Experimental investigation of glass reinforced plastic (GRP)-steel sandwich panels for marine structural applications

• Glass-reinforced plastic (GRP)-Steel (S-C-S) sandwich panels reduce stress under axial load compared to conventional steel plates. • S-C-S panels show improved load-bearing capacity under bending, with up to 54.23% increase. • Weight reduced by up to 18.05% per unit area for a specific combination using GRP-steel sandwich panels over conventional steel. • Welded S-C-S panels retain tensile strength, confirming mechanical reliability after joining. • Experimental results from tensile, hardness (Vickers and Rockwell), and bending tests validate the structural efficiency of S-C-S panels for ship hull construction. The increasing demand for lightweight, high-performance materials in the maritime industry has driven interest in composite structures as alternatives to conventional steel and aluminum. This study investigates the potential of Steel-Glass-reinforced plastic-Steel (S-GRP-S) sandwich panels as a viable replacement for conventional steel plates in marine structural applications. The proposed hybrid panels combine the strength of steel as face plates with the lightweight and corrosion-resistant characteristics of glass-reinforced plastic (GRP) cores. Three steel plate configurations of 9 mm, 11 mm and 13 mm thickness and three S-GRP-S sandwich panel configurations of 12 mm, 16 mm and 20 mm thickness were prepared and compared with their equivalent configurations. A series of experimental tests, including tensile, hardness (Vickers and Rockwell), and three-point bending were conducted on both steel plates and S-GRP-S sandwich panels of varying thicknesses. The results demonstrate that the GRP-steel sandwich panels exhibit superior axial stress response, resistance to deformation, deflection behavior while significantly reducing weight. For instance, the 5-10-5 mm sandwich panel reduced axial stress by 41.91% and the 3-6-3 mm sandwich panel reduced weight by 18.05% compared to their respective steel plate configuration. These sandwich panels also performed well in bending tests, with some setups showing over 50% improvement in flexural stiffness and load-bearing capacity. Importantly, welding did not compromise tensile properties, suggesting that welded composite structures remain mechanically sound. Overall, the S-GRP-S sandwich panels combine strength, reliability, and weight efficiency, making them a strong candidate for future ship hull construction.