- [Instructor] We have prepared two studies
ready for analysis.
We'll now run these and compare the results.
So looking at the first study, with fillets.
We check the mesh size.
We create the mesh.
We want a mesh size of .1 of an inch
which is what we have there.
So let's run the analysis
and it's carrying out the meshing at the moment.
Now it's launching the analysis.
Now it's complete and we have the results.
We're carrying down on the results
part of the menu here.
Let's right mouse click on results,
go to Chart Options,
and we want to show the maximum annotation
to see where that peak Von Mises stress is.
So I want to go to Definition.
I want to switch off the Deformed shape.
Under Settings
I want to make the Fringe Options to be Discrete
and the Boundary Options to be
set to Mesh.
If we come back to Definitions
I just want to go to Advanced Options.
I want to look at Element Values.
So with all that set let's check again.
We get a plot of un-deformed stress contours
and we can see that the maximum label
is down here where we'd expect to see the peak stresses
round about 18,000.
So let's zoom in, see that region.
It's a very familiar region that we've seen before.
Well let's also look at the quality of the mesh.
So let's go to Mesh Quality Plot,
right mouse click on here and show that.
Again we can see a maximum aspect ratio round about 19
and if we just rotate around
we'll see where that is.
And if you remember from the basic courses very familiar.
This is a problem area.
It's an element right in here
that has an aspect ratio of about 19 to one
and it could potentially be a problem for us.
Getting much bigger than that
if it heads more towards say 20, 25, 30 to one
then we could certainly start to get
numerical errors creeping into the model.
Let's switch off the Mesh Quality Plot
and actually look at the element.
And there you can see the nature
of the element we're dealing with.
It's this one here.
It's really flattened out as it's
tried to capture that run-out of the fillet.
Come back to our full view.
So a good quality mesh
but we're running into trouble at that fillet run-out.
However the stresses seem to be
occurring in the correct region
about the right levels which is just here
on the split line of the little end.
As a contrast let's go to the study
which was the fillets removed.
Now here we can see we've got still
the fillet geometry included
so it's the wrong configuration for the study.
If we right mouse click in here
there's a neat shortcut.
We could just say Activate.
What it means is the appropriate Solidworks configuration.
So I click on that and now you see the fillets have gone.
It's picked up the correct one of
the two configurations that we've got.
So again we can check that mesh
and we're looking for
a mesh size
the same as .1.
So let's run that one.
Let's run the mesh.
Meshing is complete so now let's run the analysis.
So that's the results complete translated back.
Let's go and operate on this stress result now.
Go to Chart Options.
Set up the same settings as we had before.
We want to see the maximum annotation.
We want to go to the Definition,
switch off the Deformed shape,
and in Settings I want to make sure
that this is set to Discrete
and that we are looking at
the Mesh as the boundary.
We'll also go back to Definition,
set up the Advanced Option
which was include the Elemental Values.
Check the box and that should re now set our plot.
So there's the undeformed stress contour plots.
We can see the maximum stress has moved
and it's actually being picked up by that transition
between
the conrod body
and the little end.
So there we've got a situation where
it's a good looking element shape
but it's just the abrupt transition we've got
from the conrod body into the little end.
It's producing an unrealistic change in stiffness
and hence we're producing high stress.
Let's go and look at the quality of the mesh.
You can see the quality of the mesh
right at that point is very good.
We're very close to almost perfect aspect ratio.
Close to one just at that point.
So there's nothing wrong with the element.
It's the abrupt transition from one region
to another which, again, is not realistic.
Let's go and look to see
where the worst case aspect ratio is.
It's somewhere in the body of the region here.
There's some kind of tet element
which has a transition which is creating
an aspect ratio of about five which is quite reasonable.
So let's compare all of those sets of results together.
Let's go to Compare Results
and we're going to compare all the studies
in our configuration which is just two.
We want to look at the Von Mises stress
and we also want to compare the mesh quality.
So those are the two quantities.
So the with fillet study, pick off that stress,
that mesh quality, the no fillet study,
pick off that stress, and that quality.
So check OK.
So now we can compare the different results.
I'm going to be looking at the Von Mises stresses
and looking at the aspect ratios.
So if I
click on this box here
I can de-synchronize the views
which means that I can look at the views independently.
So let's zoom in
to the little end region
and see where
the peak of the Mises stress is.
And, as we would expect,
it's in the body of the little end.
Coming to the model now with the fillet radius taken out
let's again zoom into that.
We can see it's picked up
that element in the corner.
Just zoom into that.
So there's the one rogue element, if you like,
sitting there.
So this is the one that has picked up the
abrupt transition
from stresses in the
conrod body
to stresses in the little end there.
Again nothing wrong with the element.
It's just simply that it's a
very rapid transition of stiffness.
It's on its way to a stress singularity.
Now if we look at the corresponding
aspect ratios.
We come to this model in here
we can see that this is the
rogue aspect ratio for the model with the
fillets.
And if we come around in here
we get a nice view of what that's actually looking like.
You can see what an extraordinary looking element it is
as we have that very sharp aspect ratio in there.
Not a very good quality element at all.
Come to our final plot over here.
It's just a general region.
We don't need to find the specific element.
We know it's just buried in the mass of elements over here.
So kind of summarizing the story here
we've got the model here with the fillet included.
With the fillet included there is a
slight stress region building up here
but it's not picked up at the moment.
It might do if that aspect ratio gets even worse.
At the moment the peak stress is being shown in here.
At the same time this is the same model here.
This is a view in of that element hidden just behind there.
This aspect ratio is 19 to one.
That really is beyond a limit that
we would expect, we would like.
It's usually about 15 to one
and you can imagine if we push that element much further
then we could be in numerical trouble.
Now this is the other option we chose
which was to de-feature the fillet radius
so it just simply isn't there.
Again it gets rid of that bad shaped element.
Look at our aspect ratio plot.
It's quite happy around that region.
Let's go to that region
and actually find the corresponding element.
We can zoom in here.
We can find it's a very healthy, as we mentioned,
for about one to one.
So right in there there's a good quality element.
So we kind of have to take our choice.
Good quality element high stress region
because of the abrupt transition.
Generally good quality elements in the stress area we want
but hiding behind here is lurking a problem
in that fillet radius transition.
So that's basically the story that we're dealing with here.
So let's just exit that comparison.
Just take a general view
and we can switch off the Mesh Quality Plots.
So, in this video, we've seen the effect
of de-featuring the model by excluding the fillets.
De-featuring certainly improves the mesh quality
so the aspect ratio of 19 to one
dropped to around five to one.
Aspect ratios of 19 to one and bigger
can cause numerical errors
and they might well exceed the limit
that a client has set for you.
That's often something like 10 to one or 15 to one.
So, in this case, the aspect ratios at the big end,
which is away from the area of high stress concentration,
in general we want to try
and avoid aspect ratios of more than about 10 to one.
De-featuring this model has had an adverse effect
on the local stresses at the junction
of the conrod body and the little end.
It introduces a very abrupt transition
with the re-entrant corner.
This also causes a stress singularity.
We can see that the local stress here
has reached 31,000 which is way in excess
of the peak stress at the little end region of 18,000.
This is a very localized stress.
We call it our stress singularity.
It's often discounted as an
FEA artifact in the stress report.
The two examples with and without the fillets
really illustrate the classic FEA dilemma.
Do we try and mesh difficult geometry to very fine limits
and produce very poorly shaped elements?
Or do we de-feature the model
and accept that the stresses are unrealistic?
We could be heading towards singular values
as we have these abrupt changes in geometry.
Well the answer is always to use engineering judgment
and assess how the local feature would be manufactured
and whether these high local stresses could really exist.
So, at this point, we're going to close out the model
and we're going to save it as
advanced model number eight.