# Engineering Stress increases after yield stress?

I need to know why the Engineering stress increases after reaching the yield stress point, and continues to increase until reaching the UTS. Am I correct when I remember that there is strain hardening that occurs along the region from Yield stress to the Ultimate tensile stress? Engineering stress equals Force divided by Original cross-sectional area: σ=F/Ao
My first thoughts were that obviously as you increase the force, the stress will increase (from a purely mathematical outlook, this is apparent since stress increases proportionally to force). How could this be explained in better engineering terms?
Thanks

## Answers and Replies

SteamKing
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I need to know why the Engineering stress increases after reaching the yield stress point, and continues to increase until reaching the UTS. Am I correct when I remember that there is strain hardening that occurs along the region from Yield stress to the Ultimate tensile stress? Engineering stress equals Force divided by Original cross-sectional area: σ=F/Ao
My first thoughts were that obviously as you increase the force, the stress will increase (from a purely mathematical outlook, this is apparent since stress increases proportionally to force). How could this be explained in better engineering terms?
Thanks
Does it need a better explanation than the increase in applied force leads to an increase in stress, regardless of how that stress is calculated?

The question is: "From the onset of plastic deformation up to the UTS, the engineering stress increases. Discuss in Engineering terms why."
This is for a lab report in which we performed a tensile test on three materials, steel, copper, and aluminum

Yes, thank you both for your insight. I asked the instructor also, and the answer he was looking for was regarding the cold work/strain hardening that occurs as a result of dislocations from the plastic deformation.