calculation of log. plastic strain
calculation of log. plastic strain
(OP)
Hello users,
From Nominal stress-strain curve(compressive), I calculated logarithmic plastic strain using following formula.
Log plastic stain = Nominal strain - (true comp stress/E)
Is it right? I get ascending values till 75% strain then my log plastic strain descends. Abaqus asks for a value in ascending order. What is wrong with my calculation?
My calculation:
Nominal Stress Nominal Strain Nom.strn-(true strs/E)
0,24 0,05 0,013636364
0,27 0,1 0,059090909
0,3 0,15 0,104545455
0,32 0,2 0,151515152
0,34 0,25 0,198484848
0,35 0,3 0,246969697
0,38 0,35 0,292424242
0,4 0,4 0,339393939
0,43 0,45 0,384848485
0,47 0,5 0,428787879
0,51 0,55 0,472727273
0,58 0,6 0,512121212
0,67 0,65 0,548484848
0,82 0,7 0,575757576
1,07 0,75 0,587878788
1,54 0,8 0,566666667
True stress is equal to nominal stress since my poisson ration is zero.
Thanks for your time. Any help would be nice.
From Nominal stress-strain curve(compressive), I calculated logarithmic plastic strain using following formula.
Log plastic stain = Nominal strain - (true comp stress/E)
Is it right? I get ascending values till 75% strain then my log plastic strain descends. Abaqus asks for a value in ascending order. What is wrong with my calculation?
My calculation:
Nominal Stress Nominal Strain Nom.strn-(true strs/E)
0,24 0,05 0,013636364
0,27 0,1 0,059090909
0,3 0,15 0,104545455
0,32 0,2 0,151515152
0,34 0,25 0,198484848
0,35 0,3 0,246969697
0,38 0,35 0,292424242
0,4 0,4 0,339393939
0,43 0,45 0,384848485
0,47 0,5 0,428787879
0,51 0,55 0,472727273
0,58 0,6 0,512121212
0,67 0,65 0,548484848
0,82 0,7 0,575757576
1,07 0,75 0,587878788
1,54 0,8 0,566666667
True stress is equal to nominal stress since my poisson ration is zero.
Thanks for your time. Any help would be nice.





RE: calculation of log. plastic strain
in above post, I have given the wrong formula for log plasitc strain.
Log plastic strain = True strain - (true stress/E)
= Ln(1+Nom strain) - (Truestress/E)
New log plastic strain as follows,
0,012426528
0,054401089
0,094307397
0,133836708
0,1716284
0,209333961
0,242528835
0,275866176
0,306412041
0,334252987
0,360982204
0,382124841
0,399260136
0,406385827
0,397494576
0,354453332
Here, again i have the same problem with last 2 values.
Some one can clarify me.
Thanks.
RE: calculation of log. plastic strain
Log plastic strain = Ln(1+Nom strain) - (Truestress/E)
My strength book says Truestress==nom. stress*(1+nom. strain). Is this consistent with your definition of Truestress?
Back calculating using the numbers you gave:
E=Truestress/(Ln(1+nom. strain)-Log plastic strain)
Is there agreement there? If so, I get a different "E" for every strain/stress combination. Perhaps you can tell me what I've done wrong?
engr engr Your True True My calc.
stress strain plastic strain Stress Strain E
0.24 0.05 0.01243 0.252 0.04879 6.93
0.27 0.1 0.05440 0.297 0.09531 7.26
0.3 0.15 0.09431 0.345 0.13976 7.59
0.32 0.2 0.13384 0.384 0.18232 7.92
0.34 0.25 0.17163 0.425 0.22314 8.25
0.35 0.3 0.20933 0.455 0.26236 8.58
0.38 0.35 0.24253 0.513 0.30010 8.91
0.4 0.4 0.27587 0.560 0.33647 9.24
0.43 0.45 0.30641 0.624 0.37156 9.57
0.47 0.5 0.33425 0.705 0.40547 9.90
0.51 0.55 0.36098 0.791 0.43825 10.23
0.58 0.6 0.38212 0.928 0.47000 10.56
0.67 0.65 0.39926 1.106 0.50078 10.89
0.82 0.7 0.40639 1.394 0.53063 11.22
1.07 0.75 0.39749 1.873 0.55962 11.55
1.54 0.8 0.35445 2.772 0.58779 11.88
The input data, nom. stress and nom. strain, looks interesting, as the data doesn't appear to cross the (0,0) point (that is, extrapolating back to zero nom. strain from the smallest values of nom. strain, the nom. stress at nom. strain=0 is not equal to zero). Is there an offset problem with the original data measurements?
RE: calculation of log. plastic strain
Thanks for your reply.
column 1 - Nom stress
Column 2 - Nom strain
Column 3 - True compressive stress
Column 4 - True compressive strain
Column 5 - True plastic comp. stain
True comp stress =N om.str/(1-pois rat*nom.strn)^2
True Strain = ln(1-nom.strn)
True comp. plastic strain = True.strn-(true strs/E)
In my earlier post and in your calculation, basic mistake was negative sign was not added(compressive strain) in true strain calculation.
E = 6.6 MPa
Poisson = 0
Moreover, in your forrmula of True stress, i cannot include zero poisson ratio. I have given mine also. Pls, check and correct me if i am wrong. For my case, true stress and nominal stress are same.
0,24 0,05 0,24 -0,051293294 -0,087656931
0,27 0,1 0,27 -0,105360516 -0,146269607
0,3 0,15 0,3 -0,162518929 -0,207973475
0,32 0,2 0,32 -0,223143551 -0,2716284
0,34 0,25 0,34 -0,287682072 -0,339197224
0,35 0,3 0,35 -0,356674944 -0,409705247
0,38 0,35 0,38 -0,430782916 -0,488358674
0,4 0,4 0,4 -0,510825624 -0,571431684
0,43 0,45 0,43 -0,597837001 -0,662988516
0,47 0,5 0,47 -0,693147181 -0,764359302
0,51 0,55 0,51 -0,798507696 -0,875780423
0,58 0,6 0,58 -0,916290732 -1,00416952
0,67 0,65 0,67 -1,049822124 -1,151337276
0,82 0,7 0,82 -1,203972804 -1,328215229
1,07 0,75 1,07 -1,386294361 -1,548415573
1,54 0,8 1,54 -1,609437912 -1,842771246
For your last question, I dont have any offset problem in my experimental data. If you tell me, how did you find this problem, then i could look further.
Thanks
feaplastic
RE: calculation of log. plastic strain
Why doesn't zero strain give you zero stress?
RE: calculation of log. plastic strain
truestress=nom. stress*(1+nom. strain)
But if you don't make that assumption, just calculate the deformed area of the coupon gage section using the Poisson's ratios, then you derive the equation you have written:
truestress=N om.str/(1-pois rat*nom.strn)^2
It is puzzling that these two equations give you similar results for SMALL STRAINS! That seems counterintuitive to me because you are assuming the strains are very large, which will give you incompressible deformation. But your approximation of large strains gives you
truestress=nom. stress*(1+nom. strain)
which doesn't work very well for large strains.
RE: calculation of log. plastic strain
(1 - nom. strn)
because your nom. strn is really compressive, or negative, rather than positive as you show in Column 2? If you put that negative sign there, don't you have to use a positive sign in the formula
True comp. plastic strain = True.strn-(true strs/E)
instead of the negative?
You show your 'true compressive stress' as column of positive values. If you use that sign convention, then don't you have to use the following formula?
True comp. plastic strain = True.strn+(true strs/E)
When I make those assumptions, the True comp. plastic strain monotonically increases (that is, always increases).
RE: calculation of log. plastic strain
RE: calculation of log. plastic strain
RE: calculation of log. plastic strain
RE: calculation of log. plastic strain
If you see my strs-strn curve, you could understand. Both have different slope. Imagine, for steel matl, you calculate slope from plastic region and use the same slope to calculate elastic region stress. It wont work, right?...
For your next question, i did not put negative sign in nominal strain cólumn and instead i changed the formula in my post.
I agree, your approach is professional.
For your last question, comp plastic strain should increase numerically with negative sign(compression). I dont see any confusion here. Could you clarify further. Hope, some experts will put an end to this problem.
Thanks a lot for your curiosity. I reviewed all my formulas again and hope i wont forget in future.
Thanks
feaplastic
RE: calculation of log. plastic strain
True True True True
Nom Nom Compressive True Compressive Compressive True Compressive
stress strain Stress Strain Plastic Strain Stress Strain Plastic Strain
-0.2 -0.05 0.24 -0.05129 -0.08766 -0.24 -0.05129 -0.01493
-0.3 -0.1 0.27 -0.10536 -0.14627 -0.27 -0.10536 -0.06445
-0.3 -0.15 0.3 -0.16252 -0.20797 -0.3 -0.16252 -0.11706
-0.3 -0.2 0.32 -0.22314 -0.27163 -0.32 -0.22314 -0.17466
-0.3 -0.25 0.34 -0.28768 -0.33920 -0.34 -0.28768 -0.23617
-0.4 -0.3 0.35 -0.35667 -0.40971 -0.35 -0.35667 -0.30364
-0.4 -0.35 0.38 -0.43078 -0.48836 -0.38 -0.43078 -0.37321
-0.4 -0.4 0.4 -0.51083 -0.57143 -0.4 -0.51083 -0.45022
-0.4 -0.45 0.43 -0.59784 -0.66299 -0.43 -0.59784 -0.53269
-0.5 -0.5 0.47 -0.69315 -0.76436 -0.47 -0.69315 -0.62194
-0.5 -0.55 0.51 -0.79851 -0.87578 -0.51 -0.79851 -0.72123
-0.6 -0.6 0.58 -0.91629 -1.00417 -0.58 -0.91629 -0.82841
-0.7 -0.65 0.67 -1.04982 -1.15134 -0.67 -1.04982 -0.94831
-0.8 -0.7 0.82 -1.20397 -1.32822 -0.82 -1.20397 -1.07973
-1.1 -0.75 1.07 -1.38629 -1.54842 -1.07 -1.38629 -1.22417
-1.5 -0.8 1.54 -1.60944 -1.84277 -1.54 -1.60944 -1.37610
RE: calculation of log. plastic strain
After changing the sign, i get the same values.
Thanks a lot for your help, especially the change in sign.
Regards,
feaplastic