【作者】 孙宏祥；

【导师】 许伯强；

【作者基本信息】 江苏大学 ， 光学工程， 2009， 硕士

【摘要】 本文采用有限元法建立脉冲激光在横观各向同性复合材料表面激发超声导波的数值模型,进而对横观各向同性复合材料中激光超声导波的波形及传播特征进行了数值模拟研究,主要内容如下:以平面应变理论为基础,从弹性动力学平衡方程出发,采用有限元方法,在频域中建立激光激发超声导波的数值模型。为了验证频域模型的正确性,忽略材料的粘性特征,建立有限元时域模型进行对比,频域模型与时域模型的结果基本一致,说明频域模型的正确性。在此基础上,在频域中研究激光在粘弹性薄板表面激发Lamb波的特征。数值模拟结果表明,激光激发的Lamb波主要包含低频的对称模S₀模和反对称模A₀模;材料的粘性特征会引起Lamb波的振幅衰减,且不同模态的波衰减（因子）不同。基于有限元方法,在频域中建立激光作用在横观各向同性弹性平板表面激发超声导波的数值模型。研究横观各向同性平板的各向异性及各向同性平面中超声导波的波形及传播特征,同时分析平板厚度的变化对激光超声导波模态的影响。结果表明,横观各向同性材料中垂直于纤维方向的Lamb波比沿着纤维方向的Lamb波色散特征更明显,信号频率更高;而沿着纤维方向的超声导波比垂直于纤维方向的超声导波传播速度更快。此外,激光在薄板中激发出Lamb波,随着平板厚度的增加,板中出现高阶模态的波,波形向表面波转化,得到掠面纵波和Rayleigh波。针对复合材料的粘弹性特征,采用有限元频域模型研究激光在粘弹性材料表面激发Rayleigh波的特征。比较激光在粘弹性材料与弹性材料表面激发Rayleigh波的波形及传播特征,研究材料的粘性特征对Rayleigh波的影响。进而分析材料粘性劲度参数的变化对Rayleigh波特征的影响。研究表明,Rayleigh波在粘弹性材料中的传播距离越远,波的能量耗散越多,表现为波形的衰减增大及频散特征增强。此外,随着材料粘性劲度参数的增大,波形的衰减增大,频散特征增强,且不同模态波的衰减系数不同。本文的研究结果将为激光超声应用于横观各向同性复合材料中的无损检测提供有效的理论依据,并为激光超声理论体系作有益的补充。更多还原

【Abstract】 In this paper,by using the finite element method（FEM）,a numerical model of laser-generated ultrasound on a transverse isotropic composite material is established,and the propagation and waveform characteristics of laser-generated ultrasonic waves in the transverse isotropic composite material are studied numerically.Based on the theory of plane strain and the elastic equations of dynamic equilibrium,a numerical model of laser-generated ultrasound in frequency domain is established by using FEM.To verify the correctness of the finite element model in the frequency domain,a finite element model in the time domain is established for elastic material.The calculated results in the frequency domain are in good agreement with those obtained in the time domain.And then,the waveform characteristics of laser-generated Lamb wave on a thin viscoelastic plate are analyzed in the frequency domain.The results show laser-generated Lamb waves mainly include the lower frequency components of the symmetric mode S₀ and the anti-symmetric mode A₀.In addition,the amplitudes of Lamb waves are attenuated in the viscoelastic material,and the attenuation coefficients to different modes of the viscoelastic wave are different.A numerical model of laser-generated ultrasonic waves on the transverse isotropic elastic material is established in the frequency domain by using the FEM.The propagation and waveform characteristics of ultrasonic waves in the anisotropic and isotropic planes of the transverse isotropic elastic material are studied.Moreover,the effect of the different material thickness to ultrasound mode is analyzed in details. The results indicate the Lamb waves propagating in the direction normal to the fiber direction show clearer dispersive characteristics and the frequencies are higher than those propagating in the direction parallel to the fiber direction.However,the velocities of the ultrasonic waves propagating parallel to the fiber direction are faster than those propagating perpendicular to the fiber direction.Moreover, laser-generated Lamb waves are obtained on the thin plate.When the plate thickness increases so that the higher modes can propagate in the plate,a skimming surface longitudinal wave and a Rayleigh wave are formed at the plate surface.Taking account of the viscosity characteristic of the composite material,a finite element model for simulating laser-generated Rayleigh waves on the viscoelastic material is developed in the frequency domain. Based on the numerical calculations,the differences of Rayleigh waves in the viscoelastic material and the elastic material are compared,and the effects of the material viscosity to the characteristic of Rayleigh wave are analyzed.In addition,the effect of the different viscosity modulus to the characteristics of Rayleigh waves is studied in detail.The results show Rayleigh waves are attenuated and dispersed gradually in the viscoelastic material due to energy dissipation with the increase of source-receiver distance and viscosity modulus.Furthermore,the attenuation coefficients to different modes of the viscoelastic wave are different.This work will provide a useful guidance for the use of laser ultrasonic on the characterization of transverse isotropic composite material and complement the system info of laser ultrasonic.更多还原