In summary, the two-dimensional correlation coefficient provides a quantitative
measure for how well a defect is defined with respect to some baseline value. In this
case, the baseline value was chosen at the end of heating since this represents a point at
which the defect is well-defined with respect to the surroundings. For the single-layer
specimen, correlation with the baseline image occurs relatively quickly since the time
required for the thermal front to reach the defect is small.
S ---------------------- -
0.95 -------- --- ----- A75
0.9 -- --- --- -- -- --- ----- ----- E75 -
SE50
0.85 ------I I ----- I L -----L-----
0.85 ----
0.
0.75 -'----- "------
0.67 -
0.65 t2
ATn.. image t/2 = 3
0.55 - -
0.5
0 1 2 3 4 5 6 7 8
Square Root of Time (sec1/2)
Figure 6-65. Two-dimensional correlation coefficient for Specimen A-i defects
Specimen A-2
Applying the same analysis procedure to Specimen A-2 generated markedly
different results. The threshold value for ATdef was achieved between t1/2 = 4.3 and t12
5.8 for defects A75, A50, E75, and E50. Data are provided in Figure 6-66. Defect IB
achieved the threshold value much earlier at t1/2 = 2.25. Recall that Defect IB on
Specimen A-2 contained rather large unintentional defects between the top and bottom
layer of composite. The results contained in the ATdef vs. t1/2 plot are significant for two
reasons. First, Defect IB generated a signal at a t1/2 value very close to what was