plastic strain with straus7

[giamma109]

hi to everybody!
I'm using straus7 and I have to simulate a compression test, but how can I estimate the elastic strain and the plastic strain separately?The software give the total strain but I need the plastic strain only (when applied loads are removed).



thanks!

 

[prost]

First you need to know the stress. Then for small amounts of strain, e(total)=e(plastic)+e(elastic)
where e(elastic)=stress/Young's modulus

 

[jhardy1]

Firstly, for future reference, you might want to post this question on the Strand7 Forum No. 1267 here:

http://www.eng-tips.com/threadminder.cfm?pid=1267

(Strand7 is sold as Straus7 in Europe - same product, different marketing name.)

As prost says, run a non-linear analysis, for loading up to maximum load and unloading back to zero load, with material plasticity etc implemented. The strain in the final (Unloaded) load increment will be the plastic strain.

 

[giamma109]

already done....but the difference is too little ( 0.0002 mm )....the yeld strenght is 276MPa ( aluminium 6061 T6 with Ramberg-Osgood non linear behaviour ) and the compression load is 300MPa...the compression test done in a laboratory give about 0.06 mm of plastic deformation ( with initial lenght of 45 mm ).

 

[prost]

Are those strains engineering, or 'true'?

 

[jhardy1]

I'm a bit confused about the magnitude of your claimed plastic deformations.

Assuming an elastic modulus of 69 GPa, and a yield strength of 276 MPa, your elastically recoverable deformation on a 45 mm long bar should be about 276/69000*45 = 0.18 mm.

Strain at 300 MPa is about 0.03 (from the stress-strain curve for 6061-T6 as supplied with Strand7), so the total compression of a 45 mm long bar at 300 MPa axial load should be about 0.03*45 = 1.35 mm.

Take away the elastically recoverable deformation, and I would expect a permanent deformation of about 1.17 mm after unloading.

One thing to check - the stress / strain curve for 6061-T6 is virtually horizontal at 300 MPa, so very small increments of axial load once you are up in this territory will result in large increases in displacement. (The strain almost doubles as the stress rises from 297.9 MPa to 306.8 MPa.) Or to put this in real-world terms - the actual stress strain curve for your sample will not perfectly match the supplied data, and once you are up in the range of 300 MPa, very large additional deformations can arise with very small load increments.

Strain Stress (MPa)
0.000 0.0
0.004 268.9
0.006 281.3
0.012 289.6
0.028 297.9
0.050 306.8
0.070 310.3
0.080 310.3