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.

In the application of sandwich composite materials in ship structures, to ensure the structural strength characteristics, bolt connections are usually used between the sandwich composite structure and steel main hull. However, due to local stress concentration around the holes and the boundary effects at the connection locations, the bolt connection position is often becomes the most critical area of the structure, necessitating strength and fatigue assessments.

For a full-scale steel-sandwich composite panel structure, the strength characteristics of the plate frame under a 0.1 MPa uniform load are studied using experimental and finite element methods. The accuracy of the finite element method is then verified by comparing its results with those from the experimental method. Subsequently, based on the finite element method, fatigue tests for two equivalent bolted joints are designed through structural stress equivalence, and the fatigue life of the steel-sandwich composite structure is evaluated and verified.

It is determined that the fatigue life of the panel structure under the design load meets the design requirement of 5 million cycles.

Regarding the fatigue problem of large bolted connections, equivalent joints can be designed for fatigue testing, and load equivalence can be achieved through structural stress equivalence.

To study the mechanical behavior of aluminum alloy sheets for ships under low-speed impact load, horizontal low-speed impact dynamic response tests are conducted on 5059-H116 aluminum alloy plates.

Based on the impact tests, the damage and dynamic response of test plates under different impact speeds and masses are compared. Based on a mixed hardening plastic model, a low-speed impact numerical model is established to numerically simulate the failure process under different impact velocities. The finite element method is used to analyze the influence of specimen size, impact position and impact head shape on critical failure energy, and a modified empirical formula for critical failure energy is proposed.

The results indicate that with the increase in impact velocity, the critical failure energy of the test plate increases correspondingly, but the increase is very small. Under the same impact energy, different impact masses have no effect on the critical failure energy of aluminum alloy sheets. The sensitivity of critical failure energy on the aspect ratio of the test plate is very small. The critical failure energy of blunt impact test plates with the same cross-sectional area can be considered equivalent.

The results of this study can provide references for research on the low-speed impact mechanical behavior and load-bearing capacity of aluminum alloy sheets.