Intro1 M11 Structural

8/8/2019 Intro1 M11 Structural

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In this chapter, we will describe the specifics of a structuralanalysis.

The purpose is two-fold: To reiterate the general analysis procedure. To introduce you to structural loads and boundary conditions

Chapter 11 Structural Analysis

Overview


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G eometry

Can either be created within ANSYS or imported .

Include details to improve results: G oal is to sufficiently model the stiffness of the structure Add details to avoid stress singularities (e.g. fillets) Exclude details not in region of interest (e.g. exclude small holes) Add details to improve boundary conditions (e.g. apply pressure to an

area rather than using concentrated load)

Chapter 11 A. Preprocessing

Geometry


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E lement typeThe table below shows commonly used structural element types.

The nodal DOFs may include: UX, UY, UZ, ROTX, ROTY, and ROTZ.

2-D Solid 3-D Solid 3-D Shell Line Elements

Linear PLANE42 SOLID45SOLID185SHELL63SHELL181

BEAM3BEAM4

BEAM188

Q uadratic PLANE82 PLANE2

SOLID95SOLID92SOLID186

SHELL93 BEAM189

Commonly used structura l e le me nt types

Chapter 11 A. Preprocessing

Meshing

Material properties M inimum requirement is Youngs M odulus, EX. If Poissons Ratio is

not entered a default of 0.3 will be assumed. Setting preferences to Structural limits the M aterial M odel G UI to

display only structural properties.

R eal constants and Section properties Primarily needed for shell and line elements.


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Structural loading conditions can be:

D OF Constraints Regions of the model where displacements are known.

Concentrated Forces External forces that can be simplified as a point load.

Pressures Surfaces where forces on an area are known.

Uniform Temperature Temperatures applied as a body force used with a referencetemperature to predict thermal strains.

Gravity Accelerations applied as inertia boundary conditions

Chapter 11 B. SolutionD efine Loads


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To apply a force, the following information is needed: node or keypoint number (which you can identify by picking) force magnitude (which should be consistent with the system of units

you are using) direction of the force FX, FY, or FZ

Use: Main Menu > S olution > Defi ne L oads > A pply > Structura l > F orce/M om e nt Or the commands FK or F

Q uestion: In which coordinate system are FX, FY, and FZinterpreted?

Chapter 11 B. Solution

Concentrated Forces


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Pressures

To apply a pressure: Main Menu > S olution > Defi ne L oads > A pply

Structura l > Pressure

Choose where you want to apply thepressure -- usually on lines for 2-Dmodels, on areas for 3-D models.

Pick the desired entities in the graphicswindow.

Then enter the pressure value.A positive value indicates a

compressive pressure (acting towardsthe centroid of the element). Or use the SF family of commands: SFL,

SFA, SFE, SF .


Pressure


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For a 2-D model, where pressuresare usually applied on a line, youcan specify a tapered pressureby entering a value for both the I and J ends of the line.

I and J are determined by the linedirection. If you see the taper going in the wrong direction,simply reapply the pressure withthe values reversed.

VALI = 500

500L3

500

VALI = 500VALJ = 1000

L3

1000500

VALI = 1000VALJ = 500

L3

1000 500


Pressure


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Uniform Temperature

To define uniform temperature Main Menu > S olution > Defi ne L oads > A pply >Structura l > Te mp erature > U nif or m Te mp Or use the TUNIF command.


Uniform temperature

To define reference temperature Main Menu > S olution > Load Ste p Opts > Other > Refere nce Te mp Or use the TREF command or as MP,REFT

)( ref th T T ! EI Recall,


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G ravity

To apply gravitational acceleration: Main Menu > S olution > Defi ne L oads >

Apply > Structura l > Inertia > Gravit y Or use the ACEL command.

Notes: A positive acceleration value causes deflection in the negative

direction. If Y is pointing upwards, for example, a positive ACELYvalue will cause the structure to move downwards.

Density (or mass in some form) must be defined for gravity and other inertia loads.


Gravity


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M odifying and Deleting Loads

To modify a load value, simply reapply the loadwith the new value.

To delete loads: Main Menu > S olution > Defi ne L oads > De lete When you delete solid model loads, ANSYS also

automatically deletes all corresponding finiteelement loads.


Modifying and D eleting Loads


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Static vs. Dynamic Analysis

A static analysis assumes that only the stiffness forces aresignificant.

A dynamic analysis takes into account all three types of forces.

For example, consider the analysis of a diving board. If the diver is standing still, it might be sufficient to do

a static analysis. But if the diver is jumping up and down, you will need

to do a dynamic analysis.


Solutions Options


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Inertia and damping forces are usually significant if the appliedloads vary rapidly with time.

Therefore you can use time-dependency of loads as a way tochoose between static and dynamic analysis.

If the loading is constant over a relatively long period of time, choose

a static analysis. Otherwise, choose a dynamic analysis.

In general, if the excitation frequency is less than 1/3 of thestructures lowest natural frequency, a static analysis may beacceptable.


Solutions Options


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Linear vs. Nonlinear Analysis

A linear analysis assumes that the loading causes negligiblechanges to the stiffness of the structure. Typical characteristicsare:

Small deflections Strains and stresses within the elastic limit No abrupt changes in stiffness such as two bodies coming into and

out of contact

Strai n

Stress

Elastic mo du lus(EX)


Solutions Options


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A nonlinear analysis is needed if the loading causes significantchanges in the structures stiffness. Typical reasons for stiffnessto change significantly are:

Strains beyond the elastic limit (plasticity) Large deflections, such as with a loaded fishing rod Contact between two bodies

Strai n

Stress


Solutions Options


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Reviewing results of a stress analysis generally involves: Deformed shape Stresses Reaction forces

Deformed Shape

G ives a quick indication of whether the loads were applied in thecorrect direction.

Legend column shows the maximum displacement, D M X.

You can also animate the deformation.

Chapter 11 C. Postprocessing R eview R esults


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To plot the deformed shape: Ge nera l P ost pr oc > P lot

Resu lts > Def or med Sha pe Or use the PLDISP command.

For animation: Utility Me nu > P lotCtr ls >

Animate > Def or med Sha pe Or use the ANDISP

command.

Chapter 11 C. Postprocessing

R eview R esults


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Stresses

The following stresses are typically available for a 3-D solidmodel:

Component stresses SX, SY, SZ, SXY, SYZ, SXZ (global Cartesiandirections by default)

Principal stresses S1, S2, S3, SEQV (von M ises), SINT (stressintensity)

Best viewed as contour plots, which allow you to quickly locatehot spots or trouble regions.

N odal solution: Stresses are averaged at the nodes, showing smooth,

continuous contours. E lement solution: No averaging, resulting in discontinuous contours.


R eview R esults


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To plot stress contours: Ge nera l P ost pr oc > P lot Resu lts > Con tour P lot > N oda l S olu or PLNSOL command Ge nera l P ost pr oc > P lot Resu lts > Con tour P lot > E le me nt S olu or PLESOL command

You can also animate stress contours: Utility Me nu > P lotCtr ls > A nimate > Def or med Resu lts... or ANCNTR command


R eview R esults


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A Note on Power G raphics

It is the default graphics setting ( /GRAPH,POWER ).

Plots only the visible surfaces and ignores everythingunderneath.

Advantages: Faster REPLOT, crisp graphics. Smooth, almost photo-realistic displays. Prevents stress averaging across material and real

constant boundaries.

To deactivate Power G raphics (or activate fullgraphics):

Toolb ar > POWRGRPH Or issue /GRAPH,FULL Or interactively, Utility Me nu>P lotCtr ls>St yle>

Hidde nLineO ption s> Gra phics Dis pla y Meth od is...Fu ll Mode l


R eview R esults


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It is always a good idea to do a sanity check and make sure thatthe solution is acceptable. What you need to check depends onthe type of problem you are solving, but here are some typicalquestions to ask:

Do FEA results agree with hand calculations or experimentaldata?

Is the displacement solution correct? Check the FEAdisplacement solution first since FEA stresses are second order results.

Do the reaction forces balance the applied loads?

Where is the maximum stress located? If it is at a singularity, such as a point load or a re-entrant corner, the

value is generally meaningless. Are the stress values beyond the elastic limit? If so, the load magnitudes may be wrong, or you may need to do a

nonlinear analysis.


Verify R esults


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Is the mesh adequate? This is always debatable, but you can gain confidence in the mesh by

using error estimation. Other ways to check mesh adequacy:

Plot the element solution (unaveraged stresses) and look for elements with high stress gradients. These regions are candidates

for mesh refinement.If there is a significant difference between the nodal (averaged)and element (unaveraged) stress contours, the mesh may be toocoarse.

Similarly, if there is a significant difference betweenPower G raphics and full graphics stresses, the mesh may be too

coarse.Re-mesh with twice as many elements, re-solve, and compare theresults. (But this may not always be practical.)


Verify R esults


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Chapter 11 D . Workshops

Workshops

This workshop consists of two problems:11A. Lathe Cutter 11B. 2-D Corner Bracket Tutorial

Refer to your Workshop Supplement for instructions.


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Documents

Intro1 M11 Structural