Transcript No Slide Title

Bolted Joint Assembly
Advanced Solid Body Contact Options
Contact Stiffness
• Goal
– In this workshop, our goal is to study the effect that contact stiffness specification
has on convergence and result accuracy.
• Model Description
3D bolted assembly - 4 parts:
–
–
–
–
Bracket
Bushing
Nut
Bolt
Loads and Boundary Conditions:
– One fixed support
– 45,000 N Bolt preload
Advanced Solid Body Contact Options
Contact Stiffness
Steps to Follow:
•
Start an ANSYS Workbench session. Browse for and open
“Bolted_Joint_ws03A.wbdb” project file.
–
This project contains a Design Modeler (DM) geometry file
“Bolted_Joint_ws03A.agdb” and a Simulation (S) file
“Bolted_Joint_ws03A.dsdb”.
•
Highlight the “Bolted_Joint_ws03A” file and open a Simulation Session.
•
At this point , solve the analysis.
Advanced Solid Body Contact Options
Contact Stiffness
• Review the contents of the model
Highlight each item in the
“Geometry” and
“Contact” branches of the
Project tree to become
familiar with the model.
Also, review the
specifications in the
Details Window for each
highlighted item.
Advanced Solid Body Contact Options
Contact Stiffness
Change Pretension load to:
20,000
Advanced Solid Body Contact Options
Contact Stiffness
• Review the contents of the model (cont’d):
Note especially the
“frictionless – bushing to
bracket” contact region. It
will be used to evaluate the
pressure profile at the
bushing-bracket interface
after the bolt preload closes
this gap.
This region is initially set up
as an asymmetric frictionless
pair using the Pure Penalty
method. Recall that this
algorithm depends on a
contact stiffness and a very
small penetration to generate
forces at the interface to
prevent penetration once
contact is established.
Advanced Solid Body Contact Options
Contact Stiffness
• Under the model branch, highlight
“Frictionless – bushing To bracket”
• In the Details Window:
– Change the normal stiffness specification
from “Program Controlled” to “Manual”
– Change the normal stiffness factor from the
default (1) to 10.
• Solve the analysis again
Advanced Solid Body Contact Options
Contact Stiffness
• Review the Solution Information Branch
The red lighting bolts in the
Solution branch indicates an
incomplete Solution run.
Below the worksheet
window, the message
window contains a summary
of warnings and errors
suggesting a problem with
convergence.
By double clicking on each
message with LMB, you can
expand a message into a
separate window for easier
reading.
Advanced Solid Body Contact Options
Contact Stiffness
• Review the Solution Information Branch (cont’d)
In the Details of “Solution Information” Window, switch Solver Output to Force
Convergence. This displays the same convergence data in graphical form.
Note, the force convergence value oscillates up and down between iterations well above
the acceptable convergence criteria. After two automatic bisections, substep 1
converges. However, substep 2 ultimately fails to converge.
Advanced Solid Body Contact Options
Contact Stiffness
• Review the Solution Information Branch (cont’d)
Open the Solution Information Folder and highlight the NewtonRaphson Residual Force
Clearly the source of highest residual (force imbalance) is located
along the bushing-bracket contact region.
Advanced Solid Body Contact Options
Contact Stiffness
• Review the Solution Information Branch (cont’d):
Return to the Solver Output
and scroll to near the top of
the solution information
worksheet.
In a summary listing of all the
contact pairs, find
“Frictionless – Bushing to
Bracket”. We see that this
region is associated with Real
constant sets 15 &16.
Advanced Solid Body Contact Options
Contact Stiffness
• Review the Solution Information Branch (cont’d):
Again, scroll further down the
solution information worksheet
to find the contact specifications
and calculations for this set.
Note the large default contact
stiffness (0.923e6) being used.
Given the relatively low stiffness
of the bracket feature in this
model, it is possible that the
contact stiffness being used is
too high for this application.
Advanced Solid Body Contact Options
Contact Stiffness
• We will attempt to achieve a successful
convergence by adjusting the normal
stiffness of Contact Region 6 downward
based on the feedback reviewed in the
unconverged output.
• Without changing any specifications in the
current tree, duplicate the Model branch as
follows:
– In the existing Project tree, highlight
“Bolted_Joint_ws03A” model
– RMB – Duplicate
• Rename the new model branch to reflect
the change that will be made
– “Bolted_Joint_ws03A,Norm Stiff Factor = 1e-3”
• This will enable us to run a modified
analysis without losing the existing
information.
Advanced Solid Body Contact Options
Contact Stiffness
• Under the newly created model
branch, highlight “Frictionless –
bushing To bracket”
• In the Details Window:
– Change the normal stiffness specification
from “Program Controlled” to “Manual”
– Change the normal stiffness factor from 10
to “1e-3”.
• Solve the analysis again
Advanced Solid Body Contact Options
Contact Stiffness
• The solution now converges successfully in 11 iterations and no bisections. This is ideal.
Bisections are a helpful automatic adjustment to achieve a converged solution, but they are
not efficient as all the CPU time from the last successfully converged solution leading up to
the bisection is wasted.
Advanced Solid Body Contact Options
Contact Stiffness
• Review the Solution Information of the successful run.
– Verify in the Solver Output that the modified contact stiffness was used and that
solution converged successfully.
Note: In this analysis, the first
load step calculates the
necessary interference needed
to generate the prescribed
preload. The second load step
locks the bolt pretension
element into this calculated
adjustment to achieve the bolt
pretension load.
Advanced Solid Body Contact Options
Contact Stiffness
• Under Solution Branch, RMB>Insert >Probe>Bolt Pretension
• On Bolt Pretension > RMB>”Evaluate Results”
Advanced Solid Body Contact Options
Contact Stiffness
• Review Bolt Pretension Results:
Advanced Solid Body Contact Options
Contact Stiffness
• Review Contact Results at the bushing - bracket
interface (nut side):
–
–
Open the Contact Tool Folder and Select
“Frictionless - bushing To bracket”
Review Contact Pressure and Penetration results for
this pair
Is “1e-3” an acceptable normal stiffness factor for this model?
The best way to ensure an accurate result with a standard contact pair is to perform a
sensitivity study with different stiffness values, stiffness updating schemes and algorithms
until results converge to the same “correct” answer. Too high a stiffness can produce
divergence, too low a stiffness can produce convergence but possible over penetration, an
excessive bolt pretension adjustment and ultimately an inaccurate prediction of surface contact
pressure profile.
Advanced Solid Body Contact Options
Contact Stiffness
• Consider the following sensitivity study on the effects of changes to contact stiffness:
Algorithm
Penalty
Penalty
Penalty
Penalty
Penalty
Aug-Lagrange
Normal Stiffness Factor
0.001
0.010
0.100
1.000
1.000
1.000
Stiffness Updating Scheme
never
never
never
Large Deflection
Off
Off
Off
Off
ON
ON
Cum Iterations
13
15
16
16
16
16
CPU Time
254
299
324
317
377
369
Max Contact Pressure(MPa)
138.24
141.5
198
198
196
196
Contact Penetration(mm)
0.1497
0.0153
0.0021
0.0020
0.0021
0.0021
Bolt Pretension Adjust(mm)
1.7571
1.6643
1.6539
1.6539
1.6558
1.6539
Bolt Pretension Reaction(N)
45,100
44,873
44,890
44,899
45,000
45,000
each iteration each iteration each iteration
For this model, as stiffness increases, contact penetration and the required bolt pretension
adjustment decreases while maximum pressure increases. Notice also the general trend toward
more iterations and longer run times. It is also worth noting the benefit of using the automatic
stiffness updating tool between iterations and turning on Large Deflection to achieve
convergence at the default normal stiffness factor =1.0.
Specifying the right contact stiffness is highly problem dependent and is always a balance
between quality of results (accuracy) and cost (run time). Based on this study, a normal stiffness
factor of 0.10 would be satisfactory. The Augmented Lagrange contact algorithm has proven to
provide more robust contact solutions with many applications and is recommended for
frictionless or frictional contact.