Objectives With the development of shipbuilding technology, composite materials have shown great potential in ship construction due to their excellent properties. However, the application of sandwich structures in ship main load-bearing structures has been limited due to the weak out-of-plane load-bearing capacity. The aim of this study is to design a novel foam-filled composite sandwich structure with reinforcing ribs (combined thick plate) to address this issue and explore its application prospects in ship main load-bearing structures. This research is of great significance for promoting the development of lightweight ships.

Methods In this study, a series of mechanical property tests were carried out. First, according to the anisotropic characteristics of the combined thick plate, three-point bending and tensile tests were conducted along and perpendicular to the direction of the reinforcing ribs. For the bending tests, referring to the GB/T 1456 - 2021 standard, four types of bending test specimens were designed. These specimens were divided according to the number of cell reinforcing plates in the direction perpendicular to the ribs and the bending center position in the direction of the ribs. For the tensile tests, with reference to the GB/T 1040.5 - 2008 standard, four types of tensile test specimens were designed based on the tensile center position in the direction perpendicular to the ribs and the number of cell reinforcing plates in the direction of the ribs. Each type of specimen had 6 samples tested, and the mechanical property parameters such as bending strength, bending stiffness, and tensile modulus were calculated through specific formulas.

Results The experimental results reveal the significant anisotropy of the combined thick plate. In the direction perpendicular to the ribs, the bending strength of the structure is increased by 82% compared with that in the longitudinal direction, and the tensile modulus in the longitudinal direction is 22% higher than that in the perpendicular direction. The number of cell reinforcing plates and the loading center position have a remarkable impact on the bending strength, while having a relatively small effect on the tensile modulus. The adhesive strength between the panel, ribs, and foam plays a crucial role in determining the bending strength of the structure. In terms of failure modes under bending loads, the initial failure mode in the direction perpendicular to the ribs is the local debonding of the lower panel. In the direction of the ribs, it is the shear failure of the reinforcing plate. The number of cell reinforcing plates and the bending center position have no obvious influence on the initial failure mode. After the initial failure, the damage processes of the specimens in different directions are distinct. In the specimens bent in the direction perpendicular to the ribs, the upper panel will locally debond following the lower panel, and the final failure is caused by the complete debonding of one side of the lower panel. In the specimens bent in the direction of the ribs, some specimens fail due to the complete debonding of one side of the lower panel, while in others, the lower panel does not completely debond, and the panel gradually loses its load - bearing capacity during the loading process.

Conclusions In conclusion, this research provides a technical reference for the promotion and application of composite sandwich reinforcement plates in ship main load - bearing structures. The obtained data and conclusions can guide the design and optimization of composite structures in shipbuilding, helping to improve the structural performance and safety of ships. It also offers valuable insights for further research on the mechanical properties of composite materials in marine engineering.