3. Intro
• Materials are tested to determine their basic
properties
• Determine numerical values for the
properties.
• To evaluate the material’s ability to carry or
resist mechanical forces such as tension,
compression, shear, torsion and impact.
• Testing usually results in permanent damage
to the specimen.
5. 7 Mechanical Properties : Strength
• The ability of material to
withstand compressive(a),
tensile(b) and shear
loads(c) without breaking.
100 kN
Fixed beam
Rod is being stretched
by the load
6. 7 Mechanical Properties : Hardness
• The ability of a materials to
withstand scratching or
indentation by another hard
body.
• Indicates the wear resistance of
a material.
• Press a hardened steel ball into
a hard material and then into a
soft material by the same load.
• Small indentation in the hard
material.
• Deeper impression in the
softer material.
Constant Load
Hard steel ball
Hard material Soft material
7. 7 Mechanical Properties : Elasticity
• The ability of a material to deform
under load and return to its
original size and shape when the
load is removed.
• An elastic will be the same length
before and after the load is
applied.
• All material has its own elastic
limit.
• Stress beyond this limit,
permanent deformation (plastic
deformation), and ultimately
fracture, occurs.
• Materials are only stressed within
the elastic range under normal
service conditions.
Elastic extension
Load
Load
Before and
after
loading
8. 7 Mechanical Properties : Plasticcity
• Opposite to elasticity.
• Material been loaded beyond its elastic limit causing the
material to reform permanently.
• Materials takes a permanent set and were not return to
its original sizes and shape when the load is removed.
Punch
Die
Strip before
bending
force is
applied
Bending force
End Product
9. 7 Mechanical Properties : Ductility
• Plastic deformation occurs
as the result of applying a
tensile load.
• A ductile material allows an
amount of plastic
deformation to occur under
tensile loading before
fracture occurs.
• Processes as wire drawing ,
tube drawing and cold
pressing low carbon steel
sheets into motor car body
panels.
Rod being drawn
Die
Direction of drawn
10. 7 Mechanical Properties : Toughness
• The ability of material to withstand shatter
(e.g. Glass).
• If the rod is made from a piece of high
carbon steel – for example, silver steel in
the annealed (soft) condition – it will have
only a moderate tensile strength.
• But under the impact of the hammer it will
bent without breaking, therefore it is
tough.
11. 7 Mechanical Properties : Brittleness
• Opposite of ductility and malleability.
• It is the property of a material that shows little
or no plastic deformation before fracture
when a force is applied.
• For example, a steel rod can be bent but a
grey cast iron rod snaps when you try to bend
it.
• Therefore grey cast iron is a brittle material.
14. Destructive Testing
• Part or product tested no longer maintains its
original shape or surface texture.
• Mechanical test methods are all destructive.
• Other destructive tests include
– speed testing of grinding wheels to determine
their bursting speed
– high pressure testing of pressure vessels to
determine their bursting pressure.
15. Destructive Testing—Hardness Tests
• Hardness tests with large indentations may be
regarded as destructive testing.
• Micro hardness tests may be regarded as non-
destructive because of the very small
permanent indentations.
16. Destructive Testing : Brinell
• Hardness is measured by pressing a hard steel
ball into the surface to the test piece, using a
known load.
• To ensure consistent result :
– Thickness of the specimen should be at least seven
times the depth of the indention
– The edge of the indentation should be at least three
times the diameter of the indentation from the edge
of the test piece.
– The test is unsuitable for materials whose hardness
exceeds 500 HB, as the ball indenter tend to flattened.
17. Destructive Testing : Vickers
• Preferable to the Brinell test for hard
materials.
• It use a diamond intender. (Diamond is the
hardest materials known – approximately
6000HB).
18. • Figure shows a universal hardness testing
machine suitable for performing both Brinell
and Vickers hardness test.
19.
20.
21. Destructive Testing : Shore
• The shore hardness is determined by using
selroscope.
• Works based on deference principles, and the
hardness is measured as a function of resilience.
• Scelroscope can be carried to the work piece;
useful for the testing large surfaces such as the
slideways in machine tools.
• A diamond-tipped hammer of mass 2.5g drops
through a height of 250mm.
• The height of the first rebound indicates the
hardness in a 140-divition scale.
24. Destructive Testing : Rockwell
• Not as reliable as the Brinnel and Vikers hardness tests.
• Rockwell test is widely used in industry as it quick, simple
and direct reading.
• Principles—Rockwell compares the difference in depth of
penetration of the intender when using forces of two
deferent values.
• A minor is first applied (to take up the backlash and pierce
the skin of the component) and the scales are set to read
zero.
• Then a major force is applied over and above the minor
force and the increase depth of penetration is shown on
the scales as a direct reading of hardness.
• No the need of calculation or conversion table.
26. Destructive Testing : Izod Test
• A 10mm square, notch specimen is used.
• The striker of the pendulum hits the specimen
with a kinetic energy of 162.72J at a velocity
of 3.8m/s.
• Figure shows the standard impact test
machine for both Izod and Charpy testing
occasions.
27. Destructive Testing : Izod Test
2
22 ½0 22 ½0
Root radius 0.25
Vice
100
50
Striker
70 28 22
Tes
t
Pie
ce
28. Destructive Testing : Charpy Test
• In the Izod test the specimen is supported as a
cantilever.
• But in the Charpy test it is supported as a
beam.
• It is struck with a kinetic energy of 298.3J at a
velocity of 5 m/s.
• Figure shows details of Charpy test specimen
and the manner in which it is supported.
31. NDT : Intro
• Non-destructive testing (NDT) defines a discontinuity
as an interruption in the normal physical structure or
configuration of a part, such as a crack or porosity.
• A discontinuity may or may not be detrimental to the
usefulness of a part.
• A defect is a discontinuity whose size, shape, location
or properties adversely affect the usefulness of the
part or exceed the design criteria for the part.
• The purpose of NDT can serve to analyze an existing
failure or be used to prevent future failures.
32. NDT : Penetration Test
• Penetration techniques are used to make surface
cracks visibl.
• Immerse the casting in a bath of hot paraffin.
• Heating the paraffin reduces its viscosity so that,
combined with its already high surface tension, it is
easily drawn into the finest cracks and porosity by
capillary attraction.
• The casting is removed and wiped thoroughly clean,
after which it is painted with whitewash.
• Paraffin seeping out from the cracks, will discolour the
whitewash and reveal the presence of surface cracks
and porosity.
34. NDT : Magnetic Test
• Can only be applied to ferromagnetic materials.
• Only appropriate for surface cracks and discontinuities not
more than 10 mm below the surface of the component.
• Based upon the fact that the magnetic resistance in the
region of a discontinuity is greater than the surrounding
metal.
• It distorts the magnetic flux distribution.
• The resulting distortion of the flux field is usually detected
by means of magnetic powder in suspension in light
machine oil or paraffin.
• The suspension is spread thinly over the surface of the
surface of the component and the magnetic powder
‘bunches’ in the vicinity of the fault.
36. NDT : Ultrasonic
• Pulse of high-frequency oscillations are driven
into the component by the transducer.
• When the sound waves meet any
discontinuity, such as crack, the waves are
reflected back into the transducer where they
are converted into electrical pulse which can
be display on the screen of the computer.
38. NDT : X-ray
• This is a photographic process in which the ‘illumination’ of the
component is by X-rays of the even more penetrating gamma-rays.
• These are electromagnetic radiations exactly the same as radio
waves and light waves, except they have very much shorter
wavelength.
• This enables X-ray and gamma-rays to penetrate solid objects.
• When photographic film is exposed to X-ray or gamma-ray and then
develop, the film become dark.
• If a solid object is placed between the source of radiation and the
film so that it casts a shadow on the film, the level of radiation
reaching the film in the shadow area will be reduce and the shadow
will appear on the film.
• After development, as a less dark area.
