The problems of acoustic diffraction by elastic plates or shells with edges, conjunctions, cracks, inclusions and stiffeners attract wide interest caused by the needs of shipbuilding, noise analysis, acoustics of the ocean and other sciences. Much attention is paid to problems that can be solved in a closed form of integrals or series [1, 2]. Then the analysis of physical effects is the most simple. Such are the problems using point models that are formulated by means of contact conditions fixed in a midpoint of the obstacle. However real joints, cracks, stiffeners, and other inhomogeneities have finite size. thus, for all the point models the question of applicability is of importance. partly it is discussed in [3], where the correction due to the height of a protruding stiffener is derived. In [4] the diffraction by a narrow crack is studied. For thin plates the applicability of point model is shown to be justified only for exponentially narrow cracks when kα exp (-(kh)~(8/5)), where k is the wavenumber, h is the thickness of plate and a is the width of crack.
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