According to my last post some weeks ago I now have translated the laboratory data from engineering strain to true strain with the following formula: epsilon(true)=ln(1+epsilon(engineering)). I compared then the the new values of the measurement data with the points on the graph of the ExCalibre output. And they a still different. For the Oedometer the measurement point (sigma 1/epsilon 1 true) is (1200/0,101) and on the graph of ExCalibre it is (1200/0,112). The same is for the triaxial tests. The maximum true strain epsilon 1 of the measurement is 0,197 and on the graph it goes until 0,244. Attached are the pictures of the ExCalibre output with the points and a picture of the excel table with the measurement values.
Is the transformation of strain different in ExCalibre or what can be the reason for that?
Hi Marco, my colleague Tomáš Kadlíček checked ExCalibre code, he will send you more details in a separate comment. Regards David
Hi David, thanks for the quick response.
Hi Marco, I have checked the source code of ExCalibre and it is correct. In the case of compression, you should calculate strains as follows:
epsilon(engineering) = dL/L0
epsilon(true)= – ln( 1 – epsilon(engineering))
In this way, TRUE strains will be larger then ENGINEERING strains. True strains are related to the current hight of the specimen whereas ENG strains are related to the initial hight “L0”.
Therefore, when subjected to the same amount of settlement “dL” you should always obtain larger strain increment in the case of TRUE strains in comparison to the ENG strains.
Sorry for the misunderstanding.
now everything fit together. Thanks a lot.