环形楔上楔波频散特性研究

【作者】 梁祎；

【导师】 陆建； 沈中华；

【作者基本信息】 南京理工大学 ， 光学工程， 2014， 硕士

【摘要】 本文主要从理论和实验两方面对环形楔体结构上激光激发的ASF模式楔波的传播特性进行了研究,进而研究楔顶角、曲率半径对楔波传播特性的影响。理论分析上,本文利用有限元方法建立了激光辐照于环形楔体楔尖上激发楔波的理论模型。利用二维傅里叶变换方法对时域波形作时频分析得到了楔波的频散曲线图,图中可以看出楔波的模态数随楔顶角的增加而减少。接下来通过构建有限元模型对直楔上传播的楔波进行了研究。在楔顶角相同的情况下,对比了直楔和环形楔的频散曲线图,发现：在直楔上传播的楔波没有发生色散,其模态个数与相速度值与Lagasse经验公式的理论预测值相吻合；环形楔低阶模态的色散明显,环形楔的弯曲结构导致楔波相速度的色散。在同一楔角下通过改变曲率半径来分析曲率半径对色散的影响,可以发现：对于小角度的环形楔体,低阶模态下曲率半径越小,相速度值越大,色散幅度越大；对于大角度的环形楔体,改变曲率半径不会影响相速度的色散幅度。实验方面,在归纳分析适合研究ASF模式楔形波的检测技术之后,本文搭建了基于光学干涉法的实验装置对楔波的频散特性展开研究。通过在楔尖表面进行步进扫描可以得到距离激发点不同位置的ASF模式楔形波的时间-位移图和表面扫描图,能够直观地区分出各模态；通过二维傅里叶变换,可以将时域信号转换成对应的频域信号,得到楔波的频散曲线图。由于样品楔尖是非理想的,必然会存在截断,所以实验中各模态的色散幅度明显大于数值模拟得到的结果。对于小角度的环形楔体,在保证上项截面和楔角相同的情况下,研究发现：相同模态下曲率半径越小,色散越明显,色散幅度越大；低阶模态的色散幅度明显高于高阶模态；当曲率半径较大时,高阶模态下的相速度值与理论值基本吻合。对于大角度的环形楔体,改变曲率半径不会影响相速度的色散幅度。实验上得到的结论与数值模拟的结论一致。本论文的研究成果可以为ASF模式楔形波在圆柱形楔状工程构件的无损检测和理论研究等方面提供参考。更多还原

【Abstract】 In this dissertation, ASF mode wedge waves which propagate along the cylindrical wedge tip have been researched in both experimental and numerical way. The influence of wedge wave transmission that is caused by the wedge angle and the radius of curvature is discussed.The theoretical model of laser irradiating on the tip of a cylindrical wedge which induce the wedge waves is constructed by using finite element method. By the2D-FFT method to do time-frequency analysis, the phase velocity of each anti-symmetric flexural mode is presented. In the figure the number of modes decreases with the increase of the wedge apex angle. Dispersion curves of the straight wedge and the cylindrical wedge have been compared in terms of the same wedge angle. The numerical calculation results indicate that the radius of curvature is the cause of dispersion. By changing the radius of curvature in the same wedge angle, the impact of the dispersion curves due to the radius of curvature is studied.For small-angle wedge, the dispersion magnitude of low-level mode is relatively large; under the same mode, the smaller radius of curvature is, the larger value of the phase velocity and dispersion amplitude are. For large-angle wedge, the change of the curvature radius has no influence on the dispersion amplitude.In the experimental aspect, the detection technology of ASF mode wedge waves is summarized. Based on optical interferometry method, the experimental setup is established. Through analyzing the time domain waveform and surface scanning image of the wedge wave, each mode of the corresponding wedge is separated clearly. Through the two-dimensional Fourier transform, the time domain signal is converted into a corresponding frequency domain signal, the dispersion graphs can be obtained. According to the Phase Velocity-Frequency graph, the phase velocity dispersion of each mode is gotten. Since the sample is non-ideal wedge tip with inevitable truncation, the dispersion magnitude in the experiment is significantly larger than the numerical simulation result. The experiment results match well with the conclusion of numerical calculation.The dispersion characteristics of ASF mode wedge waves in the theoretical study field can be proved by the research of this paper. The results also make it possible to apply the non-destructive evaluation to the cylindrical wedge-like engineering components.更多还原