STRESS ANS STRAIN

STRESS (N/m2) 

it is the internal  resistance offered by the body to deformation 

• Nominal stress (engineering stress) = 𝐿𝑜𝑎𝑑/𝑂𝑟𝑖𝑔𝑖𝑛𝑎𝑙 𝐴𝑟𝑒𝑎

• Actual/true stress = 𝐿𝑜𝑎𝑑/𝐶ℎ𝑎𝑛𝑔𝑒𝑑 (𝑎𝑐𝑡𝑢𝑎𝑙)𝐴𝑟𝑒𝑎

STRAIN

Deformation per unit length in the direction of deformation is known as strain.

Strain = ∆𝐿/𝐿

 

It is a dimensionless Quantity.

∆L -change in length

L - original length

ENGINEERING STRESS-STRAIN CURVE OF MILD STEEL FOR TENSION UNDER STATIC – LOADING

C’ – Upper yield point

D – Strain hardening starts

E – Ultimate point or maximum stress point

OA – straight line (proportional region, Hooke’s law is valid)

OB – Elastic region

BC – Elastic plastic region

CD – Perfectly plastic region

DE – strain hardening

EF – Necking region

A – Limit of proportionality

B – Elastic limit

C – Lower yield point

F – Fracture point

• Limit of proportionality 

It is the stress at which the stress – strain curve ceases to be a straight line.

Hooke’s law is valid up to proportional limit.

• Elastic limit

It is the point on the stress – strain curve up to which the materials remains elastic.

Upto this point there is no permanent deformation after removal of load.

• Plastic Range

It is the region of the stress – strain curve between the elastic limit and point of 

rupture.

• Yield point

This point is just beyond the elastic limit, at which the specimen undergoes an 

appreciable increase in length without further increase in the load.

• Rupture strength

It is the stress corresponding to the failure point ‘F’ of the stress-strain curve.

• Proof stress

It is the stress necessary to cause a permanent extension equal to defined percentage 

of gauge length.

Slope of OA = Modulus of elasticity

(Young’s Modulus).

It is constant of proportionality which is defined as the intencity of stress that causes 

unit strain.

Plastic strain is 10 to 15 time’s elastic strain.

Fracture strain (Ef) depends on percentage carbon in steel.

When carbon percentage increases then fracture strain decreases and yield stress 

increases