Department of Electronic Engineering, Fudan University, Shanghai 200433, China.
Ultrasonics. 2014 Jan;54(1):395-401. doi: 10.1016/j.ultras.2013.07.011. Epub 2013 Jul 18.
Mode conversions of Lamb waves can occur upon encountering damage or defect such as a notch, leading to newly-converted modes apart from wave reflection and transmission. In this paper, the transmission of the fundamental Lamb modes symmetrical S0 and anti-symmetrical A0 with anti-symmetrical notches were investigated in steel plates within the relatively short propagation distance. The group velocity and modal energy of the converted modes were analyzed using simulations and experiments. Two-dimensional finite difference time domain (2D-FDTD) method was employed to calculate the scattering field and extract numerical trends for simulation study and experimental confirmation. Both simulations and experiments revealed that the apparent group velocities of the converted modes in the transmitted signals subject to the notch positions. To describe the mode conversion degree and evaluate the notch severity, wave packets of the originally-transmitted modes and newly-converted modes were separated and corresponding mode energy percentages were analyzed at different notch severities. Frequency-sweeping measurements illustrated that the modal energy percentages varied monotonically over the notch-depth increase with a statistically consistency (R=1.00, P<0.0004).
在遇到损伤或缺陷（如缺口）时，兰姆波会发生模式转换，除了波的反射和透射之外，还会产生新的转换模式。本文研究了在相对短的传播距离内，带有非对称缺口的钢板中基本兰姆波对称模态 S0 和反对称模态 A0 的透射。利用模拟和实验分析了转换模式的群速度和模态能量。二维有限差分时域（2D-FDTD）方法用于计算散射场，并提取数值趋势进行模拟研究和实验验证。模拟和实验均表明，在接收到缺口位置的信号时，转换模式的表观群速度在传输信号中发生了变化。为了描述模式转换程度并评估缺口的严重程度，对原始传输模式和新转换模式的波包进行了分离，并在不同缺口严重程度下分析了相应的模式能量百分比。扫频测量表明，随着缺口深度的增加，模态能量百分比呈单调变化，具有统计学一致性（R=1.00，P<0.0004）。
