0.2 percent offset for computing the yield point

[svishal03]

I need to compute the yield point for an experimental true stress vs true strain plot of a material as attached in the screenshot.

See attached;

I have true stress vs true strain plot. I took some initial points on the curve and fitted a straight line which gives me the elastic part of the curve.

I constructed a line parallel to this straight line (parallel line has same slope as straight line, I got the constant of y = mx + c of the parallel line by putting x =0.002 when y = 0).

But from the graph it is evident that yield point is around 1500 MPa but the offset method gives me a yield point around 1235 MPa.

Please can anyone suggets/help what is going wrong?

 

[Mech_Engineer]

But from the graph it is evident that yield point is around 1500 MPa but the offset method gives me a yield point around 1235 MPa.

Why is it evident it occurs at 1500 MPa? It seems to me that 1500 MPa is approximately 2% yield (if the units on the x-axis are decimal percentages). Remember that 0.2% yield is a pretty small deviation from the linear region.

What material are you testing?

 

[svishal03]

The units on x-axis are not percentages.

I say that it is evident that yielding occurs at 1500MPa. because at yielding material just flows.

We can also say that one gets yield point by joining origina with breaking point (this getting a line) and then drawing a line parallel to this line such that the line drawn parallel is tangent to the stress strain curve.This point where the parallel line is tangent is the yield point (http://composite.about.com/library/glossary/y/bldef-y6168.htm).

From the graph it looks that 'flowing' occurs at around 1500MPa definitely not 1235

 

[svishal03]

The material is a metal

 

[Mech_Engineer]

The units on x-axis are not percentages.

OK, so it's strain then.

I say that it is evident that yielding occurs at 1500MPa. because at yielding material just flows.

The 0.2% yield number is meant to be the boundary of the material's linear elastic region. I would say the material is well into the yield region by 1500 MPa, because there is significant (to the eye) plastic deformation occurring. If you loaded the sample to 1500 MPa and then released, you would see permanent deformation after just one cycle.

From the graph it looks that 'flowing' occurs at around 1500MPa definitely not 1235

Be that as it may, it looks to me like there is significant plastic deformation by 1500 MPa, so I would say your estimated 0.2% offset is correct.

The material is a metal

OK, well that's not very helpful. Whatever it is it's pretty strong stuff with yield in the 1235 MPa regime (chromoly steel maybe?) I ask because it would be nice to be able to compare published values to your calculated value and see if you're in the ballpark...

 

[svishal03]

Thanks a lot for the help...I will come back after getting some more information from the experimentologist.

I shall be grateful if you can provide some insight here:

https://www.physicsforums.com/showthread.php?t=587471

Vishal

 

[kvpl]

You really should make the pic show more about the elastic area. No need to have the strain go up to 0,5 if you're studying the area of up to 0,005 or something.