Engineering Mechanics or Solid
Mechanics (also known as Strength of Materials in some institutions) is one of
the most basic subject of Engineering which deals with the engineering
properties of materials like Modulus of Elasticity, Stresses, Strain etc.
In this article we will discuss
information regarding Stresses (meaning, types, need) and Strain (types). First
of all let us recall the Newton’s Third Law which states that “To every action
there is equal and opposite reaction”. Based on the same law, Stress is induced
in the materials. Whenever any force is applied on the material, it tries to
deform the material but due to internal resistance of the material an equal
force is induced in the opposite direction of the applied force. This induced
force is the result of internal resistance of material to resist the
deformation. Just like pressure is the force applied on any material per unit
area, stress is the internal resistive force induced per unit area as the
result of applied force. Now each material has certain strength i.e. it can
take up load upto particular limit before failure. Upto that limit the material
can yield or in other words its dimensions can change due to application of
load. Strain is the ratio of change in the dimension due to applied force to
the original dimension of the body and that change in dimension can be
laterally or longitudinally.
There are different types of
forces that act on any body like Tensile force, Compressive force, Shear force,
Bending force and Torsion.
Stress depends upon the applied force as
already discussed and depending upon the type of force applied different types
of stresses are induced as shown in the figure below.
Since stress is
induced due to reaction and pressure is applied due to action, therefore stress
and pressure are numerically equal only direction is different. Strain is
another term related to body under stress. Whenever force is applied on any
body the body deforms both longitudinally and laterally as shown in figure
below. The ratio of Lateral Strain to Longitudinal Strain is termed as Poisson’s Ratio
denoted by ‘μ’.
Longitudinal Strain = 2 δL/L ; Lateral Strain = (D-d)/D
The ratio of Direct Stress to Longitudinal
Strain is known as Young’s Modulus of Elasticity denoted by ‘E’.
The ratio of Shear Stress to Shear Strain is
known as Modulus of Rigidity denoted by ‘G’ or ‘C’.
The ratio of Direct Stress to Volumetric Strain
is known as Bulk Modulus denoted by ‘K’.
There is a relationship among E, G, K and μ
as mentioned below
E = 3K (1-2
μ) and E = 2G (1+ μ)
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