in finance analysis accurately predicting stresses on
surfaces is crucial in many applications for example in fatigue life assessment
we use the stress life curve to predict an object's life
and in certain regions of the stress strain curve
the cyclic loading life is very sensitive to stress
a slight change of stress can result in hundreds of
thousands of cyclic loading differences in life prediction
although the finite element method is generally quite accurate in predicting
the body stress when the model is well defined
stress on the surface may need special treatment
that is due to the extrapolation that is used to obtain stress
on the nodes in the final method stresses are first obtained on gauss
integration points which are located inside the element and
then extrapolated to the surface for a coarse mesh with large stress
gradients extrapolation may not be sufficient to provide
accurate stresses on the surface in this video we will use an example to
demonstrate the surface stress and accuracy
caused by coarse meshes with large stress gradients
and show you how to overcome it by using the surface coating technique
this technique can be used not just for fatigue analysis
but for all analyses where accurate surface stresses are desired
ready let's go
the surface coating technology is an efficient way to obtain
accurate surface stresses it places shell elements of a specified material
on the selective face or faces of the model based on how these elements are
defined they may be used either for post-processing purposes only
or for modeling real coatings in the first case these surface coating
elements do not change the stiffness of the model and share the
same deformation as the underlying solids based on which
the stress values are calculated since no outer plane extrapolation is
needed the stress is obtained by surface coating
are quite accurate as mentioned before by
choosing the appropriate options the surface coating elements
can also be used to model real coatings and thickness is needed in that case
however we'll only focus on the post-processing function
of the surface coating in this video now let's use a simulation example to
illustrate possible inaccuracy caused by
extrapolation and how surface coatings can help in this case
we have a shaft model set up the shaft is fixed on one end and it has
a vertical loading on the other end this type of shaft with a change in
diameter is very common in engineered structures and is
typically seen in automotive components such as in the drivetrain or suspension
it experiences high cyclic loading and service
thus we must predict surface stress accurately
to help the assessment of the fatigue life in the first case
the global mesh size seems okay and if we zoom into the fillet region
we can see there are already five elements trying to capture the stress
concentration of the fillet however if we do a section cut view and
check the mesh we will see that the element size normal
to the surface direction is very large as a result the error
associated with attempting to extrapolate
large differences in stresses in the course element
can be significant let's run the simulation
and plot the equivalent stress on the fillet we see
a maximum stress of about 178 megapascals
now let's add a surface coating to the fillet and re-run the simulation
to add the surface coating right click on geometry then
insert surface coating select a region where we want to obtain
more accurate surface stress in the definition
we define the stiffness behavior to be stress evaluation only
since we want to use these surface elements only
to evaluate surface stresses and not to model a physical surface coating
next let's assign a material to the surface coating material is needed here
because it will be used to calculate the surface stress
in this case it should be the same as the base material
now we have successfully applied the surface coating let's rerun the
simulation
to obtain stress on surface coatings we need to change the contour plot scoping
method to surface coating and select the one we
just defined
in this case we see a high stress value of about
220 megapascals which is quite different from the
extrapolation value in the previous plot to verify the result of the surface
coating let's run another simulation in this case we have a finer mesh on the
fillet surface and a much smaller mesh size in the
normal to surface direction to reduce the inaccuracies of
extrapolating large stress gradients we saw
with the coarse mesh model let's first run the simulation without surface
coating
next run a simulation with surface coding
and plot the equivalent stress
we can see that the results are almost the same which seems to indicate
that this value of peak stress is accurate
now let's compare the accurate value of peak stress with the previous case
where we used a coarse mesh we see that when the mesh is coarse
and surface coating elements are used the peak stress value in the fillet
region is quite close to the one we obtained by
refining the mesh so we're able to obtain the similar
level of accuracy in surface stress even when the mesh is
coarse in the normal to surface direction
simply by using the surface coating elements
while the surface coating technique can increase the accuracy of surface
stresses keep in mind that mesh refinement
studies are also important to assure we have sufficient elements
and their associated integration points to capture the stress gradients so let's
summarize the important learnings from this video
using a coarse mesh in the normal to surface direction
can result in inaccurate surface stresses due to extrapolation
if there are large stress gradients and reducing the mesh size in that direction
can improve accuracy however a more efficient method is using
surface coating to achieve the same accurate results
at a low computational cost if you find this video helpful
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