# Dynamic Bending Behaviour of Sandwich Structures for Marine Applications
## 1 Introduction  
Fibre-reinforced plastic sandwich structures (FRPSSs) are traditionally heterogeneous materials fabricated with stronger, thin facesheets bonded to a weaker core. Recently, there has been an increased use of these materials due to increasing demand for weight reduction in design, improved efficiency, and superior performance across various engineering sectors [1; 2; 3; 4]. It is now common to find FRPSSs in the manufacture of critical components in the automotive, aerospace, defence, and maritime industries thanks to their lightweight, high strength, ease of manufacture, lower cost, as well as good corrosion resistance and buoyancy [5; 6; 7; 8]. Various researchers demonstrated that the overall mechanical behaviour of composite sandwich structures depended on the performance of its constituents (the facesheets and the cores) primarily in terms of their bond efficiency in resisting the induced damage [9]. In addition to their intrinsic properties, the operational environment of these structures during service may negatively impact their performances, potentially resulting in a decline in their mechanical performance over time [10; 11]. FRPSSs utilised in marine applications are prone to experiencing diminishing load-bearing capacity and decreasing mechanical performance over time, primarily attributed to the progressive absorption of moisture, which adversely affects the hydrophilic matrix and interface bonding at the micro, meso, and macro scales [5; 12]. One of the major loading regimes for FRPSSs in service is a low-velocity impact, which is of interest because it often leads to hard-to-notice internal damage termed _barely visible impact damage_ (BVID) [10; 13]. Thus, BVID for composite sandwich structures is of great interest to researchers as it could exacerbate the performance deterioration of the structures, leading to their catastrophic failure. Some of the commonly identified damage modes of composites subjected to seawater exposure are accelerated matrix cracking, facesheet buckling, delamination, and debonding between the facesheet andcore [14]. The literature analysis demonstrates that considerable research was performed to understand the effects of seawater on the FRPSS under quasi-static loading conditions, while their effect on impact bending is still unclear, and this study aims to bridge that gap. Experimentally, the most common means of investigating the damage behaviour of composite structures under quasi-static and dynamic loading conditions are quasi-static indentation, drop-weight, and low-velocity pendulum tests [15; 16]. For instance, drop-weight experiments were used to demonstrate that the facesheet configuration had a more significant effect on the overall energy-absorption properties of FRPSSs than the variations in the core [17]. The obtained results indicated that the typical damage modes occurring in the facesheets were fibre fracture, matrix cracking, and delamination (intralaminar and interlaminar). Similarly, mechanical characterisation of foam-core sandwich samples was performed using a pendulum test rig, and various damage modes similar to those found in drop-weight tests were observed in [18]. It was demonstrated that this method with low energy levels could accurately capture the main damage sequence and is, therefore, useful in the analysis of the low-velocity impact resistance response of composite structures. While substantial research was conducted on the low-velocity impact behaviour of FRPSSs, their bending response in cases of single and multiple dynamic loading after exposure to seawater is still unclear. Thus, in this paper, the dynamic bending behaviour of FRPSS under low-velocity impact is investigated after a prolonged period of seawater exposure. An improvised impact-bending experimental set-up is used to analyse the mechanical performance to understand the effects of moisture absorption on the main damage mechanisms.