Testing of Materials

1. OML751 TESTING OF MATERIALS
UNIT II – MECHANICAL TESTING
Mr. S. Muthu Natarajan M.E., (Ph.D.),
Assistant Professor/Mechanical
Kamaraj College of Engineering and Technology (Autonomous)
Madurai District
Mobile: +91-9566470389

2. Introduction to mechanical testing, Hardness test
(Vickers, Brinell, Rockwell), Tensile test, Impact
test (Izod, Charpy) - Principles, Techniques,
Methods, Advantages and Limitations,
Applications. Bend test, Shear test, Creep and
Fatigue test - Principles, Techniques, Methods,
Advantages and Limitations, Applications.
Syllabus

3.  Mechanical testing is a process that is used to determine the
mechanical properties of a material.
 It can be used to evaluate a material independent of its geometry and
also at defined geometrical conditions. There are a wide variety of
mechanical testing methods that can be performed when evaluating
a material
 Mechanical testing is used to determine the specific mechanical
properties of a material in a given environment. For example, if the
mechanical property of tensile strength is unknown for a material,
then a tensile test may be performed. The toughness of a material
can be quantified by performing a Charpy V-notch test.
Mechanical Testing

4.  The mechanical property of hardness can be determined by
performing a Vickers hardness test. A mechanical testing method
known as high cycle fatigue testing can be used to determine how
many load cycles a material can withstand prior to failure.
 Some mechanical tests provide information on more than just one
mechanical property. For instance, a tensile test not only provides
information about a material's ultimate tensile strength, but it also
provides quantitative data about its modulus of elasticity and its
yield strength. A tensile test can also provide insight as to how
ductile or brittle a material is based on the stress-strain curve
produced by the test and how the material fractures during the
mechanical testing.
Mechanical Testing (Contd…)

5.  There is also often more than one form of mechanical test that can
be used to evaluate a given mechanical property. An example of this
is the methods for hardness evaluation. Vickers, Brinell and
Rockwell hardness tests can all be used to determine the hardness of
a material. With toughness, a Charpy V-notch test or an Izod test
may be used to quantify how tough a material is.
 Because the environment is important, the mechanical tests should
be performed under conditions similar to those in which the
component or structure will be placed.
Mechanical Testing (Contd…)

6.  A hardness test is a method employed to measure the hardness of a
material. Hardness refers to a material’s resistance to permanent
indentation.
 There are numerous techniques to measure hardness and each of
these tests can identify varying hardness values for a single material
under testing. Hence, hardness test as a method can be dependent
and each test's outcome needs to be labeled to determine the kind of
hardness test used.
 Definition: Hardness may be defined as the ability of a material to
resist scratching, abrasion, cutting and penetration.
Hardness Test

7.  Hardness tests are extensively used to characterize a certain material
and to identify if it is appropriate for its intended purpose. All
hardness tests involve the utilization of a particularly shaped
indenter that is harder than the material under testing. The indenter
is pressed onto the test surface with the use of a certain amount of
force. The size of the depth of the indent is measured in order to
determine the hardness value.
 The major applications of hardness tests are to verify the type of
heat treatment to be used on a part and to identify if a material
possesses the required properties for its intended use. This makes
hardness tests beneficial in industrial applications.
Hardness Test (Contd…)

8. Hardness tests are beneficial because:
 The hardness test is easy to conduct.
 Results can be obtained within 30 seconds.
 Tests are relatively cost effective.
 Finished components can be subjected to testing without being
damaged.
 Any shape and surface size can be subjected to testing.
The five most common hardness scales are:
 Knoop
 Vickers
 Rockwell
 Brinell
 Shore
Hardness Test (Contd…)

9.  The hardness is measured from an indentation produced in the
component by applying a constant load on a specific indenter in
contact with the surface of the component for a fixed time.
 An indenter is pressed into the surface of the material by a slowly
applied known load, and the extent of the resulting impression is
measured mechanically or optically.
 A large impression for a given load and indenter indicates a soft
material, and a small impression indicates a hard material.
 Rockwell hardness test is probably the most widely used methods
of hardness testing.
Rockwell Hardness Test

10.  The principle of Rockwell test differs that of the others in that the
depth of the impression is related to the hardness rather than the
diameter or diagonal of the impression as shown in fig.
Rockwell Hardness Test (Contd...)

11. Rockwell Hardness Test (Contd...)

12.  The material to be tested is held on the anvil of the machine
 The test piece is raised by turning the hand wheel, till it just touches
the indenter
 A minor load of 10kg is applied to seat the specimen. Then the dial
indicator is set at zero.
 Now the major load (100 kg for B-scale or 150 kg for C-scale) is
applied to the indenter to produce a deeper indentation.
 After the indicating pointer has come to rest, the major load is
removed.
 With the major load removed, the pointer now indicates the
Rockwell hardness number on the appropriate scale of the dial.
Rockwell Hardness Test (Contd...)

13. Rockwell Hardness Test (Contd...)
Working Video

14. Advantages
 Very simple to use
 Hardness can be read directly in a single step
 Each measurement requires only a few seconds
 Zit is suitable for routine tests of hardness in mass production
 It can be used on metallic materials as well as on plastics.
Disadvantages
 Not accurate as the Vickers test
 That’s why the Vickers test is usually preferred for research and
development works.
Rockwell Hardness Test (Contd...)

15.  One of the earlier standardized methods of measuring hardness was
the Brinell test.
 In Brinell test, a hardened steel ball indenter is forced into the
surface of the metal to be tested.
 The diameter of the hardened steel (or tungsten carbide) indenter is
10mm.
 Standard loads range between 500kg and 3000kg in 500kg
increments.
 During a test, the load is maintained constant for 10 to 15 seconds.
 The diameter of the resulting impression is measured with the help
of a calibrated microscope.
Brinell Hardness Test

16. Brinell Hardness Test (Contd...)
Brinell Hardness Number:
 The measured diameter is converted into the equivalent Brinell
hardness number (BHN) using the simple relation,
 𝑩𝑯𝑵 =
𝑳𝒐𝒂𝒅 𝒐𝒏 𝒕𝒉𝒆 𝒃𝒂𝒍𝒍
𝑨𝒓𝒆𝒂 𝒐𝒇 𝒊𝒏𝒕𝒆𝒏𝒅𝒂𝒕𝒊𝒐𝒏 𝒐𝒇 𝒕𝒉𝒆 𝒔𝒕𝒆𝒆𝒍 𝒃𝒂𝒍𝒍
 𝑩𝑯𝑵 =
𝑷
𝝅𝑫
𝟐
𝑫− 𝑫𝟐−𝑫𝒊
𝟐
 P – Load applied on the intender in kg
 D – Diameter of the steel ball intender in mm
 Di – Diameter of the ball impression in mm
 If the BHN is high then the material is said to be harder, and if it is
less then the material is said to be softer

17. Brinell Hardness Test (Contd...)
 Test method consists of indenting the test material with a 10 mm
diameter hardened steel or carbide ball subjected to a load of 3000 kg.
 For softer materials the load can be reduced to 1500 kg or 500 kg to
avoid excessive indentation.

18. Brinell Hardness Test (Contd...)
Working Video

19. Brinell Hardness Test (Contd...)
Load
application
Diameter
of the ball
Duration Metals
3000 kg 10 mm 10
Seconds
Iron, steel and alloys
having hardness similar
to steel
750 kg 5 mm
500 kg 10 mm 30 seconds
Copper, Annealed
brass and magnesium
alloys etc.,
1000 kg 10 mm 15 seconds
Gun metal/ Bronze and
cold worked brass etc.,
Limitations:
 It cannot be used on very hard or very soft materials
 The test may not be valid for thin specimens
 The test is not be valid for case-hardened surfaces.

20.  The Vickers hardness test is similar to Brinell test, with a square-
based diamond pyramid being used as the indenter.
 As in the Brinell test, the indenter is forced into the surface of the
material under the action of a static load for 10 to 15 seconds.
 The standard indenter is a square pyramid shape with an angle of
136o between opposite faces. This angle was chosen because it
approximates the most desirable ratio of indentation diameter to
ball diameter in the Brinell hardness test.
 Because of shape of the indenter, this test is frequently called the
diamond-pyramid hardness test.
Vickers Hardness Test

21.  An advantage of the Vickers test over the Brinell test is that the
accuracy is increased in determining the diagonal of a square as
opposed to the diameter of a circle, as show in figure.
 The two diagonals of the indentation left in the surface of the
material after removal of the load are measured using a microscope
and their average calculated.
 The area of the sloping surface of the indentation is calculated.
Vickers Hardness Test (Contd…)

22. Vickers Hardness Test (Contd...)
Vickers Hardness Number:
 The measured diagonal is converted into the equivalent vickers
hardness number (VHN) using the simple relation,
 𝑽𝑯𝑵 =
𝑨𝒑𝒑𝒍𝒊𝒆𝒅 𝒍𝒐𝒂𝒅
𝑺𝒖𝒓𝒇𝒂𝒄𝒆 𝒂𝒓𝒆𝒂 𝒐𝒇 𝒊𝒎𝒑𝒓𝒆𝒔𝒔𝒊𝒐𝒏
 𝑽𝑯𝑵 =
𝟐𝑷 𝑺𝒊𝒏
θ
𝟐
𝑫𝟐 =
𝟏.𝟖𝟓𝟒𝟒 𝑷
𝑫𝟐
 P – Load applied on the intender in kg
 θ – Angle between the opposite faces of diamond intender
 (𝐚𝐬 𝛉 = 𝟏𝟑𝟔 𝐝𝐞𝐠𝐫𝐞𝐞𝐬)
 D – Mean diagonal length in mm

23. Advantages
 The diagonals of the square indentation can be measured more
accurately than the diameters of the circles.
 This method is suitable for hard materials as well as for soft
materials
 The Vickers indenter is capable of giving geometrically similar
impression with different loads. Thus, the hardness number is
independent of the load applied.
Disadvantages
 The impression is very small and also it requires careful surface
preparation of the specimen
 It takes a relatively long time to perform a Vickers hardness test.
Vickers Hardness Test (Contd...)

24.  Tensile test is a measurement of the ability of a material to
withstand forces that tend to pull it apart and to determine to what
extent the material stretches before breaking.
 Tensile modulus, an indication of the relative stiffness of a material,
can be determined from a stress-strain diagram.
 The tension test is the most common method for determining the
mechanical properties of materials, such as strength, ductility,
toughness, elastic modulus, and strain-hardening capability.
 This is the measure of conventional stress that can be sustained by
the metal for any deforming forces.
 It ranges from zero load (zero deformation) to ultimate strength in
tension which corresponds to the maximum load in a tension test.
Tensile Test

25.  It is usually measured by the highest point on the conventional
stress-strain curve. In engineering tension tests this strength
provides the basic design information on the materials.
 The tensile strength of a material is the maximum amount of tensile
stress that it can be subjected to before failure.
Tensile Test (Contd…)

26. There are three typical definitions of tensile strength.
Yield strength
 The stress atwhich material strain changes from elastic deformation
to plastic deformation, causing it to permanently is known as yield
strength.
Ultimate strength
 The maximum stress a material can withstand is known as ultimate
strength.
Breaking strength
 The strength co-ordinate on the stress-strain curve at the point of
rupture is known as breaking strength.
Tensile Test (Contd…)

27. A tensile test of a material is performed on ductile materials to
determine tensile properties such as :
 Limit of proportionality
 Yield point or yield strength
 Maximum tensile strength
 Breaking strength
 Percentage elongation
 Percentage reduction in area
 Modulus of elasticity
The tensile test is usually carried out with the help of a “Universal
Testing Machine” (UTM)
Tensile Test (Contd…)

28. Tensile Test (Contd...)
Working Video

29. Tensile Test (Contd...)

30.  The specimen to be tested is fastened to the two end-jaws of the
UTM.
 Now the load is applied gradually on the specimen by means of the
 movable cross head, till the specimen fractures.
 During the test, the magnitude of the load is measured by the load
measuring unit (load cell).
 A strain gauge or extensometer is used to measure the elongation of
the specimen between the gauge marks when the load is applied.
Tensile Test (Contd…)

31. Tensile Test (Contd…)

32. Tensile Test (Contd…)

33.  The impact test is performed to study the behavior of materials
under dynamic load i.e., suddenly applied load.
 Impact strength defined : The capacity of a metal to withstand blows
without fracture, is known as impact strength or impact resistance.
 The impact test indicates the toughness of the material i.e., the
amount of energy absorbed by the material during plastic
deformation.
 The impact test also indicates the notch sensitivity of a material. The
notch sensitivity refers to the tendency of some normal ductile
materials to behave like a brittle materials in the presence of
notches.
Impact Test

34.  In an impact test, a notch is cut in a standard test piece which is
stuck by a single blow in a impact testing machine. Then the energy
absorbed in breaking the specimen can be measured from the scale
provided on the impact testing machine.
 Impact strength is the resistance of a material to fracture under
sudden load.
 It is a complex characteristic which takes into account both the
toughness and strength of a material.
 It is defined as the specific work required to fracture a test
specimen with a stress concentrator in the mid when broken by a
single blow of striker in pendulum type impact testing machine.
Impact Test (Contd…)

35. Types of Impact Test
Based on the types of specimen used on impact testing machine the
impact tests can be classified into:
 Izod test
 Charpy test
 It can be noted that the impact testing machines are designated so
that both types of test can be performed on the same machine with
only minor adjustments.
Impact Test (Contd…)

36. Izod Test
 Izod test used a cantilever specimen of size 75mm x 10mm x 10mm
as shown in fig.
 The V-notch angle is 45o and the depth of the notch is 2mm.
 The Izod specimen is placed in the vise such that it is a cantilever as
shown in fig.
Impact Test (Contd…)

37. Charpy Test
 The charpy test uses a test specimen of size 55x10x10mm as shown
in fig.
 The V-notch angle is 45o and the depth of the notch is 2mm.
 The Charpy specimen is placed in the vise as a simply supported
 beam as shown in fig.
Impact Test (Contd…)

38. Impact Test (Contd...)
Working Video

39.  The specimen is placed in the vice of the anvil
 The pendulum hammer is raised to known standard height
depending on the type of specimen to be tested
 When the pendulum is released, its potential energy is converted
into kinetic energy just before it strikes the specimen
 Now the pendulum strikes the specimen. It may be noted that the
Izod specimen is hit above the V-notch and the Charpy specimen
will be hit behind the V-notch.
 The pendulum, after rupturing the specimen, rises on the other side
of the machine.
Impact Test (Contd…)

40.  The energy absorbed by the specimen during breaking is the weight
of the pendulum times the difference in two heights of pendulum on
either side of the machine.
 Now the energy i.e., the notched impact strength, in foot-pounds or
meter-kg,
 is measured from the scale of the impact testing machine.
 The pointer is set up to its maximum value (300 J)
Impact Test (Contd…)

41.  Fatigue is the phenomena of material failure for repeated pulsating
or reversing loads (or) The capacity of material to withstand
repeatedly applied stresses is known as fatigue.
 Fatigue or endurance limit is defined as the maximum stress which
a specimen can endure without failure when the stress is repeated
for a specified number of cycles/times.
 The nominal stress values that cause fatigue failure are less than the
ultimate tensile stress limit, and may be above the yield stress limit
of the material.
 Fatigue strength of a metal is usually tested using a rotating
beam machine.
Fatigue Test

42. The stress-life method: A mechanical part is often exposed to a
complex, often random of sequence of loads values of large and small
range. Types of stress life method
Rainflow analysis, Fatigue damage spectrum, S-N curve,
Miner’s rule.
The strain-life method: When strains are no longer elastic, such as in
the presence of stress concentrations, the total strain can be used
instead of stress as a similitude parameters. This is known as the strain-
life method.
The crack growth method, Probabilistic methods
Fatigue Test (Contd…)

43. Fatigue specimen is gripped on to a motor at one end to provide the
rotational motion whereas the other end is attached to a bearing and
also subjected to a load or stress.
Fatigue Test (Contd…)

44. Fatigue Test (Contd…)

45.  As the specimen rotates, there is sinusoidal variation of stress
between a state of maximum tensile stress and a state of maximum
compressive stress.
 The cycles of stress are applied until the specimen fractures. A
reduction counter records this number of stress cycles.
 Now a member of specimen of the same material (at lease 6
specimens) are tested in the same manner under different stress
levels and the results are plotted on S-N graph.
 The S-N graph is drawn on a semi-logarithmic scale with the
number of cycles (N) required to cause failure of the specimen on
the X-axis and the stress (S) on the Y-axis. The resulting curve in a
S-N graph is called as S-N curve.
Fatigue Test (Contd…)

46.  The fatigue test can tell us show long a part many survive or the
maximum allowable loads that can be applied to prevent failure.
 The fatigue test is useful in setting the design criterion with the use
of the endurance limit.
 Endurance limit is stress below which there is a 50% probability
that failure by fatigue will never occurs, which is the common
design criterion.
 Fatigue life tells us how long a component survives at a particular
stress.
Fatigue Test (Contd…)

47.  From the S-N curve, one can find the fatigue life (n) for the applied
stress. Endurance ratio = Endurance limit / Tensile strength ≈ 0.5.
 The endurance or fatigue ratio allows us to estimate fatigue
properties from the tensile test
Stages in Fatigue Failure
Stage-1 Crack initiation
Stage-2 Crack propagation
Stage-3 Sudden fracture
Fatigue Test (Contd…)

48.  Creep is the tendency of a material to deform permanently under
the influence of constant long term stresses, that are essentially
below the yield strength of the material.
 The rate of this deformation is a function of the material properties,
exposure time, exposure temperature and the applied structural
load.
 Creep is a deformation mechanism that may or may not constitute a
failure mode. Moderate creep in concrete is sometimes welcomed
because it relieves tensile stresses that might otherwise lead to
cracking.
 The continuous deformation of a metal under a steady load is known
as creep.
Creep

49.  The purpose of creep tests is to determine the creep limit. The creep
limit or the limiting creep stress is defined as the stress that will not
break the specimen when applied for an infinite period at a specific
constant temperature.
 The creep tests require the measurement of four variables stress,
strain, temperature and time.
 The creep tests are simply tension tests run at constant load and
constant temperature. Then the value of strain of the test piece is
noted as a function of time.
 The specimen for creep testing are usually the same as for
conventional tensile tests.
Creep (Contd…)

50. Objective of Creep test is to find the maximum stress that may be
applied for a long period of time at a given temperature.
Creep (Contd…)

51.  The specimen to be tested is placed in an electric furnace. At the
electric furnace, the specimen is heated to the given temperature
under the constant load.
 The strain in the specimen is measured by a strain gauge or an
optical extensometer as a function of time.
 The above test is repeated for 4 to 5 specimens at each temperature
under different loads.
 Now the creep curve i.e., the elongation versus time curves are
plotted for each specimen, as shown in fig.
Creep (Contd…)

52. Creep (Contd…)

53.  Primary Creep: The initial creep stage where the slop is rising
rapidly at first in a short amount of time. After a certain amount of
times has elapsed, the slope will begin to slowly decrease from its
initial rise.
 Secondary Creep: After the primary creep, the creep rate reaches
essentially a steady state, in which the creep rate changes little with
time. This region of approximately constant creep rate.
 During this stage, the steady state is achieved because of an
approximate balance between two opposing factors: the strain
hardening that tends to reduce the creep rate and the softening or
recovery process that tends to increase it.
Creep (Contd…)

54.  Tertiary Creep: The last stage of creep when the object that is being
subjected to pressure is going to reach its breaking point.
 In this stage, the objects creep continuously increases until the
object breaks. The slope of this stage is very steep for most
materials. During this stage, high stresses or/and at high
temperatures.
 The creep rate is greater and increases continuously till the material
undergoes fracture.
 Tertiary creep occurs when the effective cross-sectional area of the
specimen is reduced remarkably either due to localized necking or
internal void formation.
Creep (Contd…)

55.  In shear test, the shear force is the load that causes two contiguous
parts of the body to slide relative to each other in a direction parallel
to their plane of contact.
 Principle: Shear strength measures a materials ability to resist
forces that cause the materials to slide against it. The specimen is
loaded in shear fixtures, load in applied perpendicular to specimen
through plunger.
 The phenomenon of shear applies through the shear fixtures
(coupling device) is known as shear test.
Types of shear test:
 Single shear test,
 Double shear test
Shear Test

56. Shear Fixtures
 Two coupling braces which is used for both single or double shear
connection. Both are at similar position with certain distance apart.
 For single shear, specimen is routed to single brace and for double
shear, specimen is routed fully.
Shear Test (Contd…)

57.  In the double-shear method, the specimen is sheared off at two
cross sections. In the single-shear process, the specimen only shears
away at one cross section.
 Calculating the shear strength in the two processes differs in the
cross sectional area to be applied.
 The shear strength determined in the shear test is important in the
design of bolts, rivets and pins as well as for calculating the force
required for shears and presses
 The diameter is measured using the Vernier caliper
 Mount the shear fixtures on UTM and load the specimen in shear
fixture accordance to need of shear test. Operate (push) buttons for
driving the motor to drive the pump.
Shear Test (Contd…)

58. Shear Test (Contd…)

59.  Bending tests is standard test method for material of smooth bars
like flat metal spring, concrete, natural stone, wood, plastic, glass
and ceramics.
 It also called as flexural test
 Principle: Bend tests are conducted by placing a length of material
across a span and pushing down along the span to bend the material
causing a concave surfaces or a bend to form without the
occurrence of fracture and are typically performed to determine the
ductility or resistance to fracture of that material, the elastic
modulus of bending, flexural stress, and flexural strain of a
material.
Bend Test

60.  Bending tests is standard test method for material of smooth bars
like flat metal spring, concrete, natural stone, wood, plastic, glass
and ceramics.
 It also called as flexural test
 Principle: Bend tests are conducted by placing a length of material
across a span and pushing down along the span to bend the material
causing a concave surfaces or a bend to form without the
occurrence of fracture and are typically performed to determine the
ductility or resistance to fracture of that material, the elastic
modulus of bending, flexural stress, and flexural strain of a
material.
Bend Test (Contd…)

61. Bend Test (Contd…)

62. Method of Bend test Based on LOAD position
 Single point loading at the free end of a cantilever beam
 Centre point loading (or) Three point bending test
 Four point bending test
Bend Test (Contd…)

63. Method of Bend test Based on LOAD position
 Single point loading at the free end of a cantilever beam
 Centre point loading (or) Three point bending test
 Four point bending test
Bend Test (Contd…)

64.  Materials testing systems accurately and reliably measure the
flexural properties of metals, concrete, plastics, medical devices and
other products and components.
 The machines can calculate flexural modulus, flexural strength, and
yield point at maximum capacities.
Bend Fixtures
 Bend fixtures are used to determine the flexural properties of rigid
and semi-rigid materials.
 They are available in a variety of capacities, spans, and support
diameters and widths. It consists of default adjustable load pointers
based on loading position.
Bend Test (Contd…)

65. Working
 The bending fixture is supported on the platform of hydraulic
cylinder of the UTM.
 A loading beam that rests on two rollers on the top of beam to be
tested is used to apply the loads.
 A load applied to the loading beam accurately at the mid-point
between its two supporting rollers for three point loading (or) four
point loading.
 The support are generally knife-edge or convex. The load applicator
is a rounded knife-edge with an including angle of 60o, applied
wither at mid span (for 3 point testing) or symmetrically placed
from the supports (for 4 point testing).
Bend Test (Contd…)

66.  These rollers in turn must be spaced accurately at equal distances
from the supporting rollers for the beam to be tested.
 If the distance between the supporting rollers of the test-beam is L;
the supporting rollers of the loading beam are often located at L/3
or L/4 distances from the test-beam supports, although any equal
location distances can be used.
 Load and either deflection or strain are usually recorded in the test.
 Using this method, a beam mounted on supports is studied under
applied force to the beam
 The bending test demonstrates the relationship between the load of a
bending beam and its elastic deformation.
Bend Test (Contd…)