Dear Ademola,

welcome:

With reference to the stress strain curve describing the behavior of concrete under compression loading, As the stress firstly increases the stain increases linearly with the ability of the material to restore its shape which is termed the ductile range. At the end of the this range, one finds the yield point where the by increasing the stress beyond this point the rate of increase of the strain with stress increases than the ductile range. This is because the material enters in the combined elastic and plastic phase. In this region the material will not be able to restore itself to its original form barbecue the plastic deformation is not restore-able. Increasing the stress further the stress strain curve reaches to its peak point where the ultimate stress after which a very small increase in the peak stress causes the fracture of the material. So, practically, the peak stress, the ultimate stress, the fracture stress are practically equal and is a measure of the mechanical strength of the material.

For more information please refer to the link:http://www.theconcreteportal.com/cons_rel.html

For the definition of the characteristic points on the stress strain curve please refer to the link: http://physicsnet.co.uk/a-level-physics-as-a2/materials/stress-strain/

Best wishes