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# MSC Nastran: Smoothing the Way with Analytical Contact

Monday June 24, 2024 at 8:00am

The process of meshing real world geometry at a scale that can be solved practically on reasonable hardware can lead to issues with large sliding contact problems.

As an example, consider this 2D model of a housing/shaft. There’s a nominal clearance between the two of 0.2 mm on a 10 mm radius.

We can mesh this quite reasonably, and then rotate the shaft within the housing.

We’d expect to see no stress or contact between the two parts, but we do in fact see it occur and plots of contact status and stress show this clearly.

This occurs because of the mesh refinement.

If we look at the situation without contact modelled, we can see that due to the chordal deviation of the element edge from the ‘true’ geometry we get artificial penetration of the housing by the shaft.

How can we avoid this? Well, the brute force method is to use more nodes.

By adding second order elements or reducing the mesh size so that the chordal deviation is less than the gap will avoid the situation, but at a cost in computation time.

That’s fine with this simple model – the run time might increase to several minutes from a few seconds.

But if this was a 3D situation on a large assembly with multiple occurrences of this configuration… then the run times could very easily become prohibitive with finer meshes.

#### AN ALTERNATIVE TO FINER MESHES

In MSC Nastran and MSC Marc we have the concept of Analytical Contact.

Instead of using the face of the element as the contacted entity, we can elect to let the solver fit a COONS surface through the nodes on the free faces (or a spline through the edge nodes in 2D solids) and use that as the contacted entity.

This would give issues for non-trivial parts, as the patch/spline tries to maintain continuous curvature around sharp edges.

We have the option of manually defining the hard edges, or we can use automatic detection with a check on the angle between adjacent face/edge normals.

Going back to our example, we can turn on analytical contact for the housing and re-run the study.

There is now no contact detected between the parts. Looking at stress in the movie below we can see that there is no stress as a result of contact because none is taking place.

#### WHAT ELSE DOES ANALYTICAL CONTACT HELP WITH?

The support for geometric entities within the solver as one side of a contact pair has been taken further.

When we have a condition where one side of a contact interface is significantly softer than the other such that we can assume that one is effectively rigid we can actually model it as rigid geometry rather than mesh eliminating the need for meshing the stiffer part and improving run times accordingly.

A simple example of this would be modelling the interface with the floor for a quasi-static simulation of a drop test using a simple rigid planar surface contacting the falling object.

With more complex rigid surfaces, the continuous curvature eliminates the abrupt change in contact normal from face to face making convergence easier for large sliding.

This capability has been combined to great effect with the re-meshing capability in Marc to model forming type applications.

#### EXAMPLES OF ANALYTICAL CONTACT

In the first example below we are modelling a superplastic forming operation.

The tool surfaces are modelled as geometry with the workpiece as a 2D deformable meshed body. We’re plotting the element thickness here, as the part is formed to the tool using local re-meshing to fit to the surfaces more accurately.

In this example, we are modelling an extrusion process where the extrusion die is rigid and our high temperature metal is forced through it, plotting the damage criteria related to chevron cracking internally.

Lastly, we are modelling the crushing of a packer seal within a pipeline where the pipe walls are rigid.

Both Marc and Nastran are available within the MSC.One token licensing scheme which provides affordable options for SME businesses to access these and other powerful CAE solutions.

So if you want to know more about Nastran or any of the simulation software solutions or services we offer, then get in touch with us today and we can discuss your specific requirements in more detail.

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