Why Should You Simulate Fatigue?

Phil Kinnane October 8, 2012
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The release of COMSOL Multiphysics version 4.3a introduces the Fatigue Module to the world of multiphysics modeling. As the diagram below shows, the Fatigue Module is used to perform structural fatigue life computations for both strain-based and stressed-based fatigue. Since the release, I’ve come to realize that this has been a sought-after product for COMSOL users. But why should you simulate fatigue?

COMSOL Multiphysics Fatigue Module specifications
The Fatigue Module provides Findley, Matake, and Normal stress methods for high-cycle, or stress-based, fatigue and the Smith-Watson-Topper, Wang-Brown, and Fatemi-Socie methods for low-cycle, or strain-based, fatigue. In strain-based fatigue, Neuber’s rule and the Hoffmann Seeger method are available for approximate solutions of the fatigue of elastoplastic materials. In combination with the Fatigue Module with the Nonlinear Structural Materials Module, it is also possible to consider complete elastoplastic fatigue computations. Enlarge image.

No Time to Wait for Fatigue Testing

Just before the release, I was on the premises of one of COMSOL’s Certified Consultants, Veryst Engineering. There I had a great conversation with Stuart Brown who has a lot of experience with structural mechanics applications and their modeling. I asked the question “why should you simulate fatigue?”, and his simple answer was that your average doctoral student does not have the time to wait for fatigue testing. Particularly for the millions of cycles that need to be run in stressed-based fatigue.

No Time for Extensive Experimental Testing

Companies, of course, still do run these test, specifically to verify their simulations (have you seen the machines that continually open and shut drawers and doors in your local IKEA store?). Yet another reason mentioned by Stuart is that often manufacturers don’t have the time to wait for a full-scale experimental testing of fatigue to be performed. The product needs to go into service as soon as possible for a variety of reasons, such as the fact that the usefulness of the product means it should be deployed immediately.

Why you should Simulate Fatigue

Although regulatory constraints still require testing, the design cycle does not have the luxury of waiting around for ten years of testing to see when these devices should be replaced before failure. Design decisions have to be made with the extensive assistance of simulations. A good example of fatigue analysis is this model of a cylinder with a hole. Because the stress-levels do not tell the entire story in this scenario, the structural damage due to repeated loading and unloading must be analyzed.

Simulate Fatigue using COMSOL; Fatigue analysis of cylinder with a hole

Further Reading

We have a blog entry from our very first guest author, Kyle Koppenhoefer of AltaSim Technologies, with a great description of what fatigue actually is. Something I noticed when reading his entry was that he used an application from biomedical engineering to illustrate the concept; it’s not only large structures and furniture that are affected by this phenomenon, but biomedical devices too.

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