location of maximum slope of the temperature field corresponds to the edge of the defect
below the surface. This procedure will be referred to as the gradient area method in the
text below.
40 40
5 10 15 20 25 30 35 40 10 20 30 40 50 60 70
A B
Figure 6-8. Defect area computations using boundary trace method. A) Defect A75
(Specimen A-i). B) Defect IB (Specimen A-i)
A Matlab routine was developed to automate this procedure. The first step is to
identify the thermal image collected at tmax. The next step is to compute a gradient image
of the box area used to define the defect. The magnitude of the gradient for each pixel in
this area is computed using Matlab's built-in gradient operator. The gradient image
generated for Defect A75 (Specimen A-i) is provided in Figure 6-9A. Figure 6-9D
provides a surface plot of the gradient image which illustrates the location of the defect
boundary.
The next step is to locate the center of the defect by identifying the pixel with the
smallest gradient near the center of the gradient image. Once the center of the defect has
been identified, a line is constructed between the center and the upper left corner of the
area containing the defect. The location of the maximum value along this line is
determined and stored as one point on the defect's boundary. Next, a new line is
constructed from the center point to the pixel on the border just below the upper left