# Ultrasonic Attenuation Characteristics of Glass-Fiber-Reinforced Polymer Hull Structure
## 7 Conclusions  
The purpose of this study was to investigate the attenuation characteristics of ultrasonic waves according to the design and fabrication characteristics of GFRP laminates for composite ships. To this end, various combinations of GFRP laminates with _Gc_ values and thicknesses similar to the actual design values of hull plates were fabricated, and a pulse echo ultrasonic test was performed on the fabricated specimens. The conclusions of this study are as follows.  
When the _Gc_ was set to 30%, 40%, and 45% while the thickness was kept at similar values (7.56-9.87 mm), the attenuation coefficient showed a natural tendency to increase accordingly. However, some wave irregularities were identified, which are thought to have  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & & \multicolumn{3}{c}{**Mean**} & & & & \\
**Hull Plate** & **Design _Gc_ (\%)** & **Thickness** & & & & & \\  & & & & & & & & \\  & & & & & & & & \\ \hline Hull plate 1 & 30.00 & 9.87 & 0.66 & 0.92 & 0.91 & 0.79 & 0.90 & 0.84 \\ Hull plate 2 & 40.00 & 8.71 & 1.03 & 0.92 & 0.57 & 1.13 & 0.57 & 0.84 \\ Hull plate 3 & 45.00 & 7.56 & 1.49 & 1.41 & 1.33 & 1.43 & 1.72 & 1.48 \\ Hull plate 4 & 40.00 & 14.10 & 1.48 & 0.85 & 0.94 & 1.17 & 1.50 & 1.19 \\ Hull plate 5 & 40.00 & 18.34 & 1.89 & 1.55 & 1.69 & 2.13 & 1.72 & 1.80 \\ \hline \hline \end{tabular}
\end{table}
Table 6: _Vv_ values of the hull plate specimens according to burn-off testing.  
Figure 12: Comparison of void volumes within the hull plate (_Gc_ variation group vs. thickness variation group).  
originated from the increase in the GFRP laminate density caused by increasing the E-glass fiber weight; the E-glass fiber chop also increased the ultrasonic wave scattering.  
When the \(Gc\) value of the hull plate was kept constant at 40% and the thickness was changed to 10 mm, 15 mm, and 20 mm, all three measurement groups exhibited highly similar attenuation coefficients. The attenuation increased significantly, becoming approximately three times larger than that in the \(Gc\) variation group. This increase in attenuation could have occurred because the amount of polyester increased considerably when the thickness was increased at the same as the \(Gc\), and the interface change between each ply of the laminate increased.  
Considering the above results comprehensively, in the \(Gc\) range of 30-50%, which is the common range of \(Gc\) values applied in the design of laminate structures for GFRP ships, the plate thickness had a greater impact on the attenuation characteristics than the \(Gc\). In addition, the effects of inner defects such as voids on the ultrasonic waves were relatively insignificant.  
Conceptualization, D.O. and J.H.W.; methodology, D.O. and J.H.W.; funding acquisition, D.O.; manufacturing, Z.H. and J.-W.J.; test and investigation, Z.H. and S.J.; writing--original draft, Z.H. and S.J.; writing--review and editing, D.O. All authors have read and agreed to the published version of the manuscript.  
This research was a part of the project titled \"Jeonnam Sea Grant Program\", funded by the Ministry of Oceans and Fisheries, Korea (grant number 20170347).  
The authors declare no conflict of interest.