What is the difference between upper yield point and lower yield point？

 

When mild steel material is subjected to tension test, then it passes through various stages before fracture.

Discussion is limited to Plastic region only..

Beyond the elastic limit plastic deformation occurs and strains are not totally recoverable. There will be thus permanent deformation or permanent set when load is removed. These two points are termed as upper and lower yield points respectively.

From figure you can easily understand that, When ductile material is stretched beyond elastic point, material starts to show plastic behavior. A point at which Maximum load or stress required to initiate the plastic deformation of material such point is called as Upper yield point. And a point at which minimum load or stress required to maintain the plastic behavior of material such a point is called as Lower yield point.

Upper yield point is unstable in nature but lower yield point is stable in nature. so while designing the components we consider the lower yield point of material.

Upper yield point is the point after which the plastic deformation starts. This is due to the fact that the dislocations in the crystalline structure start moving. But after a while, the dislocations become too much in number and they restrict each others movement. This is called strain hardening and lower yield point is the point after which strain hardening begins.

 https://www.quora.com/What-is-the-difference-between-upper-yield-point-and-lower-yield-point-of-a-stress-strain-curve-for-mild-steel