Study On Properties Of Laser Induced Ultrasonic Waves In Solid With Different Structures And Damage Detection Simulation Via Pearson Correlation Coefficient

Laser ultrasonic technology is a novle technology which excites ultrasonic via laser.The technique facilitates non-destructive testing(NDT)and non-destructive evaluation(NDE)of materials for its non-contact feature and ability of generating broadband surface and bulk waves simultaneously in materials.Therefore,it plays an important role in the field of material evaluation and defect detection.The evaluation of materials via laser ultrasonic technology is based on the deep understanding of ultrasonic waveform in different solids.The obvious difference of waveforms in different media increases the difficulty of waveform analysis.In order to reveal the characteristics of laser ultrasonics in the medium,this paper research on the wave propagation and evolution in solids with different structures by theoretical analysis and numerical simulation.For the ultrasonic wave with obvious modal characteristics,the excitation mehod is optimized to realize beam regulation.While for the ultrasonic wave that are difficult to distinguish,a damage feature extraction scheme is proposed to improve the accuracy of material detection.The different laser absorption behavior between metallic materials and nonmetallic materials may lead to different internal sound fields.Therefore,the laser ultrasonic wave in non-metallic fused silica with different geometry is studied firstly.The two-dimensional numerical model of the pulsed laser induced ultrasonic wave in the thermoelastic mechanism is established by finite element method.The transient temperature filed of the illuminated region is analyzed,and the ultrasonic waveforms at different positions in the rear face of specimen are obtained.The waveforms in glass plates of various thickness are also calculated.The numerical results show that the temperature field in the fused silica which is considered as a buried bulk-thermal source,is different from the metallic material and lead the precursor a dipolar pulse.The ultrasonic waves are converted into surface waves by Lamb waves during the thickening of the material.The evolution is basically the same as that of metal,which means that the ultrasonic wave optimization scheme in non-metallic quartz glass can learn from metal materials.For the infinite solid material,we discuss the way to improve the focused ultrasonic beam induced by line-shaped laser phased array(LPA).The influence of geometric attenuation on the magnitude of the focused beam is analyzed in theory.An improved array with conjunction of geometric attenuation and directivity functions is designed and the superiority of the array over the one which only considers the directivity function is investigated.Numerical simulation for the generation of focused longitudinal wave in aluminum plate is implemented to reveal advantages of the improved LPA in the thermoelastic regime.It is shown that the amplitude is increased,the rise time is reduced,as well as spatial sizes are narrowed in longitudinal and transverse directions,respectively.Additionally,the effect of the thermal expansion superposition caused by adjacent laser pulse on the focused beam is studied since the excitation of ultrasonic wave is affected by the shape of the thermal expansion region.It can be seen that the appropriate spacing between adjacent elements should be twice larger than the pulse to ensure the spatial quality of the focused beam when the time lag of adjacent pulses is less than the time of the illuminated region cooling down to the initial value.This work demonstrates a promising strategy to improve the focused beam in the infinite solid material.For the thin plate material,the ultrasonic waves propagate in the form of Lamb wave.The main propose of this work is to discuss the selection and control the mode of Lamb wave for its multi-modal character and frequency dispersion.The fundamental modes are selected for analyzation since they exist at all frequencies and carry more energy than the higher-order modes in most practical situations.Two-dimensional numerical analysis model for an obliquely incident Gaussian laser pulse induced ultrasonic in an aluminum plate is established in the ablation regime.The influence of the oblique angle of force source on the waveform is calculated and the vertically incident condition is analyzed for comparation.The results show that the energy between symmetric mode and antisymmetric mode transferred under the oblique incident condition.The increased transverse component of force source enhances the amplitude of S₀ mode and the in-plane increasement is larger.On the other hand,the decreased longitudinal component of force source reduces the out-plane amplitude of A₀ mode.Our approach provides an intriguing way to control the different mode of Lamb wave in the thin plate material.The above research provides regulation methods for bulk and Lamb waves with distinct characteristics in different solids.The improved ultrasonic signal helps to the detection of different media.However,for some coating-substrate solids,the waveform is cluttered,making it difficult to obtain defect features by modal analysis.In this paper,the defect detection of the interface in the coating-substrate structure is further developed,thus extract the required information from the complex waveforms.For the coating-substrate solid material,two post processing algorithms are respectively modified and used to describe the interfacial damage for the no-intact-specimen case and intact-sample-provided case,respectively.The three-dimensional coordinate system is used in the simulation model to describe the propagation of ultrasonic waves induced by the line source of finite length in intact and damaged coating-substrate structures respectively.The influence of thickness and hardness of coating on the waveform is firstly calculated and structure in which the mode of the waveform is hard to characterize is selected for analyzation.The proper length of signal is determined by analyzing the signal of time-domain and frequency-domain at different positions of the surface.The result indicates that the proposed Pearson correlation coefficient-based methods enable the damage visualization.The frequency-domain processing algorithm is better to provide a relatively stable base value for the no-intact-specimen case while the time-domain processing algorithm is more sensitive to damage region for the intact-specimen-provided case.Moreover,the ability of the proposed method in characterizing damage of different size and shape is further investigated.Considering the merits and shortcomings of each approach,the optimum operation scheme for different materials is discussed.The proposed method clarifies the influence of a hidden damage on correlation coefficient,which provides an intriguing way to obtain the damage information via complex waveforms.Specifically,it avoides discussions of specific modalities and effectively simplifies the process of data analysis.