How to distinguish guided modes and other waves in/of structures
I'm currently working on my diploma thesis, an aspect of which is NDE of sandwich structures by means of UT. We want to use guided (lamb-like) waves to accomplish this task. My main question is what wave modes do exist in a sandwich structure. Doing literature research on this question I found different answers depending on the sandwich type and the model (i.e. first order shear plate theory or higher order sandwich theory) being used. I also know that modes can exist both in the components (facings, core) and in the sandwich plate as a whole.
So far so good. What actually confuses me now and needs to be cleared up, is that I cannot clearly distinguish between vibrations of plates (not neccessarily sandwich, but also monolithic plates) under certain boundary conditions and guided modes in plates. For that you know what I mean, I've taken this from wikipedia:
"Another question is what completely different acoustical behaviors and wave modes may be present in the real geometry of the part. For example, a cylindrical pipe has flexural modes associated with bodily movement of the whole pipe, quite different from the Lamb-like flexural mode of the pipe wall. For example, a cylindrical pipe has flexural modes associated with bodily movement of the whole pipe, quite different from the Lamb-like flexural mode of the pipe wall."
For the pipe, this distinction is quite clear. We have modes in the walls and we have modes of the whole pipe (cylinder).
But what happens if we consider a plate by analogy? Starting with the pipe again: If we have a very long pipe, we can have both propagating and standing waves of the pipe (bodily movement). Independently from that, we can have guided waves in the wall of the pipe. Now consider a plate (or better: a thin beam) by analogy. Both wave phenomena become the same then, don't they? Or where is the difference? Which of the modes are guided modes, suitable for UT?
If you can help, please reply. It's very important to me. Thank you!