| With the rapid development of modern manufacturing industry,the traditional flaw detection technology for mechanical parts has gradually been difficult to meet the detection needs in some scenes.Structural health monitoring of the device and the detection of workpiece defects put forward higher requirements for the means of nondestructive testing.In order to solve the problems of complex operation and low sensitivity of laser ultrasound and electromagnetic ultrasound(EMAT),which are two independent non-contact detection methods,and difficult to observe the influence of different parameters on the relevant physical field.Numerical analysis method is applied to the laser acoustic magnetic hybrid detection.A simulation model of multi-physical field coupling between pulsed laser and electromagnetic ultrasonic transducer was established by finite element method,and a laser-EMAT experimental system was designed for detect defects.The propagation characteristics of laser-excited sound waves in solids were studied,and the laser-electromagnetic ultrasonic detection model and the surface crack depth estimation model were established.Taking aluminum block as the research object,combined with the multi physical field coupling of heat,force,sound,magnetic and electric field,the coupling analysis based on laser acoustic magnetic energy transfer process and the experimental test of solid internal defect detection were carried out.The temperature field,stress field,magnetic flux density and corresponding waveform signals in the target were analyzed.The numerical simulation of ultrasonic wave propagation induced with laser irradiation on non-ferromagnetic metal was carried out by finite element method.The detection process of ultrasonic field stimulated by laser through photoacoustic effect and acoustic signal received by EMAT through coil was simulated.At the same time,With the help of finite element method,the influence of important parameters of electromagnetic ultrasonic transducer on physical field was analyzed,and the key components of receiving probe were optimized.The results showed that the voltage signal received by the spiral coil can correctly represent the ultrasonic displacement field caused by the thermal expansion effect of the incident laser in the solid.When the aspect ratio of the magnet is 1.5 times,the magnetic field intensity distribution at the skin layer was the best,and the influence of lifting distance on energy conversion efficiency showed a negative exponential change law.In addition,the receiving performance of electromagnetic ultrasonic transducer at different receiving spacing was evaluated,which had a certain reference significance for the research and design of process parameters of electromagnetic transducer in the future.In order to complete the quantitative detection of the crack depth on the workpiece surface,a finite element model of laser electromagnetic ultrasonic crack detection was constructed in this thesis.The propagation of ultrasonic wave and its interaction mechanism with groove crack were explored by numerical simulation.It was proposed to obtain the voltage time-domain response curve by using the coil in EMAT,and then calculated the depth of surface crack by eigenvalue.The results showed that the coupling effect between the voltage signal in the finite element model and the amplitude of the sound field excited by laser was good.The snapshots of the sound field at different times showed the phenomenon of waveform conversion in the diffracted ultrasonic field after encountering the groove crack.The error of calculating the depth of surface crack by laser electromagnetic ultrasonic testing method proposed in this thesis was less than 6.5%.In addition,the application of interferometric synthetic aperture method in the detection of internal defects of corroded pipelines was also explored.Referring to the laser pulse parameters and electromagnetic ultrasonic transducer parameters given in the optimization process of numerical simulation,a laser acoustic magnetic hybrid detection experimental platform was built.The thickness measurement experiments of 45~#carbon steel and typical experiments of aluminum block with artificial internal defects were studied.The propagation characteristics of laser excited sound wave were obtained by analyzing in time domain.After filtering and denoising,the signal-to-noise ratio is effectively improved.By changing the distance between the receiving and transmitting ends,the interaction law between the sound field and the internal defects of the test block was obtained.The research on defect nondestructive testing based on laser acoustic magnetism carried out in this thesis effectively couples the multi-physical field energy exchange process of laser and electromagnetic ultrasound.The simulation and experimental results prove the prospect of this hybrid testing method for industrial ultrasonic nondestructive testing. |
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