"We've made 'contact'...

In preparing a text for NAFEMS (www.nafems.org) on analysis for design engineers, I made the decision to include a discussion of contact conditions (natural part-part interactions) in a chapter on boundary conditions.  Since contact requires a nonlinear solution, it is typically reserved for discussions on nonlinearities. Here are some excerpts to justify my decision…

“Historically, some of the most creative load and restraint schemes were developed to avoid having to utilize nonlinear contact conditions, or actual part-part interactions, in a finite element model.”  “The tools of today allow designers to simply indicate that that inter-part contact is to be included in the model and the software defines the appropriate parameters and mesh automatically.”

“With these advancements, choosing part-to-part contact has become a viable alternative to questionable applications of loads and restraints. In fact, for most users of FEA in product design, a case could be made to say that contact conditions should be the first thought for interactions near areas or bodies of interest and should be replaced with loads or restraints only when the substitution is obvious or unavoidable, as in the case of fluid pressure in a tank.”

Some of my earliest (& most painful) FEA memories were of manually toggling nodal restraints or springs-to-ground to get a more natural response in an area where part-part interactions invalidated a simple restraint definition since employing nodal gap element contact would have driven solution times beyond tolerable, even on my SGI workstation.  Today, in my work with design engineers, I see people choosing questionable restraints over contact regularly and can’t help but feel there is some tribal, sub-conscious suppressed memory that influences this decision… misty remembrances from engineers long ago… whispering “avoid contact…avoid contact…”  OK… design analysis has only really been around for about 20 years so the ancients probably aren’t influencing today’s users.  A more likely cause is that we’re teaching the technology using out-of-date preconceptions.  If we (& I don’t just mean SolidWorks… you know who you are!) lead off our education with “restrain this” and “load that” with a discussion of contact following much later, it is inevitable that our users avoid contact unless deemed absolutely necessary.

Let this be my Manifesto, nailed to the doors of design groups around the world…  more often than not, you don’t know how a face or body will react once the system reaches equilibrium.  However, the application of a restraint implies that you do.  Consider the more natural response at faces of interest that contact, either no penetration or bonded, provides as a primary modeling method and resort to restraints, as stated above, “…when the substitution is obvious or unavoidable.” Don’t be afraid to model a few more parts into the system and explore the results. If you are inclined to be critical and diligent in your evaluation of your results at your interactions, (which I hope you are!), I think you’ll find the added solution time is far less a hindrance than the time it takes to evaluate a restraint, try a few different alternatives and assess the best route, in most cases.  As computers and solvers get faster each year, this must become the norm, rather than the exception to stay competitive. You might as well realign your thinking today!

-- Vince