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JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY
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LECTURE NOTES
ON
MECHANICS OF SOLIDS
II B. Tech I semester (JNTUH-R13)
1

UNIT – 1: SIMPLE STRESSES AND STRAINS
INTRODUCTION AND REVIEW
Preamble
Engineering science is usually subdivided into number of topics such as
1. Solid Mechanics
2. Fluid Mechanics
3. Heat Transfer
4. Properties of materials and soon Although there are close links between them in terms of the
physical principles involved and methods of analysis employed.
The solid mechanics as a subject may be defined as a branch of applied mechanics that deals with
behaviours of solid bodies subjected to various types of loadings. This is usually subdivided into
further two streams i.e Mechanics of rigid bodies or simply Mechanics and Mechanics of deformable
solids.
The mechanics of deformable solids which is branch of applied mechanics is known by several names
i.e. strength of materials, mechanics of materials etc.
Mechanics of rigid bodies:
The mechanics of rigid bodies is primarily concerned with the static and dynamic behaviour under
external forces of engineering components and systems which are treated as infinitely strong and
undeformable Primarily we deal here with the forces and motions associated with particles and rigid
bodies.
Mechanics of deformable solids :
Mechanics of solids:
The mechanics of deformable solids is more concerned with the internal forces and associated
changes in the geometry of the components involved. Of particular importance are the properties of
the materials used, the strength of which will determine whether the components fail by breaking in
service, and the stiffness of which will determine whether the amount of deformation they suffer is
acceptable. Therefore, the subject of mechanics of materials or strength of materials is central to the
whole activity of engineering design. Usually the objectives in analysis here will be the determination
of the stresses, strains, and deflections produced by loads. Theoretical analyses and experimental
results have an equal roles in this field.
Analysis of stress and strain :
Concept of stress : Let us introduce the concept of stress as we know that the main problem of
engineering mechanics of material is the investigation of the internal resistance of the body, i.e. the
nature of forces set up within a body to balance the effect of the externally applied forces.
The externally applied forces are termed as loads. These externally applied forces may be due to any
one of the reason.
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(i) due to service conditions
(ii) due to environment in which the component works
(iii) through contact with other members
(iv) due to fluid pressures
(v) due to gravity or inertia forces.
As we know that in mechanics of deformable solids, externally applied forces acts on a body and
body suffers a deformation. From equilibrium point of view, this action should be opposed or reacted
by internal forces which are set up within the particles of material due to cohesion.
These internal forces give rise to a concept of stress. Therefore, let us define a stress Therefore, let us
define a term stress
Stress:
Let us consider a rectangular bar of some cross – sectional area and subjected to some load or force
(in Newtons )
Let us imagine that the same rectangular bar is assumed to be cut into two halves at section XX. The
each portion of this rectangular bar is in equilibrium under the action of load P and the internal forces
acting at the section XX has been shown
Now stress is defined as the force intensity or force per unit area. Here we use a symbol
represent the stress.
Where A is the area of the X – section
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to

Here we are using an assumption that the total force or total load carried by the rectangular bar is
uniformly distributed over its cross – section.
But the stress distributions may be for from uniform, with local regions of high stress known as stress
concentrations.
If the force carried by a component is not uniformly distributed over its cross – sectional area, A, we
must consider a small area, ‘ A' which carries a small load P, of the total force ‘P', Then definition
of stress is
As a particular stress generally holds true only at a point, therefore it is defined mathematically as
Units :
The basic units of stress in S.I units i.e. (International system) are N / m2 (or Pa)
MPa = 106 Pa
GPa = 109 Pa
KPa = 103 Pa
Some times N / mm2 units are also used, because this is an equivalent to MPa. While US customary
unit is pound per square inch psi.
TYPES OF STRESSES :
only two basic stresses exists : (1) normal stress and (2) shear shear stress. Other stresses either are
similar to these basic stresses or are a combination of these e.g. bending stress is a combination
tensile, compressive and shear stresses. Torsional stress, as encountered in twisting of a shaft is a
shearing stress.
Let us define the normal stresses and shear stresses in the following sections.
Normal stresses : We have defined stress as force per unit area. If the stresses are normal to the areas
concerned, then these are termed as normal stresses. The normal stresses are generally denoted by a
Greek letter ( )
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