EMAT-Based Joint Measurement Method For Transverse And Longitudinal Waves Of Unidirectional Stress

Ultrasonic stress detection technology is widely used in the field of industrial nondestructive testing because it does not cause damage to the surface of the workpiece,and has the advantages of good directivity,strong penetrability,safety and controllability.The stress state of key components or structures in the equipment is the core parameter affecting its service performance,and the generation of stress will reduce the mechanical properties and fatigue life of the whole machine or functional components.The existing ultrasonic stress measuring instruments use piezoelectric ultrasonic probes to achieve high-precision detection of surface or deep stress,but it cannot overcome the influence of uneven coupling agent coating on acoustic time detection.Moreover,the existing detection of unidirectional stress often uses a transverse wave or longitudinal wave single-form waveform reflection method to measure the acoustic time to calculate the stress value.In order to achieve the non-contact measurement between the probe and the workpiece to overcome the influence of the coupling agent on the acoustic detection accuracy,design and optimize the Electromagnetic Acoustic Transducer(EMAT)with high signal-to-noise ratio,which becomes one of difficult point in the field of non-destructive testing.In addition,in the process of detecting single-direction stress by a single waveform,due to factors such as uneven thickness or axial deformation of the object to be tested,the accuracy of sound detection and the accuracy of ultrasonic stress measurement may be seriously affected.Aiming at the problem of the influence of uneven thickness and axial deformation on the detection accuracy during uniaxial stress detection,a joint measurement method of unidirectional stress transverse and longitudinal waves based on EAMT is proposed.The theory of unidirectional stress transverse and longitudinal waves joint measurement is derived based on acoustic elasticity theory formula.In order to design an electromagnetic ultrasonic transducer with high signal-to-noise ratio,the electromagnetic ultrasonic generation and reception mechanism is analyzed theoretically,and the relationship between the sound velocity of the transverse longitudinal waves and the stress is established along the stress direction,and lay the theoretical basis of electromagnetic ultrasonic probe design and detection technology.In order to improve the signal amplitude and transducing efficiency of the electromagnetic ultrasonic transducer,ANSYS simulation modeling of the electromagnetic ultrasonic transverse wave probe is performed,the ANSYS electromagnetic module is used to simulate and analyze the parameters of the permanent magnet in the transducer structure,the coil parameters,the lift distance and the distance between the permanent magnet and the coil and the influence law and conclusion of different parameter changes on the energy conversion efficiency are obtained.Based on the simulation results of ANSYS,the magnet structure,coil structure and shielding device are optimized and improved,and the output waveform quality of the electromagnetic ultrasonic transducer is effectively improved.In order to verify the reliability of the combined detection of unidirectional stress electromagnetic ultrasonic transverse and longitudinal waves,the formula of ultrasonic stressacoustic time difference is established,and the electromagnetic ultrasonic stress measurement system is built.Using the aluminum alloy 7075 bolt parts,the correlation coefficients between unidirectional stress and acoustic time difference were calibrated,and the stress test using transverse-longitudinal wave combination method were also carried out.By comparing with the measurements of the strain gauges,the reliability of the designed electromagnetic ultrasonic probe and the unidirectional stress measurement method was verified.At the same time,in order to determine the measurement accuracy of the method,the transverse and longitudinal waves joint measurement of the welding residual stress distribution of the aluminum alloy 7075 weldment is carried out,and the stress value at the sampling track is output based on the ANSYS simulation welding process,and the two are compared and verified.The experimental results show that the absolute error of the uniaxial stress electromagnetic ultrasonic transverse and longitudinal waves joint stress measurement is less than 15 MPa,which has high measurement accuracy.