Research On The Influence Of Ultrasonic Impact Treatment On Acoustic Elastic Coefficient Of Ultrasonic Nondestructive Evaluation

With the development of science and technology,the harsh working environment of engineering equipment is the main cause of equipment failure.External load and internal stress will reduce the processing accuracy of engineering equipment and the contour size under service conditions,and increase the safety hazards in daily use.The surface strengthening can significantly improve the comprehensive service performance of the material surface,and it can also accelerate the lightweight process of equipment in the fields of aerospace,high-speed trains,and aerial weaponry.With the widespread application of surface strengthening technology,it is urgent to explore a fast,accurate,and non-destructive method for evaluating the surface strengthening effect of the workpiece.Based on this,this subject takes 45 steel as the research object,The influence of impact parameters on the microstructure of 45 steel was discussed through ultrasonic impact treatment（UIT）of the samples,which provided a basis for the optimization and selection of ultrasonic impact parameters.The surface of the sample was treated with 1,4,and 7-pin ultrasonic impact gun tips,and UIT with a coverage rate of 0%to 600%at 15μm,20μm and25μm amplitude.The results show that:the time required for UIT of the same area with different numbers of needles is different and the microstructure changes are basically the same under the same coverage.Amplitude of 15μm,different UIT coverage rates has no obvious effect on the grain deformation.Amplitude of 20μm,different UIT coverage rates results in a gradient change in grain deformation.Amplitude of 25μm will cause severe plastic deformation and stacked defects will form on the surface.The reasons for the low signal-to-noise ratio of L_cr wave are analyzed by combining the microstructure and surface deformation,signal denoising method and cross-correlation step length of L_cr wave acoustic time difference calculation results are discussed.The results show that grain tilt and uneven surface are the main reasons for high noise energy and waveform distortion.The sym8 function decomposes the signals in 8 layers can meet the requirements.With the increase of the cross-correlation step length,the accuracy of time difference calculation is improved and when the cross-correlation step length reaches 1 cycle,the time difference calculation results is basically unchanged.Correspondence between different UIT coverage rates and acoustic elasticity coefficient is established by breaking the L_cr wave acoustic elasticity coefficient test.The offline acoustic elastic coefficient calibration test results show that the variation law of the L_cr wave propagation time difference of the sample with the tensile stress is basically the same at different UIT coverage rates,and the stress turning point is 337.5MPa.The propagation time difference of the L_cr wave fluctuates in the range of 0～20ns before the stress turning point and the propagation time difference of the L_cr wave increases exponentially along with the increase of the tensile stress after the stress turning point.The online acoustic elastic coefficient calibration test results show that:the L_cr wave propagation time difference-tensile stress curve shows a linear increase first and then a steady fluctuation trend in different UIT coverage rates.The acoustic elasticity coefficient increases with the increase of UIT coverage rates,first increases and then decreases,the acoustic elasticity coefficient reaches the maximum at 300%.The stress turning point of the 100%coverage rate UIT sample is266.6MPa,the stress turning point of the 200%coverage rate UIT sample is 266.6MPa,the stress turning point of the 300%～600%coverage rate UIT sample is 166.6MPa.The analysis shows that:UIT will cause plastic deformation of the surface of the sample within a certain depth,which will reduce the effective elastic bearing area within the L_cr wave detection depth,which will cause the stress turning point to move forward.The surface layer work hardened with the increase of UIT coverage rates,resulting in the thickness of the plastic deformation layer remaining constant which makes the stress turning point remain unchanged.The change of the surface roughness and the increase of the thickness of the strong plastic deformation layer are the main reasons for the increase of the acoustic elastic coefficient at first and then decrease.