Why the yield point occur in the stress-strain graph of mild steel?

Answer 1

Once material is stressed. dislocations present in it starts to move and gather near grain boundary. These dislocation are repulsive in nature and resist further movement, hence yield point occurs. Once dislocations crosses the grain boundary, there is very less amount of force required to keep them moving, hence yield point phenomenon appears i.e. less amount of force is required.

Answer 2

mild steel is an ductile material. ductile means it can be stretched to certain limits without breaking like a bubble gum. but the stretching limit is higher for ductile materials and the brittle or hard materials like cast iron have very little elongating before breaking, they mostly tend to break immediately after a very little yielding. the yield point occur in stress strain diagram because it is the point at which the ductile materials like mild steel tend to yield. while calculating the safe working limits of mild steel only the yield point is taken into account not the ultimate point or the breaking point because once the material crosses the yield point in the stress strain graph it is permanantly deformed rendering it to become useless.