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USN2ENG

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I am trying to wrap my head around this and for some reason I am blocked. I have taken Solid mechanics and strength of materials and am always used to calculating strain as ε=ΔL/L. I might be missing something easy as it has been a year since the class but in my Manufacturing Engineering class the professor went over an example that went like this:

During tensile test a specimen yields at 48KN. This is the .2% yield point.

Ao= .1m^2

Lo = .05m

Lf = .0523m

We calculate yield stress at normal with 48KN/.1m^2 = 480MPa

Then when we went to calculate Youngs Modulus (E = σ/ε), we used (ε - .002) for the ε. I feel like I am missing something. I think I know why we used this, I just wanted some clarification.

Basically E is the slope of the elastic curve. Dropping straight down on the stress strain curve gives the regular ε. If we offset this by .002 we can get the rise/run needed to get E. right?

Thanks,

Chuck