# Dynamic Bending Behaviour of Sandwich Structures for Marine Applications
## 4 Results and Discussion
### Damage Morphology Assessment  
As stated earlier, FRPSS under impact loading dissipates the impacted energies through the various damage mechanisms in the materials, and this morphology is more profound after seawater ageing as moisture uptake affects adhesion and amplifies the damage morphology. Figures 14 and 15 illustrate the CT images of the FRPSS (exposed and unexposed) samples after the bending impacts. Prominent damage characteristics such as matrix cracking, core shearing, delamination, core/factsheet debonding, and fibre breakage could be identified at varying degrees in all tested samples. Generally, the most severe damage modes characterised by facesheet rupture were located at the clamped region due to the excessive stress concentration caused by the applied boundary conditions, as excessive damage was observed at the back of the specimen. As shown in Figure 14, under catastrophic fracture (single impact regime), it is evident that seawater ageing led to more severe damage morphology in all specimens. Interestingly, core shearing was predominant in the GSP samples after seawater exposure, unlike adhesive-layered samples (GSV and GSH). This could be attributed to the absence of additional strengthening in the core, as the PVC with low shear strength bore the impact load once damage to the front facesheet began. Additionally, unlike samples with thinner cores, the GSH specimens experienced severe facesheet/core debonding due to the brittle failure of the horizontal adhesive layers and the weakening of the bonds at the interface caused by the plasticization of the matrix. A large, debonded area significantly reduces the damage resistance of FRPSS subjected to seawater under low-velocity impact. This confirms that, under out-of-plane impact conditions in marine structures, core configuration is a crucial factor in assessing the damage behaviour of FRPSS. This effect was exacerbated by moisture absorption, as exposed samples showed more extensive damage modes, with large sections of facesheet and core debonding transverse to the plane of impact in the GSH specimens.  
Figure 13: ILSS of FRPSS before and after seawater exposure.  
Figure 14: Postmortem \(\mu\)-CT analysis of FRPSS after single impact tests. Note a—fibre breakage, b—determination, c—facsheet/core debonding, and d—core shearing.  
Figure 15: Postmortem \(\mu\)-CT analysis of FRPSS after multiple impact tests. Note: \(a-\)thee breakage, b—delamination, c—foschest/core debending, and d—core shearing.  
For the multiple-impact scenario, a similar trend was observed with more severe damage to the constituents of FRPSS after seawater exposure (Figure 15). The prevalent damage modes discovered in the postmortem assessments include excessive fibre rupture, extended facesheet/core debonding, and severe delamination in the GSP, GSV, and GSH specimens. The aggressive fibre rupture in the GSP specimen could be owing to the accelerated weakening of the fibre/matrix interface of FRPSS occasioned by the polymeric action of the moisture with an increase in the number of impacts. This leads to an increase in the stress concentration at the boundary point until sudden rupture when the maximum strength of the reinforcements is exceeded. On the other hand, although adhesive layers provided some additional stiffness for the specimens (GSV and GSH), after impact, these specimens suffered more pronounced debonding at the interfaces between the facesheets and the core of the specimen. This could be attributed to the additional degradation at the interfaces because of the polymer plastisation in the adhesive layers. These damage modes, therefore, showcase the trade-off in terms of damage resistance in the introduction of adhesive layers and core variants in sandwich panels for marine applications subjected to multiple impact loading regimes. A summary of the major impact bending properties of FRPSS discussed in this study is presented in Table 6.