Specialised testing of materials
  • As the consequences of failure for aircraft is devastating, regular inspection of aircraft is necessary.
  • But due to the frequent amounts of testing done, these tests must be capable of detecting faults WITHOUT causing defects due to the test itself.
  • Tests that are able to do this are known as non-destructive tests.
  • Advantages of these tests include:
    • The same test may be repeated on the same item many times.
    • Tests may be performed on parts in service
    • Very little preparation is needed for the specimen
    • Equipment is often portable
    • Different tests can be applied to the same item simultaneously
  • Limitations include:
    • Some tests require expensive machinery
    • Results must be interpreted by a skilled and qualified technician.
  • This dot point mainly focuses on non-destructive testing of materials.

Visual Inspection

  • Before every flight, the pilot and those involved should quickly check the plane for any obvious damages.
  • For maintenance, a more detailed inspection is offered.
  • This is often used for structures which have been newly repaired or re-welded.

Dye/Fluorescent Penetrant Testing

  • This is one of the oldest non-destructive tests, and it aims to highlight external cracks/flaws in any material which aren't immediately visible.
  • The method is as follows:
    • The tested part is first thoroughly cleaned and all coatings removed.
    • The part is then painted, sprayed or soaked in a penetrant fluid and allowed to set for up to 30 minutes.
    • Heat may be used to open up pre-existing cracks
    • Excess fluid is then removed with a solvent
    • A light coloured developing powder is then applied, which draws out and reveals any penetrants in the cracks.
    • For dye penetrants, the cracks, if there is any, can be seen under normal light.
    • For fluorescent penetrants, the cracks will be visible under UV light.
  • Used on metal, polymer and composite materials.

X-Ray

  • There are two ways X-rays could be used for non-destructive testing:
    • Radiography: Where X-rays or other Radiation are put through the tested material and onto a film.
    • Fluoroscopy: Where the X-rays are instead projected onto a fluorescent screen.
  • The images created by this process show different variations within the material with different contrasts.
  • Any defects usually appear darker than the optimum state of the material.
  • The source of radiation are usually X-rays produced from an anode, or gamma rays from a Radioactive Isotope.
  • Not suitable for forgings
  • Advantages include a permanent image that can be inspected without disassembly
  • It is dangerous and evacuation is sometimes needed when performing these tests due to radiation.

Magnetic Particle Inspection

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  • NB: This method ONLY works on FERROUS materials. (e.g. Steel, Iron etc.)
  • This method involves the concept of Flux leakage. (shown in picture)
    • When a magnetised material has a crack in it, the crack itself creates a pair of magnetic poles at the edge of the crack.
    • This causes an irregularly shaped magnetic field which leaks out into the air
    • This flux leakage attracts magnetic filings and when dusted on, the areas of flux leakage become visible.
  • It involves applying magnetic particles onto a ferrous surface, and these particles can be wet(in the suspension of a liquid) or dry
  • Dry Method is as follows:
    • The surface is cleaned of any rust, scale or paint
    • The tested surface is then magnetised by induction coils or permanent magnets.
    • A layer of magnetic particles is then dusted onto the surface.
    • Excess particles are gently blown off
    • The magnetizing force is then turned off
    • Any areas where magnetic leakage occured should be visible as a cluster of particles.
  • Dry particle inspection is useful for finding sub-surface faults in forgings, castings and heavy welds.
  • Wet Suspension Method:
    • The surface is cleaned of any rust, scale or paint
    • The suspension liquid is then poured or sprayed onto the testing surface.
    • As soon as the liquid is applied, the magnetising force is applied in several short bursts
    • Check for clustered areas.
  • Wet Suspension Inspection is used for more complex shapes where dry particles can't flow properly.

Ultrasonic Testing

  • This system works by passing high frequency radio waves through a material.

PS: Check this site: http://www.ndt-ed.org/EducationResources/CommunityCollege/communitycollege.htm for a ton of information on these.