Only two basic stresses exists : (1) normal stress and (2) shear shear stress.

**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 ( s )

This is also known as uniaxial
state of stress, because the stresses acts only in one direction however, such
a state rarely exists, therefore we have biaxial and triaxial state of stresses
where either the two mutually perpendicular normal stresses acts or three mutually
perpendicular normal stresses acts as shown in the figures below :

**Tensile or compressive stresses :**

The normal stresses can be either
tensile or compressive whether the stresses acts out of the area or into the
area

**Bearing Stress : **When one
object presses against another, it is referred to a bearing stress (** **They
are in fact the compressive stresses ).

**Shear stresses :**

Let us consider now the
situation, where the cross - sectional area of a block
of material is subject to a distribution of forces which are parallel, rather
than normal, to the area concerned. Such forces are associated with a shearing
of the material, and are referred to as shear forces. The resulting force
interistes are known as shear stresses.

The resulting force intensities
are known as shear stresses, the mean shear stress being equal to

Where P is the total force and A the area over which it acts.

Stress is defined as the force per unit area. Thus, the
formula for calculating stress is:

s= F/A

Where s denotes stress, F is load
and A is the cross sectional area. The most commonly used units for stress are
the SI units, or Pascals (or N/m^{2}), although other units like psi
(pounds per square inch) are sometimes used.

Forces may be applied in different directions such as:

Tensile
or stretching

•
Compressive or squashing/crushing

•
Shear or tearing/cutting

•
Torsional or twisting

This gives rise to numerous
corresponding types of stresses and hence measure/quoted strengths. While data
sheets often quote values for strength (e.g compressive strength), these values
are purely uniaxial, and it should be noted that in real life several different
stresses may be acting.

**Tensile Strength**

The tensile strength is defined
as the maximum tensile load a body can withstand before failure divided by its
cross sectional area. This property is also sometimes referred to Ultimate
Tensile Stress or UTS.

Typically, ceramics perform
poorly in tension, while metals are quite good. Fibres such as glass, Kevlar
and carbon fibre are often added polymeric materials in the direction of the
tensile force to reinforce or improve their tensile strength.

**Compressive Strength**

Compressive strength is defined as
the maximum compressive load a body can bear prior to failure, divided by its
cross sectional area.

Ceramics typically have good tensile strengths and are used
under compression e.g. concrete.

**Shear Strength**

Shear strength is the maximum
shear load a body can withstand before failure occurs divided by its cross
sectional area.

This property is relevant to
adhesives and fasteners as well as in operations like the guillotining of sheet
metals.

**Torsional Strength**

Torsional strength is the maximum
amount of torsional stress a body can withstand before it fails, divided by its
cross sectional area.

This property is relevant for components such as shafts.

**Yield Strength**

Yield
strength is defined as the stress at which a material changes from elastic deformation
to plastic deformation. Once the this point, known as the yield point is
exceeded, the materials will no longer return to its original dimensions after
the removal of the stress.

Stress is
defined as the force per unit area. Thus, the formula for calculating stress
is:

Where s denotes stress, F is load and A is the cross sectional
area. The most commonly used units for stress are the SI units, or Pascals (or
N/m^{2}), although other units like psi (pounds per square inch) are
sometimes used.

Forces
may be applied in different directions such as:

•
Tensile or stretching

•
Compressive or squashing/crushing

•
Shear or tearing/cutting

•
Torsional or twisting

This
gives rise to numerous corresponding types of stresses and hence measure/quoted
strengths. While data sheets often quote values for strength (e.g compressive
strength), these values are purely uniaxial, and it should be noted that in
real life several different stresses may be acting.

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