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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.

  • Scaling

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

  • Remeshing

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

  • Additional option

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

  • Slideline specification

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

  • Contact cushioning

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.

  • Miscellaneous
  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. 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....).
  7. 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.
  8. Ensure that line searches have not been turned off. These typically have a benefical effect on the rate of convergence
  9. 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
  10. 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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