Tips for Contact Analysis

	Bob Williams Product Manager ALGOR, Inc. Pittsburgh, PA

Proper planning is vital for successful simulation of multiple-body contact and interaction.

This article was published in Machine Design, "Guidelines for Handling Contact in Simulations", January 11, 2007.

Users of computer-aided engineering (CAE) software often need to analyze design scenarios where parts are initially touching or come into contact later such as shrink fits, material-forming processes and impact. However, simulating contact is not always simple. Behind the scenes, there are a lot of software parameters that must be carefully specified in order to properly set up and control a contact analysis. While modern CAE software packages automate most contact parameters, some models will benefit from adjustments by the user.

The way you set up for a contact analysis can help you to get a converged, accurate solution relatively fast and easy. But, beware, less-than-optimal setup for contact analysis can cause the solution to run longer than necessary or compromise the results – or even fail.

Three models demonstrate typical applications for contact analysis: left) for a shrink fit problem, a fitted shaft expanded under a temperature load and the stresses were calculated in the surrounding material (contact between the parts was accurately simulated even though the nodes were not matched); center) the impact of a steel pinball striking a drop target was studied to determine the target's stresses and distortions (model courtesy of Stern Pinball, Inc.); right) in a planetary gear system, surface-to-surface contact was used to drive the motion of the four planet gears as the sun gear in the center is rotated.

Common Difficulties in Contact Analysis

Common difficulties encountered in contact analysis include:

• Non-smooth surfaces – When surfaces in contact have irregular edges, the non-smoothness can cause convergence problems (where the solver repeatedly tries to solve the contact problem by varying parameter values until satisfactory values are found, thus lengthening the analysis run-time). Additionally, non-smooth surfaces can lead to inaccurate results (for example, a circular ring fitted inside of a larger circular ring should produce a uniform stress contour along the contact surface; however, if the surfaces are not sufficiently smooth, stress variations can result).
   
• Soft and hard materials – When a very soft material is in contact with a hard material (such as foam and metal), the default contact stiffness calculation might lead to poor convergence (called "chattering") or unwanted penetration (where a surface incorrectly passes through another surface).
   
• Fast-moving impact – When a fast-moving part interacts with other parts, high-frequency loads are introduced (requiring the solver to take smaller time steps); high energy loss can occur (leading to over-damped behavior); and parts can penetrate (because the part moved too fast for the solver to "see" contact).
   
• Large deformation – When surfaces in contact undergo large displacement or large deformation (more than 50% strain), basic assumptions used in calculating contact parameters may no longer be valid.

Basic Steps of Contact Analysis

In order to avoid or remove these difficulties, you need to understand the basic steps for performing contact analysis:

1. Create the geometry and mesh – Reasonably smooth surfaces with a uniform mesh work best for contact analysis. Thus, it's important to know how to use the software package's tools for mesh generation and refinement.
    
2. Identify and define possible contact areas – Before starting a contact analysis, you often need to specify where contact might occur. For complex engineering systems, multiple surfaces could interact or self contact might be possible in a large deformation problem such as a rubber elasticity analysis. In such cases, you need to make certain that multiple contact areas have been defined (either automatically or by you) to cover all potential contact situations.
    
3. Determine the contact parameters – The software can often calculate contact parameter values based on the geometry, mesh and material constants. You may need to adjust values for anticipated contact problems, such as large deformation.
    
4. Determine the solution method – CAE software packages typically provide several options for the solution method for both linear and nonlinear contact scenarios. (Linear structural analysis can simulate deformation within the elastic range below the material yield point; nonlinear structural analysis can simulate geometric and material nonlinearities such as large deformation beyond the material yield point.) For nonlinear scenarios, the capture rate must be specified to control how frequently the solver should update the contact solution.
    
5. Apply necessary loads and constraints – Although it may be realistic to "leave parts free to fly around" and come into contact, some contact analyses can be greatly simplified by partially restraining parts. For example, for a pin that passes through a yoke and clevis, applying constraints to prevent the pin from rotating can reduce contact chattering.
    
6. Perform the analysis – Most processors will output messages about the status of the contact analysis during the solution. Problems, such as slow convergence, can be seen, allowing the user to stop the analysis, adjust parameters and restart.
    
7. Review the results – When two smooth bodies are in contact, a relatively smooth stress contour should result. However, for non-smooth surfaces with a coarse mesh, stress contours may be less uniform. In such cases, the user must refine or modify the mesh and analyze the model again.

CAE software packages provide convenience and flexibility by largely automating the setup and control of contact analyses while still allowing users to customize default settings as needed.

Resolving Contact Problems

There is not a single set of contact parameter values that will provide an accurate solution for all models. Many models will solve without any problems by using the automatic and default settings while some models may benefit from changes to the controlling parameters. Hence, you must have a good understanding of the functionality of each parameter to determine a proper set of values for the given problem.

When you encounter convergence problems or poor results from a contact analysis, carefully check the geometry, mesh, material properties and all contact parameters. As with all CAE analyses, simplify where possible.

For more technical information about contact analysis, see Technical Tips for Surface-to-Surface Contact Analysis in MES.