SAMPE Journal - May/June 2020 - 47

07 pixel coverage requirement for a 6.35 mm flaw
using a 640 x 512-pixel camera1. Unlike the flash,
where energy is discharged within a few milliseconds, the energy density in a step heating system is
proportional to the duration of the heating period,
so that high energy density can be achieved at the
expense of increasing the blind period, which is ½
the duration of the heating period. While this precludes the use for step heating on thin, high thermal diffusivity materials where the thermal transit
through the part is terminated before step heating
has been completed, it is quite compatible with
many composite NDT applications.
Detectability Considerations in Composite
Parts
In its simplest form, active thermographic flaw detection is based on visual identification of contrast
between a flaw and local intact regions in an IR
image sequence of the surface temperature as the
sample responds to transient thermal stimulation2.
The detectability of a flaw depends on numerous
extrinsic factors including IR camera sensitivity,
speed and resolution, excitation energy, sample
surface condition and presence of background radiation or noise sources3. However, the fundamental indicators of detectability are the flaw diameter,
depth and the degree to which the composition
of the flaw and surrounding matrix differ. Thus,
an ideal flaw is a void embedded in a solid where
the flaw diameter is significantly greater than its
depth, so that the flaw presents an adiabatic interface that completely obstructs the flow of heat
from the excited surface. In practice, an ideal flaw
is often simulated using a Flat Bottom Hole (FBH)
machined into a plate, with the understanding that
this is a "best case" scenario for detectability. Large
adiabatic flaws (with respect to their depth) may be
detectable in the unprocessed IR image sequence;
smaller flaws may require additional signal processing or considerably more excitation energy4.
A subsurface flaw can be described as a trap
formed by the sample surfaces and the flaw, which
obstructs the flow of heat, and the circumferential
boundary surface between them, through which
trapped heat escapes (Figure 2). Detectability, i.e.
the contrast between the flaw and surrounding
area, is related to the ratio of the blockage and leakage surfaces areas, e.g. for a cylindrical hole with
depth d, diameter D and radius r

Figure 2. Flat Bottom Hole (FBH) defect in an adiabatic slab.

The relationship between blocking and leakage
surfaces is characterized by the ratio of diameter
to depth (aspect ratio). Flaws with an aspect ratio
greater than 1 are considered detectable in an isotropic material1.
This relationship between aspect ratio and detectability has been demonstrated across a range
of isotropic host materials5. In composite components, detectability becomes considerably more
complex, where both perpendicular (r-z) anisotro-

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SAMPE Journal - May/June 2020

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