How do I calculate stress and strain from tensile test data for steel?

[eluu]

How would i calculate the stress and strain with the results from a lab below:

Steel’s original diameter: 8.00 mm
Steel’s original length: 49.96 mm
Steel’s final diameter: 4.64 mm
Steel’s final length: 63.17 mm
Steel’s maximum load force: 26.43 kN

I also have an MS Excel table of time, laser extensometer (mm) and load (kN) of the steel.

I have no idea where to begin :S

 

[eluu]

Ok don't worry i think i figured it out

 

[Astronuc]

One would need the elastic modulus.

Steel’s original diameter: 8.00 mm
Steel’s original length: 49.96 mm
Steel’s final diameter: 4.64 mm
Steel’s final length: 63.17 mm

Assuming the final dimensions are measured in the unloaded condition, the steel has undergone permanent (plastic deformation) and the max load corresponds to the ultimate tensile strength.

Stress is just load divided by cross-sectional area, which has changed.