# Drawing a parallel line to the straight portion of the curve

I have a curve of this form (as attached) drawn in excel.

Now, the point P upto which the red curve is (almost) a straight line is called the proportional limit.

OK. Now, I need to find the proportional limit as hown by the point p on the curve.

It is given in literature that;

As a convention, to determine the yield point and proportional limit (yield point is the point corresponding to σy on the curve) a straight line is constructed parallel to the elastic portion of the curve (elastic portion means the portion that remains a straight line) corresponding to the value of strain 0.002 as shown in the figure above.

My question is:

How to construct this parallel line to the elastic portion of the curve ?? I need to do it in excel (preferable or even Matlab if you say)- how to do it? That is; how to construct a straight line parallel to the elastic (staright) portion of the red curve above??

Please help

#### Attachments

• curve.jpg
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## Answers and Replies

pbuk
Science Advisor
Gold Member
Fit a straight line to the curve between O and P and measure its slope.

As MrAnchovy said, you have two points of the curve (O and P), which you can use to calculate the slope. Use the slope to derive the equation of the line.

Integral
Staff Emeritus
Science Advisor
Gold Member
A easy way to do this in Excell is to select a few points on the line and select a linear trend line. Choose to show the equation and you will have your line.