2D4: Integrity of Materials

1
2D4 Integrity of Materials Components

• Properties of Materials
• Failure Modes and Prevention
• Failure Prevention Strategies
• Non-destructive Testing

2
Forces on Materials
Tensile Force
Compressive Force
Shear Force
3
Bending
Tension
Compression
4
Effects of Loading

• Provided that load is not excessive, material
  itself will return to its original size when load
  is removed (material behaving in elastic manner
• At increased stresses deformation becomes
  permanent - material is said to be plastic

5
Main Modes of Failure

• Ductile failure
• Brittle failure
• Metal fatigue
• Buckling
• Corrosion
• Wear
• Creep

6
Ductile Failure

• Material moves into plastic region and loses its
  original shape
• There is a reduction in cross-sectional area
  which increases the stress (stress is
  proportional to area)
• Failure occurs at the place of reduced
  cross-sectional area
• Temperature is a major factor
• The higher the temperature, the more ductile
  materials become i.e. they can elongate more
  before failure

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Brittle Failure

• Occur very suddenly and without warning
• Occur because the structure of a material does
  not slip, either owing to the material structure
  or there is insufficient time due to the
  intensity of the load
• Small cracks spread through the material so
  quickly that massive failure is produced
• Speed of failure often results in some energy in
  material being released as sound, giving brittle
  failure a characteristic crack

8
Brittle Failure

• Note that in other failure modes, the actual
  failure may be by brittle mode, but this will
  only be part of the sequence of failure
• Some factors that promote brittle failure
• Low temperature
• Impact or Snatch Loading e.g. lifting equipment
• Residual tensile stress e.g. pre-tensioned beams
• Inherently brittle material e.g. glass and
  ceramics
• In appearance, a brittle failure shows no sign of
  any deformation - the parts could be fitted back
  together. The surface will be bright and there
  may be chevron markings across the failed
  surface

9
Metal Fatigue

• Most common type of failure where conditions
  producing mechanical vibration occur
• Fluctuating stress conditions produced by
  vibration (e.g. aircraft wings going up and down
  during flight) can cause formation of a crack
  that propagates through the material
• Crack reduces the area of material resisting the
  stresses until the remaining material can no
  longer resist the stresses and fails
• Failure is generally a brittle failure as it is
  the less ductile materials that are used to
  resist cyclic loading

10
Metal Fatigue

• Initial crack always starts from the surface and
  penetrates into body of material
• Surface blemishes e.g. machining marks and
  foreign body inclusions are likely candidates to
  set fatigue failure into action, as are holes for
  bolts, rivets, inspection hatches etc.
• The final area of metal in place when failure
  occurs is clearly identified by its colour - the
  surface of the crack will be dull due to the
  effects of air and moisture being in contact and
  setting up mild corrosion, while the newly failed
  surface will show signs of new, clean brittle
  failure
• Metal fatigue cracks can be readily detected by
  NDT
• Surface defects should be removed where possible
  by polishing

11
Buckling

• When a compressive force is put into a rod, beam
  or bar the force is resisted by the material
• As force increases, the material distorts,
  preventing a straight transfer of stress through
  the material
• Failure is due to tensile forces on the extreme
  causing ductile failure which propagates through
  the material and leads to catastrophic collapse
• Can be better resisted by a ductile material than
  a brittle material due to ability to resist
  tensile forces introduced by the bowing on the
  outside surface
• Can be limited by installing intermediate
  supports to limit movement from the straight

12
Corrosion

• Affects only metals
• Requires the presence of an electrolyte (normally
  water), potential differences between metals to
  allow flow of current
• Chemical change in which body of metal loses
  atoms - when repeated millions of times, loss of
  material can be observed as corrosion

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Corrosion

• Rate of corrosion depends on
• Stress in material
• Strength of electrolyte (pH value)
• Environment and exposure (heat accelerates
  corrosion)
• Reactivity of metal
• Metal impurities
• Can be observed by
• Localised pitting in surface of material
• Overall thinning of material
• Can be detected by discoloration or NDT methods

14
Corrosion

• Corrosion removes material, so there is less
  component to carry the stress it was designed to
  take
• Protection
• Control environment
• Selection of corrosion resistant material
• Protect metal with coating
• Sacrificial anode of magnesium, aluminium or zinc
  which is dissolved instead of the component

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Wear

• Can be produced by
• Scuffing
• Lack of lubrication between moving parts.
• Movement of parts causes friction
• Abrasion
• Occurs when small components of foreign material
  which is harder than component get between moving
  parts e.g. sand particles - surface then becomes
  scored
• Pitting
• Combination of above
• Particles produced during scuffing become
  detached and hardened, becoming the equivalent of
  foreign particles and cause abrasion

16
Creep

• When a material is under stress near to its
  elastic limit, it undergoes a process of plastic
  deformation known as creep
• Extent to which creep acts is dependant on
• Time (creep is a slow process)
• Temperature (creep accelerated in high
  temperatures)
• Not a true mode of failure as failure is often
  either brittle or ductile

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Preventing Failure in Design

• HAZOPS, FMEA etc.
• Incorporation of Safety Factors
• Correct selection of materials
• Removal of points of weakness
• Remove sharp edges
• Increase amount of material where slots or holes
  are needed

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Testing and Quality Assurance

• Checks at each stage of a process
• Management process to ensure the checks are
  carried out and recorded
• System is probably based on the BS ISO9000 Series
  of documents detailing quality assurance
• Records are important in the event of failure

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Preventing Failure in Use

• Use within manufacturers stated parameters
• Specific maintenance e.g. lubrication
• Non-destructive testing

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Failure Investigation Techniques

• Collection of samples
• Fault Tree Analysis
• Accident Investigation
• Gather samples of failed material
• Look closely at failed surfaces
• Record information
• Lab analysis may be necessary

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NDT

• Test integrity of material without destroying
  components
• Types of NDT
• Visual inspection
• Penetrant inspection
• Magnetic inspection
• Radiography
• Ultrasonic testing
• Eddy current testing
• Electrical resistivity
• Thermal imaging

Summary of NDT techniques
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Visual Inspection

• Use naked eye, microscope or magnifying glass
• Requires good light source
• Surface may require cleaning
• Cheap and easy but applies to surface defects only

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Penetrant Inspection

• Uses dye penetrant to highlight defects for
  visual inspection
• Only relates to surface defects
• Easy to use off-site as dyes are in aerosol cans

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Magnetic Inspection

• Component is magnetised and magnetic particles
  are applied
• Any defect will show as it distorts the magnetic
  field and the particles lie differently
• Only works on materials with magnetic properties
• Detects surface cracks and sub-surface cracks
  close to the surface

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Radiography

• Gamma or X-rays are passed through material and
  onto a strip of film
• Radiation triggers reaction in film which, when
  developed, shows where material is sound and
  where defects exist
• Gives permanent record
• Radiation hazard present

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Ultrasonic Testing

• Uses generator transmitting ultrasound waves into
  material and detecting them when reflected
• Equipment usually hand-held
• Results need to be interpreted by skilled
  operatives
• Can detect defects within the material
• Can be used on metallic and non-metallic objects

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Electrical Resistivity

• A property of metallic materials is that
  electrical resistance can alter when subjected to
  forces
• Uses a small loop of wire attached to the object
  (strain gauge)
• Can be used to monitor situation over long periods

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Thermal Imaging

• Cameras that can detect heat and show small
  variations onto colour screen
• Main use it to determine if a part is running hot
  I.e. lacking lubrication or rubbing where it
  isnt supposed to
• Cameras are small
• Only provides a snapshot
• Comparison between pictures is difficult by eye,
  but computer can be used

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Previous Exam Questions

• Explain the purpose of non-destructive testing
  (NDT) on items of plant (2 marks)
• Other than visual inspection, outline the
  principles, benefits and limitations of two NDT
  techniques (8 marks)
• A serious accident occurred when a threaded steel
  rod in the braking mechanism of a hoist failed.
  An investigation revealed a fatigue failure of
  the rod.
• Describe the mechanisms and characteristics of
  fatigue failure (6)
• Outline the factors that may contribute to such a
  failure (6)
• Describe the measures that could have been taken
  which might have prevented the failure (8 marks)