Finite Element Modeling

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Finite Element Modeling

• GE393
• Computer Aided Design, Analysis and Prototyping

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FEM Process Steps

• Geometry
• Create from scratch
• Import from CAD
• Select analysis approach
• Mesh
• Manual
• Automatic
• Assign material properties
• Add boundary conditions
• Supports
• Loads

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Model Geometry
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Modeling Analysis Approaches

• Planar simulations
• Plane stress
• Plane strain
• Axisymmetric
• 3D simulations
• Beam
• Symmetry (and anti-symmetry)
• Shell (or plate, surface)
• Solid

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Plane Stress

• Normal and shear stresses perpendicular to plane
  of action assumed to be zero
• Look for extrusions
• Depth small w/r/t cross section
• Slices parallel to plane stress cross section
  have same stress distribution
• Thickness input manually

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Plane Strain

• Normal and shear strains perpendicular to plane
  of action assumed to be zero
• Similar to plane stress, except
• Depth large w/r/t cross section
• Loading applied in units of load per depth
• Long pipe under pressure
• Common in civil engineering
• Retaining walls, dams

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Axisymmetric

• Revolved geometry and boundary conditions
• Base feature a revolved solid
• Some asymmetry may be acceptable bracket
  solutions
• Planar model
• Especially useful for nonlinear solutions
• Constraints
• Must be constant about parts axis
• Only need constraints parallel to axis of
  revlution
• No out-of-plane translation or rotation
• Example - pressure vessels, tanks

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Symmetry

• Reflective
• Cyclic
• Anti-symmetry

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Reflective Symmetry

• Symmetry and BCs equal across one, two, three
  planes
• Do not use for modal and dynamic analyses
• Near symmetry analyze less rigid half
• Total load divided by of symmetry planes
• Except pressure or area dependent load
• Constraints
• On solid model, prevent translation through plane
  of symmetry
• On beam and shell models, must also prevent
  rotation parallel to cut planes

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Design Simulation Frictionless Support
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Cyclic Symmetry

• Radial features
• Model as a single instance of radial feature
• Isolate instance with cut planes
• Loading must be radial
• Fan turbine blades, flywheel, motor rotors
• BCs
• Each radial feature instance must see same bcs
• Centrifugal forces, radial displacement due to
  press fit, uniform wind or fluid resistance due
  to spinning
• Gravity load on vertical wheel invalidates cyclic
  symmetry

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Beam Models

• Use for beam-like structures
• Long compared to cross-section
• If wireframe representation conveys geometry
  without ambiguity, then beam model is probably
  appropriate

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Shell (Plate) Modeling

• Used for modeling thin-walled structures
• Wall thickness small w/r/t size or surface area
• Part recognizable when modeled with zero
  thickness surfaces
• May be significantly more accurate than solid
  models in bending, with reasonable solution times
• Can be used with symmetry
• Can be mixed with other (e.g., beam, solid)
  elements
• Achieving convergence is more straightforward
  than with solids
• Mid-plane extraction

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Solid Element Modeling

• Most CPU intensive
• Bulky, low aspect ratio parts
• Potato shaped
• Inherently less ambiguous, requires fewer
  assumptions

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Contact Modeling

• Complex, nonlinear technique
• Allows two parts to touch, slide, bounce off
  and/or react against one another
• Element types
• Gap
• Slide line
• General contact

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Assembly Modeling

• Some assemblies (e.g., weldments) behave as a
  single continuous entity -gt do not require
  special modeling technique
• Use Component contribution analysis

Question when an assembly is meshed in DS, do
the parts share common nodes across parts?
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Component Contribution Analysis

• Do not start with an assembly
• Isolate each part for a separate initial analysis
• Use boundary conditions to account for
  interaction between mating parts
• Goal is to develop a simple model of the part,
  focusing on its particular analysis requirements
• Disadvantage of CCA it takes longer
• Must analyze each part individually before
  assembly analysis

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Finite Element Modeling

• GE393
• Computer Aided Design, Analysis and Prototyping