Learning Forum

[Rameez_ul_Haq]

So, you may find an attachement which shows the equations used to calculate the true strain and stress.

[1shan]

The difference between true stress/ strain and engineering stress/strain is only evident for higher values of strain, post yield point. For the elastic region the difference is negligible, as can be seen mathematically, ln(1+x)= x as x tends to zero.
Regards Ishan.

[Rameez_ul_Haq]

thank you for your quick answer. I am trying to use multilinear isotropic hardening to model plastic behavior of a ductile material. I saw the video posted by ansys on YouTube, as below.
To use multilinear hardening, I need to input elastic strain against true stress table (beginning from the yield point, onwards). However, if I compare the values of the elastic strain (formula of which is given in the video I have shared) to the true strain, I see a difference between them in the elastic range. I just want to know that is elastic strain and true strain the same thing under the elastic range or not?

[1shan]

Elastic strain builds up even after material deforms plastically. As described in the video, you need to subtract elastic strain (True stress/E) from the total strain to get the plastic strain, so that you can define the True stress vs Plastic strain data for the material. Now when you say "However, if I compare the values of the elastic strain (formula of which is given in the video I have shared) to the true strain, I see a difference between them in the elastic range." - why are you calculating the elastic strain before yield point? You need to do this post yield and yes this value will be significantly lower than true total strain = elastic strain + plastic strain.
Regards Ishan.

[Rameez_ul_Haq]

,well thank you for your answer. But I think you are missing my point here. In the elastic range, the there doesn't exist any plastic strain, right? So why should the elastic strain ( = true stress / E) be not equal to the total strain? I am mystified by this.
Morerover, you mentioned, "As described in the video, you need to subtract elastic strain (True stress/E) from the total strain to get the plastic strain", the total strain that you mentioned from which the elastic strain should be subtracted in the plastic region, that total strain is the total engineering strain or total true strain? The video doesn't say anything about this.

[Rameez_ul_Haq]

,if you could provide your opinion on the thing I asked, I would be grateful. Thank you.

[1shan]

"....that total strain is the total engineering strain or total true strain?" - Its total true strain, as mentioned by Bhargava at 4:27.
"So why should the elastic strain ( = true stress / E) be not equal to the total strain?" - They should be equal. True stain = ln (1 + engineering strain). How much of a difference are you getting between the LHS and RHS? For steel with yield strain = 0.002, this difference is 0.1%.
Regards Ishan.

[Rameez_ul_Haq]

,but the LHS in the equation which you have written (and also which I shared in the post initially) is the true strain while RHS includes the engineering strain. I want to compare the value of the LHS (i.e. true strain) with the value obtained from true stress / E (i.e. Elastic Strain). I want to know if these two would be equal or not (since there doesn't exist any plastic strain, so I was expecting that the two values should be exactly equal to each other in the elastic region).

[1shan]

How did you calculate true stress?
Regards Ishan.

[Rameez_ul_Haq]

,from the same equation which I shared in the picture.

[Rameez_ul_Haq]

,would love to hear it from you please.

[Rameez_ul_Haq]

,still waiting for your reply on this one?