# Stress vs. Strain and finding the permanent set of bar

1. Jan 20, 2010

### megr_ftw

1. The problem statement, all variables and given/known data
I am told a cirular bar of magnesium alloy is 750 mm long. The bar is loaded in tension to an elongation of 6 mm and then the load is removed.
I am given this Stress vs. Strain graph which I cant put up here since its in my book. But Stress is on the y- axis and strain is the x-axis. It looks almost like a square root curve.

2. Relevant equations
Epsilon= sigma/ E
Where E is the modulus of elasticity right? and sigma is stress and epsilon is strain

3. The attempt at a solution
I took a point on the graph to find E and got 26,666.6667 which Im not sure if I did that right. So once I have E how the heck do I find the permanent set of the bar? Also the second part wants the proportional limit once the bar is reloaded.

Luckily I have the answers in the back of the book but I want to know how to figure this out badly. turns out the permanent set is 2.97 mm and the proportional limit is 1.8 MPa