ate-shells-14218.pdf
TRANSCRIPT
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© 2011 ANSYS, Inc. July 3, 20141
Using Shell Elements in
ANSYS Mechanical
ANSYS Technical Support
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© 2011 ANSYS, Inc. July 3, 20142
Agenda
• Background
• Element Technology
– Solids
– Shells
– Solid-Shell• Geometry Modification Options
– Mid Surface Extraction
– Surface Extension
– Misc. (Virtual Topology, Repair, …)
• Shell Connections Methods
• Demo
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© 2011 ANSYS, Inc. July 3, 20143
Background
The analysis of thin and slender structures can
benefit from computationally efficient shell
modeling.
Thin structures:
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• Element topologies: shells, beams,
solid shells, connections (MPC
contact, springs, dampers, joints,
spotwelds…).
• Mid-surfacing Technology enables
3D solids to be modified so that
they can be meshed with shellelements.
Thin Structures:
Background
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© 2011 ANSYS, Inc. July 3, 20145
Conventional 3D Solid 185/6/7 elements
Element Technology
• Geometrically & spatially 3D
• Three displacement degrees of
freedom at each node
• Supported by Enhanced StrainTechnology
• Suitable for modeling 3-D bulk solid
structures
• Not typically suitable for thin
structures in bending
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© 2011 ANSYS, Inc. July 3, 20146
Conventional 3D Shell181/281 elements
• Geometrically 2D, but spatially 3D
• Six degrees of freedom (three in
translation, three in rotation) at each
node.
• Suitable for modeling 3-D thin to
moderately-thick shell structures.
Element Technology
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Thin Shells (Love/Kirchhoff)
– The cross-sectional plane initially
normal to the midsurface of the shell is assumed to
remain straight and normal to the neutral axis
during loading. This assumption excludes shear
deformations.
Moderately-Thick Shells (Mindlin/Reissner)
– The cross-sectional plane initially
normal to the midsurface of the shell is assumed to
remain straight but not remain normal to theneutral axis during loading. The shear strain, as a
result, is constant across the section.
Element Technology
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3D Solid-Shell Element (SOLSH190)
Element Technology
• Geometrically and spatially 3D
• Three translational DOF at each node.
• Has 7 internal DOF, similar to Enhanced Strain but
decoupled in bending direction.
─ condensed out at the element level
• Performs well in simulating plate structures with a wide range of
thicknesses (from extremely thin to moderate thick).
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Solid185 with enhanced strain and Solid186 show transverse shear locking, when
the structure is really thin, e.g L/t =1000
L/t = 1000, transverse shear stress
Umax=0.035, but should be 1
Solid185 Solid186
Umax=0.83, but should be 1
L/t = 1000, transverse shear stress
Element Technology
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Thin wall structure – with Solids
Remedy:
0
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1 6 11 16 21 26
Number of Elements Per Edge
N o r m a l i z e d M a x .
D e f l e c t i o n
Solid185 (enhanced strain)
1. Increase the mesh density (very expensive)
2. Use midside node Solid elements (expensive)
Umax=1
Umax=1
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Number of Elements Per Edge
N o r m a l i z e d M a x .
D e f l e c t i o n
Element Technology
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Thin wall structure – with Shells
0
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Number of Elements Per Edge
N o r m a l i z e d M a x .
D e f l e c t i o n
3. Use Shell elements - needs the mid-surface
transverse shear stress Umax=1
4. Use Solid-Shell elements - needs a swept mesh
transverse shear stress Umax=1 00.1
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Number of Elements Per Edge
N o r m a l i z e d M a x .
D e f l e c t i o n
Element Technology
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Shell vs Solid vs Solid-Shell
• Simple example of buckling of arch
shown on right
– Calculation of load required to inducebuckling
– Comparison of SHELL181, SOLID185and SOLSH190
• For thin structures, SOLSH190
matches SHELL181
– SOLID185 requires additional elements
along edge
• For thick structures, SOLSH190matches SOLID185
3rd mode thick
elem/thick elem/edge 1.00E-03 1.00E-02 1.00E-01 1.00E+00 2.00E+00
SHELL181 1 10 3.7496 3750 3.74E+06 3.09E+09 1.64E+1020 3.4509 3451 3.44E+06 2.89E+09 1.57E+10
50 3.3743 3374 3.37E+06 2.84E+09 1.55E+10
SOLID185 1 10 3533.8000 39403 4.31E+06 3.55E+09 2.23E+10
20 50.9320 4096 3.48E+06 3.23E+09 2.07E+10
50 3.7035 3386 3.38E+06 3.14E+09 2.04E+10
3 10 3534.0000 39403 4.31E+06 3.49E+09 2.13E+10
20 50.8300 4096 3.48E+06 3.18E+09 1.99E+10
50 3.6230 3386 3.38E+06 3.10E+09 1.96E+10
5 10 3533.8000 39403 4.31E+06 3.45E+09 2.04E+10
20 50.9040 4096 3.48E+06 3.14E+09 1.91E+10
50 3.6708 3386 3.37E+06 3.07E+09 1.88E+10
SOLSH190 1 10 3.7232 3722 3.72E+06 3.40E+09 2.23E+10
20 3.4530 3445 3.44E+06 3.17E+09 2.07E+10
50 3.3751 3373 3.37E+06 3.11E+09 2.04E+10
3 10 3.6055 3722 3.72E+06 3.37E+09 2.13E+10
20 3.4384 3445 3.44E+06 3.15E+09 1.99E+10
50 3.3764 3373 3.37E+06 3.09E+09 1.96E+10
5 10 3.4980 3722 3.72E+06 3.33E+09 2.04E+10
20 3.4201 3445 3.44E+06 3.12E+09 1.91E+10
50 3.2714 3373 3.37E+06 3.06E+09 1.88E+10
Element Technology
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© 2011 ANSYS, Inc. July 3, 201413
Thin wall structure – options
3. Solid-Shell:
- needs to be sweep meshed
2. Surface Body mesh (Shells):
- might need mid-surface tool
and shell mesher
1. Solid Body mesh:
- needs a robust solver
for very large models
Element Technology
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© 2011 ANSYS, Inc. July 3, 201414
How do we analyze thin structures effectively?
Assuming the structure is too
large and complex to bemeshed effectively with
conventional solid elements ,
there are two options
available
• Shells with mid-plane
extraction (right)
• Solid-Shells (Left)
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Only Surface Bodies can be meshed with SHELL elements
Surface Bodies can be generated by multiple methods:
• From Edges
• From Sketches
• From Faces
Defining Surface Bodies
Planar
surface
Non-planarsurface
Existing solid body edges
are selected for new
surface boundary.
New, frozen, surface body generated(note, solid body is hidden).
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© 2011 ANSYS, Inc. July 3, 201417
Mid-Surface ToolManual Method
– Select pair of faces, one pair at a time, in FacePairs input
– The order of selection determines the surface
normal directionDetails View of Mid-Surface
Color of faces after selectionSolid Model Color of faces during selection
Normal
Direction
1st Pick
2nd Pick
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© 2011 ANSYS, Inc. July 3, 201418
Mid-Surface Tool
Automatic Method (many control options)
• Bodies to search: Limits search to VisibleBodies, Selected Bodies, or All Bodies
• Minimum and maximum threshold sets search
range (thickness) for face pairs
•
Selection Tolerance: Allowable deviation fromperfect set
• Thickness Tolerance: Allows grouping ofmultiple pairs into one set within this value.
• Sewing Tolerance: max allowable gap between
surfaces
• Extra Trimming provides options for removing unwanted face pairs
• Preserve Bodies? allows you to save the solid bodies from which
surfaces are created (default is No)
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© 2011 ANSYS, Inc. July 3, 201419
Gap
Mid-Surface ToolTolerances
Sewing Tolerance:
• If gaps exist in adjacent face pairs, they will be sewn
together within the Sewing Tolerance
• If Gap < Sewing Tolerance, Surfaces are grouped and
connected (Conformal Mesh between them)
Selection Tolerance:
• Tolerance is used to detect face pairs in case of imperfect
offsets
Without Selection
Tolerance Selection tolerance value is
suggested to user
All the pairs detected
successfully with
Selection tolerance
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© 2011 ANSYS, Inc. July 3, 201420
Joint ToolJoint
Joins surface bodies together such that their contact
regions share common edge. Prerequisite for conformalmeshes.
Takes two or more surface bodies as input
Imprints edges on all bodies where they make contact
Surface bodies after Joint
Surface bodies to be Joined
Details View of Joint
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© 2011 ANSYS, Inc. July 3, 201422
Repair holes
Repair Holes Tool
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© 2011 ANSYS, Inc. July 3, 201423
• When to use?
– To merge together a number ofsmall (connected) faces/edges
– To simplify small features in themodel
– To simplify load abstraction for
mechanical analysis – To create edge splits for better
control of the surface mesh
control
• Virtual cells modify topology
– Original model geometry remainsunchanged
– New faceted geometry is createdwith virtual topology
Virtual TopologyWithout VT With VT
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• The “ideal” connection method for shells is to have
shared topology so the mesh is fully connected.
• In some instances, this is not possible/desirable, so
need to use:
• Contact
• Mesh Connections
• Spot Welds
Shell Connection Methods
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© 2011 ANSYS, Inc. July 3, 201426
• SHELL-to-SHELL Linear Connection (Edge/Edge)
MultiBody Part:
Shell Connection Methods
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• Types of Contact Detection available between solid and surface bodies:
– Face/Face: contact between faces of different bodies
– Face/Edge: contact between faces and edges of different bodies
– Edge/Edge: contact between edges of different bodies
• Face/Edge and Edge/Edge contact only applies to solid and surface bodies. – Contact relationships involving line bodies are not supported.
Shell Connection Methods
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• By default only Face/Face contact is considered
Global control of all connections Local control of grouped connections
Face/Edge and/or Edge/Edge contactoptions should be turned to Yes
Shell Connection Methods
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• SHELL Connectivity
• SHELL-to-SHELL Linear Connection (Edge/Edge)
Edge-to-Edge Contact :
Every formulation available
Shell Connection Methods
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• SHELL Connectivity
•SHELL-to-SHELL Linear Connection (Edge/Edge)
The default, “Target Normal, Couple U to ROT” , creates too many constraints, causing
an artificial stiffness at the connection and resulting in a discontinuity of stress
and strain distribution that should not be there
Shell Connection Methods
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© 2011 ANSYS, Inc. July 3, 201431
“Target Normal, Uncouple U to ROT” creates CEs that separate the rotational and
displacement DOFs into separate equations….
SHELL Connectivity
SHELL-to-SHELL Linear Connection (Edge/Edge)
Shell Connection Methods
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© 2011 ANSYS, Inc. July 3, 201432
- “Target Normal, Uncouple U to ROT” produces expected results
… to remove artificial stiffness at the connection,
thus improving results for special applications.
SHELL Connectivity
SHELL-to-SHELL Linear Connection (Edge/Edge)
Shell Connection Methods
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Mesh connection : Allows you to join the meshes of topologically disconnected
surface bodies:
– Previously connections such as this required a geometry application to repairgaps (e.g. DesignModeler or CAD).
– Mesh connections are made at the mesh level using either edge to edge or edgeto face configurations.
– Unlike geometry solutions, a multibody part is not required.
Mesh Connection
Example
Shell Connection Methods
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• Mesh connections use the concept of master and slave geometry to control
how the connection is made: – Master: indicates the geometry/topology onto which other geometry is projected.
– Slave: indicates the geometry that will be projected onto the master geometry.
– Master geometry can be faces or edges whereas slave geometry can only be edges.
Slave
MasterProjection
Shell Connection Methods
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© 2011 ANSYS, Inc. July 3, 201435
When to Use Mesh Connections:
• When there are disconnects in a surface model, the mesh connection feature
allows the model to be joined via a conformal mesh.
• Mid surfacing often results in situations where gaps exist in a surface model.
Mesh connections are especially useful in these situations.
Shell Connection Methods
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Mesh connections work at
part level:
• As a post mesh operation
• Base part mesh is stored to
allow for quick changes in
connections
Shell Connection Methods
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