Propagation and Scattering of Coupled Elastic Surface Waves in Curved Structure

DMS-9500723 Harris这项工作研究了表面波在壳体的弯曲弹性表面上的传播，并研究了这些波如何从一个表面耦合到另一个表面。弯曲壳体是弹性波导，其性质随波长缓慢变化，这些变化是由曲率半径和厚度的变化引起的，而其两个表面形成弱耦合表面波波导。传播的研究渐近展开的曲率半径的倒数幂的波场，随后扩展的瑞利-兰姆模式中的每一项。两个最低瑞利-兰姆模式的传播路径上的拍频产生在两个表面之间来回耦合的表面波。散射表面波的经验，当它击中一个表面附近的一个紧凑的不连续性或遇到一个区域的粗糙度沿着其中之一也进行了研究。紧凑的散射体建模分配它的反射和透射系数确定从局部分析的波场附近的不连续性。通过计算表面波的衰减来模拟粗糙度。 这项工作的结果将与爱荷华州州立大学的无损评价中心的现有数据进行比较。 高速旋转的涡轮机叶片和用于输送石油或天然气的管道是常见的弯曲结构，它们可以通过超声波检测疲劳裂纹或腐蚀等损坏。涡轮机叶片中的疲劳裂纹通常发生在其根部，该位置并不总是可见的。管道的腐蚀可能开始于其内部表面，除非关闭一段管道，否则无法检测到腐蚀。这项工作研究如何这些弯曲，弹性壳结构可以使用表面波，首先传播的外表面，然后在内表面，最终返回到外表面，他们拿起检查。这些波探测到内部表面的损坏，并将信息传送到外部。超声波检查还必须对损坏的严重性作出估计。通过模拟从一个小的表面裂纹或衰减引起的腐蚀补丁的散射，这项工作使表面波从一个表面到另一个表面所携带的信息进行解释，给出定量估计的损害。

DMS-9500723 Harris This work investigates the propagation of surface waves over the curved elastic surfaces of a shell and studies how these waves couple from one surface to another. The curved shell is an elastic waveguide whose properties change slowly with wavelength, these changes being caused by the varying radii of curvature and thickness, while its two surfaces form weakly coupled surface-wave waveguides. The propagation is studied by asymptotically expanding the wavefields in inverse powers of the radii of curvature with each term subsequently expanded in the Rayleigh-Lamb modes. The beating over the propagation path of the two lowest Rayleigh-Lamb modes gives rise to a surface wave that couples back and forth between the two surfaces. The scattering that a surface wave experiences when it strikes a compact discontinuity near one of the surfaces or encounters a region of roughness along one of them is also studied. The compact scatterer is modeled by assigning it reflection and transmission coefficients determined from a local analysis of the wavefields near the discontinuity. The roughness is modeled by working out the attenuation to the surface wave. The results of this work will be compared to existing data at the Center for Nondestructive Evaluation at Iowa State University. Turbine blades rotated at high speeds and pipelines used to carry oil or natural gas are among the common curved structures ultrasonically inspected for damage such as fatigue cracking or corrosion. Fatigue cracks in a turbine blade often occur at its root, a location not always visually accessible. Corrosion to a pipeline can begin at its interior supface where it cannot be detected unless a section of the line is shut down. This work investigates how these curved, elastic shell structures can be inspected using surface waves that propagate first over the exterior surface, then over the interior surface, ultimately returning to the exterior surface where they are picked up. Th ese waves detect damage to an interior surface and carry that information to the exterior. Ultrasonic inspections must also give an estimate of the seriousness of the damage. By modeling scattering from a small surface crack or attenuation caused by a patch of corrosion, this work enables the information carried by a surface wave from one surface to another to be interpreted to give quantitative estimates of the damage.