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.
Shell elements are usually created by meshing a surface representing the position and shape of the structure, and associating them with section data to represent shell thickness and properties.
The following additional topics concerning shell elements are available:
In addition to 3-D shell elements, axisymmetric shell elements are available for efficiently modeling axisymmetric shell structures in 2-D.
Axisymmetric shell elements, such as SHELL208 and SHELL209, may include out-of-plane translational degrees of freedom to model uniform torsion about the axis.
Only plane stress state is allowed in shell elements. Normal thickness stress may be recovered in some shell elements; however, recovered normal thickness stress is a postprocessing quantity and does not contribute to the total element strain energy.
Shell elements withhold external load with membrane, bending, and transverse shear stiffness. The in-plane rotational (drill) stiffness is added at the nodes for solution stability, as shell elements do not have a true in-plane rotational stiffness; consequently, do not expect the in-plane rotational stiffness to carry a load. For lower-order shells with uniform reduced integration, a small artificial stiffness may be included for suppressing the hourglass modes.
Some shell elements have an option allowing them to be used as membrane elements. Such elements allow deformation in the plane of the surface only (that is, stresses do not vary through the thickness). Only membrane stiffness is accounted for. Shell bending, transverse shear, and drill stiffness are excluded; therefore, only translational degrees of freedom are retained. Membrane elements are suitable for modeling extremely thin shells or composite shells with small bending stiffness. Specifying the membrane option for these structures can generally avoid ill-conditioned systems; however, because membrane elements are capable of multiple free-energy modes, using those elements alone is not recommended.
A surface-stress option is available for some shell elements, which excludes all element stiffness and mass contribution. The option allows shell elements to serve as a strain gauge for precision measurement of stresses, strains, and other element solution quantities at selected locations in the model. Similar to the membrane option, the surface-stress option requires only translational degrees of freedom.
The only shear on the free surfaces of a shell element is in-plane. Normals to the shell middle surface stay straight, but not necessarily normal. As a result, the in-plane strain variation through the thickness cannot be more complex than linear.
The assumption of linear in-plane strain variation through the thickness is invalid at the edges of layered composite shell elements that have different material properties at each layer. For accurate stresses in this area, consider using submodeling.
The program does not verify that the element thickness exceeds its width (or many times its width), as such an element may be part of a fine mesh of a larger model that acts as a shell.
If the initial shape of the model is curved, then the radius/thickness ratio is important because the strain distribution through the thickness departs from linear as the ratio decreases. With the exception of SHELL61, all shell elements allow shear deformation, important for relatively thick shells.
Various shell element types tolerate a different degree of warping before their results become questionable (as described in Warping Factor in the Mechanical APDL Theory Reference). Four-node shell elements that do not have all of their nodes in the same plane are considered to be warped. Eight-node shell elements can accept a much greater degree of warping, but unlike other midside-node elements, their midside nodes cannot be dropped.
Some shell elements, such as SHELL181 and SHELL281, use an advanced element formulation that accurately incorporates initial curvature effects.
The calculation for effective curvature change accounts for both shell-membrane and thickness strains. The formulation generally offers excellent accuracy in curved-shell-structure simulations, especially when thickness strain is significant or the material anisotropy in the thickness direction cannot be ignored.
The element coordinate system for all shell elements has the z axis normal to the plane. The element x axis is in the plane, with its orientation determined by one of the following:
The ESYS command
Side I-J of the element
Real constants
Nodes are normally located on the center plane of the element. You can offset nodes from the center plane using either of the following methods:
Use node offsets with care when modeling initially curved structures with either flat or curved elements. For curved elements, an increased mesh density in the circumferential direction may improve the results.