perpendicular to the fibers. The column labeled "Actual Size" provides the true area of
the defect based on the diameter measurements. The column labeled "Image Diameter"
provides the average diameter computed for the defect using the gradient area method.
For the camera configuration used in the pulse heating experiments, the length ratio for
each pixel was 1.1 mm/pixel. For the circular shaped air-filled defects (A75, A50, and
A25), the average diameter computed using the gradient area method was within the
distance represented by 1 pixel of the true diameter.
The COV value computed for each of these defects varied considerably. The
gradient images that were generated for defects A75, A25, E75 and E75 are provided in
Figure 6-16. These four defects all fall within a different general detectability category
based on the COV for the computed radii. The COV computed for Defect A75 was 0.10
(Category A) and the COV for Defect A25 was 0.28 (Category B). This difference in
COV would indicate that the boundary generated for A75 was more consistent than the
boundary determined for A25. This is not necessarily supported by visual inspection of
the gradient images. The higher COV for Defect A25 can be attributed to a slight
misalignment of the defect's center point. Defect E75 had a COV of 0.20 (technically a
Category A). The largest source of variation in the defect's boundary occurs on the right
side of the gradient image. This is likely due to a combination of weak signal and other
imperfections in the composite above the defect. The gradient image generated for
Defect E25 is provided in Figure 6-16D. This defect is poorly defined and the computed
radius has a high COV of 0.79 (Category C).
These results demonstrate that COV can provide important insight into how well a
defect's boundary has been defined by the gradient area method. It must be noted,