Those who are familiar with concept of Stress - Strain curve, please do continue with this post to understand about upper yeild strenght and lower yield strenght for mild steel. Those who are not familiar please take time to go through this article
From the image given here taken from etomica.org it is clear that materials like mild steel have two yield strenghs. The first called upper yield strenght and the second called lower yield strength.
Once the stress reaches the upper yield strength, the internal relaxation comes into play and the strain can be observed even at lower amount of stress. The stain is bound to osciallte between both the limits. The lower yield strength is about half the tensile strength of the material. The explaination can be summarized as follows
"At elastic limit, sudden yield happens & fall-off of load takes place. Hence material continues to defrom at lower load until material hardening sets in"
Answers.com says the reason for such behavior is Low carbon steels suffer from yield-point runout where the material has two yield points. The first yield point (or upper yield point) is higher than the second and the yield drops dramatically after the upper yield point. If a low carbon steel is only stressed to some point between the upper and lower yield point then the surface may develop Lüder bands.
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