leads to a very high COV for the computed radii. A COV of 0.98 was computed for the
defect boundary shown in Figure 6-13B.
30
29.5
2 4 6 10 12 14 16 18
A B
Figure 6-13. Reduced accuracy in area computations due to low image resolution. A)
Thermal image for small defect. B) Gradient intensity.
Proposed Method for Characterizing Detectability
This section outlines a proposed method for characterizing defect detectability.
The first distinction that will be made is based on the shape of the ATdef VS. time plot.
There were four basic shapes encountered during the IRT inspections of the Series A
specimens. Each defect in these specimens will be classified Level I, Level II, Level III,
or Level IV based on the shape of its ATdef VS. time plot. Level I defects assume a
positive slope for t > 0 and achieve a single maximum value at t = tmax. Level II curves
begin with a negative slope and reach a local minimum at t = tb. After reaching the local
minimum, the curve assumes a positive slope for t > tb until a distinct local maximum is
reached at t = tmax. Level III curves begin with a negative slope and never assume a
positive slope. There is, however, a distinct tb that is recognizable when the difference in
temperature on the perimeter of the defect's defining area is subtracted from the ATdef VS.
time plot. This can also be recognized as an inflection point in the ATdef VS. time plot.
The final classification, Level IV, is intended to describe defects that were not detected.
A graphical depiction of each classification is provided in Figure 6-14.