Research On Correction Methods And Mechanisms Of Longitudinal Critical Wave Refraction (LCR) Stress Measuring Technology

The residual stress of high speed train measured by ultrasonic method always shows a higher state than the act residual stress,even exceed the material yield strength.Faced with those measured results,it is difficult for the engineers to deal with those residual stress,because those measured results can't be efficient used to guide production.Residual stress is an important factor affecting the safety and reliability of the train,correcting the ultrasonic measured results and improving its test accuracy are very important to monitor the welding residual stress state of high speed train in service.Ultrasonic method is mainly based on the acoustic elasticity theory,within the elastic limit,the ultrasonic stress measurement relies on the linear relationship between the stress and the propagation time change.At present,the commonly used method in the industry is to fix the distance between the ultrasonic transmitter and receiver.Measuring the change propagation time of the ultrasonic instead of the variation of the velocity with fixed distance between the transducers.Finally,the liner relationship between variation of propagation time and stress are established for measuring residual stress.The variation of propagation time resulted from stress belongs to nanoseconds,but the different microstructures in weld,heat affect zone and base materials has more obvious influence on the propagation time than the stress,this is also the main obstacle to limit the engineering application of ultrasonic method to measure welding residual stress.The technique of ultrasonic method belongs to the sensitivity test.In addition to the microstructure change in different zones,the coupling state,the surface attachment,the surface curvature,the material heterosexuality and the signal processing method obviously affected the measured results for ultrasonic method.Based on the rapid,high precision and convenient testing requirements of high speed train residual stress,this paper takes A7N01 high-strength aluminum alloy material and its welded joint as the research object,and analyzes the microcosmic change,signal processing scheme,coupling state,test temperature,surface attachments,material anisotropy factors to carry out more in-depth study,mainly including the following aspects:(1)The principle of excitation of the longitudinal critical refraction(LCR)wave is introduced in detail.Based on the principle of acoustic elasticity,the linear relationship between the variation of propagation time and the change of stress is deduced.It is more suitable for engineering residual stress test.(2)Research signal processing of S355J and A7N01 aluminum alloy.Threshold wavelet analysis noise reduction is used to deal with the acoustic wave signal of S355J weathering steel and A7N01 aluminum alloy.It is found that the noise reduction treatment improved the precise accuracy for S355J weathering steel obviously,but no obvious effect for A7N01 aluminum alloy is observed.Different steps ware used to deal with the ultrasonic wave signals of S355J weathering steel with 200MPa tensile load and A7N01 aluminum alloy with 90MPa tensile load by cross correlation calculation.For the S355J weathering steel,if cross-correlation calculation starting point belongs to the LCR wave and the end point does not exceed the LCR wave range,the test accuracy is still reliable.(3)Research common influencing factors of ultrasonic methodThe transmission speed of each zone was measured,then the first critical incidence angle of each region was calculated based on the velocities of ultrasonic wave and plexiglass wedge.It was concluded that the influence of tissue change on the incident angle of LCR wave was not significant.The stress coefficient K of the parallel and vertical rolling direction of the A7N01 sheet and profile were calibrated respectively,the K of tension and compression state were also fitted individually.The results showed that the material processing directions and tensile compression stress state had significant effect on the K.The relationship between ultrasonic wave propagation time and test temperature in different zones of A7N01 established.The results showed that the different tissues had almost same relationships between ultrasonic wave propagation time and temperature,and the model is established to eliminate the temperature influence.The K of different depth of the A7N01 aluminum alloy plate were calibrated by different transducer frequency,and the K at different zones of the welded joint were calibrated.It was found that the K in weld zone changed most obviously with different transducer frequency.Comparing the K calibrated from parallel stress with that measured from vertical,it was found that the vertical stress with the test direction has little effect on the measured results.The scheme used to measure the residual stress of arc surface was analyzed theoretically,and put forward some programs to calibrate K,to,and the first critical angle of incidence.The results showed that surface paint has no little effect on K,but effect on t0 can't be neglected.The roughness and t0 of the sample polished by different sizes of sandpaper are measured,the results show when the roughness is less than 10?m and the samples can be tested directly.(4)Correcting the influence resulted from the different microstructure in A7N01 welded joint by destructing of parallel samples.For analyzing the test errors induced by the different tissue in each zone of A7N01 aluminum alloy MIG welded joint,the calibration samples of each zone were manufactured respectively.To quantitative analyze the errors of K and t0 resulted from the different microstructure in each zone,the K and t0 calibrated by the samples produced from each region to correct the measured results.To verify the higher reliability and accuracy of the corrected results,the hole-drilling measured results were used as the reference.(5)Online correcting the influence induced from different microstructure in each zone of A7N01 welded j ointIn this paper,samples with different precipitation phases and grain sizes were prepared by heat treatment,and the attenuation of each sample was tested.The relationships between attenuation and grain size,between precipitation phase and attenuation were established,in order to correct the results measured by LCR wave technology online.This paper refers to the test method for the critical refraction longitudinal wave velocity attenuation(LCR-AV)method.Similarly,the LCR-AV method was compared with the blind hole method,and it was found that the LCR-AV method was more effective.