| Advanced composite materials are widely used in the field of aerospace and the internal damage of related parts has a very important impact on the performance of the whole aircraft.Laser ultrasonic nondestructive testing technology has the characteristics of high speed,long distance and high resolution,which is suitable for the nondestructive testing of various large-size and complex-surface advanced composite material parts.The resonant frequency of the C-H and C-H2 bonds in the composite materials is equal to that of 3.4?m,which is conducive to improving the photoacoustic energy conversion efficiency.Therefore,mid-infrared laser is the preferred excitation source to generate ultrasonic field in composite materials.In this paper,based on the crystal of Zn Ge P2(ZGP)and Ba Ga4Se7(BGSe),a medium infrared excitation source of 3.4?m band was constructed,and the ultrasonic field excitated in composite materials was studied theoretically and experimentally.The model of laser ultrasonic thermoelastic excitation in composite materials was established and the physical process of thermal conduction and thermal stress coupling of laser ultrasonic thermoelastic excitation mechanism was analyzed.Based on the twolayer structure model,the finite element solution of temperature field,stress field and displacement field was carried out by using COMSOL Multiphysics multi-physical field coupling software,and the distribution of temperature field,stress field and displacement field in the composite material was calculated.Finally,the influence of laser wavelength,energy,spectral width and pulse width on ultrasonic excitation was analyzed by numerical simulation,and the threshold and equivalent optical penetration depth of laser ultrasonic were discussed.A 2.09?m pump with high energy and high repetition frequency was designed and realized.The continueous-wave and Q-switched model of Ho:YAG laser was established.The influence of crystal doping length,doping concentration,pump radius and reflectivity of output coupler was theoretically calculated.Theoretically,the heat distribution inside the crystal was analyzed,and the focal length of the thermal lens of Ho:YAG crystal under high power pump was calculated.Based on the theoretical analysis,the double-end pump Ho:YAG acoustooptic Q-switched laser was designed.Finally,the maximum output of 2.09?m laser with a repetition frequency of 1k Hz was obtained,and the minimum pulse width was 30 ns.Its beam quality factor M 2=1.2.A mid-infrared laser ultrasonic excitation source was developed.Using the theory of optical parametric oscillation and optical parametric amplifier,a high repetition frequency,tunable mid-infrared laser was designed.Based on Zn Ge P2(ZGP)crystal,the laser output of 3.2-3.5?m,pulse repetition frequency of 1k Hz and maximum single pulse energy of 5.6m J were realized.Based on Ba Ge4Se7(BGSe)crystal,the direct cavity single resonance laser output of 3.295?m and 3.936?m was designed and realized.The full width at half maximum of laser spectrum was less than 7nm,pulse repetition frequency was 1k Hz,and the maximum single pulse energy was 1m J.Finally,based on the fiber reinforced epoxy resin matrix composite,ultrasonic waves with center frequencies of 2.5MHz,5MHz,7MHz and 10 MHz were obtained simultaneously,under the excitation of 3-micron band laser in the experiment.The experimental results were in good agreement with the theoretical analysis.The effects of 1?m,2?m and 3?m lasers on ultrasonic excitation were compared.The influence of spectral characteristics of excitation source on ultrasonic field was studied.In addition,the generation of ultrasonic wave in three typical polymer matrix materials epoxy,PEEK and PI were studied under 3 micron band laser excitation.Effects of material parameters on ultrasonic waves were also studied. |
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