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Question about strain of triaxial compression simulation

Asked by Leonard

Hi,

I got a question about the engineering strain and logarithmic strain in triaxial compression test.

For example, for one model I have (in which the axial loading is along y-axis.), I got:

Before deviatoric loading, the position of top wall and bottom wall is (0.035,0.14,0.035) and (0.035,0,0.035) respectively (i.e., height of 0.14), and the triax.strain=(0,0,0).

The final position of top wall and bottom wall is (0.035,0.12,0.035) and (0.035,0,0.035) respectively. The -triax.strain=(-0.08,0.15,-0.08) at final state.

So the engineering strain is (0.14-0.12)/0.14=0.143 according to the height change of the sample.
However, since triax.strain returns the logarithmic strain, it means that the logarithmic strain is 0.15 in axial direction (y direction). And from [1], the corresponding engineering strain is exp(0.15)-1=0.16

My question is why the engineering strain obtained from the two ways are different from each other. Do I misunderstand something?

Thanks
Leonard
[1]https://answers.launchpad.net/yade/+question/660034

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Jan Stránský (honzik) said : 		#1

Hello,

> Do I misunderstand something?

yes, the signs :-)
###
from math import log,exp
L = 0.14
l = 0.12
eEng = (l-L)/L
eLog = log(1+eEng)
eEng2 = exp(eLog) - 1
eEng2wrong = exp(-eLog) - 1
print("eEng",eEng) # -0.143
print("eLog",eLog) # -0.154
print("eEng2",eEng2) # -0.143
print("eEng2wrong",eEng2wrong) # +0.166
###

cheers
Jan

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Leonard (z2521899293) said : 		#2

Thanks Jan,

haha, what a stupid mistake :(

cheers
Leonard