what is meant by yield point in stress -strain curve of mild steel?

Answer 1

If you plot a graph of stress vs strain, you'll see that as the force (stress) you use to pull your mild steel sample apart increases there will be a corresponding elongation (strain) in the metal. The plot will look like a straight line until you reach the yield point in the metal. When the metal goes past its elastic limit (gives), the plot deviates from a straight line as the metal no longer stretches in proportion to the force. In theory, if you release the force before you reach the yield point, the metal will return to its previous size.

Answer 2

If you push sideways on a vertical rod embedded in concrete (etc.) it will return to its original position when released unless you push so hard that there is permanent deformation and the material has 'yielded.' In a stress-strain test, the sample will remain essentially elastic returning nearly to its original position until you load it to the yield point causing permanent deformation. Mild steel can yield whereas brittle steels may crack under high loads.