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

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

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.

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

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

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!

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

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

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

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

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

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

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.

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!

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

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

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

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

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!

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

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]

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)]

Displacment

Von-Mises Stress

Failure Index

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