Research On Ultrasonic Measurement Equipment And Nonlinear Method Of Steel Internal Stress Detection

Residual stress is an important factor in the reliability of structure.In general,the existence of residual stress causes decrease of performance and appearance of cracks.The stress concentration effect occurring at the crack further aggravates the crack,thereby decrease the life of the structure,and even causing major production safety accidents such as brittle fracture.Ultrasonic non-destructive measurement has the advantages of safety,no pollution,deep depth of detection and good directionality in the measurement of residual stress,and is suitable for measurement of in-service equipment.The main work of this paper is as follows:First,a ultrasonic launching board is designed to stimulate transducer,use FPGA to control the pulse width,the FPGA can adjust the pulse width finely,and use the DIP switch to set the pulse width to adapt to different transducers.In the echo wave processing circuit,a channel selection circuit is added to analyze the luanch wave and the echo wave in the one-shot-receive circuit.In bandpass circuit,a high bandwidth bandpass circuit is used to pass the higher harmonic components of interest.Secondly,the data acquisition card is designed to collect ultrasonic signals.It's main controller is FPGA With the sampling frequency of 100MSPS and Quantization accuracy of 11 bits,buffered by DDR3 SDRAM,the communication interface is USB3.0.The DAQ card supports hardware trigger and software trigger mode.The state machines of FPGA internal control module are introduced in details.The initialization and starting one acquisiton flow chart of the upper computer software are introduced carefully.Finally,the pre-stressed test piece is designed,and the sound velocity and acoustic energy parameter are analyzed with the change of stress according to the collected data.It was found that the speed of sound and the acoustic energy were linear with respect to stress,but there was inaccurate test piece during the measurement.The spectral transformation of the data shows that the greater the stress,the larger the amplitude of the higher order harmonics in the spectrum.In the measurement of higher harmonics,the test pieces conform to the fitted curve,indicating that the nonlinear parameters are more stable than the linear parameters.