*Ductile metals do not have a well defined yield point. The yield strength is typically defined by the "0.2% offset strain". The yield strength at 0.2% offset is determined by finding the intersection of the stress-strain curve with a line parallel to the initial slope of the curve and which intercepts the abscissa at 0.2%.*

The question, then, is "Is there an easy way to determine the 0.2% offset strain line, without a calculator handy?" Yes, there is. Shown below is a typical stress-strain curve, in this case for a rolled steel bar.

The first step is to find the slope of the elastic portion of the stress-strain curve. You can do that very easily with the *Distance from...* command on the *Info* menu. Ensure that *Snap cursor to nearest data point* is checked, then drag the mouse from a point near the origin along the initial (hopefully linear) slope:

To avoid retyping (and transcription errors), drag your mouse over the "slope" value, right-click and select "Copy". Now select the Y=f(X) command on the *Generate* menu. The equation we want is simply (in this case) "21.6569856057*(x-2000)" (2000 microinches/inch = 0.2% strain). For the *from X* value use 2000 (or any smaller value); for *to X* use a strain value large enough that the line will be certain to cross the stress-strain curve. You can then zoom in if you overshoot the curve by a lot. The *with interval dX* entry isn't critical, since you'll be generating a straight line.

Finally, select *Find Intersections...* on the *Info* menu. (If this command is disabled you likely need to sort the data points with the *Sort* command on the *Edit* menu.) Check the *Insert data points...* and *Add labels at intersections* boxes, and you get:

... indicating a yield stress of ~70.5ksi at a strain of ~0.005.

*Thanks much to Stephen Rowell for his input on this one.*