that stress is proportional to strain, which means Hooke’s law is valid. The point A is called limit of proportionality because above this point Hooke’s law is not valid. The slope of the line OA gives the Young’s modulus of the wire. (b) Portion AB: This region is reached if the stress is increased by a very small amount.
In this region, stress is not proportional to the strain. But once the stretching force is removed, the wire will regain its original length. This behaviour ends at point B and hence, the point B is known as yield point (elastic limit). The elastic behaviour of the material (here wire) in stress-strain curve is OAB.
(c) Portion BC: If the wire is stretched beyond the point B (elastic limit), stress increases and the wire will not regain its original length after the removal of stretching force. (d) Portion CD: With further increase in stress (beyond the point C), the strain increases rapidly and reaches the point D. Beyond D, the strain increases even when the load is removed and breaks (ruptures) at the point E. Therefore, the maximum stress (here D) beyond which the wire breaks is called breaking stress or tensile strength.
The corresponding point D is known as fracture point . The region BCDE represents the plastic behaviour of the material of the wire. . .
Moduli of elasticity From Hooke’s law, stress ∝ strain stress strain = a constant, known as modulus of elasticity. Its SI unit is Nm – or pascal and its dimensional formula is ML – T – . - - - - Unit Properties of matter The normal stress or pressure is σ n = F p n ∆ = ∆ The volume strain is ε v = D V Therefore, Bulk modulus is K= − σ ε n =− ∆ ∆ ( . ) The negative sign in the equation ( .
) means that when pressure is applied on the body, its volume decreases. Further, the equation ( . ) implies that a material can be easily compressed if it has a small