Page 277 - The Jet Engine

Overhaul

matt finish which reveals surface defects that cannot fluorescent test. With the dye test, a penetrating
normally be seen with the naked eye. The metal coloured dye is induced to enter any cracks or pores
removal is normally achieved either by an electrolyt- in the surface of the part. The surface is then washed
ic process in which the part forms the anode, or by and a developer fluid containing white absorbents is
immersing the part for a short time in a special acid applied. Dye remaining in cracks or other surface
bath. Both methods must be carefully controlled to defects is drawn to the surface of the developer by
avoid the removal of too much material. capillary action and the resultant stains indicate their
locations.
22. After the components have been cleaned they
are visually and, when necessary, dimensionally 27. Fluorescent testing is based on the principle
inspected to establish general condition and then that when ultra-violet radiation falls on a chemical
subjected to crack inspection. This may include compound, known as fluorescent ink, it is absorbed
binocular and magnetic or penetrant inspection and its energy re-emitted as visible light. If a suitable
techniques, used either alone or consecutively, ink is allowed to penetrate surface cavities, the
depending on the components being inspected and places where it is trapped will be revealed under the
the degree of inspection considered necessary. rays of an ultra-violet lamp by brilliant light
emissions.
23. The non-dimensional inspections can be
divided into visual examination for general condition 28. Magnetic crack testing (fig. 25-3) can only be
and inspection for cracks. The visual examination applied to components which can be magnetized.
depends on the inspector's judgement, based on The part is first magnetized and then sprayed with, or
experience and backed by guidance from the manu- immersed in, a low viscosity fluid which contains
facturer. Although the visual examination of many ferrous particles and is known as 'ink'. The two walls
parts of the engine conform to normal engineering of a crack in the magnetized part form magnetic
practice, for some parts the acceptance standards poles and the magnetic field between these poles
are specialized, for example, the combustion attracts the particles in the ink, so indicating the crack
chambers, which are subjected to very high temper- (fig. 25-4). In some instances, the ink may contain
atures and high speed airflows in service. fluorescent particles which enable their build-up to be
viewed under an ultra-violet lamp, A part that has
24. Dimensional inspection consists of measuring been magnetically crack tested must be de-
specific components to ensure that they are within magnetized after inspection.
the limits and tolerances laid down and known as
'Fits and Clearances'. Some of the components are 29. Chromic acid anodizing may be used as a
measured at each overhaul, because only a small means of crack detection on aluminium parts, e.g.
amount of wear or distortion is permissible or to compressor blades. This process, in addition to
enable the working clearances with mating providing an oxide film that protects against
components to be calculated. Other components are corrosion, gives a surface that reveals even the
measured only when the condition found during smallest flaws.
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, particular-
ly 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 Fig. 25-3 Magnetic crack testing.

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