Nastran Finite Element Analysis and Simulation Software
Surface Contact Simulation
A Higher Standard Made Easy with NEi Nastran
For many structural Finite Element Analysis (FEA) applications, there are always decisions made by the design or analytical engineer on just how to simplify a real structure into a simulation model. The boundaries and loading conditions of a real structure can be complex, as it is oftentimes part of an assembly of many interacting components.
Some typical simplifications for an FEA to be decided upon:
The last item in the list is the topic of this article. For FE models, market driven accuracy requirements are demanding more use of real contact simulation between components so as to attain better end results. The basic question to be answered by the FE engineer is: “How do I prescribe the most appropriate load distribution onto my part of interest into the FE simulation?”
NEi Nastran is a state of the art Finite Element package with an entire suite of engineering solutions along with some easy to use and powerful contact simulation capabilities.
The FEA Requirement – Accurate Load Distribution
In deciding how a load is to be described on a certain structural component, a typical choice formerly used often in older FE codes is to simplify the load distribution as a simple point load, line load or pressure load onto certain nodes or surfaces of the part in the analysis. For example, it has been popular to “simulate” the crankshaft bearing load into a connecting rod of an engine by using a simple linear arc of loads, say 60º around the reaction force vector. (Figure 1)
Figure 1. Applied FE load distribution in rod bore assumed constant across a 60º arc and constant through the “Z” thickness; only a rough estimate yet typical of “standard” FEA practice
The load distribution assumption made in the FE model as shown in Figure 1 will be over-simplified, and hence inaccurate for a number of reasons:
First, due to the nature of the assembly between the real rod, bearing and crank journal, there will often be much more force reacting in the center of the “arc” of load developed and less force reacting at the extremities.
Second, the angle of the real load distribution “arc” is directly related to the clearance and the stiffness between the mating components; more clearance results in a smaller arc with much greater local forces at the center of contact. These differences by themselves will likely result in a different stress in the radii of interest for the connecting rod when comparing the real expected response to the over-simplified case, since the radii of interest are quite near to the load zone.
Third, even when making a so-called “A to B” comparison between designs, the very change in design can affect the load distribution. This in turn will affect the “A to B” comparison itself, so it remains wise for the FE engineer to consider the real contact conditions much more closely than what can be done with “linear load” methods or codes.
The Contact Advantage – Accuracy
Accurate load distributions between two components are obtained in an FE model by directly simulating the actual contact between them using multiple deformable bodies. This is often called “surface” contact, because the user defines which surface on “body A” is expected to come into contact with another surface on “body B”. In the connecting rod example, the user would setup surface contact between the crank journal and the rod bore.
Figure 2. With real multi-body contact conditions embedded in the FEA via contact surfaces, the skewed load distribution is accurately simulated. Color contour as shown represents contact forces with realistic load distribution. Connecting rod application is in tension combined with some crankshaft twisting.
Figure 2 shows an example of output obtained when running the connecting rod FEA using surface contact to attain an accurate load distribution in the rod bore. For the application loading as simulated, the load reaction in the rod bore is not evenly distributed at all as shown by the contour map representing contact forces. The resulting stress in the body of the rod near this area is much more accurate compared to using simple linear load assumptions.
It is clear that using contact surfaces to simulate load distribution provides a more accurate mechanical response in an analysis. However, the FE engineer still must balance the economics of spending the extra time setting up models using this approach, with the evident technical advantages. NEi Nastran offers a “best of both worlds” solution to this apparent dilemma.
The NEi Nastran Contact Advantage – Accuracy and Ease
Starting with NEi Nastran 9.0, and now with expanded capability in version 9.1, setting up contact between two components has never been easier for the user. This capability is called Automated Surface Contact Generation, or ASCG. With this feature the user now has no need to select individual nodes, elements or surfaces to come into a contact; it is done automatically.
Figure 3. Fixture on linear slides loaded with acceleration simulated with FEA. Contact conditions with NEi Nastran automatically generated.
NEi Nastran’s capability for contact modeling includes the command CONTACTGENERATE. The user can easily add this single line command directly into the NEi Nastran input data file to instruct the program to automatically find, create and activate the appropriate surface contacts.
As an example of using ASCG, a FEA result from a manufacturing application of a lens mount fixture that accelerates on a set of linear slides is shown in Figure 3 with acceleration applied. Even with dissimilar meshes set up for each component of the assembly, the ASCG easily manages each surface contact pair automatically for the solution.
With NEi Nastran’s ASCG feature, the FE engineer now has the power of an accurate and economic (i.e. “easy”) solution for multiple component models that require integrated contact simulation.
Figure 4. Linkage assembly FEA made with surface contact definitions and run with linear solution sequence, SOL 101. As of NEi Nastran 9.1, surface contact works with the linear solution sequence and runs much faster (8x in this example) than with a non-linear solution sequence while attaining similar stress results in the areas of interest.
NEi Nastran version 9.1 introduced the capability to utilize surface contact while running with the efficiency of the linear solution, SOL 101. When the FE model does not contain another nonlinearity, such as material plasticity or excess deformation, the FE engineer now has the option to run the FE model with surface contact and with the linear solver. This approach is designed to realize faster solution times while benefiting from surface contact accuracy as discussed above.
Figure 4 shows a test FEM with contact surfaces. In this test, the FEM ran over 8 times faster by utilizing the linear solution with this new capability versus running the same model with the non-linear (SOL 106) solution. Both runs were made using the same hardware: a standard Windows XP PC. Upon inspection of the output, the linear solution provided stress results similar to the non-linear solution to within 5% in the application’s area of interest.
Surface Contact Stabilization
Automated Weld Contact with Shells
Since NEi Nastran version 9.0, connecting two surfaces with dissimilar meshes has also been made easy, such as “welding” the surface of a tetrahedral mesh to the surface of a hex mesh. With NEi Nastran’s expanded suite of contact capabilities added in version 9.1, the FE engineer can now apply this powerful technique to weld shell elements together as well.
The capability to weld two dissimilar shell meshes together will save engineers many hours when working with extracted mid-plane models where it is common that meshes of the multiple surfaces do not line up by default. This feature will also prove useful in cases where CAD geometries comprising of multiple surfaces to be joined are not perfect.
Finite Element Analysis that utilizes contact simulation within a given FE model will tend to be more accurate than a simplified FE model that uses “assumed” load vectors and distributions. NEi Nastran version 10.0 expands upon existing contact simulation tools to add new features that not only provide a higher standard of accuracy, but also provide techniques that improve ease of use.
The following white paper is available in our Knowledge Base Center with sign up information.
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