Non Linear Analysis

[stefanos9891]

Hello,

I am working on a non-linear contact problem in Ansys Apdl. In the material models i am trying to input strain and stress values for the multilinear plasticity. In the project description we were given several points ploted on a graph of "engineering stress and strain". The 0.2% yield stress has a value of 735 MPa and the young's modulus 200GPa. Now if I want to put the points of "stress Vs plastic strain" should i convert the "engineering stress and strain" points from the graph given to "true stress and strain"? If that's so then here are my calculations for one point.
The next "engineering stress" point provided after the yield stress is 738Mpa and the equivalent strain point is 0.848%
i know that eps(true)=ln(1+eps(eng))
and that Sigma(true)=Sigma(eng)*(1+eps(eng))

therefore eps(true)=ln(1+0.848)=0.614%
and Sigma(true)=738*(1+0.00848)=744.26MPa

The true eps(elastic)=Sigma(true)/E=744.25/200000=0.00372

The true eps(plastic)=eps(true)-eps(elastic)=0.00614-0.00372=0.00242 or 0.242%

Is my thinking correct?

Thank you in advance

 

[coolbreeze]

No conversion necessary, Plastic strain is given in the graph of points that are greater than 0.2% strain or yield point. So if you have 0.848% strain of a metallic material, it is by definition in plastic strain. 0.2% is the point where many metallic materials are no longer elastic, meaning it won't go back to its original shape...the material is permanently (plastically) deformed. Many materials do not behave this way, so be careful if you are considering non-metals.

True Stress/True Strain is rarely used in engineering applications and is generally confined to the laboratory and the scientific endeavors of PhD's.

 

[Drej]

There are many posts on Eng-Tips related to this topic, the best thing to do is search (try searching within the ANSYS forum).

I will say two things here. Firstly, depending on the level of plastic strain you observe in your analysis, you may have to convert from Engineering to True strain (generally > 5% as a rule of thumb). Run your analysis to determine this first using your Engineering data - if the total strain is greater than 5% then consider converting. Generally I would not call the use of True stress/strain a rarity, and for some applications this would be considered extremely important (such as metal forming).


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