Research On Quantitative Characterization Of Cracks In Metallic Components Based On Laser Ultrasonics

In this paper,it has been carried out that the research work which quantitative characterization technology of cracks in metallic components based on laser ultrasonics,aiming at the urgent need of non-contact quantitative detection of cracks in key metallic components of weapons and equipment,such as aero-engine blades,turbine disks.In order to solve these technical problems of "undetectable" and "inaccurate" in the quantitative detection of microcracks at present,these key technologies have been studied emphatically,such as timedomain sensitive feature extraction of laser ultrasonic wave,laser nonlinear harmonics feature extraction based on bispectrum,and laser nonlinear modulated waves feature extraction based on sideband peak counting(SPC).(1)A thermal-structural coupling finite element model of laser ultrasonic has been established by using Newmark integral method and finite element software ANSYS.On this basis,a model of detecting surface cracks using Rayleigh wave excited by laser thermoelasticity and a model of detecting internal cracks using longitudinal wave excited by laser ablation have been established,which are used to simulation study the feature extraction methods of Rayleigh wave characterizing surface cracks and longitudinal wave characterizing internal cracks,respectively.A completely non-contact laser ultrasonic testing system has been built by integrating and synchronizing the high-energy sub-nanosecond pulse laser and double-wave hybrid nonlinear laser interferometer based on photorefractive crystal,which is used to experimental research the feature extraction methods of surface cracks and internal cracks in metallic components.(2)In order to realize the accurate quantitative characterization of surface microcracks,it has been studied that the crack time-domain sensitive feature extraction method of laser Rayleigh waves.Firstly,on the basis of analyzing the generation mechanism of Rayleigh waves caused by surface cracks,three linear characteristics of Rayleigh waves,i.e.,reflection coefficient,arrival time difference and transmission coefficient,have been extracted.The mapping relationship between these characteristics and surface crack depth characteristics has been studied,and the range of their quantitative characterization of surface crack depth has been discussed.Then,the influence of crack inclination angle on the reflection coefficient characteristic has been studied when the depth and length of surface cracks are constant,and the influence generation mechanism has been analyzed.It is found that the characteristic is more sensitive to the depth and angle of surface cracks than to the crack length.Finally,these features of reflection coefficient and arrival time difference have been used to test the surface microcracks in aluminum plate.It is found that the reflection coefficient feature quantitatively characterizes the surface crack depth in the range of 0.5?2 times of the detection wavelength,and the arrival time difference feature quantitatively characterizes the range of 0.2mm?0.8mm,which is independent on the detection wavelength.(3)In order to realize the accurate quantitative characterization of surface microcracks in early stage,it has been further studied that the laser ultrasonic nonlinear harmonic feature extraction method based on bispectrum.Firstly,the nonlinear harmonic spectrum generated by the interaction between surface crack and Rayleigh wave has been extracted by bispectrum estimation method,and the characteristic of bispectrum damage coefficient(BDF)is defined as the ratio of the bispectrum peak difference between damage signal and intact signal to the bispectrum peak of intact signal at the frequencies F,F.Then,it has been studied that the relationship between BDF feature extracted from reflected and transmitted Rayleigh wave and surface crack depth,length and angle feature.And it has been discussed the influence of detection position and component reflection boundary on the feature extraction.It is found that the nonlinear feature is more sensitive to the depth and angle of surface crack,but insensitive to the length and detection position of surface crack,and the existence of component reflection boundary has a negative impact on the BDF feature extraction.Finally,the BDF feature extracted from the reflected Rayleigh wave has been used to test the surface microcrack in aluminum plate.It is found that the BDF feature quantitatively characterizes the surface crack depth in the range of 0.1?0.5 times of the detection wavelength.(4)In order to verify the applicability of linear and nonlinear harmonic features extracted from Rayleigh wave,it has been carried out that the research work of internal crack length detected by ultrasonic longitudinal wave.The relationship between transmission coefficient,BDF feature of transmitted longitudinal wave and internal crack length has been studied,and the influence of normalized amplitude of transmitted longitudinal wave on BDF feature extraction has been discussed.It is found that two features can well realize the quantitative characterization of internal crack length.Finally,the BDF feature extracted from transmitted longitudinal waves has been used to detect the real internal closed cracks of steel members,and it is found that the feature can detect the internal closed cracks well.(5)Aiming at the internal closed microcracks in complex shape components that is not easy to be detected by traditional detection methods,it has been studied that the feature extraction method of laser ultrasonic nonlinear modulated waves based on sideband peak counting(SPC).Firstly,the characteristics of laser ultrasonic guided waves in metallic rods has been analyzed,and the dispersion curve has been drawn.It is found that the ultrasonic guided waves excited by pulse laser in metallic rods have dispersion and multi-modal characteristics,which leads to the aliasing of ultrasonic guided waves which is difficult to distinguish.Then,the SPC method has been used to extract the nonlinear modulated wave features in the ultrasonic guided wave spectrum,and the SPC difference feature has been defined.Finally,the nonlinear modulated wave feature has been used to test the real single and multiple internal closed cracks in steel rods and the real single internal closed crack in steel curved surface member,and it is found that SPC difference feature can effectively detect the real internal closed cracks.Finally,the surface crack with the minimum depth of 0.1mm and the internal closed crack with the minimum radial length of 1.5mm have been successfully tested by using the completely non-contact laser ultrasonic testing system in this paper.