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Stress-strain curve requirements, outlined

Question asked by Justin Strempke on Feb 26, 2014
Latest reply on Feb 27, 2014 by Jared Conway

I would like some help straightening out what exactly SW requires for material information in order to do NL analysis.


Based on the Help, the majority of the stress-strain curves by analysis need to be input in the fashion of True Stress, Engineering Strain (see below):


SW Sim S-S curve input info.JPG


That's not a huge issue, as finding Engineering stress-strain curves and transforming the stress portion is just an equation.  My issue is how to actually enter them into SW.  It seems NL elastic pulls the yield, Young's mod, etc from the S-S curve, which implies it needs the full range of stress and strain, from zero.  Plasticity models require an elastic modulus and yield strength (though I'm not certain if the yield is used in calcs or just for FOS plots), and literature/pop-up errors state the FIRST stress point should be the yield stress (very bottom bullet in the above picture).  First question:


1) What should the correlating strain be?  Does it start at zero, essentially requiring an offset from a derived chart for all points, or should the first strain point be a calculated value of Sy/E?  This would be an idealized model, but may not fit most published data for materials that use a 0.2% offset to determine yield.  Is this just a discrepency that must be accepted?


2) Next for some models Log-strain is required to be entered vs. True stress, this may be a stupid question but Log-strain and True-strain are the same, correct?  I'm not familiar with the Log-strain term, and the transforms I've seen are the same as for Engr strain to True strain...


3) Also is there a heirarchy in SW's use of available material data?  Say a stress-strain curve is input, but (for plasticity models) a tangent modulus and hardening factor are also input.  SW takes the plot first, correct?  Reading this:


SW Sim S-S curve input info 2.JPG

does tell me the curve will be used first, based on the wording "Material properties like elastic modulus, Yield strength, etc will be taken from the stress-strain curve when it is available and not from the material properties table."  So I tend to think the first stress/strain point calculates the elastic modulus for the linear portion...  But if the material is plastic-like (literally, like PVC, etc), the low-strain portion can have a very humped look which would take multiple point to properly define.  Will SW still use the first point, effectively the next point from 0,0, as the yield?  Is it pretty much irrelevant again beside FOS?


4) Lastly, it is possible to use various material models in a single simulation?  Say you have a brittle glass which can run linear elastic, and metal which uses plasticity von-mises.  Can they be combined in this fashion, or do the glass need to be at least NL elastic, and can all NL model types be mixed?


Sorry for the run-around questioning, I just would like this straight in my head once-and-for-all!  I'm pretty sure I have it lined out but then I'll read something that throws me off...  Thanks!