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Computational Science CPDin the School of Computing, Engineering and Physical

Sciences

ANSYS Fatigue Analysisby Dr J. Whitty

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Lessons structure• The lessons will in general be subdivided

in to eight number of parts, viz.:1) Statement of learning objectives2) Points of orders3) Introductory material (ANSYS-Workbench)4) Concept introduction (Static analyse)5) Development of related principles (Fatigue)6) Concrete principle examples via –

reinforcement examination type exercises7) Summary and feedback8) Formative assessment, via homework task

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Learning objectivesAfter the session you will be able to:

– Utilize all phases of the FEM within commercial software, viz

• Solid modelling• Pre-processing (meshing)• Solution (inc. setting boundary conditions)• Post-processing (evaluation of required results)• Validation

– Use suitable static failure criteria to determine the suitability of a engineering components with in FEM software

– Use FEM software to evaluate the fatigue life and failure of components

– Answer examination questions pertain to the interpretation of Finite element results.

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ANSYS Workbench• Native CAD import and

interface• All simulation work completed

in one environment inc.1. Solid Modelling

Fundamentals  2. Placed Features, Assembly   3. Modelling Techniques  4. Simulation, Wizards & Tools 5. Heat Transfer & Thermal

Stress  6. Surface & Line Models  7. Natural Frequencies 8. Buckling Loads9. CFD

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Learning Check: ANSYS Workbench: Example

usage• In the lecture we simulated the stress and deflexion in a cantilever, as a learning check we will consider a learning check, we will consider a very simple problem for the first 20 minutes of this lecture – Example: Consider a simple benchmarking example of a cantilever

beam of 500mm long and a cross section of 20mm by 40mm, which is exposed to a load P. Use the ANSYS workbench software in order to predict the failure load on the component and determine the maximum displacement of the beam.

• Re call from last session the displacement and stress are calculated.

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6M WL

Z bd

Take 5mins to evaluate these as we will be needing them later

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33 4

3 Ebd

WL

EI

WL

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Using ANSYS Workbench v12

• Using to software the following steps are applied, thus:– Drag and drop the structural

static from the analysis systems menu into the work area (as shown)

– Then work the list provided, viz.

• Geometry• Model• Set-up• Solution• Results

– Each of these steps will be taken in turn!

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Geometry creation• Here you will need to right mouse-

click on geometry and then create the solid model, via extrusion a 20mm by 40mm rectangle

500mm

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Mesh creation• Right mouse click on mesh and issue a

generate command to produce the mesh as shown.

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Apply boundary conditions (set-up)• Left mouse click on Static Solution in the project

tree then apply the following BCs. Then simply hit the solve button to obtain the solution

Apply a load of 500N on surface Completely fix

one end

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ANSYS Workbench Results• Once an

analysis is complete is usual to observe the following field data, in this order: Check

displacement

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6M WL

Z bd Check stress

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3

2040000,200

5005004

= 3.91mm

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Workbench approximate stress

• It is important that the correct stress theory is applied in order to check the results. In this case the Equivalent (Huber-Mises) stress is plotted. Since the component is in pure bending then the stress value can be calculated given the standard bending formulae

2

6M WL

Z bd

So in this case:

or:

22max N/mm75.39

2040

5005006

93.75MPa

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Re-design the geometry• Last week we redesigned the component • Here is it usual to loose materials in the blue areas i.e. So long that the

safety factor is large enough.• You should try this as an exercise yourself

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Optimized static design• There are various solutions to this however after a small amount of

iterations you should find that a 3.25 circular slot by 225mm long (cross centres) should be ample to reduce the minimum safety factor to about 2.1.

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Fatigue Modelling• Now for something new.

– You can activate the Fatigue properties based on an SN curve resident in the ANSYS software using the menu path Tools>Fatigue Tool

– Insert life to start off with and generate the results as normal

Note ANSYS has predicted the

fatigue failure point very well!

The calculated life is170,490 cycles of

500N the load

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Fatigue Safety Factor• Just as with the stress tool the fatigue safety factor can also

be evaluated thus:– Activate the Fatigue Tool by left mouse clicking on it: insert safety factor

The safety factor now reports failure of the component

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Class exercise• Remove the slot to see if the component is still fit for purpose

under fatigue conditions– Simply delete the extrude feature in the ANSYS DesignModeller tree then

go back into the results module and revaluate.

The safety factor still reports failure of the component

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Class exercise: Increase depth• Increase the depth of the beam by 10mm and resolve

– Simply change the dimension of the initial sketch, click on the solid under geometry in the model-tree regenerate then re-solve

The safety factor still reports failure of the component

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New static safety factor • Simply plot this (left mouse button) is the static analysis

– Simply change the dimension of the initial sketch, click on the solid under geometry in the model-tree regenerate then re-solve

The static safety factor has increased

three-fold

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SummaryHave we met our learning objectives in particular are you

will be able to:– Utilize all phases of the FEM within commercial

software, viz• Solid modelling• Pre-processing (meshing)• Solution (inc. setting boundary conditions)• Post-processing (evaluation of required results)• Validation

– Use suitable static failure criteria to determine the suitability of a engineering components with in FEM software

– Use FEM software to evaluate the fatigue life and failure of components

– Answer examination questions pertain to the interpretation of Finite element results.If so when and were within the

workshop were these met!

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Class exercise• The component shown is made

from 5mm thick sheet steela) Use ANSYS DesignModeller (or

SolidWorks) b) If the 75mm edge is to be

clammed determine the maximum load P if a static safety factor of 3 is required for the component

c) Increase the mesh density using the Relevance setting in the mesh object and comment on the density

d) Explain the observations made in part(c).

e) Evaluate the value P if an additional fatigue safety factor of 2 is required.

f) Is this the most efficient method of modelling this particular component.

P

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Examination Type Question

• Explain two processes of descertization used in FEA [6]a) For the component shown why

has the designer plotted the quantities shown [4]

b) When should the minimum principal stress be considered [3]

c) If the yield stress of the material is 50kpsi explain if the component is fit for purpose [6].

d) How will the results differ if the mesh density is increased [3]

e) What other type of analysis would be useful to determine if the component is fit for purpose [3]

[25 marks]

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Past Examination Question

• The Figures (a) Displacement, (b) Von-mises stress and (c) the Failure Index, shows a FEA of a component conducted on ProMechanica. Explain, why the Von-mises stress is plotted? [2]– Is this component safe for use? [4]– How would a multi-pass solution affect

the displacement and stress values? [5]– How would changing the diameter of

the hole affect the stress values? [4]– Is the Von-mises stress the highest

stress value? [5]– How would the data presented here

change after a Fatigue analysis [5](25 marks)

[University of Bolton 2007, BEngII(CAA01)]

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Displacment

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Von-Mises Stress

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Failure Index

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Solution Tips• This was covered earlier in the lecture,

think about what are the easiest things to check for and how the materials fail.a) Think about the symbols in the general torsion

and bending formulae and the solution should present itself

b) Evaluate the standard determinate and rearrange accordingly

c) Each of the criteria have been used earlier in the lecture:i. Use the result from part (b) in order to evaluate

the difference in the principal stressii. Let the third principal stress equal zero in Huber-

Mises formula then use the result from c(i) and the first principal stress value from the derivation in (b) to obtain the required result