Spring 2008, MET 415 - FEA Applications I

Prof. Dave Johnson, dhj1@psu.edu, Penn State - Erie, The Behrend College

3D SHELL ELEMENTS

Thin-walled structures - “Thin” in ONE dimension

Surface area / thickness > 10 : 1

ANSYS & WBE test 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

Shell mesh is centered on the mid-plane of the geometry (by default)

Often with large thin structures, either the inner or outer face may be close enough to the mid-surface to use

Shell elements can "overlap" at corners.

Warped shells: error increases as deviation from flat grows.

Error also occurs at non-flat intersections or changes in thickness.

Shell elements require real constants - to define thickness.

Finding the mid-plane of a thin, solid model

Pro/ENGINEER – Wildfire has a midsurfacing tool in its Mechanica module:

          Applications > Mechanica > Structural > Create “Pairs” …

ANSYS-Workbench has a mid-surfacing tool in DesignModeler which can be used to “compress” the faces of a solid to a mid-plane.  Faces must be offset by a constant dimension.

When a shell model is appropriate – it will have a much shorter solution time and better accuracy than a solid element mesh.

3D SOLID Body (0.075” plate) vs. 3D SURFACE Body

46,121 tetrahedrals / 94,639 nodes   11,245 shells / 11,469 nodes