Research On Nondestructive Testing For Surface Crack In Metallic Components Based On Surface Acoustic Waves Generated By Laser Pulse

The research on NDT for crack in metallic components by using surface acoustic waves(SAW)generated by laser pulse is carried out in this doctoral dissertation supported by National Natural Science Foundation of China(No.51375434).On the basis of analyzing the research status and development tendency of ultrasound NDT and laser ultrasound NDT,and understanding the crack detection mechanism of SAW generated by laser pulse,the optimization analysis of propagation characteristics of the SAW,the crack feature extraction method based on nonlinear modulations of SAW,and the detection of surface crack by using scanning laser source(SLS)technique are mainly researched.Mean while,by combining the state space predictive model(SSPM),the crack detection results are analyzed and the object to quantitative NDT of surface crack in metallic components is achieved.Furthermore,a surface crack detection system by using SAW generated by laser pulse is built to meet the NDT requirement of modern industry.The scheme of this dissertation is organized as follows:In Chapter 1,the great importance of metallic components to the development of national defense and national economy is elaborated,and the signification of developing NDT for metallic components is outlined.By stating the research status and development tendency of ultrasound NDT and laser ultrasound NDT,the research direction of this dissertation is clarified.Meanwhile,the research framework of this dissertation is established.In Chapter 2,the theoretical bases of crack detection by using SAW generated by laser pulse are researched.First of all,a thermal solid coupling physical model is established to simulate the interaction between high power laser pulse and metallic component.The temperature field and ultrasound field in the component generated by laser pulse are obtained by solving the heat conduction equation and acoustic wave equation using Green Function method and finite element method,respectively.Then,the generation mechanisms of SAW,longitudinal waves and transverse waves are discussed systematically.On these bases,the nonlinear modulation mechanism between SAW and surface crack is mainly researched.These contents provide a theoretical basis for the following study.In Chapter 3,the optimization of propagation characteristics of the SAW generated by laser pulse is researched.Based on the generation mechanism of SAW,the effects of the rising time,the width of the exciting laser pulse as well as the variable cross-section of the metallic component on the propagation characteristics of the SAW are analyzed.On these bases,the optimization of propagation characteristics of the SAW is realized.The research results lay a theoretical foundation for the NDT application of the SAW generated by laser pulse.In Chapter 4,a crack sensitive feature extraction method based on nonlinear modulations of SAW generated by laser pulse is proposed.The SSPM technique is used to reconstruct the one-dimensional vibration signal of the metallic component into the high-dimensional state space.The dynamic characteristic of the component can be described by geometric figure in the state space and crack is shown to change the properties of the geometric figure.The normalized nonlinear prediction error(NNPE)feature is proposed in this study to identify the changes in the geometric figure-and hence the crack.Experimental results reveal that the NNPE feature outperformed the usually used linear features.In Chapter 5,a crack quantification method based on SLS and SSPM is proposed.The SLS technique is introduced to scan the surface of the metallic component to improve the real-time performance of the proposed technique.The SSPM technique is used to reconstructing the state space and the NNPE feature is used to indentifying the crack.Then the bilinear interpolation method is used to improving the detection accuracy of the proposed technique.Experimental results reveal that the surface crack in metallic components can be successfully detected and quantified with high detection accuracy and real-time performance by using the proposed technique.In Chapter 6,a reference-free crack sensitive feature extraction method based on the SLS technique and SSPM technique is proposed to detect the surface crack in metallic components,without using any reference data from undamaged component.Experimental results show that the surface crack can be successfully detected and quantified by using the proposed reference-free crack quantification method.In Chapter 7,the conclusions and the innovation points of this dissertation are summarized,and some advanced topics for future work are also forecasted.