Research On Imaging Of Dynamic Ultrasonic Wavefields Of Internal Defects In Micro Mechanical Structure Based On Digital Holographic Interferometry

With the development of micro nano manufacturing technology,product quality control has become a research hotspot.A new method of ultrasound-laser holography composite nondestructive testing is proposed for the detection of internal defects in micro mechanical structures.It is proposed that the ultrasonic excitation micro mechanical structure is used to measure the dynamic ultrasonic wavefields of the micromechanical structure with the digital holographic interferometry,and the digital hologram is reconstructed to restore the three-dimensional ultrasonic wavefields images.In this thesis,the imaging of dynamic ultrasonic wavefield of internal defects in micro mechanical structures based on digital holographic interferometry is studied.In order to measure the dynamic ultrasonic wavefields,the technology of digital holography,including the complex amplitude distribution of light field,the mode of reproducing light and the setting of diffraction light,are studied and analyzed.The mathematical model of three-dimensional dynamic ultrasonic wavefields measurement based on digital holography is established,and the average intensity of the digital holographic interferogram is obtained,and the distribution of the interference field of the ultrasonic wavefield is revealed.The characteristics of three-dimensional dynamic ultrasonic wavefields are analyzed,and a dynamic acoustic wavefields imaging detection method based on digital holographic interferometry is proposed.The digital hologram of the transient ultrasonic w avefields of the micromechanical structure is obtained by the time delay,and the digital holographic interferograms of the transient ultrasonic wavefields are calculated,and the three-dimensional dynamic ultrasonic field is recovered.The method can effectively acquire transient ultrasonic wavefields and detect 3D ultrasonic wavefields.The imaging principle of the reflective off-axis digital holography is studied and analyzed.The pulsed laser reflected digital holography system is designed,and the light path of the reflective digital holographic microscope is optimized.The short coherent distance of the pulse laser is guaranteed,the influence of the stray light on the system is reduced and the quality of digital hologram is improved.The relationship between the frequency of the ultrasonic wavefields,the pulse width of the pulsed laser and the exposure time of the high speed camera is studied and analyzed.The pulse width of the pulsed laser and the exposure time of the high speed camera are determined,and the high precision measurement of the high frequency dynamic ultrasonic wavefields is realized.In order to verify the detection accuracy of the system,the resolution test targets is detected.The results show that the imaging precision is higher than the traditional reflective system.The Fresnel diffraction integral algorithm,convolution algorithm and angular spectrum method are studied and analyzed.The influence of the reconstructed distance on the reconstructed image quality is analyzed.The method of auto adjusting reconfiguration distance based on the average gradient is proposed,which makes the reconstructed image quality to be the best.A reference light mathematical model including tilt factor is established,and the influence mechanism of tilt factor on reconstructed image quality is revealed.The unwrapping algorithm and phase distortion compensation algorithm are fused,and an optimized least square curve fitting phase mask method is proposed,which improves the imaging efficiency and quality.The mathematical model of computer-generated hologram and ultrasonic wavefields is studied and analyzed.The simulation process of digital holographic interferogram of the dynamic ultrasonic wavefields is designed,the ultrasonic wavefields measurement process is simulated,and the digital holographic interferograms of the ultrasonic wavefields are calculated.The function relation between the phase change and the displacement is obtained by analyzing the digital holographic interferogram of the ultrasonic field with different amplitude,and the law of the interference fringe changes with the amplitude of the ultrasonic field is revealed.By analyzing the digital holographic interferograms of the ultrasonic wavefields of different amplitude,the function relation between the phase change and the displacement is obtained,and the law of the interference fringe variation with the amplitude of the ultrasonic wavefields is revealed.The phase unwrapping algorithm based on discrete cosine transform and the phase unwrapping algorithm based on reliability ordering are studied.The phase unwrapping experiment of the digital holographic interferograms of ultrasonic wavefields shows that the phase unwrapping algorithm based on the reliability sorting can effectively suppress the propagation of the error.The reliability function and path in phase unwrapping based on reliability sorting are analyzed,and an optimization algorithm based on reliability sorting phase unwrapping is proposed,which reduces the effect of noise on phase.Simulation experiments show that the method can recover the three-dimensional ultrasonic field better.In order to verify the correctness and reliability of the dynamic imaging method and system proposed in this thesis,two experimental schemes are proposed.First,the ultrasonic wavefields of the piezoelectric wafers is detected,the digital holograms of the transient ultrasonic wavefields are captured and the three-dimensional images of the ultrasonic wavefields are reconstructed,and the correctness of the imaging of dynamic ultrasonic wavefield based on digital holographic interferometry and tsystem of digital holographic microscopy is verified.Secondly,an aluminum alloy with micro defects is designed,and the digital holograms of the transient ultrasonic wavefields are obtained,and the three-dimensional image of the ultrasonic wavefields are reconstructed.The experimental results show that the imaging of dynamic ultrasonic wavefields based on digital holographic interferometry and system of digital holographic microscopy have high sensitivity and resolution.