Thermal testing of ceramic components using a thermal gradient

Non-destructive evaluation of a ceramic nozzle is accomplished by generating a thermal gradient across the nozzle while it is mounted in a test rig. The thermal gradient is then measured and compared to a preselected gradient that is representative of the harshest condition to which the nozzle will be exposed during its operating life. The thermal gradient is adjusted until it approximates the preselected gradient. The nozzle is then observed for any cracks which reveal a weakness in the ceramic.

TECHNICAL FIELD 
The present invention relates to non-destructive evaluation methods, and in 
particular to such a method for ceramic components used in gas turbine 
engines. 
BACKGROUND OF THE INVENTION 
Microfocus X-ray inspection, fluorescent penetrant inspection and visual 
inspection are conventional, non-destructive evaluation techniques used on 
metal components before the component is mounted in a gas turbine engine. 
These techniques reveal flaws or cracks in the metal that would result in 
the component failing during its operating life. Thus, the component can 
be rejected before it is mounted in the engine. For a number of reasons, 
these techniques have not been very reliable when used on ceramic 
components. First, a ceramic component may have hidden cracks or flaws 
that are related to a weakness in the ceramic, but which cannot be 
detected due to resolution limitations of these techniques. Second, these 
techniques may detect flaws or cracks in the ceramic that are not 
indicative of weakness in relation to the anticipated operational stress 
for the part, resulting in the rejection of a perfectly good part. 
Accordingly, there is a need for a non-destructive evaluation method for 
ceramic components that only detects flaws or cracks in the ceramic 
indicative of a weakness in relation to the anticipated operational stress 
that the material will be exposed to during engine operation. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a non-destructive 
evaluation method for ceramic components that detects flaws or cracks in 
the ceramic indicative of a weakness in relation to the anticipated 
operational stress that the material will be exposed to during engine 
operation. 
The present invention achieves this objective by providing a method in 
which a thermal gradient is generated across a ceramic component. The 
thermal gradient is then measured and compared to a preselected gradient 
representative of the largest thermal induced stresses the component will 
likely be exposed to during its operating life. The thermal gradient is 
adjusted until it approximates the preselected gradient. The ceramic 
component is then observed for any cracks which reveal a weakness in the 
ceramic.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a test rig 10 is mounted on a table 12 and includes a 
holding fixture 14 disposed between two support posts 16, 18. The holding 
fixture 14 slides backwards, and the support posts 16, 18 swing apart. A 
ceramic turbine nozzle 20 is supported by the fixture 14. Mounted to the 
support posts 16, 18 are a plurality of impingement tubes 22. Some of the 
tubes 22, referred to as torches, deliver a jet of hot gas generated by 
igniting a mixture of oxygen and propylene. Alternatively, numerically 
controlled lasers can be used to apply heat. Other of the tubes 22 deliver 
cooling air. The flow of gas or air through each of the tubes 22 is 
controlled by a valve 26. 
To calibrate the rig 10, the fixture 14 is positioned in its back position 
and the support posts 16, 18 are spaced apart. The torches are then lit, 
and the flow adjusted until the cone of each flame is about 3/16 inches 
long. The cooling air tubes are slightly opened to prevent the rig 10 from 
over heating. The nozzle 20 is mounted on the fixture 14 which is then 
moved forward. The posts 16, 18 are brought together so that the gas flows 
and air flows from the tubes 22 impinge on the surface of the nozzle 20. 
For the nozzle 20 two torches are directed at its leading edge and two at 
its trailing edge. Cooling air is directed to its fillet radii and to the 
outer surface of its inner and outer shrouds. After a few seconds 
exposure, the temperature distribution or gradient across the nozzle 20 
stabilizes to steady state. 
An optical fiber thermometer (OFT) is then used to measure this temperature 
gradient. The tip of the OFT is held as close to the nozzle surface as 
possible, preferably at a distance of about 0.025 inches to 0.05 inches. 
It is important to avoid touching the nozzle surface with the pyrometer as 
it will conduct energy away from the ceramic, locally cooling it. To 
ensure that the surface temperature of the nozzle 20 does not exceed 
2200.degree. F., measurements should first be made at the locations where 
the maximum temperatures are anticipated. A sufficient number of locations 
are measured so that a comparison can be made between the measured 
temperature gradient and a preselected temperature gradient. 
FIG. 2 shows a typical preselected temperature gradient for the gas turbine 
engine first turbine stage ceramic nozzle 20 in which regions of different 
temperature are highlighted by different colors, though FIG. 2 is in black 
and white with different shades representing different colors. For the 
nozzle 20 the highest temperatures occur at the leading and trailing edges 
30,32, and the coolest temperature at the outer and inner shrouds 34,36. 
The preselected temperature distribution is generated by using computer 
models and represents the conditions under which the nozzle 20 will 
experience the largest thermally induced stresses during its operating 
life. 
The flow of gas and air through the torches and cooling jets are adjusted 
until the measured temperature gradient agrees with the preselected 
gradient. In addition, the contours of the different temperature regions 
should also agree. This is easily checked as the nozzle 20 becomes 
translucent when heated. 
With the impingement jets 22 calibrated to reproduce the preselected 
gradient, the posts 16, 18 are moved apart and the holding fixture 12 is 
moved backward. The rig 10 can now be used to test a plurality of nozzles 
one nozzle at a time. Each nozzle is mounted to the holding fixture 12, 
exposed to the jets 22 for about 10 seconds and observed. Cracks will 
appear on the surfaces of the nozzle, if there are any weakness related 
flaws in the ceramic. 
Though the preferred embodiment has been described with reference to a 
ceramic nozzle used in a gas turbine engine. The subject invention is 
applicable to any ceramic component wherever it may be used. 
Various modifications and alterations to the above described preferred 
embodiment will be apparent to those skilled in the art. Accordingly, this 
description of the invention should be considered exemplary and not as 
limiting the scope and spirit of the invention as set forth in the 
following claims.