Reliability of Upper/lower yield point in tensile test.

[Ravi Singh choudhary]

Some metal give two yield point in stress strain diagram. One is called Upper yield point and Lower yield point. Why lower yield point is considered as more reliable. I can think of Upper yield point is much more dependent upon experimental apparatus settings. So why lower yield point is much more reliable as considered as actual material property of metal.

 

[PhanthomJay]

The 2 yields are not due to experimental error, rather, it is due to the molecular structure of the material (mild steel in particular) which exhibits an initial upper yield value over a small range of time under load, which quickly lowers and flattens to the lower value with increasing strain. More reliable to use the lower value over the wider range of strain, but also, more conservative for engineering.