TESTING OF METALS: Tensile test, Compression Test, Testing of Impact Strength, Testing of fatigue, Testing of Creep

TESTING OF METALS

Metal testing is accomplished for the purpose
of for estimating the behavior of metal under loading (tensile, compressive,
shear, tortion and impact, cyclic loading etc.) of metal and for 
providing necessary
data for the product designers, equipment designers, tool and die designers
and system designers. The material behavior data under loading is used by
designers for design calculations and determining weather a metal can meet the
desired functional requirements of the designed product or part. Also, it is
very important that the material shall be tested so that their mechanical
properties especially their strength can be assessed and compared. Therefore
the test procedure for developing standard specification of materials has to be
evolved. This necessitates both destructive and non-destructive testing of
materials. Destructive tests of metal include various mechanical tests such as
tensile, compressive, hardness, impact, fatigue and creep testing. A standard
test specimen for tensile test is shown in Fig. 1 Non-destructive testing
includes visual examination, radiographic tests, ultrasound
test, liquid penetrating test and magnetic
particle testing.

Tensile test

A tensile test is carried out on standard
tensile test specimen in universal testing machine.
Fig. 2 shows a schematic set up of
universal testing machine reflecting the test specimen griped between two cross
heads. Fig. 
shows the
stress strain curve for  ductile material. Fig. shows
the properties of a ductile
material. Fig. 5
 shows the
stress strain curves for wrought
iron
and steels. Fig. 6 shows the stress
strain
curve for
non
ferrous material.

Tensile test specimen
Fig 1: Tensile test specimen



Schematic universal testing machine
Fig 2: Schematic universal testing machine



Stress strain curve for ductile material
Fig 3: Stress strain curve for ductile material



Properties of a  ductile  material
Fig 4: Properties of a 
ductile  material




Stress strain curves for wrought iron and steel
Fig 5: Stress strain curves for wrought iron and steel



Stress strain curves for non-ferrous material
Fig 6: Stress strain curves for non-ferrous material 

Compression Test

Compression test is reverse of tensile test.
This test can also be performed on a universal testing machine. In case of
compression test, the specimen is placed bottom crossheads. After that,
compressive load is applied on to the test specimen. This test is generally
performed for testing brittle material such as cast iron and ceramics etc. Fig.
7 shows the schematic compression test set up on a universal testing machine.
The following terms have been deduced using figures pertaining to tensile and
compressive tests of standard test specimen.


Schematic compression test set up on a universal testing machine
Fig 7: Schematic compression test set up on a universal
testing machine


Hook’s Law

Hook’s law states that when a material is
loaded within elastic limit (up to proportional limit), stress is proportional
to strain.

Strain

Strain is the ratio of change in dimension to the
original dimension.

Tensile Strain

The ratio of increase in length to the original length is
known as tensile strain.

Compressive
Strain

The ratio of decrease in length to the original length is
known as compressive strain.

Modulus of
Elasticity

The ratio of tensile stress to tensile strain
or compressive stress to compressive strain is called modulus of elasticity. It is denoted by E. It is also
called as
Young’s modulus of elasticity.
E = Tensile Stress/Tensile Strain

Modulus of
Rigidity

The ratio of sheer
stress to shear strain is called modulus of rigidity.
It is denoted by G.  G = Shear
Stress/Shear Strain

Bulk Modulus

The ratio of direct stress to the volumetric
strain (ratio of change in volume to the 
original volume is known as volumetric strain) is called Bulk modulus
(denoted by K).

K = Direct stress/volumetric strain

Linear and
Lateral Strain

When a body is subjected to tensile force
its length increases and the diameter
decreases. So when a test specimen of metal is stressed, one deformation
is in the direction of force which is called linear strain and other
deformation is perpendicular to the force called lateral strain.

Poisson’s Ratio

The ratio of lateral strain to linear strain
in metal is called poisson’s ratio. Its value is constant for a particular
material but varies for different materials.

Proof
Resilience

The maximum amount of energy which can be
stored in an elastic limit is known as proof
resilience.

Modulus of
Resilience

The proof resilience per unit volume of a material is
modulus of resilience or elastic toughness.

 Testing of
Hardness

It is a very important property of the metals
and has a wide variety of meanings. It embraces many different properties such as resistance to wear, scratching, deformation and machinability
etc. It also means the ability of a metal to cut another metal. The
hardness of a metal
may  be determined by the following tests. 

1. Brinell hardness test
2. Rockwell hardness test
3.Vickers hardness
(also called Diamond Pyramid) test
4. Shore
scleroscope

Rockwell hardness testing machine
Fig 8:Rockwell hardness
testing machine


Testing of Impact Strength

When metal is subjected to suddenly applied
load or stress, it may fail. In order to assess the capacity of metal to stand sudden impacts, the impact test is
employed. The impact test measures the energy necessary to fracture a standard
notched bar by an impulse load
and  as
such is an indication of the notch toughness of the material under shock loading.
Izod test and the Charpy test are commonly performed for determining impact
strength of materials. These methods employ same machine and yield a
quantitative value of the energy required
 
to fracture a special V notch shape metal. The most common kinds of
impact test use notched specimens loaded as beams. V notch is generally used
and it is get machined to standard specifications with a special milling cutter
on milling machine in machine shop. The beams may
be simply loaded (Charpy test) or loaded as cantilevers (Izod test). 

The function of the V notch in metal is to ensure that the
specimen will break as a result of the impact load to which it is subjected.
Without the notch, many alloys would
simply bend without breaking, and it would therefore be impossible to determine
their ability to
absorb energy.
It is therefore important to observe that the blow in Charpy
test is delivered at a point directly behind the notch and in the Izod test the blow is struck on the same
side of the notch towards the end of the
cantilever.
Fig. 9
 shows the impact testing set   up arrangement for charpy test. The
specimen is held in a rigid vice or support and is struck a blow by a traveling
pendulum that fractures or severely deforms the notched specimen. The
energy input in this case is a function of
the height of fall and the
weight of
the pendulum used in the test setup. The energy remaining after
fracture
is determined from the height of rise of the pendulum
due to inertia and its weight. The difference between the
energy input and the energy remaining represents the energy
absorbed by
the standard metal specimen. Advance testing setups of
carrying out such experiments are generally
equipped
with scales and
pendulum- actuated pointers, which provide direct readings
of energy absorption.

Schematic impact testing machine setup
Fig 9: Schematic impact testing machine setup


Testing of fatigue

Material subjected to static and cyclic
loading, yield strength is the main criterion for product design. However for
dynamic loading conditions, the fatigue strength or endurance limit of    a material is used in main criteria
used for designing of parts subjected to repeated alternating stresses over an extended
period of
time. Fig  10 shows a fatigue test set up
determining
  the fatigue strength of
material. The fatigue test determines the stresses which a sample of material
of standard dimensions can safely endure
for a given number of cycles. It is performed on a test specimen of standard
metal having a round cross-section, loaded at two points as
 a rotating simple beam, and supported at
its ends. The upper surface of such a standard test specimen is always in
compression and the lower surface is always in tension. The maximum stress in
metal always occurs at the surface, halfway along the length of the standard
test specimen, where the cross section is minimum. For every full rotation of
the specimen, a
point in the surface originally at the top centre goes alternately from a maximum
in compression to a maximum in tension and then back to the same
maximum in compression. Standard test specimens are tested to failure using
different loads, and the number of cycles before failure is noted for each load.
The results of such tests are recorded on graphs of applied
stress against the logarithm of the number
of cycles to failure. The curve is known as S-N curve. 


Schematic fatigue test setup
Fig 10:Schematic fatigue
test setup


Testing of Creep

Metal part when is subjected to a high constant stress at high temperature for a longer period of time, it will undergo a slow and permanent deformation (in form of a crack which may further propagate further towards creep failure) called creep. Creep is time dependent phenomena of metal failure at high constant stress and at high temperature such subjecting of at steam turbine blade. A schematic creep testing setup is shown in Fig. 11 Test is carried out up to the failure of the test specimen. A creep curve for high temperature and long time creep is shown in Fig. 12. The curve shows different portions of the primary secondary and tertiary creep which ends at fracture in metals.
Schematic creep testing setup

Fig 11:Schematic
creep testing setup

 



Creep curve for a high temperature and long time creep test
Fig 12:Creep curve for a high temperature and long time
creep test


CHOICE OF MATERIALS FOR THE ENGINEERING APPLICATIONS

The choice of materials for the engineering purposes
depends upon the following factors:
1. Availability of
the materials,
2. Properties needed for meeting
the functional requirements,
3. Suitability of the materials
for the working
conditions in service
4. The cost of the materials.

 

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