激光在金属楔形板中激发超声导波传播规律的研究

【作者】 王嘉宇；

【导师】 沈中华；

【作者基本信息】 南京理工大学 ， 光学工程， 2010， 硕士

【摘要】 本文从理论方面对脉冲激光在楔形板中激发超声导波的规律和超声导波在楔形板中传播的频散特性进行了研究。基于弹性力学的基本理论,建立了楔形板导波传播的理论模型,采用位移势函数法求解Navier-Stokes方程,并结合自由楔形板的零应力边界条件推导出楔形板的频散方程,通过数值方法求解频散方程得到频散曲线。比较并分析了几种不同角度楔形板的频散曲线,得到频散特性随楔形板角度变化的规律。小角度楔形板和大角度楔形板内导波的频散特性有较大区别,小角度楔形板厚度变化缓慢,导波频散曲线和平板中的情况相近；大角度楔形板厚度变化明显,导波频散特性较平板中有较大差异,随楔形结构角度不同频散特性也有明显变化。最后对楔形板内导波频散行为变化的原因给出了解释。基于激光超声的热弹激发理论,考虑脉冲激光作用于楔形板表面激发超声导波,并忽略热扩散效应与光学穿透效应,将激光作用等效为表面脉冲点力源。将脉冲激光作用等效力视为边界条件,采用双积分变换法求解波动方程,并结合边界条件确定出积分变换解(?)(k,ω)的四个系数,再对(?)(k,ω)采用二维IFFT数值运算得到导波波形。编程求解了几种不同角度楔形板内导波波形,通过比较分析不同位置的波形,得到导波在楔形板中传播时波形的变化规律。导波在楔形板内传播时,波形随板厚度变化而变化,在板厚度小的位置波形有明显的Lamb波特征,随着导波传播波形逐渐发生改变,厚度大的位置波形出现Rayleigh波的特征,并且随楔形板角度增大,这种转变趋势越明显。最后通过实验研究了楔形板中导波波形的变化情况,和理论计算结果取得较好的符合。本文的研究成果为金属楔形板和楔形结构材料的激光超声无损检测提供了理论依据。更多还原

【Abstract】 Dispersion Properties of guided-wave and waveforms of guided-wave excited by pulsed laser in metal wedge are studied theoretically in this paper.Established a theoretical model of guided wave propagation in metal wedge based on Elasticity. Using displacement potential function method for solving Navier-Stokes equations, derived dispersion equation of guided-wave in metal wedge combined with free stress boundary conditions. Using numerical method for solving the dispersion equation, and dispersion curves for different wedge angles are compared and analyzed. Small-angle wedge and wide-angle wedge dispersion properties of guided-wave are quite different. Guided-wave dispersion curves are similar to the case in flat in small-angle wedge with slowly linearly varying thickness. Dispersion characteristics of guided-waves in the wide-angle wedge have quite different than the flat, and waveform varies with the wedge angle. Finally, reasons for the changes of guided wave dispersion characteristics are given to explain.Based on the thermoelastic mechanism of laser ultrasound, and ignore the thermal diffusion and optical penetration effect, consider the pulsed laser as a point force source excitation guided waves in surface of metal wedge. Solving wave equation in elastic media under the point force source boundary conditions using double Fourier transform method. Determine four coefficients of integral transform solution with the boundary condition, and out-of-plane displacement(waveform)is calculated using double IFFT method. Programming to solve guided-wave waveform with several different angles metal wedge, and waveforms on different positions are compared and analyzed. Guided-wave shape varies with the thickness of wedge, the Lamb wave wavform characteristics significantly in the small thickness position, and show the Rayleigh wave characteristics in the location of large thickness. And with the wedge angle increases, this change more obvious trend. Finally, changes of the waveform in aluminium wedge are studied experimentally, and consistent the theoretical calculation result.This study result provide a theoretical basis for laser-ultrasonic nondestructive testing of metal wedge and wedge-shaped metal structure.更多还原