Sæther M M, Midtbø S H, Lunde P
University of Bergen, Department of Physics and Technology, Bergen, Norway.
University of Bergen, Department of Physics and Technology, Bergen, Norway.
Ultrasonics. 2022 Sep;125:106795. doi: 10.1016/j.ultras.2022.106795. Epub 2022 Jun 17.
Axial radiation from leaky Lamb waves propagating in a 6.05 mm water-immersed steel plate being excited by a sound beam normally incident to the plate, is investigated as a function of axial distance, z, and frequency, f, over the 350-1000 kHz frequency band of the S, A, and A Lamb modes in the plate. For certain leaky Lamb modes, prior literature has revealed complex characteristics in the transmitted pressure field close to the plate, caused by diffraction due to the finite angular spectrum of the incident beam. The present work extends earlier work by bringing insight into the changes of these field characteristics in the near- and far-field of the transmitted beam, over the frequency band of leaky Lamb modes, for normal beam incidence to the plate. A baffled piston source in a full-wave angular spectrum propagation model is used to analyze the phenomena involved. Maxima and minima that can not be described with plane wave theory are observed in the frequency spectrum of the axial pressure transfer function through the plate. At very long ranges the normalized transmitted sound beam tends to attain characteristics of the plate's plane-wave transmission coefficient, for two of the leaky Lamb modes. Near-field interference phenomena not described in prior literature are identified. For the leaky Lamb mode associated with a backward-wave branch close to the fundamental thickness-extensional resonance in the plate, TE1, the axial near-field is shown to extend to very far ranges. Supplementary measurements add confidence to the simulation results and findings. Besides of their fundamental significance in the study and understanding of sound beams transmitted through a fluid-immersed solid plate, the results are of importance e.g. in immersion applications where material characterization is made using fluid-coupled ultrasonic transducers in a through-thickness resonant transmission setup, such as plate thickness or material properties measurements.
研究了由垂直入射到6.05毫米水浸钢板上的声束激发的泄漏兰姆波在钢板中传播时的轴向辐射，该辐射是轴向距离z和频率f的函数，频率范围为钢板中S、A和A兰姆波模式的350 - 1000kHz频段。对于某些泄漏兰姆波模式，先前的文献表明，由于入射波束的有限角谱导致的衍射，在靠近钢板处的透射压力场中呈现出复杂的特性。本工作通过深入了解泄漏兰姆波模式频段内、透射波束近场和远场中这些场特性的变化，扩展了早期的工作，研究了垂直波束入射到钢板的情况。使用全波角谱传播模型中的带障活塞源来分析所涉及的现象。在通过钢板的轴向压力传递函数的频谱中观察到了无法用平面波理论描述的最大值和最小值。对于两种泄漏兰姆波模式，在非常远的距离处，归一化的透射声束趋于达到钢板平面波传输系数的特性。识别出了先前文献中未描述的近场干涉现象。对于与钢板中接近基本厚度扩展共振的反向波分支相关的泄漏兰姆波模式TE1，轴向近场被证明可以延伸到非常远的距离。补充测量增加了对模拟结果和发现的可信度。除了在研究和理解通过流体浸没固体板传输的声束方面具有基本意义外，这些结果在例如浸没应用中也很重要，在这种应用中，使用流体耦合超声换能器在全厚度共振传输设置中进行材料表征，如板厚或材料特性测量。
