SAMPE Journal - May/June 2020 - 51

the local filter area was subtracted from a larger
outer kernel, comprising a 25x25 mean filter iterated 20 times. The resulting SRC matrix was summed
over the time range 2.5s to 12.5s. Contrast results
for both the close-proximity, flash thermography
experiment and the large-standoff, step thermography experiment are presented in Figure 6.

Figure 6. Sum SRC images of close-proximity flash thermography experiment (left) and large-standoff step thermography experiment (right).

Signal-to-Noise Ratio Comparison
The signal-to-noise ratio (SNR) of each defect was
calculated by comparing the signal generated by
the FBH from the sum SRC image to the signal
generated by a known sound area. Horizontal profile lines were drawn to across the defects at each
depth to display the graphs shown in Figure 7.
Spatial Resolution Discussion
In the experimental setups used for this paper, the
close-proximity flash thermography setup provided a higher spatial resolution (2.1 px/mm) when
compared to the large-standoff step thermography
setup (1.4 px/mm), which was designed to measure
a minimum of a 6.35 mm defect. When analyzing
the larger defects, the difference in resolution did
not significantly affect the measured SNR values.
However, camera resolution becomes a critical factor when analyzing smaller defects (i.e. the 2.2mm),
where lower resolutions can cause the number of
pixels that fall on smaller defects to fall at or below
the recommendations provided by ASTM E258207. In the close-proximity flash thermography experiment, the 2.2mm defects cover approximately
4.6 pixels, compared to the large-standoff step
thermography experiment, where these same defects only cover 3.1 pixels, falling right at the recommend minimum limit. This, combined with the
clutter generated by the weave of the sample and
the small aspect ratios of the defects, cause issues
with detectability.
CONCLUSIONS
When compared to the SNR results provided by
a close-proximity flash thermography system, a
large-standoff step thermography system maintains detectability required for identification of
subsurface defects in composite components while
providing a considerable increase in inspection
rate. The large-standoff step thermography system
provides lower SNR values for the shallow defects,
due both the use of the summed SRC filter along
with the longer blind period, but offers comparable
performance when analyzing deeper defects.
Replacing flash thermography with stepped
thermography produces an extended blind period, making it less effective for inspection of thin,

w w w. s a m p e . o r g

Figure 7. Horizontal profile lines measured across
the FBH defects, comparing SNR generated by
the close-proximity flash thermography system
(gray, solid) and the large-standoff step thermography system (black, dotted) at depths 1.5 mm
(top), 2.3 mm (middle), and 3.0 mm (bottom).

M AY/J U N E 2 0 2 0

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SAMPE Journal - May/June 2020

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