Experimental Study On The Mechanism Of Photoacoustic Nonlinear Mixing-frequence Based On "clapping" Theory

In recent years,nonlinear ultrasonic crack detection technology has attracted much attention because of its much higher crack detecting ability than linear ultrasonic crack detection technology.By utilizing various nonlinear effects produced by the interaction between acoustic waves and microcracks in materials,microcracks with a long size far less than ultrasonic can be detected.At the beginning of the 21 st century,non-linear ultrasonic crack detection technology began to use laser technology to excite ultrasound and modulate cracks for crack detection.This technology of laser-induced ultrasound and photothermal-assisted modulation crack detection is called photoacoustic non-linear crack detection technology.As a kind of photoacoustic non-linear crack detection technology,photoacoustic non-linear mixing-frequence detection technology has the advantages of high signal-to-noise ratio and simple implementation.Based on the clapping theory,this paper studies the influence of the resonance peak frequency shift caused by the power change of the heating source on the amplitude of the main peak signal of the nonlinear mixing.The results show that the resonance frequency of the material will shift and the amplitude of the main peak signal of the nonlinear mixing-frequence will change due to the power change of the heating source.Then the relationship between the stabilization time of the amplitude of the non-linear mixing-frequence signal,i.e.the pre-modulation time,and the power of the heating source is studied.The results show that the shorter the pre-modulation time is,the larger the power of the heating light source is.At last,the phenomenon of the disappearance of the non-linear mixing-frequence signal caused by the change of the power of the heating source is studied.This phenomenon can be used to retrieve some information about cracks in materials,such as crack width,stiffness at cracks,etc.At present,the photoacoustic non-linear mixing-frequence crack detection technology mostly irradiates the crack by the combination of the excitation light source and the heating light source.In order to study in detail the contribution of two light sources to the mixing-frequence phenomenon in the laser-ultrasonic non-linear mixing-frequence crack detection technology,this paper separates the excitation light source from the heating light source in space,and discusses the contribution of two light sources to the non-linear mixing-frequence signal in acoustics and thermology by studying the corresponding changes of the non-linear side lobe signal.In the experiment,keeping one laser beam at the crack and moving another laser beam near the crack,the change of the amplitude of the non-linear mixing-frequence signal with the position of the moving light source was studied.The experimental results show that the scanning laser spot is in a certain range near the crack(usually 2-6 times the diameter of the laser spot),and the phenomenon of non-linear mixing-frequence occurs;furthermore,the heating light source is irradiated at the crack,and the excitation light source is moved to the area where the mixing-frequence signal can not be produced.The non-linear mixing-frequence signal can not be produced by compensating the power of the heating light source.This phenomenon shows that,based on the generation mechanism of photoacoustic non-linear mixing-frequence,it is necessary to consider the role of high-frequency acoustic wave of excitation source in the generation of non-linear mixing-frequence signal.