Stress strain and tensile strength diagram

[akkshaya]

why does stress decrease after it reaches its maximum tensile strength after its plastic range ?

 

[Mapes]

Hi akkshaya, welcome to PF. What are your thoughts? We don't simply provide answers here, because that wouldn't help anybody learn; you have to present your own attempt at a solution.

 

[xxChrisxx]

Look up necking and strain hardening, that should give you the information to understand why stress reduces after UTS.

 

[akkshaya]

all i know is that its due to the necking or the neck formation .when i sarched the info , it said about two tangents and "draw ratio".. what is a draw ratio??. necking is when large amount of strain is experienced if a little stress is applied .. is this right ? am not clear in this necking concept .. can you help me out ?

 

[Mapes]

I agree that it corresponds to necking. How does necking affect the assumptions of an engineering stress - engineering strain curve? (Hint: compare with a true stress - true strain curve.)

 

[Emreth]

necking means your sample gets much thinner at some point than the rest. Imagine most of the cylindrical sample diameter is say 1 inch, length is 5 inches, there's a region which is 1 inch wide in the middle that's only half an inch in diameter. that region is said to show necking, deformation is not uniform in your tensile test sample.

 

[akkshaya]

thanks for the necking concept ... and i saw that true stress-true strain curve and its explanations... i still don't uderstand how the load falls off if the area of cross section decreases after the application of a lot of stress..

 

[Mapes]

i still don't uderstand how the load falls off if the area of cross section decreases after the application of a lot of stress..

[STRIKE]The load didn't fall off! That's absolutely not what's plotted on the y-axis of an engineering stress-strain diagram.[/STRIKE]

The load decreases from its maximum point, but what is happening to the true stress (vs. the engineering stress) in the material?

EDIT: Corrected my mistake.

 

[nvn]

The load decreases if the engineering stress-strain curve y coordinate decreases.

 

[Mapes]

The load decreases if the engineering stress-strain curve y coordinate decreases.

Yep, I misspoke there, since the engineering stress is the load normalized to a constant value. Revised to draw a comparison between the engineering stress and the true stress.

 

[Emreth]

The load falls off because the amount of area reduction in the necking area is faster than the work hardening in the material. So although stress increases, the load still drops. Engineering stress doesn't take the area decrease into account so it shows a decrease whereas true stress relates to the actual cross section area, so it always increases until fracture.

 

[akkshaya]

oh k now i get it .. thanx.. and also what is a "draw ratio"?