Elasticity Of Materials

Elasticity is that property of a material by virtue of  which deformation caused by applied load disappears upon removal of the load. In other words, elasticity of a material is its power of coming back to its original position after deformation when the stress or load is removed. Elasticity is a tensile property of the material. 

Proportional Limit , It is maximum stress under which a material will maintain a perfectly uniform rate of strain to stress. Even though this value is difficult to measure, it is used in important applications such as precision instruments ,springs, etc. 

Elastic Limit , Most materials can be stressed slightly above the proportional limit without taking a permanent set., The greatest stress that a material can endure without taking up some permanent set is called elastic limit. Beyond elastic limit , therefore the material does not regain its original form and permanent set occurs.

Yield Point . At a certain stress, ductile materials particularly cease offering resistance to tensile forces, i.e., they flow and a relatively large permanent set takes place without a noticeable increase in load. This point is called yield point. Some materials exhibit a definite yield point, in which case the yield stress or yield strength is simply the stress at this point. Mild steel is an instance of this.

Proof Stress : Most ductile materials exhibit progressive yield and another measure of yield stress, usually known as proof stress.

Proof stress is defined as the amount of stress a material can withstand without taking more than a small amount of set , common measures being 0.1 or 0.2 percent of the original gauge length.