Text: Building Better Products with FEA,
by V. Adams & A. Askenazi, (Read pp. 141-151)

Represent thin-walled structures. 
(ANSYS: SHELL63, 93, 43, and others)
(Workbench: SHELL181)

Shell elements can be quad. or triangles. Triangle shells are not as bad as triangular solid elements. They still behave well in bending; are constant strain only in membrane stiffness and stress.

Warped shells lose accuracy as they deviate from planar. (p. 246)

(Default) The shell mesh is centered on the mid-plane of the geometry

Element Coord. System: "x" and "y" are in-plane; "z" is normal

Effects pressure load directions (Fig. 4.35, p. 144)

Shells have "TOP" and "BOTTOM" faces (Important in Postprocessing):

• In ANSYS with PowerGraphics (/GRAPH,POWER), you see the stress on the side of the element that you are looking at.  The contour legend scale includes the max. and min. from BOTH sides of ALL shells. (default in WBE)
• Without PowerGraphics (/GRAPH,FULL), you must switch from TOP to BOTTOM. The stress you see is not related to the side of the elements you see.  The contour legend scale ONLY includes the chosen shell element faces. (Can be achieved in WBE with a Command Object)
• Also, you can request "MID" mid-plane stress on some shell elements. (Can be achieved in WBE with a Command Object)
• (Fig. 4.36, p. 145) Inconsistent shell normals (top and bottom problem)

What makes a thin-walled structure ? Surface area / thickness > 10:1

• ANSYS/WBE tests radius/thickness (r/t) ratio:
  • Warning: SHELL93 with r/t <= 5; may be inaccurate
  • Error: SHELL93 with r/t <= 0.5; may not be used
  • (when r is close to t, consider using solids)

Shell elements can overlap at corners. (Fig. 4.37, p. 146)

(ANSYS: /ESHAPE uses thickness data to show shells as solids; WBE: automatic display of thickness)

Shell models can be more accurate than solid models and can have a shorter run time.

• (Fig. 4.42, p. 149) is a 2D model which can’t capture out-of-plane response
• (Fig. 4.43, p. 149) is a 3D solid model, 1 element through the wall, can’t calculate stress accurately
• (Fig. 4.44, p. 150) is a 3D shell model, does the best job for this part.

SPECIAL SHELL ELEMENTS or OPTIONS

• membrane-only (cloth behavior)
• bending-only
• shear-panel

COMPATIBILITY WITH OTHER PROGRAMS:  Moldflow

IN ANSYS, TYPE:   HELP,181

• Why are real constants required ?
• Any unusual data in real constants or material properties ?
• Any surprises in the types of loads permitted ?
• Look over the list of Special Features
• Check out the elements options.

Finally, check out SHELL63, SHELL43, and SHELL93