Yield Strength versus Ultimate Strength

Question

What is the qualitative difference between these two:

As seen on the table Typical yield and ultimate strengths.

I am trying to resolve the meaning of the phrase "contact yield stress" from C. Thorrton 1997 to real world values.

Answer 1

If you look at a stress-strain diagram, the difference becomes clearer.

The initial slope is where stress is directly proportional to strain (like a spring) and the material behaves like this up to its elastic limit where it reaches its yield strength.

Beyond this the material deforms permanently (like an overstretched spring that won't return to its original shape). The material then becomes strain hardened until you reach the ultimate strength and necking starts to occur and the material becomes weaker again until it breaks apart.

Answer 2

Yield stress is the stress at which that the material deforms permanently, ultimate tensile stress is the stress at which it breaks.

There is probably some official ISO/ASME definition of how much it has to deform for it to count as having yielded.

Materials first deform elastically - when you release the stress they return to their original shape, this is what a metal spring does.
Then with more force they deform plastically - when you release the stress they have permanently been stretched into a new shape, this is yield.
Finally they break, this is ultimately tensile stress, or breaking point

Answer 3

The deformation amount is 0.2% to count as yielded for steel

Hard steels and non-ferrous metals do not have defined yield limit, therefore a stress, corresponding to a definite deformation (0.1% or 0.2%) is commonly used instead of yield limit. This stress is called proof stress or offset yield limit (offset yield strength):

$\sigma t= \frac{F_S}{S_0}$