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Transcript of 75806934 ANSYS Workbench Simulation Static
Static Structural Analysis
Chapter Four
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Training Manual
Linear Static Structural Analysis
April 22, 2004
Inventory #002071
4-2
Chapter Overview
• In this chapter, performing linear static structural analyses
in Simulation will be covered:
– Geometry and Elements
– Contact and Types of Supported Assemblies
– Environment, including Loads and Supports
– Solving Models
– Results and Postprocessing
• The capabilities described in this section are generally
applicable to ANSYS DesignSpace Entra licenses and
above.
– Some options discussed in this chapter may require more
advanced licenses, but these are noted accordingly.
– Free vibration, harmonic, and nonlinear structural analyses are
not discussed here but in their respective chapters.
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April 22, 2004
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4-3
Basics of Linear Static Analysis
• A linear static structural analysis is performed to obtain the
response of a structure under applied static loads
– Displacements, reaction forces, stresses, and strains are
usually items of interest that the user wants to review
• The general equation of motion is as follows:
where [M] is the mass matrix, [C] is the damping matrix, [K]
is the stiffness matrix, {x} is displacements, and {F} is force
• Because this is a static analysis, all time-dependent terms
are removed, leaving the following subset:
tFxKxCxM
FxK
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4-4
Basics of Linear Static Analysis
• For a linear static structural analysis, the displacements {x}
are solved for in the matrix equation below:
This results in certain assumptions related to the analysis:
– [K] is essentially constant
• Linear elastic material behavior is assumed
• Small deflection theory is used
• Some nonlinear boundary conditions may be included
– {F} is statically applied
• No time-varying forces are considered
• No inertial effects (mass, damping) are included
• It is important to remember these assumptions related to
linear static analysis. Nonlinear static and dynamic
analyses are covered in later chapters.
FxK
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Linear Static Structural Analysis
April 22, 2004
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4-5
A. Geometry
• In structural analyses, all types of bodies supported by
Simulation may be used.
• For surface bodies, thickness must be
supplied in the ―Details‖ view of the
―Geometry‖ branch.
• The cross-section and orientation of line bodies are defined
within DesignModeler and are imported into Simulation
automatically.
– For line bodies, only displacement results are available.
ANSYS License Availability
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April 22, 2004
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4-6
… Elements Used
• In Simulation, the following elements are used:
– Solid bodies are meshed with 10-node tetrahedral or 20-node
hexahedral elements
• SOLID187 and SOLID186
– Surface bodies are meshed with 4-node quad shell elements
• SHELL181 using real constants
• Section definition (and offsets) are not used
– Line bodies are meshed with 2-node beam elements
• BEAM188 (with 3rd orientation node)
• Section definition and offsets are supported
Advanced ANSYS Details
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4-7
… Point Mass
• A Point Mass is available under the Geometry branch to
mimic weight not explicitly modeled
– A point mass is associated with surface(s) only
– The location can be defined by either:
• (x, y, z) coordinates in any user-defined Coordinate System
• Selecting vertices/edges/surfaces to define location
– The weight/mass is supplied under ―Magnitude‖
– In a structural static analysis, the point mass is affected by
―Acceleration,‖ ―Standard Earth Gravity,‖ and ―Rotational
Velocity‖. No other loads affect a point mass.
– The mass is ‗connected‘ to selected surfaces
assuming no stiffness between them. This is
not a rigid-region assumption but similar to a
distributed mass assumption.
– No rotational inertial terms are present. ANSYS License Availability
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… Point Mass
• A point mass will be displayed as a round, grey sphere
– As noted previously, only inertial loads affect the point mass.
– This means that the only reason to use a point mass in a linear
static analysis is to account for additional weight of a
structure not modeled. Inertial loads must be present.
– No results are obtained for the Point Mass itself.
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… Point Mass (ANSYS Details)
• Internally, the Point Mass is modeled as a concentrated
mass connected to surfaces with RBE3 constraints
– A translational-only MASS21 (KEYOPT(3)=2) has given mass
– RBE3-type of surface constraint is enabled with CONTA174,
which are generated on associated ‗surfaces‘
• KEYOPT(2)=2 for MPC algorithm
• KEYOPT(4)=1 for nodal detection (contact)
• KEYOPT(12)=5 for bonded contact
– A pilot node TARGE170 is generated at the
same node as the MASS21.
• KEYOPT(2)=1 for user-supplied constraints
• KEYOPT(4)=111111 for all DOF active
– Note that RBE3 has 6 DOF but MASS21 only has 3 DOF and no
rotary inertia. Also, since RBE3-type of surface constraint
used (rather than CERIG-type of surface constraint), there is
no stiffness between point mass and rest of structure.
Advanced ANSYS Details
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… Material Properties
• The required structural material properties are Young’s
Modulus and Poisson’s Ratio for linear static structural
analyses
– Material input is under the ―Engineering Data‖ branch, and
material assignment is per part under the ―Geometry‖ branch
– Mass density is required if any inertial loads are present
– Thermal expansion coefficient and thermal conductivity are
required if any thermal loads are present
• Thermal loading not available with an ANSYS Structural license
• Negative thermal expansion coefficient may be input (shrinkage)
– Stress Limits are needed if a Stress Tool result is present
– Fatigue Properties are needed if Fatigue Tool result is present
• Requires Fatigue Module add-on license
– Specific loading and result tools will be discussed later
ANSYS License Availability
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DesignSpace x
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Structural /
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… Material Properties
• Worksheet view of sample material shown below:
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B. Assemblies – Solid Body Contact
• When importing assemblies of solid parts, contact regions
are automatically created between the solid bodies.
– Surface-to-surface contact allows non-matching meshes at
boundaries between solid parts
– Tolerance controls under ―Contact‖ branch allows the user to
specify distance of auto contact detection via slider bar
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… Assemblies – Solid Body Contact
• In Simulation, the concept of contact and target surfaces
are used for each contact region.
– One side of the contact region is comprised of ―contact‖
face(s), the other side of the region is made of ―target‖ face(s).
– The integration points of the contact surfaces are restricted
from penetrating through the target surfaces (within a given
tolerance). The opposite is not true, however.
• When one side is the contact and the other side is the target, this
is called asymmetric contact. On the other hand, if both sides are
made to be contact & target, this is called symmetric contact since
neither side can penetrate the other.
• By default, Simulation uses symmetric
contact for solid assemblies.
• For ANSYS Professional licenses and
above, the user may change to
asymmetric contact, as desired.
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… Assemblies – Solid Body Contact
• Four contact types are available:
– Bonded and No Separation contact are basically
linear behavior and require only 1 iteration
– Frictionless and Rough contact are nonlinear
and require multiple iterations. However, note
that small deflection theory is still assumed.
• When using these options, an interface treatment
option is available, set either as ―Actual Geometry
(and Specified Offset)‖ or ―Adjusted to Touch.‖
The latter allows the user to have ANSYS close the
gap to ‗just touching‘ position. This is available
for ANSYS Professional and above.
Contact Type Iterations Normal Behavior (Separation) Tangential Behavior (Sliding)
Bonded 1 Closed Closed
No Separation 1 Closed Open
Frictionless Multiple Open Open
Rough Multiple Open Closed
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… Assemblies – Solid Body Contact
• For the advanced user, some of the
contact options can be modified
– Formulation can be changed from ―Pure
Penalty‖ to ―Augmented Lagrange,‖ ―MPC,‖ or
―Normal Lagrange.‖
• ―MPC‖ is applicable to bonded contact only
• ―Augmented Lagrange‖ is used in regular ANSYS
– The pure Penalty method can be thought of as
adding very high stiffness between interface of
parts, resulting in negligible relative movement
between parts at the contact interface.
– MPC formulation writes constraint equations
relating movement of parts at interface, so no
relative movement occurs. This can be an
attractive alternative to penalty method for
bonded contact.
ANSYS License Availability
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DesignSpace
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… Assemblies – Solid Body Contact
• Advanced options (continued):
– As explained in Chapter 3, the pinball
region can be input and visualized
• The pinball region defines location of near-
field open contact. Outside of the pinball
region is far-field open contact.
• Originally, the pinball region was meant to
more efficiently process contact searching,
but this is also used for other purposes,
such as bonded contact
• For bonded or no separation contact, if gap
or penetration is smaller than pinball region,
the gap/penetration is automatically
excluded
– Other advanced contact options will be
discussed in Chapter 11. In this case, the gap between
the two parts is bigger than the
pinball region, so no automatic
gap closure will be performed.
ANSYS License Availability
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4-17
… Assemblies – Solid Body Contact
• Internally, the solid face contact regions are modeled in
ANSYS as CONTA174 and TARGE170 elements
– By default, pure penalty method is used with relative contact
stiffness of 10 with symmetric contact pairs being generated
– For bonded and no separation contact, any geometric
penetration or gap is ignored if within the pinball region.
– For frictionless and rough contact, considering ―actual
geometry‖ makes any initial gap or penetration ramped
whereas ―adjust to touch‖ closes gap with auto CNOF
– NEQIT is set to 1 for if only bonded or no separation contact
exist; it is set higher otherwise (20-40, depending on model).
Contact Type KEYOPT(2) KEYOPT(5) KEYOPT(9) KEYOPT(12)
Bonded 1 0 1 5
No Separation 1 0 1 4
Frictionless, Actual Geometry 1 0 2 0
Frictionless, Adjusted to Touch 1 1 1 0
Rough, Actual Geometry 1 0 2 1
Rough, Adjusted to Touch 1 1 1 1
Advanced ANSYS Details
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… Assemblies – Surface Body Contact
• For ANSYS Professional licenses and above, mixed
assemblies of shells and solids are supported
– Allows for more complex modeling of assemblies, taking
advantage of the benefits of shells, when applicable
– More contact options are exposed to the user
– Contact postprocessing is also available (discussed later)
ANSYS License Availability
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DesignSpace
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… Assemblies – Surface Body Contact
• Edge contact is a subset of general contact
– For contact including shell faces or solid
edges, only bonded or no separation
behavior is allowed.
– For contact involving shell edges, only
bonded behavior using MPC formulation is
allowed.
• For MPC-based bonded contact, user can set
the search direction (the way in which the
multi-point constraints are written) as either
the target normal or pinball region.
• If a gap exists (as is often the case with
shell assemblies), the pinball region can be
used for the search direction to detect
contact beyond a gap.
ANSYS License Availability
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… Assemblies – Surface Body Contact
• Internally, any contact including an edge (solid body edge
or surface edge) results in asymmetric contact with
CONTA175 for the edge and TARGE170 for the edge/face
– Contact involving solid edges default to pure penalty method
– Contact involving surface edges use MPC formulation.
Instead of ―target normal,‖ if search direction is ―pinball
region,‖ KEYOPT(5)=4 set on companion TARGE170 element.
– For bonded contact (default), both use KEYOPT(12)=5 and
KEYOPT(9)=1.
• For surface faces in contact with other
faces, standard surface-to-surface
contact is used, namely CONTA174
and TARGE170 Example of Simulation-
generated edge-to-edge
contact, which results in
CONTA175 on one edge and
TARGE170 on the other.
Advanced ANSYS Details
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… Assemblies – Contact Summary
• A summary of contact types and options available in
Simulation is presented in the table below:
– This table is also in the Simulation online help. Please refer to
this table to determine what options are available.
• Note that surface body faces can only participate in bonded or no
separation contact. Surface body edges allow MPC-based bonded
contact only.
Contact Geometry Solid Body Face Solid Body Edge Surface Body Face Surface Body Edge
All types Bonded, No Separation Bonded, No Separation Bonded only
All formulations All formulations All formulations MPC formulation
Symmetry respected Asymmetric only Symmetry respected Asymmetric only
Bonded, No Separation Bonded, No Separation Bonded only
All formulations All formulations MPC formulation
Asymmetric only Asymmetric only Asymmetric only
Bonded, No Separation Bonded only
All formulations MPC formulation
Symmetry respected Asymmetric only
Bonded only
MPC formulation
Asymmetric only
Solid Body Face
Solid Body Edge
Surface Body Face
Surface Body Edge
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… Assemblies – Spot Weld
• Spot welds provide a means of connecting shell assemblies
at discrete points
– For ANSYS DesignSpace licenses, shell contact is not
supported, so spotwelds are the only way to define a shell
assembly.
– Spotweld definition is done in the CAD software. Currently,
only DesignModeler and Unigraphics define spotwelds in a
manner that Simulation supports.
– Spotwelds can also be created in
Simulation manually, but only at
discrete vertices.
ANSYS License Availability
DesignSpace Entra
DesignSpace x
Professional x
Structural x
Mechanical/Multiphysics x
DesignModeler x
Pro/ENGINEER
Unigraphics x
SolidWorks
Inventor
Solid Edge
Mechanical Desktop
CATIA V4
CATIA V5
ACIS (SAT)
Parasolid
IGES
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… Assemblies – Spot Weld
• Internally, spot welds are defined as a set of BEAM188
elements. The spot weld is defined with one beam element,
and the top and bottom of the spot weld is connected to the
shell or solid elements with a ‗spider web‘ of multiple
beams.
– The BEAM188 elements use
same material properties as
underlying materials but
with an appropriate circular
cross-section with radius=
5*thickness of underlying
shells
– Figure on right shows spot-
welds between two sets of
shell elements, which are
made translucent for clarity.
Advanced ANSYS Details
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C. Loads and Supports
• There are four types of structural loads available:
– Inertial loads
• These loads act on the entire system
• Density is required for mass calculations
• These are only loads which act on defined Point Masses
– Structural Loads
• These are forces or moments acting on parts of the system
– Structural Supports
• These are constraints that prevent movement on certain regions
– Thermal Loads
• Structurally speaking, the thermal loads result in a temperature
field, which causes thermal expansion on the model.
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… Directional Loads
• For most loads/supports which have an
orientation, the direction can be defined by
components in any Coordinate System
– The Coordinate System (CS) has to be
defined prior to specifying the loading. Only
Cartesian coordinate systems may be used
for loading/support orientation.
– In the Details view, change ―Define By‖ to
―Components‖. Then, select the appropriate
Cartesian CS from the pull-down menu.
– Specify x, y, and/or z components, which are
relative to the selected Coordinate System
– Not all loads/supports support use of CS:
ANSYS License Availability
DesignSpace Entra x
DesignSpace x
Professional x
Structural x
Mechanical/Multiphysics x
Load Supports Coordinate Systems
Acceleration No
Standard Earth Gravity No
Rotational Velocity No
Force Yes
Remote Force Location of Origin Only
Bearing Load Yes
Moment Yes
Given Displacement Yes
Loads/Supports not
listed in the table do not
have direction
associated with it, so
Coordinate Systems are
not applicable.
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… Acceleration & Gravity
• An acceleration can be defined on the system
– Acceleration acts on entire model in length/time2 units.
– Users sometimes have confusion over notation of direction. If
acceleration is applied to system suddenly, the inertia resists
the change in acceleration, so the inertial forces are in the
opposite direction to applied acceleration
– Acceleration can be defined by Components or Vector
• Standard Earth Gravity can also be applied as a load
– Value applied is 9.80665 m/s2 (in SI units)
– Standard Earth Gravity direction can only be defined along
one of three World Coordinate System axes.
– Since ―Standard Earth Gravity‖ is defined as an acceleration,
define the direction as opposite to gravitational force, as noted
above.
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… Rotational Velocity
• Rotational velocity is another inertial load available
– Entire model spins about an axis at a given rate
– Can be defined as a vector, using geometry for axis and
magnitude of rotational velocity
– Can be defined by components, supplying origin and
components in World Coordinate System
– Note that location of axis is very important since model spins
around that axis.
– Default is to input rotational velocity in radians per second.
Can be changed in ―Tools > Control Panel > Miscellaneous >
Angular Velocity‖ to revolutions per minute (RPM) instead.
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… Inertial Loads in ANSYS
• Inertial loads are modeled in ANSYS as follows:
– Acceleration and Standard Earth Gravity are represented via
ACEL command
– Rotational velocity is defined via CGLOC (defines origin) and
CGOMGA (defines rotational velocity about CGLOC)
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… Forces and Pressures
• Pressure loading:
– Pressures can only be applied to surfaces and always act
normal to the surface
– Positive value acts into surface (i.e., compressive)
negative value acts outward from surface (i.e., suction)
– Units of pressure are in force per area
• Force loading:
– Forces can be applied on vertices, edges, or surfaces.
– The force will be distributed on all entities. This
means that if a force is applied to two identical
surfaces, each surface will have half of the force
applied. Units are mass*length/time2
– A force is defined via vector and magnitude or by
components (in user-defined Coordinate System)
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… Bearing Load
• Bearing Load (was called ―Bolt Load‖ in prior releases):
– Bearing Loads are for cylindrical surfaces only. Radial
component will be distributed on compressive side using
projected area. Example of radial distribution shown below.
Axial component is distributed evenly on cylinder.
– Use only one bearing load per cylindrical surface. If the
cylindrical surface is split in two, however, be sure to select
both halves of cylindrical surface when applying this load.
– Load is in units of force
– Bearing load can be defined
via vector and magnitude or
by components (in any
user Coordinate System).
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… Moment Load
• Moment Load:
– For solid bodies, a moment can be applied on any
surface
– If multiple surfaces are selected, the moment load
gets apportioned about those selected surfaces
– A vector and magnitude or components (in user-defined
Coordinate System) can define the moment. The moment acts
about the vector using the right-hand rule
– For surface bodies, a moment can also be applied to a vertex
or edge with similar definition via vector or components as
with a surface-based moment
– Units of moment are in Force*length.
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… Remote Load
• Remote Load:
– Allows the user to apply an offset force on a surface or edge of
a surface body
– The user supplies the origin of the force (using vertices, a
cylinder, or typing in (x, y, z) coordinates). A user-defined
Coordinate System may be used to reference the location.
– The force can then be defined by vector and magnitude or by
components (components for direction is in Global CS)
– This results in an equivalent force on
the surface plus a moment caused by
the moment arm of the offset force
– The force is distributed on the surface
but includes the effect of the moment
arm due to the offset of the force
– Units are in force (mass*length/time2) ANSYS License Availability
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… Structural Loads in ANSYS
• Structural loads are modeled in ANSYS as follows:
– Pressures are applied directly on surfaces via SF,,PRES
– Forces on vertices and edges are applied as nodal loads via
F,,FX/FY/FZ
– Forces on surfaces are applied as pressures on face 5 of
surface effect elements SURF154 with KEYOPT(11)=2
• KEYOPT(11)=2 to use full area, including tangential component
– Bearing loads are applied as pressures on face 5 of surface
effect elements SURF154. Two sets are created for axial and
radial components of bearing load:
• Axial component uses KEYOPT(11)=2 for pressure on full area
• Radial component uses KEYOPT(11)=0 for pressure (which is
applied on compressive part of cylinder only) on projected area w/
tangential component
– Moments on vertices or edges of shells are applied as nodal
loads via F,,MX/MY/MZ
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… Structural Loads in ANSYS
• Moment load on surface is defined by surface constraint
– Surface constraint is RBE3-type of distributed loading
– Pilot node at surface CG defined by TARGE170 with
KEYOPT(2)=1 and KEYOPT(4)=xxx000
– Surface is defined by CONTA174 with KEYOPT(2)=2,
KEYOPT(4)=1, KEYOPT(12)=5
– Moment applied as nodal load on pilot node
• Remote force load is defined by surface constraint
– Surface constraint is RBE3-type of distributed force
– Pilot node at force origin defined by TARGE170 with
KEYOPT(2)=1 and KEYOPT(4)=000xxx
– Surface is defined by CONTA174 or CONTA175 with
KEYOPT(2)=2, KEYOPT(4)=1, KEYOPT(12)=5
– Force applied as nodal load on pilot node
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… Supports (General)
• Fixed Support:
– Constraints all degrees of freedom on vertex, edge, or surface
– For solid bodies, prevents translations in x, y, and z
– For surface and line bodies, prevents translations and
rotations in x, y, and z
• Given Displacement:
– Applies known displacement on vertex, edge, or surface
– Allows for imposed translational displacement in x, y, and z (in
user-defined Coordinate System)
– Entering ―0‖ means that the direction is constrained.
– Leaving the direction blank means that the entity is free to
move in that direction
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… Supports (Solid Bodies)
• Frictionless Support:
– Applies constraint in normal direction on surfaces
– For solid bodies, this support can be used to apply a
‗symmetry‘ plane boundary condition since ‗symmetry‘ plane
is same as normal constraint
• Cylindrical Constraint:
– Applied on cylindrical surfaces
– User can specify whether axial, radial, or tangential
components are constrained
– Suitable for small-deflection (linear) analysis only
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… Supports (Solid Bodies)
• Compression Only Support:
– Applies a compression-only constraint normal to any given
surface. This prevents the surface to move in the positive
normal direction only.
– A way to think of this support is to imagine a ‗rigid‘ structure
which has the same shape of the selected surface. Note that
the contacting (compressive) areas are not known beforehand.
– Can be used on a cylindrical surface to model a
―Pinned Cylinder Support‖, which was available
in 7.1 but is a special case of the ―Compression
Only Support‖. Notice the example on the right,
where the outline of the undeformed cylinder is
shown. The compressive side retains the shape
of the original cylinder, but the tensile side is free to deform.
– This requires an iterative (nonlinear) solution.
• Compressive behavior is not known a priori, so an iterative
solution is required to determine what sides exhibit
compressive behavior.
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… Supports (Line/Surface Bodies)
• Simply Supported:
– Can be applied on edge or vertex of surface or line bodies
– Prevents all translations but all rotations are free
• Fixed Rotation:
– Can be applied on surface, edge, or vertex of surface or line
bodies
– Constrains rotations but translations are free
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… Structural Supports in ANSYS
• The following are applied internally in ANSYS:
– Fixed support constraints result in D,,ALL for given entity
– Given displacement is D,,UX/UY/UZ for specified direction (if
CS is supplied, nodes are rotated in that local CS)
– Frictionless surface involves nodal rotation such that UX is in
normal direction, and D,,UX is applied
– Cylindrical constraint rotates nodal coordinates in cylindrical
CS and constrains appropriate direction with D,,UX/UY/UZ
– Simply supported constraints apply D on UX, UY, and UZ on
shells or beams
– Fixed rotation constraints apply D on ROTX, ROTY, and ROTZ
on shells or beams
– For compression-only supports, the surface mesh is copied to
form a rigid target surface (TARGE170) on top of the original
surface (CONTA174). Standard contact behavior is used to
model this support, and that is why it is a nonlinear solution.
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… Summary of Supports
• Supports and Contact Regions may both be thought of as
being boundary conditions.
– Contact Regions provides a ‗flexible‘ boundary condition
between two existing parts explicitly modeled
– Supports provide a ‗rigid‘ boundary condition between the
modeled part an a rigid, immovable part not explicitly modeled
• If Part A, which is of interest, is connected to Part B,
consider whether both parts need to be analyzed (with
contact) or whether supports will suffice in providing the
effect Part B has on Part A.
– In other words, is Part B ‗rigid‘ compared to Part A? If so, a
support can be used and only Part A modeled. If not, one may
need to model both Parts A and B with contact.
Type of Support Equivalent Contact Condition at Surfaces of Part
Fixed Support Bonded contact with a rigid, immovable part
Frictionless Support No Separation contact with a rigid, immovable part
Compression Only Support Frictionless contact with a rigid, immovable part
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… Thermal Loading
• Temperature causes thermal expansion in the model
– Thermal strains are calculated as follows:
where a is the thermal expansion coefficient (CTE), Tref is the
reference temperature at which thermal strains are zero, T is
the applied temperature, and eth is the thermal strain.
– Thermal strains do not cause stress by themselves. It is the
constraint, temperature gradient, or CTE mismatch that
produce stress.
– CTE is defined in the ―Engineering‖
branch and has units of strain per
temperature
– The reference temperature is defined in the
―Environment‖ branch
ref
z
th
y
th
x
th TT aeee
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… Thermal Loading
• Thermal loads can be applied on the model
– Any temperature loading can be applied (see Chapter 6 on
Thermal Analysis for details)
– Simulation will always perform a thermal solution first, then
use the calculated temperature field as input when solving the
structural solution.
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… Thermal Loading in ANSYS
• In ANSYS, for any thermal loads present in the model:
– ANSYS will always solve a thermal solution first
• Even if a uniform temperature field is applied, a thermal solution
will be performed. This is why temperature body loads in a
structural analysis is not possible with an ANSYS Structural
license.
– Reference temperature is defined with TREF (not MP,REFT)
• TREF and TUNIF commands are set to the same value, as specified
under ―Reference Temp‖ of the Environment branch Details view
– Coefficient of thermal expansion per material is supplied with
MP,ALPX (not MP,CTEX or MP,THSX)
– Temperature loading is input via BF commands after thermal
solution
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D. Solution Options
• Solution options can be set under the ―Solution‖ branch
– The ANSYS database can be saved if ―Save
ANSYS db‖ is set
• Useful if you want to open a database in ANSYS
– Two solvers are available in Simulation
• The solver is automatically chosen, although some
informative messages may appear after solution
letting the user know what solver was used. Set
default behavior under ―Tools > Options … >
Simulation: Solution > Solver Type‖
• The ―Direct‖ solver is useful for models containing
thin surface and line bodies. It is a robust solver
and handles any situation.
• The ―Iterative‖ solver is most efficient when solving
large, bulky solid bodies. It can handle large models
well, although it is less efficient for beam/shells.
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… Solution Options
– Weak springs can be added to stabilize model
• If ―Program Controlled‖ is set, Simulation tries to
anticipate under-constrained models. If no
―Fixed Support‖ is present, it may add weak springs
and provide an informative message letting the user
know that it has done so
• This can be set to ―On‖ or ―Off‖. To set the default
behavior, go to ―Tools > Options … > Simulation:
Solution > Use Weak Springs‖.
• In some cases, the user expects the model to be in
equilibrium and does not want to constrain all
possible rigid-body modes. Weak springs will help
by preventing matrix singularity.
• It is good practice to constrain all possible rigid-body
motion, however.
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… Solution Options
– Informative messages are also present:
• The type of analysis is shown, such as ―Static
Structural‖ for the cases described in this section.
• If a nonlinear solution is required, it will be indicated
as such. Recall that for some contact behavior and
compression-only support, the solution becomes
nonlinear. These type of solutions require multiple
iterations and take longer than linear solutions.
• The solver working directory is where scratch files
are saved during the solution of the matrix equation.
By default, the TEMP directory of your Windows
system environment variable is used, although this
can be changed in ―Tools > Options … > Simulation:
Solution > Solver Working Directory‖. Sufficient free
space must be on that partition.
• Any solver messages which appear after solution can
be checked afterwards under ―Solver Messages‖
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… Solving the Model
• To solve the model, request results first (covered next) and
click on the ―Solve‖ button on the Standard Toolbar
– By default, two processors (if present) will be used for parallel
processing. To set the number, use ―Tools > Options … >
Simulation: Solution > Number of Processors to Use‖
– Recall that if a ―Solution Information‖ branch is requested, the
contents of the Solution Output can be displayed.
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… Solution Options in ANSYS
• The solver selection for direct vs. iterative:
– The solvers used are either the direct sparse solver
(EQSLV,SPARSE) or the PCG solver (EQSLV,PCG)
– A simplified discussion between the two solvers: • If given the linear static case of [K]{x} = {F}, Direct solvers factorize [K] to solve for [K]-1.
Then, {x} = [K]-1{F}.
– This factorization is computationally expensive but is done once.
• Iterative solvers use a preconditioner [Q] to solve the equation [Q][K]{x} = [Q]{F}. Assume
that [Q] = [K]-1. In this trivial case, [I]{x} = [K]-1{F}. However, the preconditioner is not
usually [K]-1. The closer [Q] is to [K]-1, the better the preconditioning is, and this process
is repeated - hence the name, iterative solver.
– For iterative solvers, matrix multiplication (not factorization) is
performed. This is much faster than matrix inversion if done entirely in
RAM, so, as long as the number of iterations is not very high (which
happens for well-conditioned matrices), iterative solvers can be more
efficient than sparse solvers.
– The main difference between the iterative solvers in ANSYS — PCG,
JCG, ICCG — is the type of pre-conditioner used.
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… Solution Options in ANSYS
• Weak spring option:
– If used, weak springs are added to the mesh. These are
modeled with COMBIN14 with small stiffness and added to the
extreme dimensions of the part.
• Solver working directory:
– The ANSYS input file is written as ―ds.dat‖ in the solver
directory. The output file is ―solve.out‖ and can be viewed in
the ―Solution Information‖ branch of the ―Solution‖ branch.
– ANSYS is executed in batch mode (-b) as a separate process.
During solution, the results file .rst is written. The results are
also read in and XML results files are generated in batch
mode. The XML files are then read into Simulation.
– All associated ANSYS files have default jobname of ―file‖ and
are deleted after solution, unless changed in ―Tools > Options
… > Simulation: Solution > Save Ansys Files‖.
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… Solution Options in ANSYS
• Various defaults in ANSYS are turned off when solving in
Simulation:
– Solution control (SOLCON,OFF) is turned off
– Multiframe restart is turned off (RESCON,,NONE)
– ANSYS shape checking is turned off (SHPP,OFF)
– Number of equilibrium iterations (NEQIT) is set to 1 if contact
is not present or if all contact is bonded or no separation.
• Otherwise, it is automatically determined, such as NEQIT,20
(frictionless contact) or NEQIT,40 (rough contact). NSUBST,1,10,1
is also set in these cases.
– Only requested results is output with OUTRES, not everything
by default
• Results are later written to XML files in /POST1, which are then
read back into Simulation. Hence, Simulation does not directly
read the results from the .rst file
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E. Results and Postprocessing
• Various results are available for postprocessing:
– Directional and total deformation
– Components, principal, or invariants of stresses and strains
– Contact output
• Requires ANSYS Professional and above
– Reaction forces
• In Simulation, results are usually requested before solving,
but they can be requested afterwards, too.
– If you solve a model then request results afterwards, click on
the ―Solve‖ button , and the results will be retrieved. A
new solution is not required if that type of result has been
requested previously (i.e., total deformation was requested
previously but now direction deformation is added).
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… Plotting Results
• All of the contour and vector plots are usually shown on the
deformed geometry. Use the Context Toolbar to change the
scaling or display of results to desired settings.
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… Deformation
• The deformation of the model can be plotted:
– Total deformation is a scalar quantity:
– The x, y, and z components of deformation can be
requested under ―Directional.‖ Because there is
direction associated with the components, if a
―Coordinate System‖ branch is present, users can
request deformation in a given coordinate system.
• For example, it may be easier to interpret displacement for a
cylindrical geometry in a ‗radial‘ direction by using a cylindrical
coordinate system to display the result.
– Vector plots of deformation are available.
Recall that wireframe mode is the easiest
to view vector plots.
222
zyxtotal UUUU
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… Deformation
• Deformation results are available for line, surface, and solid
bodies
– Note that ―deformation results‖ are associated with
translational DOF only. Rotations associated with the DOF of
line and surface bodies are not directly viewable
– Because deformation (displacements) are DOF which
Simulation solves for, the convergence behavior is well-
behaved when using the Convergence tool
– Vector deformation plots cannot use―Alert‖ or ―Convergence‖
tools because they are vector quantities (x, y, z) rather than a
unique quantity (x or y or z). Use Alert or Convergence tools
on ―Total‖ or ―Directional‖ quantities instead.
– ―Total‖ deformation is an invariant, so ―Coordinate Systems‖
cannot be used on this result quantity. Also, ―Vector‖
deformation is always shown in the world coordinate system.
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… Stresses and Strains
• Stresses and strains can be viewed:
– ―Strains‖ are actually elastic strains
– Stresses and (elastic) strains are
tensors and have six components
(x, y, z, xy, yz, xz) while thermal
strains can be considered a vector
with three components (x, y, z)
– For stresses and strains, components can be
requested under ―Normal‖ (x, y, z) and ―Shear‖
(xy, yz, xz). For thermal strains, (x, y, z) components are under
―Thermal.‖
• Can request in different results coordinate systems
• Thermal strains not available with an ANSYS Structural license
• Only available for shell and solid bodies. Line bodies currently do
not report any results except for deformation.
• ―Equivalent Plastic‖ strain output is covered in Chapter 11 ANSYS License Availability
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… Stresses and Strains
• Safety Factors can be used to evaluate designs:
– Because stress is a tensor, it is hard to evaluate
the response of the system by looking solely at
stress components
– The ―Stress Tool‖ allows the user to
have Simulation calculate scalar
results related to factors of safety
– In the next slides, stress results will
be discussed, along with different
criteria of evaluating material response,
as available from the Stress Tool.
– The ―Stress Tool‖ branch controls
what theory will be used and what
type of stress limit will be used.
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… Principal Stresses
• Principal Stresses and Strains:
– From basic mechanics review, the stress tensor can
be rotated such that only normal stresses appear.
These are the three principal stresses s1 < s2 < s3.
– Principal values of stress and strain results can be requested.
The three principal values also have direction associated with
them, and a ―Vector Principal‖ output can be selected.
• Principal values can be exported to Excel with Euler angles
– In the example shown on the right, one
can easily see the three principal
stresses (white=max, blue=min). From
this, one can see that the part is under-
going bending with one side in tension
and the other in compression.
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… Principal Stresses
• Maximum Tensile Stress Theory:
– The maximum tensile stress theory can be used for the
―Stress Tool‖. It utilizes the maximum principal stress and is
generally suitable for brittle materials.
– The criterion can be thought of as the following:
where st is the ultimate (or yield) tensile strength
– If plotted in two-dimensional principal stress space, the failure
surface results in a square as shown below. A stress state
lying inside the square is assumed to be fine but any stress
state lying on the edges of the square will fail.
– The max tensile stress criterion, as its
name implies, only considers the tensile
behavior. For many brittle materials, the
compressive strength is much greater,
so this assumption may be valid. s1
s2
st
st
1s
s tsafetyF
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… Principal Stresses
• Mohr-Coulomb Theory:
– The Mohr-Coulomb theory can be used for the ―Stress Tool‖.
It utilizes the maximum and minimum principal stresses and is
suitable for brittle materials.
– The criterion is as follows:
where st and sc are the ultimate (or yield)
tensile and compressive strengths.
– The failure surface is plotted in two-dimensional principal
stress space below. Unlike the maximum tensile stress
theory, the Mohr-Coulomb theory considers the
effects of the compressive strength.
1
31
ct
safetyFs
s
s
s
s1
s2
st
st
sc
sc
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… Equivalent Stress
• Equivalent Stress:
– The von Mises or equivalent stress se is defined as:
– This criterion is commonly used for ductile metals.
– When uniaxial tensile tests of specimens are performed to
determine the yield strength and stress-strain relationships,
the engineer needs a way to relate the uniaxial data to the
stress state (tensor). Hence, the equivalent stress is a
commonly used scalar invariant for this purpose.
2
13
2
32
2
212
1sssssss e
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… Equivalent Stress
• Maximum Equivalent Stress Theory:
– The Maximum Equivalent Stress Theory can be used for the
―Stress Tool‖. It compares the equivalent stress with the yield
(or ultimate) strength and is suitable for ductile materials.
– The criterion is as follows:
where sy is the tensile yield (or ultimate) strength.
– The failure surface is plotted in two-dimensional principal
stress space below.
– A stress state can be separated into hydrostatic and
distortional terms. The hydrostatic term
contributes to volume change but the
distortional term is associated with
yielding. Hence, the maximum equivalent
stress criterion is also known as the
distortion energy criterion.
e
y
safetyFs
s
s1
s2
sy
sy
sy
sy
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… Maximum Shear Stress
• Maximum Shear Stress:
– The maximum shear stress tmax is defined as
which results in the largest principal shear stress
– This value can be compared to the yield strength to predict
yielding for ductile materials
• Stress Intensity:
– The stress intensity is twice the value of the maximum shear
stress.
– The stress intensity provides the value of the largest
difference between principal stresses
2
31max
sst
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… Maximum Shear Stress
• Maximum Shear Stress Theory:
– The Maximum Shear Stress Theory or the Tresca criterion can
be used for the ―Stress Tool‖. It is suitable for ductile
materials.
– The criterion is as follows:
where sy is the tensile yield (or ultimate) strength
and f is a factor (default=0.5)
– The failure surface is plotted in two-dimensional principal
stress space below with the von Mises criterion superimposed
on in with a thin line. The two criteria are
quite similar, although the Tresca criterion
is slightly more conservative (maximum
difference between the two does not
exceed 15%).
maxt
s y
safety
fF
s1
s2
sy
sy
sy
sy
ANSYS License Availability
DesignSpace Entra x
DesignSpace x
Professional x
Structural x
Mechanical/Multiphysics x
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… Contact Results
• Contact Results:
– Contact results can be requested for selected
bodies or surfaces which have contact elements.
– Contact elements in ANSYS use the concept of
contact and target surfaces. Only contact surfaces
report contact results. MPC-based contact, the
target surfaces of any contact, and edge-based contact do not
report results. Line bodies do not support contact.
• If asymmetric or auto-asymmetric contact is used, then contact
results will be reported on the ‗contact‘ surfaces only. The ‗target‘
surfaces will report zero values, if requested.
• If symmetric contact is used, then contact results will be reported
on both surfaces. For values such as contact pressure, the actual
contact pressure will be an average of both surfaces in contact.
– Contact results are first requested via a ―Contact Tool‖ under
the Solution branch.
ANSYS License Availability
DesignSpace Entra
DesignSpace
Professional x
Structural x
Mechanical/Multiphysics x
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… Contact Results
• The user can specify contact output under ―Contact Tool‖
– The Worksheet view easily allows users to select which
contact regions will be associated with the ―Contact Tool‖
– Results on ‗contact‘ or ‗target‘ sides (or both) can be selected
from the spreadsheet (symmetric vs. asymmetric contact)
– Specific contact results chosen from Context Toolbar
ANSYS License Availability
DesignSpace Entra
DesignSpace
Professional x
Structural x
Mechanical/Multiphysics x
Select contact regions you want to
review (add more ―Contact Tool‖
branches to look at contact region
output separately).
Right-click on the worksheet to see
other available options.
For the ―Contact Tool‖, then
request contact output results, and
those results will correspond to
selected contact regions.
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… Contact Results
• Types of Contact Results available:
– Contact Pressure shows distribution of normal contact
pressure
– Contact Penetration shows the resulting amount of
penetration whereas contact Gap shows any gap
(within pinball radius).
– Sliding Distance is the amount one surface has slid with
respect to the other. Frictional Stress is tangential contact
traction due to frictional effects.
– Contact Status provides information on
whether the contact is established (closed
state) or not touching (open state).
• For the open state, near-field means that it is
within pinball region, far-field means that it is
outside of pinball region.
ANSYS License Availability
DesignSpace Entra
DesignSpace
Professional x
Structural x
Mechanical/Multiphysics x
Contour results are plotted with the
rest of the model being translucent
for easier viewing.
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… Contact Forces
• If ―Reactions‖ are requested for ―Contact Tool‖, forces and
moments are reported for the requested contact regions
– Under the ―Worksheet‖ tab, contact forces for all requested
contact regions will be tabulated
– Under the ―Geometry‖ tab, symbols will show direction of
contact forces and moments.
ANSYS License Availability
DesignSpace Entra
DesignSpace
Professional x
Structural x
Mechanical/Multiphysics x
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… Reaction Forces at Supports
• Reaction forces and moments are output for each support
– For each support, look under the ―Details‖ view
after solution. Reaction forces and moments are
printed. X, y, and z components are with respect
to the world coordinate system. Moments are
reported at the centroid of the support.
– The reaction force for weak springs, if used, is
under the ―Environment‖ branch Details view
after solution. The weak spring reaction forces
should be small to ensure that the effect of weak
springs is negligible.
ANSYS License Availability
DesignSpace Entra x
DesignSpace x
Professional x
Structural x
Mechanical/Multiphysics x
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… Reaction Forces at Supports
• The ―Worksheet‖ tab for ―Environment‖ branch has a
summary of reaction forces and moments
– If a support shares a vertex, edge, or surface with another
support, contact pair, or load, the reported reaction forces may
be incorrect. This is due to the fact that the underlying mesh
will have multiple supports and/or loads applied to the same
nodes. The solution will still be valid, but the reported values
may not be accurate because of this.
ANSYS License Availability
DesignSpace Entra x
DesignSpace x
Professional x
Structural x
Mechanical/Multiphysics x
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… Fatigue
• If the Fatigue Module add-on license is available, additional
post-processing involving fatigue calculations is possible
– The ―Fatigue Tool‖ provides stress-based fatigue calculations
to aid the design engineer with evaluating the life of
components in the system
– Constant or variable amplitude loading, proportional or non-
proportional loading is possible
ANSYS License Availability
Fatigue Module x
Damage Matrix at Critical Location Contour of Safety Factor
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… Results (ANSYS Details)
• Postprocessing calculations are performed in /POST1 after
the solution as part of the ANSYS input file
– All contour result plots in Simulation are the same as nodal
(averaged) solution with Full Graphics
• Viewing Simulation contour plots would be similar to using
PLNSOL with /GRAPH,FULL commands in ANSYS
• No plotting is actually done in input file – this is to give an idea of
equivalent plotting commands in ANSYS
– Reaction forces for supports as well as nodal result data is
sent to Simulation via XML files
• XMLOPT and /XML commands are used
– Contact (reaction) force calculations are performed by
selecting contact surfaces and performing FSUM about
centroid. This is repeated for target surfaces.
Advanced ANSYS Details
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• Workshop 4 – Linear Structural Analysis
• Goal:
– A 5 part assembly representing an impeller type pump is
analyzed with a 100N preload on the belt.
F. Workshop 4