Keith Frankie

Stess / Strain curves; engineering vs true values

Discussion created by Keith Frankie on Jan 13, 2014
Latest reply on Jan 15, 2014 by Jared Conway

This study is interesting.  I've been working through some exercises to gain a better understanding of simulation and materials.  Please point out any mistakes I make!


Imagine you perform a standard uniaxial tension test on a sample of 'alloy steel' in a materials lab.  Your machine grabs both ends of the sample and pulls at a constant elongation rate.  Your material is magical though - it follows the SW linear material model and hence has a fixed modulus of elasticity and never breaks.  You would expect the stress/strain curve to be a straight line, right?


So lets do it in SW.  My bar is 1m square x 10m long.  I use a nonlinear static study.  I set the material as 'alloy steel' and mesh it as a beam.  To keep computation time down a mesh control gives us 5 elements.  One end is fixed. A prescribed displacement fixture pulls the other end 10m over the length of the study.  (Tip: create a second solid body so that you can add this fixture type to the beam.  Then exclude this second body from the analysis - you can't add this type of fixture if SW thinks you only have beams).


When your results come up add a time history plot showing the axial beam stress.  Here it is:


Stress Strain.png

That's not linear!  Note that by running the study from 0 to 100 seconds the x scale can be read as 'Engineering Strain [%]'


What's going on here?  Here's my explination: Young's Modulus (or, as it turns out bilinear or multi linear) material data is interpreted by SW as using true strain.


I saved this data ('file' -> 'save as' in the response graph) and converted the 'percentage engineering strain' to 'engineering strain' and then to true strain.  Re-graphing gives this:


True Conversion.png


Very linear!


The SW help file ( ) says that for most studies engineering strain is used.  It's only for large strain studies that true strain needs to be input.  It seems to me that this chart is incorrect - really SW just always uses true strain.  Because true and engineering strain are so close for small strain values it doesn't really matter.  I also originally interpreted this chart as indicating true strain was needed only when using the 'large strain option' in the study properties, but again I think that is wrong.  (The SW help file is very sparten about the large strain option, but further reading indicates that when the volume of the deformed body changes significantly, as with large strain, this option needs to be used for accurate results.  This is the next step beyond large deformations, where although the shape of the body changes significantly, the volume doesn't necessarily change much.)