1. ANSYS Fatigue Analysis
Computational Science CPD in the School of Computing, Engineering and Physical Sciences
ANSYS Fatigue Analysis
by Dr J. Whitty

2. Lessons structure
The lessons will in general be subdivided in to eight number of parts, viz.:
Statement of learning objectives
Points of orders
Introductory material (ANSYS-Workbench)
Concept introduction (Static analyse)
Development of related principles (Fatigue)
Concrete principle examples via – reinforcement examination type exercises
Summary and feedback
Formative assessment, via homework task

3. Learning objectives
After 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.

4. ANSYS Workbench Native CAD import and interface
All simulation work completed in one environment inc.
Solid Modelling Fundamentals 
Placed Features, Assembly  
Modelling Techniques 
Simulation, Wizards & Tools 
Heat Transfer & Thermal Stress  
Surface & Line Models  
Natural Frequencies
Buckling Loads
CFD

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

6. 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!

7. 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

8. Mesh creation
Right mouse click on mesh and issue a generate command to produce the mesh as shown.

9. 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

10. ANSYS Workbench Results
Once an analysis is complete is usual to observe the following field data, in this order:
Check displacement
= 3.91mm
Check stress

11. 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
So in this case:
or:
93.75MPa

12. 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

13. 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.

14. Note ANSYS has predicted the fatigue failure point very well!
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
The calculated life is
170,490 cycles of 500N the load
Note ANSYS has predicted the fatigue failure point very well!

15. The safety factor now reports failure of the component
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

16. The safety factor still reports failure of the component
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

17. 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

18. 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

19. Summary If so when and were within the workshop were these met!
Have 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!

20. Class exercise The component shown is made from 5mm thick sheet steel
Use ANSYS DesignModeller (or SolidWorks)
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
Increase the mesh density using the Relevance setting in the mesh object and comment on the density
Explain the observations made in part(c).
Evaluate the value P if an additional fatigue safety factor of 2 is required.
Is this the most efficient method of modelling this particular component. P

21. Examination Type Question
Explain two processes of descertization used in FEA [6]
For the component shown why has the designer plotted the quantities shown [4]
When should the minimum principal stress be considered [3]
If the yield stress of the material is 50kpsi explain if the component is fit for purpose [6].
How will the results differ if the mesh density is increased [3]
What other type of analysis would be useful to determine if the component is fit for purpose [3]
[25 marks]

22. 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)]

23. Displacment

24. Von-Mises Stress

25. Failure Index

26. Solution Tips
This was covered earlier in the lecture, think about what are the easiest things to check for and how the materials fail.
Think about the symbols in the general torsion and bending formulae and the solution should present itself
Evaluate the standard determinate and rearrange accordingly
Each of the criteria have been used earlier in the lecture:
Use the result from part (b) in order to evaluate the difference in the principal stress
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