a certain time (tT in Figure 6-53) and then diverge. The process is illustrated for two
points on Specimen A-i (one-layer FRP) in Figure 6-54. Point 1 is located very close to
the heat source and experiences an overall AT of 11.50C after 60 seconds of heating.
Point 2 is located farther from the heat source, which results in a lower AT of 60C after
the same heating duration. Co is computed for each curve by measuring the slope of the
line through the points collected for t < tT. For both of the curves shown in Figure 6-54A,
tT occurs at some time between t1/2 = 2 and t1/2 = 3. Once Co is known, ATnorm can be
computed using equation 5-9. The resulting curves for ATnorm are provided in Figure 6-
54B. With the exception of some minor deviation that begins at t1/2 = 6, the two points
display a similar ATnorm response.
It was observed that the initial slope of AT vs. t1/2 plots did not pass through the
origin. This was likely a result of poor synchronization between the time that the lamps
were turned on and the time the first image was recorded. If this were the case, the AT
response would actually be zero for some time interval between t = 0 and t = 1 sec. This
observation could also result from the fact that some time is required for the lamps to
reach their maximum output level. In this case, the initial slope of the AT vs. t1/2 curve
would start at a lower value and increase as the lamps reach a constant output level.
In order to apply this method to the data collected during each experiment, the
initial slope and corresponding y-intercept was determined using a least-squares
regression for the data points obtained between t = 1 sec and t = 4 sec. Since the top
layer of FRP was essentially the same for all of the specimens, this initial window proved
adequate to capture the trend of the initial response for each pixel.