Aniket
Ansys Employee

One can use shell elements to model thin structures (where one dimension is much smaller than the other two dimensions). Shell elements decouple the deformation on the surface and the deformation in the normal direction, allowing for a simple and efficient simulation of a thin structure.

No firm rules exist to indicate when it is best to use shell elements. If the structure acts as a shell, you can use shell elements.

Please refer to: 

5.2. Shell Elements (ansys.com) 

-Aniket

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