Tensile Testing (Determining Young's Modulus)

[stpmmaths]

Question:
Why is it not sensible to calculate the Young's modulus of the material (steel) when the experiments was conducted?

But I thought it should be possible to calculate the Young's modulus after plotting the stress/strain curve. Young's modulus can be found by finding the gradient of the straight line on the curve. But what with the question?

 

[stpmmaths]

Is it related to the slipping which occurs during the test?