Elastic limit:
Most of the solid objects which when subjected to an applied stress will return to their original shape and dimensions provided the stress is below the characteristic threshold for the material of which they are made. This limit is commonly known as elastic limit. Beyond this stress level, deformation of plastic occurs, wherein the atomic or molecular structure changes shape permanently. The elastic limit is the maximum stress which can be applied to a metal without producing permanent deformation in it. When external forces act upon the material they tend to form internal stresses within it which cause deformation. If the stresses are not too great the material will come back to its original shape and dimension when the external stress is removed. It is the maximum stress per unit area within the solid material which can arise before the onset of permanent deformation. When stresses up to the elastic limit are removed, the material takes back its original size and shape. Stresses beyond elastic limit cause the material to flow. And at this point material behaves like a plastic and won't be elastic any more. It can be defined as the maximum stress up to which a material can exhibit the property of elasticity is known as elastic limit. If the deformation forces the residual strain left in it. Hooke's law states that the stress is proportional to strain up to the elastic limit. When a specimen is loaded beyond the elastic limit the stress increases and reaches a point at which the material starts yielding this stress is known as yield stress. Ultimate load can be defined as maximum load which can be placed prior to the breaking of specimen. Stress corresponding to the final load is known as ultimate stress. Elastic limit, also known as yield point, is an upper limit for the stress which can be applied to a material before it deforms permanently. The elastic limit is measured in Newtons per square meter, also called as pascals (Pa).