Elements and Meshing

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Elements and Meshing
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Elements and Meshing

• Element Types
• Meshing

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Element Types

• Point Element - Mass
• Line Elements - Rods, Spars, Beams
• Shells
• 3-D solids

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Line Element Types

• Rod or Spar Elements
• beam elements that cannot carry moments.
• transmit axial loads only (tension/compression)
• used to model trusses

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Line Element Types

• Beam Elements
• defined by end points, material, x sectional
  area, orientation vector, area moments of
  inertia, torsional stiffness
• angular orientation of cross section must be
  specified
• used to model beam sections e.g. longitudinals,
  t-bars

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Plate and Shell Elements
Surface-like elements used to represent
thin-walled structures (e.g. structures major
dimensions should be 10 times its thickness) Can
be quadrilaterals or triangles (usually quads
with triangles used to aid modelling in certain
areas)

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Plate and Shell Elements

• 1st order triangle elements only capable of
  calculating a single strain across entire element
  (rarely used)
• Linear quad elements have linear strain
  distribution from one node to next. Also able to
  add mid nodes to improve strain gradient

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Plate and Shell Elements
Tip Aspect ratio ratio of longest to shortest
dimension Best accuracy when AR 1.0

AR 10.0 40 error
AR 5.0 15 error
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3-D Solid Elements

• Use for complex structures where simpler elements
  cannot be used.
• Three types in common use
• brick
• wedge
• tetrahedron

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3-D Solid Elements
Tip Use brick/wedge where possible for 3-d
meshing for increased computational efficiency
and accuracy Except where geometry cannot be
broken into brick/wedge without loss of geometric
integrity

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Special Elements

• Mass elements - idealise mass of component
• Spring elements - model springs in structure
• Damper elements - may be used in dynamic
  analysis
• Rigid elements - connect degrees of freedom of
  one node/entity with degrees of freedom of other
  node/entity

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Meshing

• Every element has the following attributes
  associated with it
• Element type (TYPE)
• Real constants (REAL)
• Material properties (MAT)

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Meshing

• Since structure usually has multiple attributes,
  need to ensure that element is assigned correct
  attributes
• assign to geometry prior to meshing
• active global settings before meshing
• modify attributes after meshing
• Tip
• verify attributes by activating attribute
  numbering

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Mesh Density

• Smart sizing (fine to coarse)
• Global element sizing
• Keypoint sizing - controls element size at
  keypoints, useful for stress concentrations
• Line sizing - controls element size at lines
• Area sizing - controls element size at areas

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Changing a Mesh
1. Clear mesh and re-mesh 2. Refine mesh
(available for all area elements and only tet 3-d
elements) using mesh tool

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Meshing

• Free Mesh (default)
• no element restrictions
• mesh does not follow pattern
• suitable for complex shaped areas and volumes
• Mapped Mesh
• restricts element shapes to quads for areas and
  bricks for volumes
• typically regular pattern and hence only
  suitable for regular areas

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Meshing

• Mapped Mesh
• 1. Ensure regular shapes e.g. areas with 3 or 4
  sides. Can be reduced by concatenating or
  slicing.
• 2. Specify size and shape controls
• 3. Generate mesh
• Advantages
• lower number of elements
• lower number of dof