Yield stress of aluminum

When you are all calculating the yield stress of aluminum from the tensile test that we did, there are two things I want you to keep in mind:

1) I mentioned this in lab, but in case anyone forgot, you can just use the same diameter of the aluminum cylinder as we used for the brass cylinders last week when you are converting from load to stress...nominally they are machined to the same dimensions.

2) When you are finding your yield stress, I want everyone to quote the yield stress with a 0.2% offset. This means you will NOT report the 'classical' yield stress at the inflection point of the stress strain curve, but rather you have to draw an offset line that starts from 0.2% strain on the x-axis and is drawn up to intersect the stress-strain curve with a slope equaling the elastic modulus. The intersection of the offset line and the stress-strain curve will give you the yield stress with a 0.2% offset. Make sure in your report when you mention the yield stress you found that you are quoting yield stress with a 0.2% offset. Because aluminum doesn't really exhibit much in the way of work hardening and the curve stays quite flat after yielding, you'll find the 'classical' value and the 0.2% offset will be pretty close, but I want everyone to follow what is considered 'standard operating procedure' in industry when quoting properties of materials. After all, everyone's report is a reflection of an industry standard test for fracture toughness, so we might as all be as rigorous as possible.