The signal-to-noise ratio depends on the heating of the sample,which in turn depends on the power and the heating time. Therefore, the greater heating time can lead to the higher signalto-noise. However, thermal diffusion process can lead to the blurring of the image over time. As the power of the generator is limited, a compromise is necessary to get enough heat into the material under test and to have a good contrast in the image. This compromise is quite easy for most materials and results in heating time of about 50–200 ms[30]. In the experiments under reflection mode, the heating
time is set after optimisation and comparison as 200 ms and the cooling time is set as 300 ms. Firstly, the front side of 10 J impact sample is tested. Fig. 7 shows the thermograms for front side and rear side of 10 J impacted laminate at 200 ms. The unit for x-axis and y-axis is pixel and the unit of temperature is digital level (DL).In Fig. 7a, there is a circle shape of higher temperature around impact. However, the middle area (concavity with thinner thickness) does not show the higher temperature. In Fig. 7b, the higher temperature distribution is concentrated.
The signal-to-noise ratio depends on the heating of the sample,which in turn depends on the power and the heating time. Therefore, the greater heating time can lead to the higher signalto-noise. However, thermal diffusion process can lead to the blurring of the image over time. As the power of the generator is limited, a compromise is necessary to get enough heat into the material under test and to have a good contrast in the image. This compromise is quite easy for most materials and results in heating time of about 50–200 ms[30]. In the experiments under reflection mode, the heatingtime is set after optimisation and comparison as 200 ms and the cooling time is set as 300 ms. Firstly, the front side of 10 J impact sample is tested. Fig. 7 shows the thermograms for front side and rear side of 10 J impacted laminate at 200 ms. The unit for x-axis and y-axis is pixel and the unit of temperature is digital level (DL).In Fig. 7a, there is a circle shape of higher temperature around impact. However, the middle area (concavity with thinner thickness) does not show the higher temperature. In Fig. 7b, the higher temperature distribution is concentrated.
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