MET 415 Lecture Notes

Text: Building Better Products with FEA,
by V. Adams & A. Askenazi, (Read pp. 235-40, 246-49)

"Resist the temptation to simply blast an automesh at a part" (p.235)

Always start by PLANNING ! (Based on analysis goals, then determine b.c.)

• Ask yourself, 2D or 3D ? Solid or shells or beams ?
• Consider resources (limitations on RAM, disk space, model size, and processor speed vs. allotted time)

Element Selection (p.237)

• In 2D models:
  • Quadrilateral elements have linear shape functions (and strain distribution)
  • First-order triangles (like PLANE182 or PLANE42) are constant strain elements (only have a single, constant value of strain) - poorer accuracy
  • Higher-order triangles (PLANE 183 or PLANE82) still have 2^nd-ordershape functions
• In 3D models:
  • Bricks have linear or quadratic shape functions. (SOLID185/186 or SOLID 45/95)
  • Higher-order tetrahedral are used, still with 2^nd-ordershape functions (SOLID187 or SOLID92)
  • First-order tetrahedral are constant strain, not recommended
  • However, more tetrahedral are needed than bricks (5:1), longer runs

Manual Meshing (FEA-2) vs. Automeshing (p.238-40)

• Manual meshing - much higher modeling time, faster solutions
• longer to converge the manual mesh / bricks are more accurate
• Neat tricks:
  • Mesh an area and extrude/sweep it ,plus the mesh, into 3D.
  • Mesh area on face of volume, extrude mesh through volume.
• Automatic meshing - shorter modeling time, longer solutions.
• Perception: Some "bosses" just don’t like triangles or tetrahedra
• P-elements vs. H-elements: can’t be mixed in ANSYS
  • Don’t rely on large p-elements to adequately capture stress

Element Shape Quality (p.246)

• Ideal Shapes: equilateral triangle or square quadrilateral
• Beware of:
  • high aspect ratio (> 5:1)
  • Large taper
  • Skewed angles (< 45, > 135 degrees)
  • In 3D; warped faces
• Mapped meshing (FEA-2): used to force all quads (or bricks)
  • The user must set divisions along all edges of a region.
  • Areas must be "regular" i.e. 3 or 4 sides
  • Available, in the ANSYS MeshTool
• Biasing a Mesh: refine in a region of interest, usually with a gradual variation to the larger elements.
  • Smoother than we see using "Refine at Elements", or Areas, etc.
  • Compare Figures 7.11 and 7.12 (a)