Transcript MATERIALS TESTING

MECHANICAL TESTING
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Why are metals tested ?
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Ensure quality
Test properties
Prevent failure in use
Make informed choices in using materials
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Factor of Safety is the ratio comparing
the actual stress on a material and the
safe useable stress.
Two forms of testing
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• Mechanical tests – the material may be
physically tested to destruction. Will
normally specify a value for properties
such as strength, hardness, toughness,
etc
• Non-destructive tests (NDT) – samples or
finished articles are tested before being
used.
HARDNESS TESTING
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Hardness is the ability to withstand
dents or scratches
Hardness testing machine
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• The indenter is
pressed into the metal
• Softer materials leave
a deeper indentation
Brinell hardness test
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• Uses ball indentor.
• Cannot be used for
thin materials.
• Ball may deform on
very hard materials
• Surface area of
indentation is
measured.
Vickers hardness test
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• Uses square pyramid
indentor.
• Accurate results.
• Measures length of
diagonal on
indentation.
Rockwell hardness tests
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• Gives direct reading.
• Rockwell B (ball) used for soft materials.
• Rockwell C (cone) uses diamond cone for
hard materials.
• Flexible, quick and easy to use.
Impact Tests
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Toughness of metals is the ability to
withstand shock load and impact.
It will not fracture when twisted.
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Izod test
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• Strikes at 167 Joules.
• Test specimen is held
vertically.
• Notch faces striker.
Charpy impact test
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• Strikes form higher
position with 300
Joules.
• Test specimen is held
horizontally.
• Notch faces away
form striker.
Tensile Testing
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• Uses an extensometer to apply measured
force to an test specimen. The amount of
extension can be measured and graphed.
• Variables such as strain, stress, elasticity,
tensile strength, ductility and shear
strength can be gauged.
• Test specimens can be round or flat.
Extensometer
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Producing graphs
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• Two basic graphs:
• Load – extension graph.
• Stress – strain graph.
Load - extension graph for low
carbon steel
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Draw graph for this
tensile test?
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Identify the straight line part of the graph.
Youngs Modulus (E)
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E = Stress
Strain
• Stress = Load
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Cross section area
• Strain = Extension
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Original length
Youngs Modulus for
stress – strain graph
• Select point on elastic
part of graph
• Calculate Youngs
Modulus with this
point
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E = Stress
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Strain
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Youngs Modulus for Load
– extension graph
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Proof Stress
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• The stress that causes a % increase in
gauge length.
• It can be found by drawing a line parallel
to the straight part of the graph.
• A value can be taken from the vertical
axis.
Proof stress for Load –
Extension graph
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Proof stress for Stress –
Strain graph
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Tensile Strength
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Tensile strength =
Maximum Load
Cross section
area
• Maximum load is the highest point on the
graph.
• Often called Ultimate Tensile Strength (UTS)
Creep
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When a weight is hung from a piece of lead and left for a
number of days the lead will stretch. This is said to be
creep.
Problems with creep increase when the materials are
subject to high temperature or the materials themselves
have low melting points such as lead.
Creep can cause materials to fail at a stress well below
there tensile strength.
Fatigue
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• Fatigue is due to the repeated loading and
unloading.
• When a material is subjected to a force acting in
different directions at different times it can
cause cracking. In time this causes the material
to fail at a load that is much less than its tensile
strength, this is fatigue failure. Vibration for
example is a serious cause of fatigue failure.
Fatigue
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• Fatigue can be prevented with good design practice.
• A smooth surface finish reduces the chance of surface
cracking.
• Sharp corners should be avoided.
• Corrosion should be avoided as this can cause fatigue
cracks.
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