# Possibilities of Detecting Damage Due to Osmosis of GFRP Composites Used in Marine Applications
## 4 Results and Discussion  
The performed numerical calculations showed that the best imaging of all defects was obtained at the frequency f = 1 kHz (this is the frequency of the sound wave) and the amplitude A = 0.1 mm. Figure 6 shows the graphs of temperature changes over time on the sample surface over the defects. The simulation time of generating vibrations (heating) was t\({}_{\rm h}\) = 3 s, and the total simulation time t = 5 s. The point of application of the center of the vibration transducer (with an area of 1 cm\({}^{2}\)) on the sample surface was: x = 50, y = 50 (Figure 5). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Value** & **Unit** \\ \hline Specific heat & 1.62 & J\(\cdot\)kg\({}^{-1}\cdot\)K\({}^{-1}\) \\ Density & 2000 & kg\(\cdot\)m\({}^{-3}\) \\ Thermal conductivity & 0.38 & W\(\cdot\)m\({}^{-1}\cdot\)K\({}^{-1}\) \\ Poisson’s coefficient & 0.24 & - \\ Young’s modulus & 36 & GPa \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Thermal and strength properties of the composite [26]. Figure 5: Model of composite where 1, 2, and 3 correspond to the defects 1, 2, and 3, respectively. Figure 7 shows the temperature distribution after 3 s from the start of vibration generation. Then, as shown in Figure 5, the increases in temperature values above the defects were the highest. The shape of defects cannot be determined in Figure 7, which shows the thermogram simulation. The Fourier transform was used to improve this visualization (Figure 8). Such an operation is possible thanks to the ThermoSon software used in the simulation. The principle of processing thermograms using the Fourier transform is described in the paper [27]. As you can see in Figure 8, the shape of the defects is very visible and consistent with the model shown in Figure 5. The image shown in Figure 5 was obtained for the 14 th harmonic. Figure 6: The course of temperature changes on the sample surface over the defects (location of the center of the vibration transducer x = 50, y = 50). Figure 7: The temperature distribution after 3 s from the start of vibration generation (location of the center of the vibration transducer x = 50, y = 50). Determining the shape and dimensions of the defect, especially when it is not visually visible, is important to repair the damaged part of the hull of the watercraft. The example of using the Fourier transform presented in the article shows how helpful the thermogram processing methods (described in [27]) can be in detecting defects using infrared thermography. Figures 9 and 10 show how the temperature rise changes with time during the heating and cooling phase over the defects when the position of the center of the vibration transducer changes. Figure 8: Image of the thermogram from Figure 6 after analysis using the Fourier transform for the 14 th harmonic. Figure 9: The course of temperature changes on the sample surface over the defects (location of the center of the vibration transducer x = 40, y = 50). The presented simulation results clearly show that the impact on the detection of the defect (increase in the temperature value on the surface of the sample above the defect), with the same thermal forcing parameters (vibration transducer operating parameters), is significantly influenced by three factors: the size of the defect area, the distance between the defect, the transducer application point, and the depth under the surface where the defect is located. The defect D1, which has the largest surface area, generates the highest increase in temperature, as it can be seen in the graphs in Figures 7 and 9. Figure 7 shows the effect of the depth at which the defect is located under the sample surface. The transducer application point is at a similar distance from the D2 and D3 defects. The defect D2 is located deeper under the surface of the sample and the value of the temperature rise on this defect is lower than that on the defect D3, even though the surfaces of these defects are the same.