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Creating an SN Curve from a Test and Using it in Simulation on Similar Loading Setups

Question asked by John Parker on Jul 25, 2012
Latest reply on Jan 24, 2014 by Jared Conway

Hello everyone,


I am working on what seems like an easy concept but having trouble making it work.  This is most likely due to my lack of understanding of what the fatigue simulation is actually doing to get it's "Life" information.  Here's the gist:


We have a series of different sizes of a part that are basically just scaled versions of eachother.  We have had fatigue testing done on the larger of two sizes and have enough data to produce  a basic load vs. # of cycles curve.  I want to use this information to create an SN curve that is applicable for this particular loading configuration and then predict the life of the smaller of the the two sizes at varius loads using this curve.  First I want to create a curve that can predict the number of loads on the same size part that was tested as proof of concept.


Here's what I did:

  1. Took the model of the part that had had the real life fatigue testing done and set it up in a static study at each of the loads used to build the load vs. Cycles curve. 
  2. I used the stress data from this to create a S-N curve with stress vs. cycles.  It looks pretty much the same using either VM or P1 stress in this loading configuration, which is primarily bending.
  3. Saved this curve into solidoworks and ran fatigue on the same part that I got th loading data from as a check.  Both the curve and the "event" in simulation have an R value of .13


This is where it stops working.  I was expecting that if I define my SN curve like this, I should be able to run fatigue with the loads from the curve and see fatigue failure at a given stress/# of cycles as defined on the SN curve.  This is not at all what happened, it seems to be predicting about twice the life for the part as what was defined in the curve.  Any ideas why?



Thanks to anyone who can help!  I hope I'm doing something simple wrong.