# Stress and strain

Stress

• No engineering material is perfectly rigid and

hence, when a material is subjected to external

load, it undergoes deformation.

• While undergoing deformation, the particles of

the material offer a resisting force (internal

force). When this resisting force equals applied

load the equilibrium condition exists and hence

the deformation stops.

• These internal forces maintain the externally

applied forces in equilibrium.

Contd…

• Stress = internal resisting force / resisting

cross sectional area = R/ A

• The internal force resisting the deformation

per unit area is called as stress or intensity of

stress.

• SI unit for stress: N/m2

• Also designated as a pascal (Pa) Pa = N/m2

Contd…

• gigapascal, 1GPa = 1×109 N/m2 = 1×103 MPa=

1×103 N/mm2

• kilopascal, 1kPa = 1000 N/m2

• megapascal, 1 MPa= 1×106 N/m2 =

1×106N/(106 mm2) = 1 N/mm2

• 1 MPa= 1 N/mm2

Direct or Normal Stress: Intensity of resisting force

perpendicular to or normal to the section is called the

normal stress. Normal stress may be tensile or

compressive.

Tensile stress: stresses that cause pulling on the surface of the section, (particles of the materials tend to pull apart

causing extension in the direction of force)

Compressive stress: stresses that cause pushing on the

surface of the section, (particles of the materials tend to

push together causing shortening in the direction of force)

Strain

• If a bar is subjected to a direct load, and hence

a stress, the bar will changes in length. If the bar

has an original length L and change in length by

an amount δL, the linear strain produced is

defined as,

Linear strain,

ε=Original length (L) / Change in length (δL )

• Strain is a dimensionless quantity.

Stress- Strain curve for mild steel

• Elastic limit : It is the stress beyond which the material will not return to its original shape when unloaded but will retain a permanent deformation called permanent...

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