# Dynamic Bending Behaviour of Sandwich Structures for Marine Applications
## 5 Conclusions  
This paper examined the effect of seawater exposure on the impact bending properties of FRPSSs and internal damage modes and morphology, with the moisture intake monitored with an AE approach. It contributes to the understanding of the global impact bending response of FRPSSs used in marine structures when subjected to low-velocity loading, leading to possible optimisation of composite structures. The major findings are as follows:  
* Specimens of Type A exhibited a higher moisture uptake than the GSV and GSH samples, with absorption rates of 4.8%, 4.4%, and 4.3%, respectively. This was ascribed to the effects of the adhesive layers in Type B. * All specimens showed a reduction in damage resistance properties after seawater exposure under both single and multiple impact loading regimes. For single-impact loads, GSP experienced the largest reduction in force at 39.1% after seawater exposure, while GSV and GSH exhibited reductions of 7.2% and 15.9%, respectively. However, there was minimal change in absorbed energy between exposed and unexposed specimens. In multiple impact scenarios, GSV samples exhibited the greatest reduction in impact resistance, increasing with the number of impacts from 48.7% to 54%, whereas GSH samples had the least reduction, ranging from 6.3% to 10.9% in load-bearing capacity. In terms of energy absorption, GSV had the largest decrease, ranging from 51.5% to 58.8%, while GSH and GSP specimens showed a relatively lower decrease after exposure. * Seawater exposure led to a reduction in bending stiffness by 8.9%, 45%, and 19.2% for GSP, GSH, and GSV, respectively. For ILSS, a similar reduction was observed by 18.57%, 17.43%, and 6.62% for GSP, GSV, and GSH samples, respectively. In terms of the theoretical failure modes, GSP decreased by 18%, 12.2%, and 18.6%, while GSH reduced by 2.4%, 17.1%, and 21.2% for facesheet fracture, indentation, and shear core \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Force** & **Energy \({}^{*}\)** & \(K_{bs}\)\({}^{*}\) & **ILSS** \\ \cline{2-5}  & **(N)** & **(J)** & **(N/mm)** & **(MPa)** \\ \hline GSPu & 122.52 \(\pm\) 13.75 & 1.81 \(\pm\) 0.05 & 22.04 \(\pm\) 4.36 & 16.10 \(\pm\) 1.67 \\ GSPe & 74.54 \(\pm\) 10.09 & 1.81 \(\pm\) 0.04 & 20.07 \(\pm\) 5.10 & 13.34 \(\pm\) 1.55 \\ GSVu & 138.11 \(\pm\) 9.85 & 1.74 \(\pm\) 0.13 & 43.72 \(\pm\) 9.11 & 17.89 \(\pm\) 1.17 \\ GSVe & 128.2 \(\pm\) 18.76 & 1.70 \(\pm\) 0.14 & 35.34 \(\pm\) 7.07 & 15.19 \(\pm\) 2.33 \\ GSHu & 149.45 \(\pm\) 5.86 & 1.85 \(\pm\) 0.02 & 42.14 \(\pm\) 4.26 & 5.03 \(\pm\) 0.19 \\ GSHe & 125.69 \(\pm\) 19.05 & 1.75 \(\pm\) 0.06 & 23.19 \(\pm\) 6.86 & 3.91 \(\pm\) 0.62 \\ \hline \hline \end{tabular} Note. \({}^{*}\) value for single impact at fracture energies only.  
\end{table} 
Table 6: Summary of major impact bending performance of FRPSS exposed to seawater. failures, respectively. GSV followed the same trend except for indentation failure, which saw an increase of 11.4%, which could be due to the cross-linking behaviour of the vertical epoxy layer with the top facesheet. These indicators provide a better understanding of the aggravated effect of moisture uptake on the damage sequence of FRPSS. * Under single impact loading, intensified core shearing, delamination, core/facesheet debonding, and fibre breakage were identified as the principal damage modes in the postmortem assessment of the samples due to sweater ageing. Furthermore, core shearing was predominant in the GSP samples after seawater exposure, unlike adhesive-layered samples (GSV and GSH). The prevalent damage modes for multiple impacts are excessive fibre rupture, extended facesheet/core debonding, and severe delamination in the GSP, GSV, and GSH specimens. These modes were caused by a combination of moisture uptake and an increase in the number of impacts. For the specimen with adhesive layers, the additional stiffness provided by the adhesive layer (GSV) and a thicker core (GSH) weakened interlaminate and facesheet/core interfaces. Conceptualization, N.O.-u., V.V.S.