system realize the real-time measurement and transmission of ultrasonic time of flight,bolt axial stress,bolt tightening torque,environmental temperature and other parameters.Based on Lab VIEW platform,the software system was designed to facilitate the control of the whole calibration process and to realize the automatic processing,display,recording and output of the calibration results.After the development of the prototype of the calibration test system,a lot of experimental research and data analysis were carried out,and the influence of bolt clamping length and temperature change on the ultrasonic time of flight and calibration results was analyzed.The test analysis results show that the developed system can easily and quickly calibrate the sensitivity coefficient of high-strength bolts with the length range of 65-120 mm,the specification of m8-m12 and the accuracy level of 8.8.The resolution accuracy of ultrasonic time of flight is 1ns,the accuracy of stress measurement is ± 0.05% FS,and the calibration repetition accuracy is ± 2%.The performance index meets the expected technical requirements.