Method and an apparatus for inspecting articles

An apparatus (42) for inspecting an as cast turbine blade (40), with impingement cooling passages (36) (see FIG. 2 ), comprises a vacuum pump (52) arranged to evacuate the interior of the turbine blade (40) through pipe (48) and valve (54) and a supply of steam (46) arranged to supply steam to the interior of the turbine blade (40) through the pipe (48) and a valve (50). An infrared camera (56) is arranged to view the outer surface (32) of the turbine blade (40) to detect hot spots produced on the outer surface (32) of the turbine blade (40) by jets of steam impinging on the inner surface (35) of the turbine blade (40) from the cooling passages (36). A processor (60) records the images produced by the camera (56) and analyses the images to determine if the cooling passages (36) have been formed and if they have been formed in the correct position.

The present invention relates generally to a method and an apparatus for 
inspecting articles. The invention is particularly suitable for inspecting 
impingement cooling passages, or film cooling passages, of hollow turbine 
blades or hollow turbine vanes of gas turbine engines to determine whether 
such cooling passages are blocked or if they are in an incorrect position. 
The invention is also suitable for inspecting articles to determine 
whether there are cracks, debonds, voids, delaminations, oxidation or 
corrosion in the article and also if there is insufficient adhesion 
between the article and a coating on the article. 
The turbine blades and turbine vanes of gas turbine engines usually 
comprise cooling passages for single or multi pass convection cooling, 
impingement cooling or film cooling purposes. The film cooling passages 
extend through the wall of the turbine blade from a first, interior, 
surface to a second, exterior, surface of the turbine blade. The 
impingement cooling passages extend through structure within the turbine 
blade and are spaced from the first, interior, surface of the turbine 
blade. The convection cooling passages generally extend longitudinally in 
the wall of the turbine blade. Other cooling passages extend through 
structure within the turbine blade. The turbine blades and turbine vanes 
may comprise various combinations of any one or ore of convection cooling 
passages, impingement cooling passages and film cooling passages. 
These turbine blades and turbine vanes are cast using the investment 
casting technique in which ceramic: cores, located in ceramic shell 
moulds, are used to define the impingement cooling passages and other 
cooling passages within the structure of the turbine blade. The film 
cooling passages are generally formed by machining through the walls of 
the turbine blades to connect with other cooling passages within the 
structure of the turbine blade. 
However, there is a requirement to inspect the turbine blades and turbine 
vanes to ensure that the impingement and/or other and/or film cooling 
passages have been formed without blockages and/or that the cooling 
passages have been formed in the correct position. The blockages in the 
impingement and other cooling passages may be formed because not all of 
the ceramic cores have been removed, or the ceramic cores may have cracked 
during casting to allow metal to block the cooling passage and the 
impingement and other cooling passages may have been formed in the wrong 
position because the cores moved within the ceramic shell mould. Blockages 
in the film cooling passages may be formed by incorrect machining etc. 
If one or more of the other cooling passages has been displaced from its 
nominal position, a subsequently formed film cooling passage 
interconnecting with the other cooling passage may only graze or even miss 
the other cooling passage, it may interconnect at the wrong position or 
the tool, may pass across the other cooling passage and damage the 
opposite wall of the other cooling passage. These effects may reduce the 
strength of the turbine blade and are difficult to detect. Thus it is 
desirable to detect any discrepancies in the impingement and/or other 
cooling passages before the film cooling passages are formed by expensive 
machining processes. 
It is known to use borescopes to inspect the interior of cast turbine 
blades and turbine vanes to ensure that the cooling passages have been 
correctly formed. However, the use of borescopes is a time consuming and 
labour intensive process. Furthermore it has the disadvantage of not being 
able to detect some specific forms of blockage unless specific lighting 
techniques or light guides are used. 
It is known to use X-ray computer tomography to inspect articles and this 
would be possible for cast turbine blades and turbine vanes to ensure that 
the cooling passages have been correctly formed. However, X-ray computer 
tomography equipment is very expensive and the cost of generating 
tomographs of a large number of sections from each turbine blade or 
turbine vane would make the process uneconomical. Furthermore it not 
possible to detect all the cooling passages in some portions of the 
turbine blade or turbine vane and there is a lower limit to the size of 
feature that can be resolved by X-ray computer tomography and some 
blockages may be to thin to be detected. 
It is also known to use thermography to inspect, turbine blades to 
determine if the cooling passages are blocked. 
U.S. Pat. No. 3,566,669 passes a hot gas through and out of the cooling 
passages of a turbine blade and through and out of a reference passage 
having different wall thicknesses. An infrared camera views the external 
surface of the turbine blade and the reference passage to determine if the 
cooling passages are at the correct position relative to the outer surface 
of the wall and/ or to determine if any of the cooling passages are 
blocked. 
UK patent GB2164746B passes a hot gas through the film cooling passages of 
a turbine blade. An infrared camera views the film cooling passages and 
determines the relative intensities of the black body radiation emitted by 
the film cooling passages to determine if any of the film cooling passages 
are blocked. 
U.S. Pat. No. 5,111,046 is similar to GB2164746B but subsequently passes a 
cold gas through the film cooling passages of the turbine blade to 
determine the presence of certain type of blockage. 
These patents do not disclose how to detect blockages in cooling passages 
if the hot gas cannot exit the turbine blade through film cooling 
passages. Also any hot gas passing into the cooling passages in the 
turbine blade, as used in the above patents, may not produce effective 
heat transfer in the areas of the turbine blade to be inspected and thus 
the inspection technique is not sensitive enough to determine any 
blockages in impingement cooling passages or other cooling passages. 
Accordingly the present invention seeks to provide a method of inspecting 
an article which reduces the above mentioned problems. 
Accordingly the present invention provides a method of inspecting an 
article, the article having a first surface and a second surface facing in 
the opposite direction to the first surface, comprising the steps of: 
(a) forming an enclosed volume at least partially defined by the first 
surface of the article, 
(b) removing air from the enclosed volume, 
(c) supplying a condensable heating fluid to the enclosed volume, 
(d) directing a flow of the condensable heating fluid onto or over the 
first surface of the article to cause the temperature of the first surface 
to change, 
(e) viewing the second surface of the article during the temperature change 
to produce a series of images, and 
(f) analysing the series of images to determine the presence or absence of 
defects in the article. 
Preferably the method comprises injecting a small quantity of the 
condensable heating fluid into the enclosed volume after step (a) and 
before step (b). 
Alternatively the method may comprise injecting a small quantity of the 
condensed heating fluid into the enclosed volume after step (a) and before 
step (b). 
Preferably step (c) comprises supplying steam to the enclosed volume. 
Step (e) may comprises applying a thermochromic material to the second 
surface of the article and viewing the second surface of the article with 
a camera sensitive to radiation in the visible light band. Step (e) may 
comprise viewing the second surface of the article with a video camera, a 
cine camera or a CCD camera. 
Preferably step (e) comprises viewing the second surface of the article 
with a camera sensitive to radiation in the infra red band. 
A calibrated emissivity coating may be applied to the second surface of the 
article. 
Preferably step (d) comprises directing a plurality of jets of the 
condensable heating fluid onto the first surface of the article. 
Preferably step (a) comprises forming the enclosed volume solely by the 
first surface of the article. 
Preferably step (d) comprises directing a plurality of jets of the 
condensable heating fluid through passages in the article onto the first 
surface of the article. 
Preferably step (b) comprises evacuating the enclosed volume. 
Preferably the article is a turbine blade or a turbine vane. 
Alternatively step (a) may comprise forming the enclosed volume partially 
with a vessel. 
Step (d) may comprises supplying a plurality of jets of the condensable 
heating fluid through passages in the vessel onto the first surface of the 
article. 
The article may be a composite material article, a laminated material 
article or a coated article. 
The present invention also provides a method of inspecting an article, the 
article having a plurality of passages extending from a first surface to a 
second surface facing in the opposite direction to the first surface, 
comprising the steps of: 
(a) forming an enclosed volume at least partially defined by the second 
surface of the article and a first surface of a vessel, 
(b) removing air from the enclosed volume, 
(c) supplying a condensable heating fluid to the enclosed volume, 
(d) directing a flow of the condensable heating fluid through the passages 
onto the first surface of the vessel to cause the temperature of the first 
surface of the vessel to change, 
(e) viewing the second surface of the vessel during the temperature change 
to produce a series of images, and 
(f) analysing the series of images to determine the presence or absence of 
defects in the article. 
Preferably the method comprises injecting a small quantity of the 
condensable heating fluid into the enclosed volume after step (a) and 
before step (b). 
Alternatively the method comprises injecting a small quantity of the 
condensed heating fluid into the enclosed volume after step (a) and before 
step (b). 
Preferably step (c) comprises supplying steam to the enclosed volume. 
Step (e) may comprise applying a thermochromic material to the second 
surface of the article and viewing the second surface of the vessel with a 
camera sensitive to radiation in the visible light band. Preferably step 
(e) comprises viewing the second surface of the vessel with a video 
camera, a cine camera or a CCD camera. 
Preferably step (e) may comprise viewing the second surface of the vessel 
with a camera sensitive to radiation in the infra red band. 
A calibrated emissivity coating may be applied to the second surface of the 
vessel. 
Preferably step (d) comprises directing a plurality of jets of the 
condensable heating fluid onto the first surface of the vessel. 
Preferably step (d) comprises directing a plurality of jets of the 
condensable heating fluid through passages in the article onto the first 
surface of the vessel. 
Preferably step (b) comprises evacuating the enclosed volume. 
Preferably the article is a turbine blade or a turbine vane. 
The present invention also provides an apparatus for inspecting an article, 
the article having a first surface and a second surface facing in the 
opposite direction to the first surface, the first surface at least 
partially defining an enclosed volume, comprising means to remove air from 
the enclosed volume, means to supply a condensable heating fluid to the 
enclosed volume, means to direct a flow of the condensable heating fluid 
onto or over the first surface of the article to cause the temperature of 
the first surface to change, means to view the second surface of the 
article during the temperature change to produce a series of images, and 
means to analyse the series of images to determine the presence or absence 
of defects in the article. 
The present invention also provides an apparatus for inspecting an article, 
the article having a plurality of passages extending from a first surface 
to a second surface facing in the opposite direction to the first surface, 
comprising a vessel having a first surface to define an enclosed volume 
with the second surface of the article, means to remove air from the 
enclosed volume, means to supply a condensable heating fluid to the 
enclosed volume, means to direct a flow of the condensable heating fluid 
onto or over the first surface of the vessel to cause the temperature of 
the first surface to change, means to view the second surface of the 
article during the temperature change to produce a series of images, and 
means to analyse the series of images to determine the presence or absence 
of defects in the article. 
Preferably there are means to inject a small quantity of the condensed 
heating fluid into the enclosed volume. 
Preferably the means to supply a condensable heating fluid comprises a 
supply of steam. 
The second surface of the article, or the second surface of the vessel, may 
have a thermochromic material. The means to view the second surface of the 
article, or the second surface of the vessel, may comprise a camera 
sensitive to radiation in the visible light band. The means to view the 
second surface of the article, or the second surface of the vessel, may 
comprise a video camera, a cine camera or a CCD camera. 
Preferably the means to view the second surface of the article, or the 
second surface of the vessel, comprises a camera sensitive to radiation in 
the infra red band. 
The second surface of the article, or second surface of the vessel, may 
have a calibrated emissivity coating. 
Preferably the enclosed volume is defined solely by the first surface of 
the article. 
Preferably the article comprises a plurality of passages to direct a 
plurality of jets of the condensable heating fluid onto the first surface 
of the article. 
Alternatively the article may comprise a plurality of passages to direct a 
plurality of jets of the condensable heating fluid onto the first surface 
of the vessel. 
Preferably the article is a turbine blade or a turbine vane. 
Alternatively the enclosed volume is defined by the first surface of the 
article and a first surface of a vessel. 
The vessel may comprise a plurality of passages to direct a plurality of 
jets of the condensable heating fluid onto the first surface of the 
article. 
Preferably the means to remove gas from the enclosed volume comprises means 
to evacuate the enclosed volume. 
The article may be a composite material article, a laminated material 
article or a coated article.

A turbine blade 10, as shown in FIGS. 1 to 3, comprises a root portion 12, 
a shank portion 14, a platform portion 16 and an aerofoil portion 18. The 
turbine blade 10 is suitable for use in a gas turbine engine (not shown). 
The aerofoil portion 18 has a number of lengthwise extending chambers 20, 
22, 24, 26 and 28. The chamber 20 is adjacent the trailing edge of the 
aerofoil portion 18 and the chamber 30 is adjacent the leading edge of the 
aerofoil portion 18. The aerofoil portion 18 of the turbine blade 10 has a 
first, inner, surface 30 and a second, outer, surface 32. The inner 
surface 30 defines the chamber 26. The second surface 32 has a pressure 
surface 32A and a suction surface 32B. 
The chamber 26 is the main chamber and there are a plurality of further 
chambers 34 which are narrow relative to the main chamber 26 which 
surround the main chamber 26 and which are positioned between the first 
surface 30 and the second surface 32. The chambers 34 also extend 
lengthwise of the aerofoil portion 18. Each of the chambers 34 is 
interconnected with the main chamber 26 by one or more cooling passages 
36. The chambers 34 between the chamber 26 and the pressure surface 32A 
are interconnected to the pressure surface 32A by one or more film cooling 
passages 38. The chambers 34 have first, inner, surfaces 35 adjacent the 
second, outer, surface 32 of the aerofoil portion 38. A more complete 
description of the turbine blade is present in UK patent GB2283538B. 
In operation, cooling air is forced up the chambers 34 adjacent the suction 
surface 32B of the aerofoil portion 18 from the root portion 12. The 
cooling air then flows through the cooling passages 36 into and across the 
main chamber 26. The cooling air then flows from the main chamber 2E 
through the cooling passages 36 into the chambers 34 adjacent the pressure 
surface 32A. The cooling air flowing through the cooling passages 36 
impinges upon the first surfaces 35 of the chamber 34 to provide 
impingement cooling of the wall of the aerofoil portion 18. The cooling 
air then flows through the film cooling passages 38 to exit the aerofoil 
portion 18 and to provide a cooling film of air on the pressure surface 
32A of the aerofoil portion 18. 
As discussed earlier the turbine blades 10 are generally manufactured by 
investment casting using ceramic shell moulds and ceramic cores to define 
the chambers 20, 22, 24, 26 and 28, the further chambers 34 and the 
passages 36. After the metal has been cast in the ceramic shell mould the 
ceramic shell mould and ceramic cores are removed. The passages 38 are 
generally produced by electrochemical machining, electodischarge machining 
or laser machining etc. 
It is desirable to inspect the as cast turbine blade, or turbine vane, 
before the film cooling passages 38 are formed, to ensure that the 
chambers 34 and passages 36 have been formed and/or that they are in the 
correct position. 
An apparatus 42 for inspecting the as cast turbine blades 40, or turbine 
vanes, is shown in FIGS. 4. the apparatus 42 comprises a supply of steam 
46, for example boiler, which is supplied with water via a pipe 44. The 
supply of steam 46 is interconnected to the main chamber 26 in the 
aerofoil portion 18 of the as cast turbine blade 40 via a pipe 48. The 
pipe 48 has a valve 50 to control the supply of steam along the pipe 48 to 
the chamber 26. The pipe 44 is also interconnected to the chamber 26 via 
the pipe 48 and the pipe 48 has a valve 55 to control the supply of water 
along the pipe 48 to the chamber 26. A vacuum pump 52 is also 
interconnected to the main chamber 26 in the aerofoil portion 18 of the as 
cast turbine blade 40 via the pipe 48. A valve 54 is provided to control 
the removal of gas from the chamber 26. 
A camera 56 is provided to view the second, outer, surface 32 of the 
aerofoil portion 18 of the as cast turbine blade 40. The camera 56 
supplies electrical image signals along line 58 to a processor 60, which 
comprises an image recorder 62 and an image analyser 64. The processor 60 
supplies electrical signals along line 66 to an indicator 68 and/or along 
line 70 to a display 72. The processor 60 may comprise a personal computer 
or other computer. The indicator 68 may for example comprise a lamp which 
Lights to indicate that the passages 36 in the as cast turbine blade 40 
are present and are in the correct position. The display 72 may be a 
television screen, or monitor to show the images of the as cast turbine 
blade 40 to an operator. 
The camera 56 is preferably a camera sensitive to radiation in the infrared 
band, but the camera 56 may be a camera sensitive to radiation in the 
visible band if the turbine blade 40 is coated with a thermochromic 
material, for example a paint, or coating, which contains thermochromic 
liquid crystals or a self adhesive tape having thermochromic liquid 
crystals. The camera may be a thermal imaging camera, a video camera, a 
cine camera or a CCD camera. 
In order to inspect the as cast turbine blade 40 the pipe 48 is fitted and 
sealed to the portion of the chamber 26 in the root portion 12 of the 
turbine blade 40. Initially all of the valves 54, 55 and 56 are closed. 
The turbine blade 40 and camera 56 are so arranged to enable one surface 
of the turbine blade 40 to be viewed by the camera 56, in particular the 
turbine blade 40 may be mounted in a fixture (not shown). 
When the turbine blade 40 is connected to the pipe 48 and the chamber 26 is 
sealed, the valve 54 is opened to allow the vacuum pump to evacuate the 
chamber 26 and hence the chambers 34 and cooling passages 36, assuming the 
chambers 34 and cooling passages 36 have been correctly formed. After the 
chamber 26 has been evacuated the valve 54 is closed and the valve 50 is 
opened to allow steam to be supplied to the chamber 26. As the steam is 
admitted to the chamber 26 the temperature of the second surface 32 of the 
turbine blade 40 is viewed by the camera 56 and recorded by the image 
recorder 62 of the processor 60. The analyser 64 of the processor 60 
analyses the images of the stored in the image recorder 62 to determine 
whether the turbine blade 40 is satisfactory or not. 
In the turbine blade 40 the steam flows through the chamber 26 and some of 
the steam flows through the cooling passages 36 and the steam impinges on 
the first surfaces 35 of the chambers 34, in the form of discrete jets. 
The first surfaces 35 of the chambers 34 are those adjacent the second 
surface 32 of the aerofoil portion 18 of the turbine blade 40. Each jet of 
steam creates a very localised condensing heat transfer to the wall of the 
turbine blade 40 which in turn generates a thermal wave that passes 
through the thickness of the wall. Each thermal wave creates a transient 
hot spot on the second surface 32 of the aerofoil portion 18 of the 
turbine blade 40, the region around each transient hot spot heats up more 
slowly to the same temperature as the transient hot spot. 
The camera 56 is arranged to view the second surface 32 of the aerofoil 
portion 18 of the turbine blade 40 to detect the transient hot spots on 
the surface 32 of the aerofoil portion 18 of the turbine blade 40. Thus 
the presence of transient hot spots at the appropriate positions in the 
images of the aerofoil portion 18 of the turbine blade 40 indicates that 
the cooling passages 36 have been correctly formed. 
On the other hand the lack of transient hot spots at the appropriate 
positions, or transient hot spots at unexpected positions, in the images 
of the aerofoil portion 18 of the turbine blade 40 indicates that the 
cooling passages 36 have been incorrectly formed. The turbine blades 40 
with incorrectly formed cooling passages 36 may be discarded before any 
more expensive machining processes are performed. 
The processor 60 sends appropriate signals to the display 68 to show 
whether a particular turbine blade 40 has correctly or incorrectly formed 
cooling passages 36. If the thermal diffusivity of the material of the 
turbine blade 40 is known a time:temperature history of a transient hot 
spot enables the processor 60 to determine the local wall thickness of the 
aerofoil 18, or if the thickness is known the thermal diffusivity may be 
calculated. 
The use of steam to heat the turbine blade 40 is advantageous because it 
provides the condensing heat transfer to the turbine blade 40. The 
condensation of the steam in the chambers 34, when the steam from the 
cooling passages 36 contacts the wall, allows more steam to be drawn into 
the chambers 34 from the chamber 26 to create impingement jets of steam 
which produce significant flow of heat into the wall of the aerofoil 
portion 18. These impingement jets of steam occur until the pressure ratio 
across the cooling passages 36 falls below a critical value. The critical 
value depend upon the geometry of the chamber 34 and the cooling passage 
36. The pressure ratio may fall due to a decrease in pressure in chamber 
26 or an increase in pressure in chamber 34. 
In order to aid the removal of gas, or air, from the chamber 26 the above 
method may be modified by firstly opening the valve 50 to allow a small 
quantity of steam into the chamber 26 which will condense on the walls of 
the chambers 26 and 34 to form a film of water. The valve 50 is then 
closed and valve 54 is opened to evacuate the chambers 26 and 34. The film 
of water in the chambers 26 and 34 evaporates at low pressure displacing 
the gas, or air, during the evacuation process. Air is a non-condensable 
gas., which inhibits the flow of the impingement jets of steam onto the 
surface of the wall, and may reduce the rate of heat transfer. The small 
quantity of steam also heats up the root portion 12 and the shank portion 
14 of the turbine blade 10 through which the steam has to flow to reach 
the chambers 34 and cooling passages 36. The heating up of the root, 
portion 12 and shank portion 14 by the steam reduces the amount of 
condensation occurring in the chambers 34 and cooling passages 36. 
Alternatively to aid the removal of gas, or air, from the chamber 26 the 
above method may be modified by firstly opening the valve 55 to allow a 
small quantity of water into the chamber 26. The valve 55 is then closed 
and valve 54 is opened to evacuate the chambers 26 and 34. The water in 
the chambers 26 and 34 evaporates at low pressure displacing the gas, or 
air, during the evacuation process. An advantage of using water is that it 
does not heat the turbine blade and so does not reduce the achievable 
amplitude of the temperature step that can be created. 
It is also possible to preheat the root portion 12 and the shank portion 14 
of the turbine blade 10 in an oven to approximately 90.degree. C. 
Following preheating of the root portion 12 and shank portion 14 of the 
turbine blade 10, the aerofoil portion 18 is cooled, for example by a flow 
of cool gas, to enable transient hot spots with a large temperature 
difference from the surrounding regions to be produced. The cooling is 
performed quickly to minimise the possibility of cooling the root portion 
12 and the shank portion 14 of the turbine blade 10. 
The pipe 48, as shown more clearly in FIG. 9, preferably comprises a thick 
outer wall 47 to contain the steam pressure and a thin inner wall 49 of 
low thermal mass and low thermal conductivity to separate the flow of 
steam from the thick outer wall 47, which is at ambient, room, 
temperature. The thin inner wall 49 quickly heats up when steam flows 
through the pipe 48 so minimising the amount of condensation which is 
drawn into turbine blade 10. The pipe 48 includes a steam trap chamber 51 
to prevent condensate, which has formed on the thick outer wall 47, from 
entering the chamber 26 of the turbine blade 40. The steam flowing through 
the inner pipe 49 flows radially outwardly into the chamber 51 to prevent 
condensate on the outer pipe 47 flowing into the turbine blade 40. A 
further steam pipe 53, or other heating apparatus, is positioned in 
proximity to the chamber 51 and pipe 48 to minimise condensation of steam. 
The turbine blade may be coated with an emissivity coating to assist the 
resolution of transient hot spots or to provide a calibrated emissivity 
surface to allow accurate measurement of the temperature to be made. 
The advantage of the present invention is that it allows the cooling 
passages 36 of the turbine blade 40 to perform their required function, 
but under different circumstances. The present invention also allows the 
turbine blades be inspected more quickly than using a borescope or X-ray 
computer tomography. The present invention uses evacuation and 
condensation heating to create a very high rate of temperature increase on 
the inside of the turbine blade unlike conventional thermography which 
uses hot gases which have a very low rate of temperature increase. The 
advantage of the very high rate of temperature increase is that it makes 
it possible to resolve smaller features, i.e. the impingement cooling 
holes. The present invention does not require a flow of the steam through 
and out of the turbine blade unlike conventional thermography, enabling 
the turbine blade to be inspected before all their cooling passages have 
been formed and thus enables defective turbine blades to be rejected 
before being subjected to expensive machining processes. It is also 
possible to determine the distance of hot spots from datum features on the 
turbine blades for each individual turbine blade. This information may be 
used in subsequent machining processes to guide the machining process to 
eliminate the problem of machining film cooling holes in turbine blades in 
a position which is incompatible with the actual position of the internal 
cooling passages, thus saving on scrap and eliminating the need for 
further inspection. 
The use of thermochromic liquid crystals and a video camera sensitive to 
radiation in the visible band is particularly suitable for low volumes of 
turbine blades, and an inspector may view each image of a recording made 
by the video camera, this avoids the expense of a thermal imaging camera. 
The use of thermochromic liquid crystals and a CCD camera sensitive to 
radiation in the visible band is particularly suitable for moderate 
volumes of turbine blades and qualitative inspection only is required, a 
processor is preferred to analyse the images made by the CCD camera. 
The use of a camera sensitive to radiation in the infrared band is 
particularly suitable for high volumes of turbine blades, to eliminate the 
expense of the thermochromic liquid crystals. To improve resolution a 
non-calibrated emissivity coating may be applied to the turbine blades. If 
quantitative information is required a calibrated emissivity coating is 
applied to the turbine blades. 
An apparatus 82 for inspecting turbine blades 86 is shown in FIG. 5, 6 and 
7. The apparatus 82 is substantially the same as the apparatus 42 but 
differ because the turbine blades 86 have film cooling passages 92 
extending through the wall of the turbine blades 86 from chamber 90. 
The apparatus 80 differs in that a vessel 84, having an aerofoil shape in 
cross-section, is placed over the aerofoil portion of the turbine blade 
86. The vessel 84 is spaced from the wall of the turbine blade 86 by a 
uniform clearance 88 to define an enclosed volume between the vessel 84 
and the wall of the turbine blade 86. The open end of the vessel 84 is 
sealed to the platform portion of the turbine blade 86 by a seal strip 82. 
The pipe 48 is arranged to interconnect with the chamber 90. 
The process of inspecting the turbine blade 86 is the same as that 
described with reference to FIG. 4, however the steam flows through the 
film cooling passages 92 as a plurality of impingement jets which impinge 
upon the first, inner, surface 83 of the vessel 84. The camera 56 is 
arranged to view the second, outer, surface 85 of the vessel 84, to detect 
transient hot spots on the outer surface 85 of the vessel 84. 
An apparatus 114 for inspecting an article 100 is shown in FIG. 8. The 
apparatus 114 is substantially the same as the apparatus 42 but differs 
because the article 100 is not a turbine blade, is not hollow and does not 
have film cooling passages extending through the article 100. 
The apparatus 114 differs in that a vessel 102 is placed over the article 
100. The interior of the vessel 102 is divided into a first chamber 104 
and a second chamber 110 by a plate 106. The plate 106 is spaced from the 
article 100 by a uniform clearance 110 to define the second chamber 110 
between the plate 106 and the article 100. The open end of the vessel 114 
is sealed to the article 100 by one or more clamps and/or seals 112. The 
plate 106 is provided with a plurality of passages 108 to direct jets of 
steam onto a first surface 99 of the article 100. The pipe 48 is arranged 
to interconnect with the first chamber 104 for the supply of steam to the 
first chamber 104. A pipe 49 is arranged to interconnect with the first 
chamber 104 for the evacuation of the first chamber 104. A camera is 
arranged to view the second surface 101 of the article 100. 
The process of inspecting the article 100 is the same as that described 
with reference to FIG. 4, however the steam flows through the plurality of 
passages 108 as impingement jets which impinge upon the first surface 99 
of the article 100. The camera is arranged to view the second surface 101 
of the vessel 100 to detect transient hot spots on the second surface 101 
of the article 100. The technique is used in this embodiment to detect 
variations in the thermal diffusivity in the article 100 and hence to 
detect defects in the article 100, for example voids, cracks, areas of 
corrosion or areas of oxidation. If the article is a composite or 
laminated material the technique may detect areas with debonds or 
delaminations. If the article has a coating the technique may detect areas 
with poor adhesion of the coating. The resolution may be achieved by 
increasing the number of passages 108 per unit area in the plate 106. 
Although the invention has described the use of steam to inspect the 
articles, it is also possible to use any other suitable condensable 
heating fluid. 
Although the invention has described the use of a vacuum pump to evacuate 
the articles, or enclosed volume, to remove gas, or air, from the article, 
or enclosed volume, it may be possible to remove the gas, or air, in other 
ways. For example ammonia, or other gas that is highly soluble in the 
condensable heating fluid, may be supplied to the enclosed volume to purge 
the gas, or air, from the enclosed volume. However, this is not preferred 
because it is necessary to provide an exit pipe and valve to allow the 
displaced gas, or air, out of the enclosed volume. Additionally there are 
problems of handling the ammonia and any waste product gases but the need 
for a vacuum pump would be removed. 
Although the invention has referred mainly to turbine blades it is equally 
applicable to turbine vanes and other articles. 
Although the invention has described the use of jets of steam directed to 
impinge upon the surface of the article or vessel to provide impingement 
heat transfer to the article or vessel, it may be possible to arrange for 
the steam to flow over the surface of the article if the surface of the 
article has other heat transferring features on its first, inner, surface 
such as ribs, strips, pedestals. These features will transfer heat into 
the wall of the turbine blade to create hot spots on the second, outer, 
surface of the aerofoil portion of the turbine blade.