Composite materials are widely used for ship superstructures for their lightweight and high-performance merits. Bolt-mounted metallic equipment bases introduce bolt-hole stress concentration under cyclic marine loads, triggering multi-mode composite damage and potential structural failure. Research on ultimate-strength and progressive-failure characteristics of large-scale ship superstructure steel-composite bolted joints remains insufficient. This paper investigates bolted connections between equipment steel bases and composite stiffened plates.

Combined structural test and ABAQUS finite-element simulation are carried out. Test specimens are vacuum-molded composite sandwich stiffened plates with PVC foam core and carbon-glass-fiber face sheets, fitted with Q355 steel flanges fastened by M16 high-strength bolts. Horizontal tensile loading fixtures and multi-point strain monitoring are deployed for tests. In simulation, SC8R, COH3D8 and C3D8R elements are used for composite skins, cohesive adhesive layers and solid components respectively. Hashin failure criterion and Tserpes stiffness-degradation model are implemented via VUSDFLD subroutine to capture progressive composite damage; bolt pre-load and equivalent equipment gravitational mass are applied.

Tests yield an ultimate bending moment of 50.92 kN∙m. Elastic response maintains below 24.58 kN∙m; adhesive debonding initiates afterwards, and tensile-side stiffener-panel debonding occurs at 44.78 kN∙m. Dominant failure modes comprise stiffener-panel interface debonding, bolt-hole fiber breakage and X-shaped matrix damage near flange bolt holes. Simulated ultimate bending moment reaches 52.86 kN∙m, with a relative error of only 3.81 % against experiments. Numerical results well reproduce observed multi-region damage features, with core failure thresholds of 2.05 MPa (sandwich panel) and 2.4 MPa (stiffener).

The progressive-damage simulation framework integrating Hashin criterion, Tserpes degradation model and ABAQUS can accurately predict failure progress of such bolted hybrid joints, as verified by 3.81 % ultimate-strength deviation and reproduced damage patterns. It fills the research gap for large ship superstructure steel-composite connections and supplies validated analysis tools for safety assessment and design optimization of marine composite structures. Future studies will target structural stiffness promotion and bolt-joint integrity improvement for dynamic ship-borne equipment.