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PERMAS-CA Contact Analysis

Coupling under trosional load, growing slipping regions (red) when load increases (by courtesy of Voith Turbo)
Coupling under trosional load, growing slipping regions (red)
with increasing load (by courtesy of Voith Turbo)

Static analyses with non-linear boundary conditions (contact problems) can be analyzed using the PERMAS-CA module.
Contact boundary conditions may be present between elastic bodies or between elastic bodies and a rigid counterpart. The bodies may behave also non-linearly.
There are several methods to describe contacts:

  • by specification of the contact nodes in pairs.
  • by specification of node sets for each contact zone (the node pairs are detected automatically).
  • assignment of nodes / node sets to surfaces (incompatible meshes).
  • general surface-to-surface contact (incompatible meshes).

The feature to define contact with incompatible meshes allows the independent meshing of the contacting bodies. This simplifies the modeling of complex contact surfaces (like tooth contact between gearwheels) essentially.
The direction of contact and the initial gap width may be specified explicitly or determined automatically from the geometry. Any press fit is easily modeled by the specification of a negative gap width.

Drive gear and crown gear.

The contact analysis can handle isotropic or anisotropic frictional contact with sticking or slipping according to Coulomb’s law. For large contact problems and for complex frictional tasks module PERMAS-CAX provides a suitable extension of contact analysis (see next section). Both modules together cover a wide application area for contact analysis. This combination is complemented by module PERMAS-CAU (see section after next section) which takes into account large relative displacements of contacting bodies.

  • The specification of a load history allows the correct simulation of assembly and working loads and any contact situation with slipping and sticking friction. This facilitates the convenient simulation of such situations in a quasi-static analysis. A postscript plot file of the load history can be exported to view its graphical representation (see Fig. 95).
  • The load history can be amended by pretensioning (e.g. of bolts), where the contact analysis is used to describe the pretension. In this way, the screw tightening torque is modeled by a known contact force in the barrel of the bolt.

A generalized concept for bolt pretension is provided. Beside the classical approach using a cutting plane with pretension in normal direction, a new approach using cylindrical thread coupling with pretension in axial direction is available. This highly innovative feature offers a convenient definition which can take into account the detailed effects of radial spreading and axial torque caused by the thread’s flank and pitch geometry without the need of modeling the flank shape or thread line explicitly (see Fig. 96).

Shaft-hub connection with friction, change of the contact status during rotating bending (read: sliding, blue: sticking)
Shaft-hub connection with friction, change of the
contact status (read: sliding, blue: sticking)

Comprehensive checks allow the verification of contact models like type of contact, its geometry (gapwidth and normal vector, see also Fig 85), and the contact coordinate system (for normal and frictional force directions). In addition, the contact status is available in all iteration steps for checking purposes.

For frictional contact the quality of surfaces is of utmost importance. Therefore, PERMAS can smooth contact surfaces in order to essentially improve frictional behavior.

The analysis procedure uses a reduced flexibility model which is derived from the set of contact degrees of freedom. This procedure has the following advantages:

  • The iteration is very efficient making it best suited for extremely large models with an arbitrary number of contact nodes.
  • The accuracy of the results is fully preserved, because no additional stiffnesses are introduced by the modeling of contacts.

The simultaneous analysis of an arbitrary number of loading cases is possible. The contact parameters, i.e. gap width and coefficients of friction, may be different for each loading case. The contact boundary conditions are taken into account automatically by the static analysis procedure. No additional user request is required for a contact analysis.

For efficient calculation of successive contact variants contact status files are available for easy job recovery and considerable run time reductions.

In addition to all results usually derived from a static analysis the contact analysis provides for the contact status, the contact forces, the contact pressure (see Fig. 97), the gap widths, and the relative gap displacements.

For subsequent analyses, contact states can be locked. This contact locking leads to linear constraints according to the current contact state. To achieve this, the active contacts are automatically transformed into kinematic constraints. With this new model various kinds of subsequent analyses are possible like eigenvalue analysis, heat transfer analysis (see Fig. 98), or submodeling).