Thermal diffusivity imaging of continuous fiber ceramic composite materials and components Page: 4 of 13
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methods are primarily utilized for specimens whose thermal properties are assumed
uniform [2-5]. The critical issue in most CFCC applications, however, is the
distribution of thermal properties within a given component. In these situations,
thermal infrared imaging methods, which are of greater utility, have been employed
as a nondestructive testing method for CFCCs with the limitation that the test
methods employed are either not portable or very expensive. CFCCs emit energy in
different directions, parallel and perpendicular to the two dimensional weave.
Transmissivity, the ratio of energy transmitted by the material to the incident energy,
is used to determine the wavelength at which a body transmits infrared energy and is
important in infrared detection.
Nondestructive techniques such as ultrasonic testing have limitations due to the
requirement that the test specimen be submerged in water, while CT scanning is
effective but cannot be portable and inexpensive. Presently, acoustic emission and
resonance are being researched, but more time is needed in order to determine their
workability as test methods. Thermographic techniques are full-field and
simultaneously and independently measure thermal properties over a 2-D distribution
of locations. Since the detection and thermal excitation required is accomplished
radiometrically, the method is inherently noncontact and can be performed at elevated
This paper describes a modified thermographic imaging technique to the
measurement of thermal diffusivity of a number of SiC/SiC CFCC specimens and
components. Currently, the available commercial equipment for infrared analysis
limits the size of components to 10 to 15 mm diameter and 6 mm thickness requiring
high spatial resolution and long acquisition times . The approach of the
experiment is to use a commercially available infrared camera, a suitable thermal
excitation device, and locally written software to automate the entire test setup.
Complete thermal history is recorded for the specimen area of interest from the time
of thermal excitation until the surface reaches its maximum temperature. From this
temperature-time relationship, thermal diffusivity is calculated by two methods. By
imaging at a higher resolution than required and spatially averaging the resulting
images before computation of the diffusivity, sufficient noise reduction is achieved in
a single heating cycle while maintaining the required spatial resolution.
The thermal imaging system used for this work is illustrated in Fig. 1. This system
uses the method of Parker et al.  to calculate thermal diffusivity requiring a thermal
pulse of short duration to be incident upon the front surface of a specimen and the
temperature of the back surface to be monitored as a function of time. This was
accomplished by heating the front surface of the specimen using a 6 U photographic
flash lamp with a pulse duration of less than 8 ms and monitoring the back-surface
temperature using a commercially available scanning radiometer infrared camera.
The camera was equipped with a 3-12 pm optical band-pass lens system and a liquid-
nitrogen-cooled HgCdTe detector. Images were acquired using a Mac II with an on-
board frame grabber capable of storing 128 images in real time at 512 x 512 pixel 8-
bit resolution. The frame grabber board received standard RS-170 signals from the
infrared camera and digitized each received image, which was then processed with
locally developed software. The software extracted the average gray scale value
representing the temperature of the specimen. Specimens were mounted in
diatomaceous earth having low thermal conductivity and minimizing lateral heat flow.
By determining the position of the scanning mirrors at the time of the flash (e.g., by
determining which of the 512 image rows was excited by the flash) the exact flash
time could be determined with sub-millisecond resolution.
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Ahuja, S.; Ellingson, W.A.; Steckenrider, J.S. & King, S. Thermal diffusivity imaging of continuous fiber ceramic composite materials and components, article, December 31, 1995; Illinois. (digital.library.unt.edu/ark:/67531/metadc678631/m1/4/: accessed December 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.