Structural AnalysisChapter 10

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• In this chapter, we will describe the specifics of a structural analysis.

• The purpose is two-fold:– To reiterate the typical analysis steps that were introduced in Chapter 4.– To introduce you to structural loads and boundary conditions

Chapter 10 – Structural AnalysisOverview

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Geometry

• Can either be created within ANSYS or imported.

• Include details to improve results:– Goal 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 10 – PreprocessingGeometry

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• Element type• The table below shows commonly used structural element types.• The nodal DOF’s may include: UX, UY, UZ, ROTX, ROTY, and ROTZ.

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

Linear PLANE42 SOLID45 SHELL63 SHELL181

BEAM3, BEAM4

Quadratic PLANE82 PLANE2

SOLID95 SOLID92

SHELL93

Commonly used structural element types

Chapter 10 – PreprocessingMeshing

• Material properties– Minimum requirement is Young’s Modulus, EX.– Setting preferences to “Structural” limits the Material Model GUI to

display only structural properties.

• Real constants– Primarily needed for shell and line elements.

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• Structural loading conditions can be:

DOF 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 reference temperature to predict thermal strains.

Gravity Accelerations applied as inertia boundary conditions

* Not covered in this course

Chapter 10 – SolutionDefine Loads

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Displacement Constraints

• Used to specify where the model is fixed (zero displacement locations).

• Can also be non-zero, to simulate a known deflection.

• To apply displacement constraints :– Solution > -Loads- Apply > Displacement

• Choose where you want to apply the constraint.

• Pick the desired entities in the graphics window.

• Then choose the constraint direction. Value defaults to zero.

– Or use the D family of commands: DK, DL, DA, D.

• Question: In which coordinate system are UX, UY, and UZ interpreted?

Chapter 10 – SolutionDisplacement Constraints

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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:– Solution > -Loads- Apply > Force/Moment– Or the commands FK or F

• Question: In which coordinate system are FX, FY, and FZ interpreted?

Chapter 10 – SolutionConcentrated Forces

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Pressures

• To apply a pressure:– Solution > -Loads- Apply > Pressure

• Choose where you want to apply the pressure -- usually on lines for 2-D models, on areas for 3-D models.

• Pick the desired entities in the graphics window.

• Then enter the pressure value. A positive value indicates a compressive pressure (acting towards the centroid of the element).

– Or use the SF family of commands: SFL, SFA, SFE, SF.

Chapter 10 – SolutionPressure

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• For a 2-D model, where pressures are usually applied on a line, you can specify a tapered pressure by entering a value for both the I and J ends of the line.

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

VALI = 500

500L3

500

VALI = 500VALJ = 1000

L3

1000500

VALI = 1000VALJ = 500

L3

1000500

Chapter 10 – Solution…Pressure

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Uniform Temperature

• To uniform temperature– Solution > -Loads- Apply > Temperature > Uniform Temp– Or use the TUNIF command.

Chapter 10 – SolutionUniform temperature

• To define reference temperature– Solution > -Load Step Opts > Other > Reference Temp– Or use the TREF command or as MP,REFT

LTT refth )( • Recall,

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Gravity

• To apply gravitational acceleration:– Solution > -Loads- Apply > Gravity– 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 ACELY value will cause the structure to move downwards.

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

Chapter 10 – SolutionGravity

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Modifying and Deleting Loads

• To modify a load value, simply reapply the load with the new value.

• To delete loads:– Solution > -Loads- Delete >– When you delete solid model loads, ANSYS also

automatically deletes all corresponding finite element loads.

Chapter 10 – SolutionModifying and Deleting Loads

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Static vs. Dynamic Analysis

• A static analysis assumes that only the stiffness forces are significant.

• 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.

Chapter 10 – SolutionSolutions Options

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• Inertia and damping forces are usually significant if the applied loads vary rapidly with time.

• Therefore you can use time-dependency of loads as a way to choose 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 the structure’s lowest natural frequency, a static analysis may be acceptable.

Chapter 10 – SolutionSolutions Options

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Linear vs. Nonlinear Analysis

• A linear analysis assumes that the loading causes negligible changes to the stiffness of the structure. Typical characteristics are:

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

out of contact

Strain

Stress

Elastic modulus(EX)

Chapter 10 – SolutionSolutions Options

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• A nonlinear analysis is needed if the loading causes significant changes in the structure’s stiffness. Typical reasons for stiffness to change significantly are:

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

Strain

Stress

Chapter 10 – SolutionSolutions Options

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

Deformed Shape

• Gives a quick indication of whether the loads were applied in the correct direction.

• Legend column shows the maximum displacement, DMX.

• You can also animate the deformation.

Chapter 10 – PostprocessingReview Results

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• To plot the deformed shape:– General Postproc > Plot

Results > Deformed Shape– Or use the PLDISP command.

• For animation:– Utility Menu > PlotCtrls >

Animate > Deformed Shape– Or use the ANDISP command.

Chapter 10 – Postprocessing…Review Results

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Stresses

• The following stresses are typically available for a 3-D solid model:

– Component stresses — SX, SY, SZ, SXY, SYZ, SXZ (global Cartesian directions by default)

– Principal stresses — S1, S2, S3, SEQV (von Mises), SINT (stress intensity)

• Best viewed as contour plots, which allow you to quickly locate “hot spots” or trouble regions.

– Nodal solution: Stresses are averaged at the nodes, showing smooth, continuous contours.

– Element solution: No averaging, resulting in discontinuous contours.

Chapter 10 – Postprocessing…Review Results

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• To plot stress contours:– General Postproc > Plot Results > Nodal Solu… or PLNSOL command– General Postproc > Plot Results > Element Solu… or PLESOL command

• You can also animate stress contours:– Utility Menu > PlotCtrls > Animate > Deformed Results... or ANCNTR command

Chapter 10 – Postprocessing…Review Results

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A Note on PowerGraphics

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

• Plots only the visible surfaces and ignores everything “underneath.”

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

constant boundaries.

• To deactivate PowerGraphics (or activate “full graphics”):

– Toolbar > POWERGRPH– Or issue /GRAPH,FULL

Chapter 10 – Postprocessing…Review Results

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Reaction Forces

• The sum of the reaction forces in each direction must equal the sum of applied loads in that direction.

• Best viewed as a listing:– General Postprocessor > List Results > Reaction Solution… or PRRSOL

command

Chapter 10 – Postprocessing…Review Results

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• It is always a good idea to do a “sanity check” and make sure that the solution is acceptable.

• What you need to check depends on the type of problem you are solving, but here are some typical questions to ask:

• Do FEA results agree hand calculations or experimental data.

• Is the displacement solution correct? Check the FEA displacement 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. (We will discuss more about this in Chapter 5.)

• 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.

Chapter 10 – PostprocessingVerify Results

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

using error estimation data (discussed in Chapter 14).– 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 too coarse.

• Similarly, if there is a significant difference between PowerGraphics and full graphics stresses, the mesh may be too coarse.

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

Chapter 10 – Postprocessing…Verify Results

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Stress AnalysisH. Workshop• This workshop consists of two problems:

10A. Lathe Cutter10B. 2-D Corner Bracket Tutorial

Refer to your Workshop Supplement for instructions.