Mechanical behaviour of composite sandwich structures for marine applications

The use of fibre-reinforced plastics (FRPs) is increasing due to their strength-to-weight ratio, impact resistance, and versatility in modern designs. FRP sandwich structures (FRPSS), widely used in maritime applications, offer additional buoyancy and weight advantages. Composed of reinforced facesheets bonded to a weaker core, FRPSS optimises weight-to-stiffness performance. This study evaluates composite panels with glass-fibre/epoxy facesheets and different polyvinyl chloride foam core configurations (GSP, GSH, GSV). The mechanical performance of these configurations is analysed under seawater exposure (3.5% salinity) in quasi-static and dynamic loading, with the intention to inform material optimisation for marine use. The study investigates how moisture uptake affects the mechanical behaviour of FRPSS after seawater exposure. Moisture led to plasticization, polymerisation, and swelling, weakening the interface adhesion. An innovative acoustic emission (AE) monitoring framework was introduced, effectively tracking seawater degradation with 31% and 48% degradation in samples without (Type A) and with epoxy layers (Type B), respectively. This novel AE method shows promise for detecting polymerisation at the interface between the facesheets and the core. To the authors' knowledge, this is the first use of this AE approach to monitor moisture uptake and polymerisation in synthetic composite sandwich structures. In terms of the penetration performance, samples were subjected to three types of indenter geometries (sharp, semi-blunt and blunt) before and after seawater exposure. It was observed that samples exposed to sharp indentation exhibited the greatest decrease in load-bearing capacity (48.9%, 51.5%, and 34.1%) while those for blunt indentation had the least (21%, 46.1%, and 22.5%) for GSP, GSV and GSH configurations, respectively. This was due to reduced penetration forces resulting from matrix plasticisation and degraded matrix/fibre interface, exacerbated by a smaller indenter geometry. Also, early damage initiation and intensified damage progression were observed for sharp samples after seawater exposure. Interestingly, by comparison, the core configuration significantly influenced localised damage for samples indented by conical and square indenters, unlike those subjected to hemispherical indenters, with limited impact. Seawater exposure adversely affected energy absorption and penetration performance, enhancing macroscale damage mechanisms.

The dynamic investigation showed a reduction in damage resistance after single and multiple impacts following seawater exposure. Under single impact, GSP had the largest force reduction (39.1%), while GSV and GSH showed decreases of 7.2% and 15.9%, respectively. In multiple impacts, GSV had the greatest reduction in impact resistance, rising from 48.7% to 54%, while GSH had the smallest decrease (6.3% to 10.9%). Energy absorption dropped most significantly in GSV (51.5% to 58.8%), with lower decreases in GSP and GSH. Seawater also reduced bending stiffness by 8.9%, 45%, and 19.2% for GSP, GSH, and GSV, respectively. Theoretical failure modes in GSP were reduced by 18%, 12.2%, and 18.6%, while GSH dropped by 2.4%, 17.1%, and 21.2% for facesheet fracture, indentation, and shear failures, respectively. GSV followed a similar pattern, except for an 11.4% increase in indentation failure, likely due to cross-linking of the vertical epoxy layer with the facesheet. Post-mortem analysis revealed intensified core shearing, delamination, debonding, and fibre breakage in single impacts, with core shearing predominant in GSP. For multiple impacts, the main damage modes were fibre rupture, facesheet/core debonding, and severe delamination in GSP, GSV, and GSH. Lastly, a viscoelastic analysis was carried out to capture and understand the effect of seawater on their damping properties, after which theoretical prony series parameter was derived. Results showed that moisture uptake reduced load-bearing capacity, with tensile strength and elastic modulus reducing by 35.4% and 8.4%, respectively. Type B specimens showed a more significant decrease by 38% in storage modulus when compared to Type A, while Tan δ increased by 10% and 5.7% for Type A and Type B, respectively, indicating higher strain energy dissipation. Furthermore, Type A specimens showed greater stiffness and energy dissipation after seawater exposure. It should be noted that the increased Tan δ reflected intensified degradation and better energy absorption. Prony-series parameters were derived from the data, which could be used to develop numerical viscoelastic models to optimise FRPSS structures. These findings add to the understanding of FRPSS by facilitating more insights into the material properties of composite sandwich structures and their subsequent optimisation for marine applications to improve out-of-plane damage resistance capabilities. It is worth noting that the investigation in this study is limited to experimental analysis and restricted to facesheets made from E-glass fibre as reinforcements, while the manufacturing technique is restricted to the hand-layup technique with its inherently reduced constituents precision, especially with the adhesives in the core. However, it provides sufficient methodology to ensure the repeatability of the results in this thesis.