| As a new non-destructive testing method,laser ultrasound has advantages that traditional testing techniques cannot achieve.Therefore,it has achieved rapid development in the fields of engineering machinery,rail transit,aerospace,and other fields.In order to achieve longdistance,non-contact non-destructive testing of laser ultrasound in the industrial field,it is necessary to propose reasonable analysis methods,deeply study the characteristics of laser ultrasound sound field under different conditions,and establish a method for enhancing laser ultrasound signal under the thermoelastic mechanism based on this.Therefore,the following work has been carried out in this article:Firstly,based on the principle of laser ultrasonic excitation,the influence of light source during laser ultrasonic excitation is analyzed according to the finite element theory.Based on the basic theory of laser ultrasound excitation,the theoretical sound field distribution of ultrasonic waves excited by an ideal point light source and considering the size of the light source is discussed.Next,the basic method,parameter selection,and solution process for analyzing laser ultrasound using finite element method are presented.In addition,the finite element simulation software Comsol was used to numerically simulate and analyze the ultrasonic sound field under different parameters,improving the theory of laser ultrasonic sound field distribution.Secondly,the propagation path and acoustic field directivity of laser induced ultrasound body waves were studied through theoretical experiments.Through a large number of experiments,the propagation path and sound field distribution of the body wave excited by a single pulse laser with varying size were analyzed.Based on existing theories,the relationship between the arrival time of the body wave and the size of the single pulse laser was corrected,and ideas and methods for analyzing the directivity of the body wave sound field were proposed.On the one hand,theoretical and technical guidance was provided for some applications that allow laser micro ablation testing,On the other hand,it provides a reference for the setting of light source parameters and the selection of sound field to improve the efficiency of laser ultrasound excitation.Finally,a pulse modulation based enhancement scheme for laser ultrasound bulk wave sound field is proposed,and the feasibility and effectiveness of this scheme in product internal defect detection are verified through theoretical models and simulation tests.Based on the previous research on the directionality of the acoustic field,aiming at the low efficiency of laser ultrasound excitation under the thermoelastic mechanism,and taking advantage of the characteristics of the ultrasonic field with a certain angle range,a method for enhancing the thermoelastic laser ultrasound acoustic field based on multi-pulse modulation is proposed.The basic principle of its enhancement is introduced in detail,and a finite element model is established using the Comsol simulation software,The sound field enhancement was systematically explained and analyzed by extracting the amplitude of the characteristic waveform on the sound field enhancement surface and the lower surface of the sample.After changing the parameters of various light sources in multi-pulse modulation for simulation and selection optimization,an implementation scheme for defect detection based on enhanced sound field is presented,and good results have been achieved. |
