concrete results in a relative surface temperature decrease (F = 0.22). The previous
section on pulse thermography analysis was primarily concerned with monitoring the
difference in temperature between a defect area and a surrounding defect-free area. The
fact that the "defect-free" area actually consists of a multi-layer material that responds
differently than a homogeneous material to thermal stimulation was not considered.
Two factors must also be considered when applying the step heating model to
results from the current study: two-dimensional heat flow around defects and non-
uniform heating. If the thermal front traveling from the surface does encounter a defect,
Equation 5-9 will only apply for as long as the heat flow remains one-dimensional
(Osiander et al. 1996). If a thermal gradient is present between the defect area and the
surrounding defect-free area, the "backed-up" heat that is stored above the defect drain
off into the surroundings by traveling around the defect. Once this occurs, the magnitude
of the divergence away from the homogeneous case is reduced.
Non-uniform heating can be addressed by applying a normalization procedure to
each pixel in the series of thermal images (Osiander et al. 1996). Normalized AT is
computed with the following equation:
AT (t) =AT(t) 1 (6-9)
C, I
ATnorm = Normalized change in temperature
Co = Initial slope of AT vs. t1/2 curve (oC/sec1/2)
t = time
ATnorm provides an indication of how far the change in temperature for a single
point has drifted away from what would be expected for a homogeneous material. Note
that for a homogeneous material, Equation 6-9 results in a value of zero for ATnom, at all
values of time. For the case of a non-homogeneous material, ATnomn will equal zero up to