Elastic limit is the highest deforming stress under which a material remains elastic
Hooke’s law states that the extension produced in the wire is directly proportional to the load applied within elastic limit.
i.e. within elastic limit, extension ∝ load applied.
Later on it was found that this law is applicable to all types of deformations such as compression, bending, twisting etc. and thus a modified form of Hooke’s law was given as stated below.
Within elastic limit, the stress developed is directly proportional to the strain produced in a body
i.e. stress ∝ strain or, stress = E × strain
where E is constant and known as modulus of elasticity.
Modulus of elasticity
According to Hooke’s law, within elastic limit,
stress ∝ strain
or, stress = E × strain
Where E is known as the coefficient of elasticity or, the modulus of elasticity of a body.
It depends upon the nature of material of the body and the manner in which the body is deformed.
Thus modulus of elasticity is the ratio of the stress to the corresponding strain produced, which is a constant, within the elastic limit.
Types of moduli of elasticity
Corresponding to three types of strain, there are three elastic moduli, described below :
(a) Young’s Modulus of elasticity (Y)
It is the ratio of stress to the longitudinal strain within the elastic limit. Thus
Consider a metal wire AB of length l, radius r and of uniform area of cross-section a. Let it be suspended from a rigid support at A, as shown in the figure below.
Let a normal force F be applied at its free end B and let its length increase by Δl (= BB’ )
Then, longitudinal strain = Δl/l
Normal stress = F/a = F/πr2
(b) Bulk Modulus of elasticity (k)
It is the ratio of normal stress to the volumetric strain, within the elastic limit. Thus
Consider a spherical solid body of volume V and surface area a. In order to compress the body, let a pressure p be applied on the body and suppose that its volume decreases by ΔV as shown in figure.
Then, volumetric strain = – ΔV/V
Here negative sign shows that volume is decreasing when pressure is applied.
Normal stress = P
K = – P/(ΔV/V)
K = – PV/ΔV
(c) Modulus of rigidity (η)
It is the ratio of tangential stress to the shearing strain, within the elastic limit. It is also called shear modulus of rigidity.Thus
Question : A steel wire of length 4m and diameter 5 mm is stretched by 5 kg-wt. find the increase in its length, if the Young’s modulus of steel of wire is 2.4 × 1012 dyne/cm2.