Elastic limit - the point till which the wire retains its original length after the force is withdrawn.
Yield point - the point where there is a large permanent change in length with no extra load force.
This is how these two terms are defined in my A Level book and also stated by my teacher.
In Wikipedia, yield point is stated as follows:
A yield strength or yield point is the material property defined as the stress at which a material begins to deform plastically. Prior to the yield point the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible.
By that definition, shouldn't yield point and elastic limit have the same point on a stress-strain graph?
I know that below the elastic limit, the material will only show elastic behaviour. After the yield point, the material will exhibit plasticity. Nevertheless, the gap between these two is very small.
So, in the picture below, why are they put in two different points which are pretty far apart from each other?
P.S. If the picture is wrong in anyway, please mention why it's wrong.