Why calculate true stress and strain?

[Heimerdinger]

I am taking a lab course in mechanical engineering and in an experiment wherein we graph a stress strain curve, the TA mentioned true stress and strain. I know what these values are and I even know how to calculate it given the engineering stress and strain, but I just don't understand why it is necessary in the first place to calculate the true stress and strain. Can anyone help?

 

[FAlonso]

Because as the load increases, the cross-section over which the force is applied changes. So if you use engineering stress (which takes into account the initial area), the value of stress will be under-predicted (lesser than what the value actually is). True stress solves this issue by considering the instantaneous area over the course of application of load so that as the cross-section changes, the value of stress is calculated using the new area of cross-section. Hope it helps