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0.2 % offset for yield point computation

I have got some experimental readings of true strain vs true stress. I have fitted a straight line to the elastic part of the curve as shown in the attached figure.

As seen, the fitted straight line starts not from the origin (as the data was noisy here) but at an intermediate point (here where stress = 400 MPa)

Now, I want to draw a lien parallel to this line at an offset 0f 0.002. My question is should the offset be measured (horizontally)from the origin or (horizontally) from the point corresponding to sigma = 400 MPa?

 

Please help

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0_pt_2_percent_offset_1.JPG63.88 KB
offset_3.JPG44.69 KB

From figure, I found theat the experimental data is engineering strain vs true stress.  the offset be measured (horizontally)from the origin.

See atatched offset_3.jpg

 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?
Why is the 0.2% offset method not working here?

See atatched offset_3.jpg

 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?
Why is the 0.2% offset method not working here?

Further, I'm sure many would have heard of the Meredith's method : Meredith's construction, which sets the yield point as the point at which the tangent to the curve is parallel to the line joining the origin and the breaking point- is it a good substitute than 0.2% offset method ? Please can anyone advise?

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