MET 415 Lecture Notes

Text: Building Better Products with FEA,
by V. Adams & A. Askenazi, (Read pp. 152-55, 249-54)

Resources: the most CPU intensive (and memory and disk space too)

Popularity: automeshing tets requires the least amount of thought

Identify structures for solid modeling: "chunky", "bulky", low aspect ratio
    Don’t forget to take advantage of symmetry !!!

Common shapes:  Brick, Wedge, Pyramid, Tetrahedral

Solid Modeling Tips: (P. 154)

• inherently less ambiguous (fewer assumptions)
• model everything: welds, fillets, chamfers, etc.
• Results data are simpler to interpret

Manual Meshing (FEA-2)  (p. 250)

• divide complex volumes into small "regular" shapes (4, 5, or 6 faces)
• Also, extrude or revolve a meshed area into a meshed volume
• The first task in manual meshing is PLANNING !

Automeshing Solids (p. 251)

• Plan to use parabolic tetrahedral (higher-order elements, SOLID187 or SOLID92).
• These elements are more resource intensive, but provide the best accuracy.
• Avoid linear tetrahedral (constant strain = lower-order, no mid-side nodes).

Element Quality Issues  (p. 252)

Factors which control the quality of the automeshed solid model:

1. analyst:  clean geometry (no slivers/short edges)
2. analyst:  mesh refinement (smooth transitions, no distorted tets)
3. software:  auto-correction of poor element shapes
   • Position of mid-side nodes on the higher-order element edges:
   • effects accuracy, follows curved surfaces closely (Fig. 7.14, p. 253)

"The ability to specify local mesh refinement is critical to the automeshing process"
Remember: uniform, global refinement = LARGE models

MESHING FAILURES (actually do happen)   (p. 254)

When it happens:

• locate the problem area, edge, or corner
• make geometry corrections
• mathematical problems in surface representation may occur (NURBS), fluctuations in tangency along an edge.  Re-define surfaces.
• mesh surfaces with shells.  Examine results for problems.

ANSYS 3D SOLID ELEMENTS:

SOLID185 (SOLID45)

• 3D STRUCTURAL SOLID
• used for linear and many non-linear applications
• lower-order element: 8 nodes, 6 faces
• 3 DOF/node (UX,UY,UZ)
• (not recommended in wedge or tet shape)

SOLID186 (SOLID95)

• 3D 20-NODE STRUCTURAL SOLID
• used for linear and many non-linear applications
• higher-order element: 20 nodes, 6 faces
• 3 DOF/node (UX,UY,UZ)
• (can be used in wedge, pyramid, or tet shape)

SOLID187 (SOLID92)

• 3D 10-NODE TETRAHEDRAL STRUCTURAL SOLID
• used for linear and many non-linear applications
• higher-order element: 10 nodes, 4 triangular faces
• 3 DOF/node (UX,UY,UZ)

SOLID72 (still works, but is no longer documented)

• 3D 4-NODE TETRAHEDRAL STRUCTURAL SOLID with ROTATIONS
• used for linear and a few non-linear applications (not plasticity)
• intermediate-order element: 4 nodes, 4 triangular faces
• 6 DOF/node (UX,UY,UZ,ROTX,ROTY,ROTZ)
• designed for mesh import from CAD programs
• not as accurate as SOLID92

SOLID75 (still works, but is no longer documented)

• 3D 8-NODE STRUCTURAL SOLID with ROTATIONS
• used for linear and a few non-linear applications
• intermediate-order element: 8 nodes, 6 faces
• 6 DOF/node (UX,UY,UZ,ROTX,ROTY,ROTZ)
• not as accurate as SOLID95