Linear Structural Analysis - Lawrence Berkeley National …als/FEA/Workbench_Sim… · PPT file ·...

56
Static Structural Analysis Chapter Four

Transcript of Linear Structural Analysis - Lawrence Berkeley National …als/FEA/Workbench_Sim… · PPT file ·...

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Static Structural Analysis

Chapter Four

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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• For a linear static structural analysis, the displacements {x} are

solved for in the matrix equation below:

Assumptions:– [K] is 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.

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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

– 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 surfacesassuming no stiffness between them.

– No rotational inertial terms are present.

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… Point Mass• A point mass will be displayed as a round, grey sphere

– a point mass in a linear static analysis can be used to account for the additional mass of a structure not modeled

– No results are obtained for the Point Mass itself

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… Material Properties• Young’s Modulus and Poisson’s Ratio are required for

linear static structural analyses– Material input is handled in the “Engineering Data” application– Materials are assigned per part in 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– 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

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… Material Properties• Engineering Data view 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.– 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 a contact region is referred to as a contact

surface, the other side is referred to as a target surface– The contact surfaces are restricted from penetrating through

the target surfaces however the opposite is not true• When one side is the contact and the other side is the target, this is

called asymmetric contact. If both sides are made to be contact & target this is called symmetric contact

• 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 and require only 1 iteration

– Frictionless and Rough contact are nonlinear and require multiple iterations (small deflection theory is still assumed)

• An “interface treatment” option is available:– “Add Offset”: input zero or non-zero value for

initial adjustment– “Adjusted to Touch”: ANSYS closes any gap to a

just touching position (ANSYS Professional and above)

Contact Type Iterations Normal Behavior (Separation) Tangential Behavior (Sliding)Bonded 1 Closed ClosedNo Separation 1 Closed OpenFrictionless Multiple Open OpenRough 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 classic ANSYS

• Note: the MPC formulation writes constraint equations relating the movement of parts at an interface. This can be an attractive alternative to penalty method for bonded contact.

• Additional information on advanced contact is presented in the Workbench nonlinear course as well as the ANSYS documentation

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… Assemblies – Solid Body Contact• Advanced options (continued):

– Pin Ball Region:• Inside pinball = near-field contact• Outside pinball = far-field contact• Allows the solver to more efficiently

process contact calculations

• 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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… Assemblies – Surface Body Contact• For ANSYS Professional licenses and above, mixed

assemblies of shells and solids are supported– More contact options are exposed to the user– Contact postprocessing is also available (discussed later)

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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

• If a gap exists the pinball region can beused to detect contact beyond the gap.

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… Assemblies – Contact Summary• Summary of contact types and options available in

Simulation:

Contact Geometry Solid Body Face Solid Body Edge Surface Body Face Surface Body EdgeAll types Bonded, No Separation Bonded, No Separation Bonded onlyAll formulations All formulations All formulations MPC formulationSymmetry respected Asymmetric only Symmetry respected Asymmetric only

Bonded, No Separation Bonded, No Separation Bonded onlyAll formulations All formulations MPC formulationAsymmetric only Asymmetric only Asymmetric only

Bonded, No Separation Bonded onlyAll formulations MPC formulationSymmetry respected Asymmetric only

Bonded onlyMPC formulationAsymmetric 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– 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.

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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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. . . Time Type• A time type option is available at certain license levels

• The default time type for loading is “static”

• “Transient” and “harmonic” time types are available as options

• Sequence loading allows a series of static time steps to be set up in advance and solved at once

• Sequenced results can be reviewed step by step

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. . . Time Type• Specify the desired number of sequence

steps in the “Steps” field in the main toolbar.

• Enter the value of the load for each step by first highlighting the desired step in the graphics window.

• The chart in the graphics window displays the variation of the load while a legend in the graphics window displays the current view setting.

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. . . Time Type• Results of a sequenced simulation can be reviewed by highlighting

the solution of interest on the graph, “RMB > Retrieve Results”.

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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– In the Details view, change “Define By” to

“Components”. Then, select the appropriate 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 SystemsAcceleration NoStandard Earth Gravity NoRotational Velocity NoForce YesRemote Force Location of Origin OnlyBearing Load YesMoment YesGiven 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• Acceleration:

– Acts on entire model in length/time2 units.– Note, inertial forces are in the opposite direction to applied

acceleration– Acceleration can be defined by Components or Vector

• Standard Earth Gravity: – 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– “Standard Earth Gravity” is defined as an acceleration. Define

the direction opposite to gravitational force

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… Rotational Velocity• Rotational velocity:

– Entire model spins about an axis at a given rate– Define by vector using geometry for axis of rotation– Define by components supplying origin and components in

global or local coordinate system– Input can be in radians per second (default) or RPM

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… Forces and Pressures• Pressure loading:

– Applied to surfaces, acts normal to the surface– Positive value into surface, negative value acts

out of surface– Units of pressure are in force per area

• Force loading:– Forces can be applied on vertices, edges, or

surfaces.– The force will be evenly distributed on all entities.

Units are mass*length/time2

– Force can be defined via vector or component methods

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… Bearing Load• Bearing Load (cylindrical surfaces only):

– Radial component will be distributed on compressive side using projected area (see below)

– Axial component is distributed evenly on cylinder– Use only one bearing load per cylindrical surface. If the

cylindrical surface is split 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 or component method

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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 is evenly distributed

– Vector or component method can be used. 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

– Units of moment are in Force*length.

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… Remote Load• Remote Load:

– applies an offset force on a surface or edge of a surface body– The user supplies the origin of the force (vertices, cylinder or

x, y, z coordinates). A user-defined Coordinate System may be used to reference the location.

– Can be defined using vector or component method– Applies an equivalent force and moment on the surface– Units are in force (mass*length/time2)

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Load transfer surface

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. . . Bolt Load• Bolt Load:

– Applies a pretension load to a cylindrical section simulating bolt tightening

– Apply pretension load (force) or adjustment (length) as initial condition

– For sequenced loading additional options are available (see next page)

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. . . Bolt Load• For static analyses the preload is applied in an initial solution and

external loads are applied in a subsequent solution– Note this 2 step sequence is automatic and transparent

• The bolt load is automatically locked during the second solution

• For sequence analyses users may choose to lock or open the bolt load in any load step except the first

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. . . Bolt Load• Bolt Load Tips:

– Can be used only in 3D simulations– Can be applied to cylindrical surfaces or bodies. Bodies

require a local coordinate system with the Z axis representing the pretension direction

– A refined mesh is recommended (must be more than 1 element along the bolt section)

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… Supports (General)• Fixed Support:

– Constraints all degrees of freedom on vertex, edge, or surface• Solid bodies: constrains x, y, and z• Surface and line bodies: constrains x, y, z, rotx, roty and rotz

• 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 constraint in the normal compressive direction– Can be used on a cylindrical surface to model a

pin (referred to as “Pinned Cylinder” 7.1)

– Notice in the example above 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.ANSYS License AvailabilityDesignSpace Entra xDesignSpace xProfessional xStructural xMechanical/Multiphysics x

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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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… Thermal Loading• Temperature causes thermal expansion in the model

– Thermal strains are calculated as follows:

where is the thermal expansion coefficient (CTE), Tref is the reference temperature at which thermal strains are zero, T is the applied temperature, and th is the thermal strain.

– CTE is a material property defined in “Engineering Data” and has units of strain per temperature

– The reference temperature is defined in the“Environment” branch

refzth

yth

xth TT

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… Thermal Loading• Thermal loads:

– 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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• Workshop 4.1 – Linear Structural Analysis

• Goal:– A 5 part assembly representing an impeller type pump is

analyzed with a 100N preload on the belt.

D. Workshop 4.1 – Linear Structural Analysis

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E. Solution Options• Solution options can be set under the “Solution”

branch– Save ANSYS db:

• Useful if you want to open a database in ANSYS– Two solvers are available in Simulation:

• The “Direct” solver is useful for models containingthin surface and line bodies. It is a robust solverand handles any situation (Sparse solver in ANSYS)

• The “Iterative” solver is most efficient when solvinglarge, bulky solid bodies. It can handle large modelswell, although it is less efficient for beam/shells (PCG solver in ANSYS)

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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 userknow that it has done so

• This can be set to “On” or “Off”.

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… Solution Options– Solution Information:

• Analysis Type: Static Structural, thermal, etc • Nonlinear solution: indicated when options cause the

solution to be nonlinear. These type of solutions require multiple iterations and take longer than linear solutions.

• Solver Working Directory: – Location where scratch files are saved during

solution– Default is the TEMP directory on Windows

systems – Can be changed in “Tools > Options … >

Simulation: Solution > Solver Working Directory”– Solver Messages: review messagges triggered

during the solution by clicking “Solver Messages” field

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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”

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F. 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 for linear analyses

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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.”– if a “Coordinate System” branch is present, users

canrequest directional results in a given coordinate system.

– Vector plots of deformation are available.

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… Deformation• Deformation results are available for line, surface, and solid

bodies– Because deformation (displacements) are DOF which

Simulation solves for, the convergence behavior is well-behaved when using the Convergence tool

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… Stresses and Strains• Stresses and strains:

– Stresses and (elastic) strains have six components(x, y, z, xy, yz, xz) while thermal strains have 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.”

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… Stress Tools• Safety Factors (choose from 4 failure

theories):– Ductile Theories:

• Maximum Equivalent Stress• Maximum Shear Stress

– Brittle Theories:• Mohr-Coulomb Stress• Maximum Tensile Stress

– Within each stress tool safety factor, safety margin and stress ratio can be plotted

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… Contact Results• Contact Results:

• 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

ANSYS License AvailabilityDesignSpace EntraDesignSpaceProfessional xStructural xMechanical/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• Contact Results:

– Pressure: shows distribution of normal contact pressure

– Penetration/Gap: shows the resulting amount of penetration or gap (within pinball radius).

– Sliding Distance: distance one surface has slid with respect to the other.

– Frictional Stress: is tangential contact traction due to frictional effects.

– Status: open or closed• 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 therest of the model being translucentfor easier viewing.

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… Contact Forces• Contact Reactions: 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

– Reaction forces and moments are shown in the details for each support. X, Y, and Z components are reported with respect to the world coordinate system. Moments are reported at the centroid of the support.

– The reaction force for weak springs, if used, are listed under the “Environment” branch Details. 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.

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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 AvailabilityFatigue Module x

Damage Matrix at Critical Location Contour of Safety Factor

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G. Workshop 4.2 – 2D vs 3D Analysis• Workshop 4.2 – Comparing 2D and 3D Structural Analysis

• Comparing 2D and 3D structural analyses.

• Shown here are the 3D sector model and the 2D axisymmetric model.

Pressure Cap

Retaining Ring