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practice, for some parts the acceptance standards
are specialized, for example, the combustion
chambers, which are subjected to very high temperatures
and high speed airflows in service.
24. Dimensional inspection consists of measuring
specific components to ensure that they are within
the limits and tolerances laid down and known as
’Fits and Clearances’. Some of the components are
measured at each overhaul, because only a small
amount of wear or distortion is permissible or to
enable the working clearances with mating
components to be calculated. Other components are
measured only when the condition found during
visual inspection requires dimensional verification.
The tolerances laid down for overhaul, supported by
service experience, are often wider than those used
during original manufacture.
25. The detection of cracks that are not normally
visible to the naked eye is most important, particularly
on major rotating parts such as turbine discs, since
failure to detect them could result in crack
propagation during further service and eventually
lead to component failure. Various methods of
accentuating these are used for inspection, the two
principal techniques being penetrant inspection for
nonmagnetic materials and electro-magnetic
inspection for those parts that can be magnetized.
26. Two forms of penetrant inspection in common
use are known as the dye penetrant and the
fluorescent test. With the dye test, a penetrating
coloured dye is induced to enter any cracks or pores
in the surface of the part. The surface is then washed
and a developer fluid containing white absorbents is
applied. Dye remaining in cracks or other surface
defects is drawn to the surface of the developer by
capillary action and the resultant stains indicate their
locations.
27. Fluorescent testing is based on the principle
that when ultra-violet radiation falls on a chemical
compound, known as fluorescent ink, it is absorbed
and its energy re-emitted as visible light. If a suitable
ink is allowed to penetrate surface cavities, the
places where it is trapped will be revealed under the
rays of an ultra-violet lamp by brilliant light
emissions.
28. Magnetic crack testing (fig. 25-3) can only be
applied to components which can be magnetized.
The part is first magnetized and then sprayed with, or
immersed in, a low viscosity fluid which contains
ferrous particles and is known as ’ink’. The two walls
of a crack in the magnetized part form magnetic
poles and the magnetic field between these poles
attracts the particles in the ink, so indicating the crack
(fig. 25-4). In some instances, the ink may contain
fluorescent particles which enable their build-up to be
viewed under an ultra-violet lamp, A part that has
been magnetically crack tested must be demagnetized
after inspection.
29. Chromic acid anodizing may be used as a
means of crack detection on aluminium parts, e.g.
compressor blades. This process, in addition to
providing an oxide film that protects against
corrosion, gives a surface that reveals even the
smallest flaws.
Overhaul
267
Fig. 25-3 Magnetic crack testing.
30. When the requirement for a detailed inspection
on a component such as a turbine disc is necessary,
etching of the disc surfaces would be followed by
binocular inspection of the blade retention areas. The
whole disc would then be subjected to magnetic
crack test, followed by re-inspection of the disc
including a further binocular inspection of the blade
retention areas.
Repair
31. To ensure that costs are maintained at the
lowest possible level, a wide variety of techniques
are used to repair engine parts to make them suitable
for further service. Welding, the fitting of interference
sleeves or liners, machining and electro-plating are
some of the techniques employed during repair.
32. The welding techniques detailed in Part 22 are
extensively used and range from welding of cracks
by inert gas welding to the renewing of sections of
flame tubes and jet pipes by electric resistance
welding.
33. On some materials now being used for gas
turbine engine parts, different techniques may have
to be employed. An example of this is the high
strength titanium alloys which suffer from brittle
welds if they are allowed to become contaminated by
oxygen during the cooling period. Parts made in
these alloys, which have to withstand high stress
loadings in service, are often welded in a bag or
plastic dome that is purged by an inert gas before
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