Safety factor calculation in Fatigue Wizard

[bjorn_fallqvist]

It seems to me that the calculation of a fatigue safety factor in the strain (and -stress) based fatigue calculation behaves strangely. According to the official site, the safety factor should simply the factor of the desired cycles against the fatigue life, e.g. a safety factor of 1 if I want 500 000 cycles, and the calculated fatigue life is 500 000 cycles. However, this is not what happens.

For example, I have run an analysis that calculates the fatigue life to 9.75E+7 cycles. If I then set a desired fatigue life of 1E+6, I would think I get a safety factor of about 97.5, right? Instead, the safety factor is calculated to 1.69.

Is the information on fatiguewizard.com outdated? What exactly does the safety factor calculate?

[ilyasramdzan]

Hi,

I remember reading somewhere that the factor of safety calculation in Fatigue Wizard calculates the ratio of the stress value at your desired life cycle to the stress value currently acting on your model (Instead of model life cycle/desired life cycle). But I'm not sure where I have read this. Still can't find it. I'll let you know when I find it.

[John_Holtz]

Hi Bjorn,

If you are using the Fatigue Wizard in one of the recent Simulations Mechanical or Multiphysics (2012 or 2013), then you should not rely on the documentation at fatiguewizard.com. The current documentation is in the Mechanical documentation.

If you are using older software, then that website would be applicable. But note this introduction on the website:

FatigueWizard Support website for existing customers
As of August 2009 FatigueWizard is no longer owned and operated by FatigueWizard Limited.

This website now serves as a support site for existing customers with active maintenance.

I happened to be doing some testing with the Fatigue Wizard yesterday in preparation for a future project, so perhaps this information will solve your question. I was testing the stress-based fatigue only, so the strain-based may be a little bit different. What I found out is:

1) When calculating the "Cycles to failure" (step 5 of the wizard), the Visualizer within the Fatigue Wizard will show the factor of safety (FOS) based on the number of cycles. In particular, FOS = log10(calculated life at node) / log10(desired life). I think that this gives a different result than (calculated life at node) / (desired life), but I did not sit down and prove it mathematically.

2) When calculating the "Cycles to failure", the Results environment within Mechanical does not show the FOS.

3) When calculating the "Safety Factor" (step 5 of the wizard), this result can be viewed from either the Visualizer or the Results environment. FOS = multiplier for all loads (step 4) to give a calculated life equal to the desired life.

P.S. I just noticed that Bjorn's original post was from 2010, which explains why I do not remember reading it!