216
medium saturation specimens in Series B were constructed using the amount of epoxy
recommended by the FRP system manufacturer.
Normalized temperature results for the single-layer specimens appear to be very
similar. The mean value of ATnorm for Specimen B-MC-1 was -0.277 (a = 0.023). The
mean value of ATnorm for Specimen A-i was -0.283 (c = 0.018). A Student t-test was
performed to validate the hypothesis that the two means are equal (Table 6-23). The
corresponding t-test value was 6.66. This value exceeds the tabulated t-value for p =
0.001 (degrees of freedom = oo), which indicates that the means are significantly
different. The t-test value obtained when comparing the two means for the two-layer
specimens was 0.99, which indicates that the means are essentially the same.
Table 6-23. Normalized temperature response @ t = 60 sec for properly saturated
specimens
Series B: medium saturation Series A: medium saturation
Standard Standard
Mean @ deviation @ Mean @ deviation @ t-test
Layers NP t=60 sec t = 60 sec NP t=60 sec t = 60 sec value
1 1350 -0.277 0.023 770 -0.283 .018 6.66
1a 50 -0.277 0.022 50 -0.279 .019 0.49
2 1196 -0.093 0.023 780 -0.092 .021 0.99
3 1092 +0.008 0.017 864 +0.023 .019 18.16
4 990 +0.027 0.025 851 +0.084 .020 54.31
a Defect free areas on Specimen A-i and Specimen B-MC-1 were randomly sampled for 50 data
points
The requirements for claiming significance based on Student's t-test are very
stringent if a large number of data points are considered. A better test for significance
may be to randomly sample each of the areas for 50 data points and then compute the
mean and standard deviation of the sample. This operation was performed for the defect-
free areas on Specimen A-i and Specimen B-MC-1. The resulting means and standard
deviations were essentially the same but the t-value was reduced to 0.48. This t-value
indicates that the means are not significantly different. Additional investigation is needed