75806934 ANSYS Workbench Simulation Static

Static Structural Analysis

Chapter Four

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Linear Static Structural Analysis

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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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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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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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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

DesignSpace Entra x

DesignSpace x

Professional x

Structural x

Mechanical/Multiphysics x

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… 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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… 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

DesignSpace Entra x

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Structural x


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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.


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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


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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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• 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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• 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.


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• 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.


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• 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


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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)


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• 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.


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• 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.


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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.


DesignSpace Entra

DesignSpace x

Professional x

Structural 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.


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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:


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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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• 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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• 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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• 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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• 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.


where sy is the tensile yield (or ultimate) strength.


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.


where sy is the tensile yield (or ultimate) strength

and f is a factor (default=0.5)


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


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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.


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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


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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.


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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.


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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.


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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.


DesignSpace Entra x

DesignSpace x

Professional x

Structural 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


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.


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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

workshops/AWS81_Workshop_04.ppt




Documents

75806934 ANSYS Workbench Simulation Static