Shell, Solid, Beam or Connector: Choosing the Right Element
Best-practice guidance on selecting element types — what each represents, what it cannot represent, and the failure modes of poor element selection.
Principle
The element type must represent the physical behaviour the engineering question requires. Each element type captures certain deformation modes and cannot capture others. The choice is not a matter of preference — it is a matter of structural mechanics.
Why it matters
Using the wrong element type can produce results that look plausible but are fundamentally wrong. A beam element cannot capture local stress at a cross-section change. A first-order tetrahedron with full integration is artificially stiff in bending. A shell element cannot capture through-thickness stress. These are not minor inaccuracies — they can change the answer by orders of magnitude.
Good practice
| Element Type | Represents Well | Cannot Represent | Typical Use |
|---|---|---|---|
| Beam (B31/B32) | Axial, bending, torsion of slender members | Local stress at cross-section changes, warping | Frames, trusses, stiffeners, ribs |
| Shell (S4R) | In-plane and bending of thin structures | Through-thickness stress, transverse shear detail | Skin panels, webs, thin-walled sections |
| Solid (C3D8R) | Full 3D stress state | Thin walls efficiently (needs many elements through thickness) | Lugs, fittings, thick brackets |
| Solid (C3D10) | Complex geometry, accurate stress | Not efficient for thin structures | Complex castings, stress concentrations |
| Connector | Fastener behaviour, preload, stiffness | Local stress around the hole | Bolts, rivets, spot welds |
| Rigid | Infinite stiffness link | Any flexibility | Rigid links, reference point coupling |
Warning signs
- First-order tetrahedral elements (C3D4) used for stress analysis — these are constant-strain elements and cannot represent bending
- Shell elements used for a component with thickness greater than 1/10 of the span — through-thickness stress is not captured
- Beam elements used where local stress at a cross-section transition is the output of interest
- A single layer of solid elements through a thin wall — shear locking produces artificial stiffness
Verification checks
- For shells: check that the thickness-to-span ratio is within the shell theory assumption (typically t/L < 0.1)
- For solids: verify at least 3 elements through the thickness for bending-dominated problems (first-order) or 1-2 (second-order)
- For beams: verify that the cross-section dimensions are small relative to the length and that local effects are not required
- Compare element behaviour against a known analytical solution for the same loading condition