20
18 -
16 *
14
S10 *
8 8
6 -O o-
24 _'.--e--------< --
<. **
0 0.2 0.4 0.6 0.8 1
radii COV
Figure 6-42. Maximum signal vs. radii COV for all detected defects in Series A
In this equation, a is the thermal diffusivity of the material above the defect. This
model assumes that the volume of material above the defect acts like a "heat trap".
Excess heat that builds up in this volume can only escape through the curved surface of
the "imaginary cylinder" above the defect. tmax corresponds to the amount of time
required to fill-up this volume. As a result, smaller defects at the same depth will have a
shorter time to ATmax. Similarly, if two defects of the same size are considered, the one
closer to the surface will have a smaller tmax.
Since this model is based on filling a volume above a defect with heat, the time
associated with the pulse duration will have an impact on the time at which ATmax occurs.
As a result, the data generated using the different pulse durations can not be analyzed
using this model. This is illustrated by the bar charts in Figure 6-43. For the single-layer
specimen (Figure 6-43A), the tmax recorded for Defect IB during the flash heating
experiment was 12 sec. The same defect resulted in a tmax of 2 sec for the 60 sec pulse
duration. A similar trend is observed for the remaining defects in all four specimens.