Simple Stresses and Strains
• 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.
• 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
• SI unit for stress: N/m2
• Also designated as a pascal (Pa) Pa = N/m2
• gigapascal, 1GPa = 1×109 N/m2 = 1×103 MPa=
• 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
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)
• 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
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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