toughness is defined as the total area under the stress over strain curve from zreo to rupture.
With stress in units of force/area(y-axis) over,
strain in units of lenght/change in lenght)(technicly unit-less)(x-axis)



In this pic/graph that I (shamelessly) stole from wikipeida, we see point 2 the point where it goes from elastic deformation(non-perment) to plastic deformation (perment).
In reigon 4 is the work hardening phase there the material gets stronger (it can take more force/energy with less deformation) untill point 1 where reaches its max strenght or ultimate strenght, after in drops slightly until point 3 where it finally fails .