Stress vs Strain for mild steel 
After Yield point on stress strain diagram the under curve occurs what does it mean what will happen for the mild steel at that particular time and again why the curve goes to up and reaches ultimate point  i am not getting that WHY that under curve comes
 A: I think you're getting confused between stress and engineering stress. As a material has more and more strain, it starts to thin (called necking), and the engineering stress declines. However, measured relative to cross sectional area, the stress continues to increase.
A: The physics here get quite tricky. At point E the material has not changed in its molecular structure then there’s a small transition gap to point Y. You can compare this gap to the ice to liquid water transition. I make this analogy because from point Y on the material will act like elastic seamy liquid. The other thing that happens is the molecular alignment changes and the material actually gets harder in layman’s terms we call it “Forging”. This is why the material is getting thinner “necking” yet it takes more force until it gets to the U point. At point U the material is at its hardest and past this the molecular cohesion starts to brake down. After U we apply more force to break it “doted line” in actuality the material takes less force to failure R.
