Research On Bolt Axial Stress Detection System Based On Acoustoelastic Effect

As a commonly used connector,bolts are widely used in the connection of key equipment in ships,vehicles,bridges,aerospace and other fields.The axial stress of the bolt has great influence on the use state,performance and lifespan of the engineering structure.Loose connection of bolts can cause structural instability and serious safety accidents.In order to ensure the reliability and safety of the whole equipment,it is of great engineering significance and application value to scientifically and accurately detect the axial stress of the bolt,and then to judge the connection state.According to the analysis of a large number of references,and aiming at the limitations of conventional detection methods,ultrasonic longitudinal wave detection method based on acoustic elastic effect was established to measure bolt's axial stress.The method realized indirect measurement of bolt's axial stress by measuring ultrasonic time-of-flight.In order to achieve accurate measurement of bolt stress,it is necessary to establish the high-performance data acquisition,transmission and processing system.The whole system mainly consists of two parts: the calibration test system and the bolt stress detection system.The former is mainly to determine the function relationship between the axial stress of the bolt and the ultrasonic time-of-flight,and the latter mainly realizes the real-time detection or monitoring of the bolt connection state.First,the paper established the mathematical model of the relationship between the axial stress of the bolt and the difference of ultrasonic time-of-flight based on Hooke's law and acoustic elastic effect,and analyzed the influence of the temperature on the measurement results.On this basis,an effective calibration test system was established.The test system of a high speed rate and a high resolution detected the ultrasonic echo signal,and used the wavelet denoising method to improve the measurement accuracy of ultrasonic time-of-flight by eliminating the influence of high frequency noise in the measured signal.Through the actual test and data processing of two different material bolts of 8.8 grade carbon steel and 12.9 grade alloy steel,the relationship between the axial stress of the bolt and the difference of ultrasonic time-of-flight was established,which had a highly significant linear relationship.The hardware and software design of bolt stress detection system based on the field programmable gate array-FPGA were completed.And the paper focused on the analysis,design and debugging of high-speed data acquisition module,FPGA control module,and Gigabit Ethernet data transmission module based on UDP protocol.The paper used the time staggered sampling method to increase the sampling frequency,used the FIFO buffer to store the data,and realized the logic design of Ethernet transmission by the finite state machine.Finally,the on-line logic analyzer SignalTap II of Quartus II software was used to debug the system.The debugging results showed that the data transmission in the physical layer did not lose packets,and the system worked stably.Finally,the PC's terminal user interface based on LabVIEW was designed,and the actual performance of bolt stress detection system was tested.The paper used LabVIEW to complete the receiving,displaying and subsequent processing of data.The axial stress of the 8.8 grade carbon steel bolt was measured by the developed detection system,and the three spline interpolation method was used to process the measured data.The results of repeated tests and data analysis showed that the whole detection system works steadily,the resolution of ultrasonic time-of-flight reaches 2ns,and the resolution of bolt axial stress bolt is about 2.131 MPa.