Elastic limit:

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