MECHANICAL TESTING

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MECHANICAL TESTING
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Why are metals tested ?

• Ensure quality
• Test properties
• Prevent failure in use
• Make informed choices in using materials
• Factor of Safety is the ratio comparing the
  actual stress on a material and the safe useable
  stress.

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Two forms of testing

• 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.

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HARDNESS TESTING

• Hardness is the ability to withstand dents or
  scratches

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Hardness testing machine

• The indenter is pressed into the metal
• Softer materials leave a deeper indentation

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Brinell hardness test

• Uses ball indentor.
• Cannot be used for thin materials.
• Ball may deform on very hard materials
• Surface area of indentation is measured.

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Vickers hardness test

• Uses square pyramid indentor.
• Accurate results.
• Measures length of diagonal on indentation.

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Rockwell hardness tests

• 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.

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Impact Tests

• Toughness of metals is the ability to
  withstand shock load and impact.
• It will not fracture when twisted.

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Izod test

• Strikes at 167 Joules.
• Test specimen is held vertically.
• Notch faces striker.

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Charpy impact test

• Strikes form higher position with 300 Joules.
• Test specimen is held horizontally.
• Notch faces away form striker.

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Tensile Testing

• 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.

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Extensometer
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Producing graphs

• Two basic graphs
• Load extension graph.
• Stress strain graph.

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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.
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Youngs Modulus (E)

• E Stress
• Strain
• Stress Load
• Cross section area
• Strain Extension
• Original length

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Youngs Modulus for stress strain graph

• Select point on elastic part of graph
• Calculate Youngs Modulus with this point
• E Stress
• Strain

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Youngs Modulus for Load extension graph
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Proof Stress

• 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.

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Proof stress for Load Extension graph
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Proof stress for Stress Strain graph
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Tensile Strength

• Tensile strength Maximum Load
• Cross section area
• Maximum load is the highest point on the graph.
• Often called Ultimate Tensile Strength (UTS)

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Creep
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.
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Fatigue

• 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.

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Fatigue

• 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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