Transcript you - STEM CPD Module

Computational Science CPD
in the School of Computing, Engineering and Physical Sciences
ANSYS Fatigue Analysis
by Dr J. Whitty
1
Lessons structure
• The lessons will in general be subdivided
in to eight number of parts, viz.:
1)
2)
3)
4)
5)
6)
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
7) Summary and feedback
8) Formative assessment, via homework task
2
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.
3
ANSYS Workbench
•
•
Native CAD import and interface
All simulation work completed in
one environment inc.
1.
2.
3.
4.
5.
6.
7.
8.
9.
4
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.
3
3
WL
4
WL
• Re call from last

 
session the displacement
3EI
Ebd3
and stress are
M
6WL
calculated.
 
 
Z
bd 2
Take 5mins to evaluate these as we will be
5needing 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!
6
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
7
Mesh creation
• Right mouse click on mesh and issue a generate
command to produce the mesh as shown.
8
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
9
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
4  500 5003

200,000 40 203
Check stress
10
= 3.91mm
M
6WL

 
Z
bd 2
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

M
6WL
 
Z
bd 2
So in this case:  max 
6  500  500
 93.75 N/mm 2
2
40  20
or: 93.75MPa
11
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
12
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.
13
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!
14
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
15
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
16
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
17
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
18
Summary
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!
19
Class exercise
• The component shown is made
from 5mm thick sheet steel
a) 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.
20
P
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]
21
Past Examination
Question
•
The Figures (a) Displacement, (b) Vonmises stress and (c) the Failure Index,
shows a FEA of a component conducted
on ProMechanica. Explain, why the Vonmises 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)]
22
Displacment
23
Von-Mises Stress
24
Failure Index
25
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 stress
ii. 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
26