J.Cugnoni LMAF / EPFL 2012. Vertices (0D): ◦ Coordinates & coordinate system Edges (1D): ◦...
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Transcript of J.Cugnoni LMAF / EPFL 2012. Vertices (0D): ◦ Coordinates & coordinate system Edges (1D): ◦...
J.Cugnoni LMAF / EPFL 2012
Vertices (0D): ◦ Coordinates & coordinate system
Edges (1D): ◦ several Vertices => line / curve
Surfaces (2D): ◦ closed loop of edges (shared vertices),
parametric 2D space, normal = orientation Volumes (3D):
◦ a closed set of surfaces (shared edges), unified normal orientation
3D CAD volume: all edges are shared between boundary faces =>no « free » edges => surface is closed => it’s a volume!
3D CAD volume: all edges are shared between boundary faces =>no « free » edges => surface is closed => it’s a volume!
Several formats are supported by Abaqus CAE:◦ STEP : universal format, good for volumes & assemblies◦ IGES : universal format, good for surfaces, ok for volumes◦ SAT : ACIS engine, native geometry format of Abaqus
CAE, good for nearly everything◦ CATPart: CATIA v5 format, can be imported with a
specific module (1 licence)
Always check the geometry:◦ Free edges / invalid entities: tools => query => geom.
diagnostic◦ If free edges, stitch the surfaces: tools => geom. repair
=> part => stitch◦ If meshing problems, convert to « precise »: tools =>
geom. repair => part => convert to precise◦ Check the dimensions / units !!◦ If you have problems with geometric operations (like
partition), try to Convert to Precise and Convert to Analytical representation
Mesh generation in 3D is based on the same
hierarchy as the CAD model:◦1D: meshing of the edges, starting from a
user-defined element size / distribution
◦2D: propagation of 1D mesh to 2D surface;
structured or free (advancing front or
medial axis).
◦3D: propagation of 2D mesh to the 3D volume;
structured, semi-structured (sweep), free
Method:◦ Use the curvilinear parameter to distribute nodes
along edges => create 1D elements Definition:
◦ Constant size: number of elements on edge or element size
◦ Variable size: number of elements and bias Bias = ratio of largest to smallest elem. size Pick the edge close to the end to be refined
Constant element
sizeDefault (global)
element size
Biased element size distribution
Methods:◦ Propagate 1D mesh on the surface◦ Curved surface:
Nearly planar: use projection on the best plane General: mesh in Parameter space
◦ Algorithms: Structured / mapped meshing Delaunay triangulation Advancing front meshing Medial axis
Definition:◦ Just select the meshing algorithm◦ Automatically inherits the mesh size from the
edges
Mapped meshing (works for surfaces having 3 to 5 corners)
Advancing front meshing Medial axis meshing
Methods: Propagate 2D mesh in the volume Algorithms:
Structured / mapped meshing : map volume to a simple case (hexa)
Semi-structured: « extrusion » / « sweep » of a free 2D mesh (tri or quad) Generates either hexa or prisms (wedges)
Free meshing: Delaunay or Advancing Front tetrahedralization
Definition: Just select the meshing algorithm Automatically inherits the mesh size from the surfaces
& edges
Mapped meshing for hexa: any extrusion of mapped quad. mesh
Mapped meshing for hexa: « simple » 3D primitives here 1/8 of a sphere
Sweep meshing for hexa.: free quad mesh + extrusion
Sweep meshing for wedges : free tri. mesh + extrusion
Free tetrahedral meshing: free advancing front 2D meshing + 3D adv. front tetrahedralization the most general meshing algorithm in Abaqus/CAE
Goal◦ Decompose the geometry into simpler volumes / faces
Method:◦ Cut edges, faces or volumes by planes, extrusions,
sketch… Useful to:
◦ Use structured or sweep meshing on certain region of the part
◦ Enhance mesh quality & assign local refinements◦ Create new faces / edges for boundary conditions or
output Drawback:
◦ If not used correctly: create a lot of small faces and edges => generate very small elements of bad quality
Example: see demo & tutorial
Continuous Displacement field => need congruent mesh on the boundaries with shared nodes at the interface
Continuous mesh if and only if shared face or edge => When working with “imported” geometry, need to « merge » boundary faces & edges!! => always check for “Free edges” !!
Incompatible meshing methods can create “hanging” nodes or displacement jumps which are not linked across boundary; for example, linear to quadratic or tetra to hexa transitions are not “compatible” => discontinuous displacement
If not possible to have shared boundaries, one need to impose displacement compatibility through kinematic constraints => additional equations (to avoid whenever possible!!)
Two disconnected CAD regions
Presence of two bounding faces=> Duplicated nodes & no continuity of displacement
Two connected CAD regions
only one shared bounding face=> shared nodes so displacement is continuous at nodes
Duplicated bounding
face / edge
Duplicated bounding
face / edge
Shared bounding
face / edge
Shared bounding
face / edge
QuadraticTetrahedral
Mesh
Linear Hexahedra
l mesh
Linear Quadrangular
facesQuad.
Triangular faces
Hanging nodes!!
Tetrahedral mesh regions can only be linked to prismatic (wedge) regions.
Prismatic regions can be linked to both hexa (along structured faces) and tetra.
Criteria◦ Geometry : Distortion ,aspect ratio, minimum angle,
maximum angle, …◦ FE-based: jacobian
Influence:◦ Low quality = bad mesh convergence◦ Large stress field discontinuities ◦ Some elements may « lock » for high aspect ratio◦ Create numerical « round-off » errors & singularities◦ May completely « crash » the solver if jacobian is
negative ! Advice:
◦ It is usually better to have « good quality » quadratic tetrahedra than « highly deformed » hexahedra !!
◦ Small edges & nearly tangent junction surfaces can be problematic because they require too small or too sharp elements => use virtual topology
In CAD: ◦ Create CLEAN parts for FEA:
Avoid creating small surfaces & edges Avoid « tangent » connections (very small angles) Try to minimize the number of faces present in the
model Prefer a single « sweep » / « loft » to complex cut /
extrude operations (=> can use structured meshing)◦ Remove unsignificant geometric details:
ask yourself what is important (abstraction) for the goal of the modelling !!!
Typical details: fillets / chamfers, small holes, unsignificant components (bolts & nuts, rivets)
For complex parts / assemblies, it is usualy very time consuming to try to « fix » the geometry & meshing problems, you should better
completely reconstruct a clean 3D CAD model just for FE analysis
In FEA pre-processor / mesher:◦ Always check imported geometry (free edges?)◦ If necessary: repair geometry or try a different
format◦ Partition to create simpler volumes
( symmetries ? )◦ Choice of meshing method (if possible): Hex
structured > Hex swept > Wedges swept > Tetra free
◦ Use compatible meshes at the interface !!!◦ Check mesh quality: at least no Analysis Error◦ Define local refinements where necessary◦ Use virtual topology if necessary