How Do I Correctly Calculate and Plot Stress-Strain for a Physics Test?

[MCooltA]

I need to draw a stress and strain graph. New to physics, so would like to know if i am on the right line and require a bit of help for the last few questions

These are the values I have:

Diameter - 10.02mm
Original Length - 50mm
Extension -10mm
Diameter After Test - 6.76mm
Max Load - 42.7 kN
Break Point - 31 kN

Found the cross sectional area to be - 78.85

So for the maximum tensile stress, i done

42700/78.85 = 541.5 MPa ... Is this correct?

How do i go about calculating the strain for this value so i can plot it on my graph. I know the formulas, but wasnt given a extension at the Max Load?

Thanks

 

[PhanthomJay]

There is a differenve between the engineering stress-strain curve and the true stress-strain curve. The engineering stress-strain curve does not take into account the 'necking down' of the cross sectional area, whereas the true stess -strain curve does. The engineering curve is most often used. Your calc for the max stress is correct. if you weren't given any intermediate extension values as a function of stress, you can't do much with a plot of the curve. Was the the 10 mm extension at the break point, or max load point, or other point?

 

[MCooltA]

10mm was the final extension, so a total of 60mm, after it had broken.

And i am trying to calculate the engineering stress and engineering strain. Thanks

 

[PhanthomJay]

10mm was the final extension, so a total of 60mm, after it had broken.

If that's all the info you have, you can compute the stress-strain only at the break point (engineering stress at break point = 31000/78.85 MPa, and strain at break point = 10/50). You don't have much else to work with.