User Area
> Advice
How to Obtain Slideline Convergence
- Use separate iterative loops
If contact and naterial
nonlinearity (MNL) is present then the use of separate
iterative loops is recommended. This will sort out the contact
first before then analysing the MNL. This facility can
be found in the load case properties -"solution
parameters" section and in "advanced" button.
It may be necessary to scale the dimension
of the model (and all associated quantities, such as Young's
modulus etc.) from metres to millmetres (say) to avoid
round-off errors causing problems since some penetrations
may be numerically very small in magnitude when metres
are being used
The mesh surrounding the contact area
should be modified to minimise potential node-on-node
situations. With normal penetration a formulation has
been implemented to deal with such situations, but with
friction the effects of the problem are rather more difficult
to deal with in a consistent formulation, although they
have been minimised. Make sure, therefore, that the meshes
are not the same on the slave and master surfaces - as
mismatched as possible is the aim
Option 353 does help convergence problems
with frictional slidelines - it invokes a fixed penalty
parameter definition to avoid potential problems with
the variable penalty parameter definition at node-on-node
situations. This is not currently available from the MODELLER
menu system - it can be invoked from the command bar using
the command SET OPTION 353
The slideline definition should be extended
along the boundaries of each body sufficiently far to
ensure that contact is picked up during the iterative
process when the contact status can be significantly different
to the status at the final point of equilibrium
One of the reasons for poor convergence
can be the chatter of contact nodes in and out of contact
in this job. Contact cushioning can be invoked from the
slideline definition form to stabilise the contact conditions
and give an improved convergence performance.
- The overall stiffness experienced by
the load path in a slideline analysis can change significantly
due primarily to the number of contact nodes active
in any iteration. The nonlinear logfile parameter CSTIF
(current stiffness parameter) is a measure of the overall
stiffness of a structure and is monitored to determine
the automatic invocation of the arc-length procedure.
In a geometrically nonlinear analysis in which snap-through,
snap-back, buckling and the like are experienced, this
is appropriate. However, slideline analyses rarely invoke
such behaviour and do not need this facility. To suppress
the arc-length facility for a slideline analysis, select
the appropriate load case properties and go to the advanced
form in the incrementation section set the “stiffness
ratio to switch to arc-length” to zero. Search
for “Nonlinear Solution Procedures” in the
online help for more information.
- Slideline analyses permit the arbitrarily
large relative deformations to occur between two or
more bodies. Inevitably some analyses will result in
deformations that exceed the small deformation assumption
of a geometrically linear analysis (the tangent of the
maximum rotation experienced should be approximately
equal to the angle itself measured in radians). If this
is the case, invoke an appropriate geometrically nonlinear
strain measure (select “options” in the nonlinear
section of the load case properties form). The measures
supported by each element type can be seen in the Element
Reference Library. In general, Total Lagrangian is the
most robust method if available.
- The default slideline stiffness factors
are recommended for general use. These factors control
the amount of interpenetration at the slideline interface
increasing their magnitude will reduce the interpenetration
experienced, although too large a value will cause “chatter”
at the interface and corresponding convergence problems.
There are circumstances, however, in which these may
need to be reduced, such as very few contact nodes active
or significantly different material properties between
the master and slave surfaces. The use of frictional
slidelines may also require this reduction. Typically
the reduction would be an order of magnitude at a time,
e.g. 0.1, 0.01 etc.
- If only a few contact nodes are active
during the analysis it is possible for the contact to
be lost during the iterative process if a large load
increment is being applied (i.e. the bodies pass completely
through each other). A fine mesh is recommended on both
sides of the slidelines to eliminate this problem.
- It is possible to use higher order
elements (i.e. midside nodes present) in conjunction
with slidelines but we normally suggest working with
low order elements (only corner nodes present) since
higher order elements are susceptible to element mechanisms.
For more information see Using
Higher Order Elements with Slidelines.
- For curved geometries, it can be helpful
to suppress the initial penetration check to permit
initial forces to be created at the slidelines interface
in places where there is initial penetration as a result
of using straight-sided elements (File>Model Properties>Solution>Nonlinear
Options....).
- Make sure that the pre-contact detection
switch is not invoked. This is not recommended in general
and can cause considerable additional forces to be developed
in the initial increment that can cause numerical problems.
- Ensure that line searches
have not been turned off. These typically have a benefical
effect on the rate of convergence
- By default, slideline nodes that are
in contact initially will have their coordinates modified
in order to relocate them normally onto their opposing
surface. However, if the option to "suppress initial
slideline penetration check" have been invoked
(File> Model properties> Solution> Nonlinear
options…), the initial penetration can cause significant
initial straining of the model and result in convergence
problems
- The close contact detection distance
specified in the slideline properties attribute may
not be sufficient for the gap that is physically between
one or more slidelines. Search for "Close Contact"
in the online help for more information
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