Front-flash thermal imaging characterization of continuous fiber ceramic composites. Page: 5 of 9
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3
indicated as peak difference time and peak slope time, respectively, in Fig. 2, and can be used to
correlate the depth of the flaw.a
v
C
as
s.
A
a
x.
r.+
C
it
d
c.
a
FPeak Slope
Peak Slope
TimePeak Difference
Peak Difference
TimeTime
Fig. 2. Temperature-difference curve showing characteristic times.
RESULTS
Front-flash thermal imaging tests were conducted on three flat-plate specimens, i.e., a
carbon/carbon (C/C) specimen and two CFCC specimens. These specimens were machined with
flat-bottom holes of various diameters and depths at the back surface. They were used to simulate
subsurface delaminations of different sizes and depths from the front surface.
C/C Specimen
This specimen is a 25-mm-thick C/C composite plate machined with six flat-bottom holes at
the back surface. These holes all have the same diameter (12 mm) but different lengths. When
viewed from the front surface, the depths of the hole bottoms below the front surface ranged from
0.5 to 3 mm with an increment of 0.5 mm, as illustrated in Fig. 3.Cl
Hole
A
B
C
D
E
FDepth (mm)
0.5
1.0
1.5
2.0
2.5
3.0A B C
D E F
Fig. 3. Diagram of C/C specimen with six flat-bottomed holes at back surface.
Figure 4 shows an infrared thermal image (at spatial resolution of 0.3 mm/pixel) of the
front surface after the thermal flash has been applied. It shows the thinnest material layer (0.5 mm)
at left with the highest temperature rise and the thickest layer (3 mm) at right with the smallest
temperature rise. The temperature difference for each of the six holes was determined by
subtracting the average temperature over the flawed area from that of two adjacent areas with no
subsurface flaws. Use of two (rather than one) unflawed areas may reduce the effect of
nonuniform heating. The corresponding difference curves as a function of time for the six holesi
I
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Deemer, C. Front-flash thermal imaging characterization of continuous fiber ceramic composites., article, April 23, 1999; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc620168/m1/5/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.