Research On Detection Method Based On Laser Thermoelasticitv For Surface Defect Of Mechanical Parts

Mechanical parts are the inseparable basic units of machinery.They are widely used in aerospace,energy,chemical,transportation,weaponry,manufacturing facilities and so on.Some mechanical parts may work in harsh environments such as high temperature,high pressure,high load,corrosion and radiation.During their service,defects will generate on the surface or inside of the parts.If the defects cannot be identified in time and effectively,they will deteriorate and eventually lead to the failure of parts.The failure of mechanical parts seriously reduces the stability and reliability of mechanical equipments,and may even lead to high-risk and destructive accidents,resulting in huge economic losses and endangering people's lives.Therefore,research on fault diagnosis methods for mechanical parts to evaluate their quality and safety and ensure their reliability has great social and economic benefits.This research is supported by National Natural Science Foundation of China(No.51375434)-A New Dection Method of Laser Ultrasonic Techniques for Fatigure Cracks of Mechanical Components.In this dissertation,we study the detection method based on laser thermoelasticity for surface defect of mechanical parts.This dissertation mainly includes the following five parts.(1)The basic principle of laser thermoelasticity is analyzed,and the excited model of laser ultrasonic is established.A method based on Romberg numerical integration and De-Hoog numerical inversion for solving laser ultrasonic field is studied.For the flat and cylindrical mechanical parts,the fully coupled finite elment models are established.The explicit finite element method is used to solve these models.Based on the results of the models,the detection methods for surface defects of mechanical parts under different excitation modes of laser-induced surface waves and their application characteristics are analyzed.The analysis results provide a theoretical basis for the detection method based on laser thermoelasticity for surface defect of mechanical parts.(2)The characteristics of surface waves excitated by single-point laser are analyzed.A wavelet packet-singular value decomposition(WPT-SVD)method is proposed to identify the surface defect in mechanical parts under single-point excitation mode.A parameter kr is defined to characterize the depth of the surface defect.Firstly,the time of flight of surface waves is calculated by the wavelet analysis to identify the position of the surface defect.Secondly,the transmission signals of cylindrical surface defects are decomposed by WPT method.The characteristic matrices are constructed to represent different states of the defect signals.Thirdly,the non-zero singular value group of characteristic matrices is obtained by SVD,which can represent the transmission signals under different defect depths.Finally,the parameter kr is defined to characterize the depth of the surface defect.A laser ultrasonic surface defect detection system for cyliderical mechanical parts is built.The experimental results show that the method proposed in this part can be used to identify the position and depth of the surface defect.(3)The mechanism of the signal enhancement of the surface waves generated by a pulsed laser for cylindrical surface defects is analyzed.Based on the mechanism,a method using the surface waves excitated by line-scanning laser is proposed to identify the position and depth of the cylindrical surface defect.According to the the variation of the peak-to-peak value of the surface waves excited by the line-scanning laser at different detection positions,the optimized method for the detection position of the cylindrical surface waves at any diameter is proposed.The relationship between SLLSP of surface waves generated by line-scanning laser and the depth of the surface defect is studied,which provides a basis for depth identification of surface defects.(4)A method for the identification of surface defects is proposed based on the frequency-wavenumber(F-W)analysis of wide-band surface waves generated by the surface-scanning laser.The surface defect is imaged by extracting the defect scattering signals in each frequency.The three-dimensional scanning laser ultrasonic detection system is built with the surface-scanning program.The resuts show that the proposed method can effectively filter the refected waves from the boundary,and the image resolution of the proposed method is obviously higher than that of RMS imaging method.The proposed method can identify the position,orientation and the severity of surface defects.(5)A quantitative method is studied based on the S-F-W imaging of the wide-band laser surface waves.Different images are obtained by S-F-W method under different sliding window size and frequency bands.By comparing these images,the optimal S-F-W sliding window size and frequency bands of surface waves are selected according to the signal-to-noise ratio and the resolution of S-F-W images.The image segmentation method for the S-F-W images of surface defects is studied,the quantitative method of the surface defect is given.On the basis of S-F-W image segmentation,geometric features of surface defects such as length,width,circumference,area and inclination angle are extracted.