stress-strain curve

Welcome! I applaud your resourcefulness to seek out this forum to get your questions answered as well as your spirit in not being satisfied with the textbook answers. You have the makings of a fine engineer. Now, to your questions!

1. What you are referring to as upper and lower yield I believe is actually the yield point and the ultimate strength. Up to the yield point, the material is purely elastic and will return to original shape after the load is removed. After the yield point dislocations caused by Cottrell atmospheres within the material cause the stress to dip slightly before continuing to rise to the ultimate strength. The material has been permanently deformed at this point. This final rise is the area of strain hardening and area will decrease uniformly due to Poisson contractions. After the ultimate strength, a neck will form in the specimen and failure will occur soon after. All ductile materials will exhibit these characteristics, but they are less well-defined in less-ductile materials like aluminum than for materials like mild steel. (By defined, I mean easy to see on the chart, not the actual definition.)

2. You are absolutely correct! Most stress-strain curves are not area compensated! They chart apparent stress not true stress. In a true stress-true strain curve, stress will continue to rise until failure, but the yield points are not as well defined.

I hope this is helpful. If you have more questions, please ask. Good luck in your studies!

the answer to ur 1st question is: stress strain curve shape for almost all ductile material is very much similar.

for 2nd one:Pl refer again the defination of stress:it is the resistance force applied by the material gainst deformation.Once the necking strats i.e yielding the bonding becomes weak.This implies that the material now cant be able to resist the deformation.Thats why afetr yield point stress value will decrease.

No, stress is load divided by area. After necking, and the x-section area decreases, true stress (area-compensated) will continue to increase until failure.

Thanks to every one for clearing my doubt..