How to report true strain in IDEAS or other FEA programs 1

[EngineerDave]

Does anyone know how to specify the results as true strain in FEA? If you look through the menus I can't find any reference to the results as true strain.

In ANSYS I can't find anything either to report it this way.

Any help would be greatly appreciated.

Engineer Dave

 

[EdR]

Dave:

The "true" strain is related to "engineering" strain by:

eps(true) = ln (1+eps(engr))

where ln is the natural logarithm..

You may need to check the users manual for Ansys, etc. to see what strains are output....could be "engineering" strains or "Greens" strains....but will NOT be "true" strains

Unless you are doing some specialized work true strains are generally not of much use....

Hope this helps

Ed.R.

 

[grisa]

Hi Dave,
"True" strain is important when you are solving the nonlinear analysis. It is need to describe mechanical properties of materials. Difference in "True" and "Engineering" strains practically begins above Yield point. In the linear analysis it haven't differences.

 

[tld23]

* If you do a non-linear analysis you should get true strains.
* A linear analysis will usually give engineering strains.

Another important bit to remember is not the outputs but the inputs of the analysis. If you are doing a non-linear analysis the material properties input will ususlly need to be in true strain (but check the documentation in case the program does the calculation). Often test results are reported in engineering strain.

TERRY

 

[trainguy]

tld23,

I am looking at a non-linear analysis in which the material property function is input as shown, for EN 10025 (355 MPa yield strength) material on some European rail equipment.

stress Strain
0. 0.
200. 0.001
355. 0.00378
466. 0.1
490. 0.2

In your opinion, is this true strain, and therefore correctly input?
The work was done on FEMAP/MSC Nastran, and I'm having trouble relating stresses and strains.

I'm betting you need much more info., but please realize that I am a neophyte at all things non-linear.

Thanks,
GA

 

[brad]

Some observations:
For your table of stresses, with E=200GPA:
stress Strain El Strain (s/E) Delta strain
0. 0. 0 0
200. 0.001 .001 0
355. 0.00378 .00178 .002
466. 0.1 .00233 .0967
490. 0.2 .00245 .198

Elastic strain is simply Stress/E. It is noteworthy that
at 355 MPA, the difference is .002, 0.2% offset (the
classic definition of "yield&quot. This appears that somebody has created the 355 MPa data point from this information. The fact that this data point yields precisely the 0.2% offset leads me to believe that your data is true stress and total strain.
e(pl) = e(total) - e(elastic) = e(total) - s/E

e(pl) = true plastic strain
e(total) = true total strain
e(elastic) = true elastic strain
s = true stress
E = Young's modulus.

From this, it appears that what I earlier referred to as
"Delta strain" in the table is in fact true plastic strain
(often the required measure for nonlinear analysis).

Short answer, it appears that this is true stress and true total strain; whether this is the appropriate input for you is code-dependent (many nonlinear codes take true stress and plastic strain).

Brad

 

[trainguy]

Thanks, Brad.

GA