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What is the difference between thin and thick shell formulations?

Answer: The inclusion of transverse shear deformation in plate-bending behavior is the main difference between thin and thick shell formulation. Thin-plate formulation follows a Kirchhoff application, which neglects transverse shear deformation, whereas thick-plate formulation follows Mindlin/Reissner, which does account for shear behavior. Thick-plate formulation has no effect upon membrane (in-plane) behavior, only plate-bending (out-of-plane) behavior.

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In general, the contribution of shear deformation becomes significant when ratio between the span of plate-bending curvature and thickness is approximately 20:1 or 10:1. The formulation itself is adequate for ratio down to 5:1 or 4:1. In that this ratio is dependent upon the projected span of curvature, shell thickness may be greater than the actual plan dimensions of a shell object.

Stiffness for pure-bending deformation

The statement that thick shells tend to be stiffer than thin shells applies only to the bending components of shells, and to models in which meshing is too coarse.

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Stresses may be of greater concern than deflections. When shear deformation is expected to be important, we recommend the thick-shell element because it will better capture the stress distribution. This is the case not only for thicker shells, but also for regions near openings and other geometric discontinuities in which transverse shear deformation develops.