Uncertainty calculations

How would you calculate uncertainty for the yield strength and PQ since you graphed the plots and used a line to find where it intersects the graph to get these values. Would you include the error in finding the point using matlab and the error with using the equation for the line which doesn't give you an exact value for the yield strength or PQ or do you just assume that the error in these methods is zero?

Using the (max slope - min slope)/2 formula can give you the uncertainty in the slope (see ctools "error analysis" last page). Using that I was able to get PQ with around 1% uncertainty, which agrees with what the ASTM E399 (section 8.4) wanted. I'm not sure the recommended way we are supposed to find the uncertainty, but that should be one way to do it...

Yep, the error analysis stuff on Ctools talks about finding the uncertainty in the slope of a line which will factor into your uncertainty in Pq.

Remember for the yield strength you can actually get a precision error, because you have data from three lab sections.