This study investigates the influence of internal pore defects on the quasi-static compressive behavior and mechanical response of metal lattice structures fabricated using selective laser melting (SLM) additive manufacturing. The findings aim to provide a reference for evaluating the mechanical properties of new ship protection structures created through SLM additive manufacturing.

Quasi-static compression tests and simulation calculations were conducted on 3D-printed lattice structures with porous defects. The Gurson−Tvergaard−Needleman (GTN) porous metal plasticity model, which accounts for internal pores, was employed for the simulation calculations. The damage parameters of the GTN model were calibrated using the Central Composite Design (CCD) and Response Surface Methodology (RSM), with experimental validation.

The results show that under quasi-static compression, micro-voids within 3D-printed lattice structures tend to coalesce in stress-concentrated regions, forming larger voids. This leads to significant reductions in structural strength, load-bearing capacity, and energy absorption efficiency. The computational model, which incorporates pore defects, can predict the mechanical failure behavior and energy absorption characteristics of 3D-printed lattice architectures with high accuracy, showing a deviation of only 0.25% from the experimental energy absorption results.

The research findings provide a reference for evaluating the mechanical properties of new ship protection structures made through SLM additive manufacturing. The model that incorporates pore defects effectively predicts the energy absorption capability of lattice structures with defects, and the compression performance of the structures significantly decreases as porosity increases.

To improve the cooperative detection efficiency and mission stability of AUVs in complex underwater environments, this study proposes an improved MADDPG algorithm combined with the artificial potential field (APF) method.

To overcome the limitations of conventional APF in path planning, such as local optima, and the drawbacks of MADDPG, such as poor convergence and training instability due to random early-stage exploration, this study proposes the APF−MADDPG algorithm, which integrates APF's attractive field to guide AUVs' initial movement. The key innovations include: 1) constructing a dynamic, time-varying potential field model that adjusts the field strength coefficient in real time to enhance early-stage exploration; 2) employing Monte−Carlo simulations to generate possible target trajectories, statistically analyzing their spatiotemporal distribution in the operational area, and establishing a probabilistic model to predict the dynamic movements of underwater targets; and 3) integrating sonar detection probabilities at varying distances into the reward function design and path evaluation metrics with the use of the cumulative detection probability (CDP) formula. Comparative simulations were conducted for cooperative detection tasks involving 2 and 3 AUVs under identical initial conditions to evaluate the performance differences between the APF−MADDPG and conventional MADDPG algorithms.

The experimental results demonstrate that: In terms of detection performance, APF−MADDPG achieves a CDP of 80.93% in the 2-AUV scenario, representing a 7% improvement over conventional MADDPG, while in the 3-AUV scenario, it reaches 92.67%, showing a 0.6% increase;

Regarding algorithmic performance, APF−MADDPG exhibits superior initial convergence speed and final convergence stability in both scenarios; In stability tests, the improved algorithm shows less performance fluctuations across repeated trials, confirming its superior robustness.

In the context of the increasing demand in the shipbuilding industry for lightweight and high-energy-absorbing structures, this study aims to enhance the mechanical properties of lattice structures under lightweight conditions. A novel and improved body-centered cubic (BCC) lattice structure (BCCZ) is designed based on a multi-configurational design approach. This is crucial because traditional BCC lattice structures have limitations in load-bearing capacity and energy absorption capacity. The newly designed structure is expected to overcome these drawbacks, providing better solutions for shipbuilding and related fields.

To achieve this goal, a series of methods are adopted. Firstly, compression tests are conducted. BCC and BCCZ lattice structures with specific parameters (L₁ = 15.0 mm, θ = 45.0°, L₂ = 10.6 mm, D = 2.5 mm) are fabricated using selective laser melting (SLM). The mechanical properties and energy absorption characteristics of these two structures under quasi-static compression are compared and analyzed. Secondly, numerical simulations are conducted. By changing the diameter-to-length ratio (D/L) while keeping other parameters constant, 12 different structures are designed. The finite element method is used to study the effect of D/L on the mechanical properties and energy absorption characteristics of BCC and BCCZ lattice structures. This combined experimental and simulation approach ensures a comprehensive and in-depth understanding of the structural behavior.

The results show significant differences between the two structures. In terms of deformation modes, the deformation processes of both structures can be divided into three stages: the initial linear elastic stage, the nonlinear damage stage, and the densification stage. However, the BCC structure is more stable during the nonlinear damage stage, while the introduction of vertical struts in the BCCZ structure changes its deformation mode from a bending-dominated structure to a coupled structure of vertical strut tension-dominated and inclined strut bending-dominated. With respect to mechanical properties, as the D/L ratio increases, the elastic modulus, compressive strength, and specific energy absorption of both structures increase. The BCCZ structure consistently exhibits better performance in these three aspects. Specifically, the compressive strength of the BCCZ structure is greater than that of the BCC structure, with an improvement rate of more than 100%, although the overall rate of increase in compressive strength shows a downward trend as D/L increases. In contrast, the improvement in specific energy absorption efficiency becomes more pronounced. In terms of energy absorption efficiency, the BCC structure is superior to the BCCZ structure, but this difference diminishes gradually as D/L increases.

In conclusion, this study provides valuable insights for the design of new energy-absorbing and protective structures in ships. The design of the BCCZ structure and the study of its performance under different D/L ratios offer a theoretical basis and practical reference for optimizing lattice structures in engineering applications. It also lays the groundwork for further research on lattice structures, such as exploring more complex multi-configurational designs and studying their behavior under dynamic loading conditions. The results can guide the selection of appropriate lattice structures in different engineering scenarios, promoting the development of lightweight and high-performance structures in the shipbuilding industry and other related fields.

This study focuses on the quasi-static pressure characteristics of a confined cabin in a water mist environment.

First, on the basis of the theoretical model of quasi-static pressure in a confined cabin in adiabatic ideal gas, two new assumptions are added to establish a theoretical model of quasi-static pressure in a confined cabin in a water mist environment. Internal explosion experiments in a confined cabin with and without water mist are then performed with the charge as an independent variable, and the quasi-static pressure is measured and used to verify the mitigation effect of water mist on quasi-static pressure and obtain its empirical formula.

Combined with the theoretical model and experimental data analysis, it is concluded that the ability of water mist to reduce quasi-static pressure increases with the increase in charge-volume ratio, but the increase rate gradually decreases. Both the theoretical formulas of the afterburning correction term and empirical formulas can be used to estimate the quasi-static pressure peak in the cabin.

The experimental results verify the mitigation effect of water mist on quasi-static pressure. The experimental data of this paper and others shows good agreement with the empirical formula of the quasi-static pressure peak of an internal explosion in a water mist environment, and can be used to estimate the quasi-static pressure peak.

This paper aims to analyze the explosion loading characteristics in a confined ship cabin with a pressure relief hole, and the impact of the pressure relief hole in propagating shock loads from the explosion affected cabin to the adjacent cabin.

First, a numerical simulation of the explosion venting effect on a double cabin is conducted, and the pressure drop of the explosion affected cabin is analyzed with the Baker empirical formula. On the basis of the pressure distribution characteristics in the adjacent cabin, the empirical formula for the initial peak pressure in the adjacent cabin is deduced using the least squares principle.

The results indicate that the increase in the size of the pressure relief hole will change the initial state of the pressure field in the explosion affected cabin and adjacent cabin during the explosion venting process, weakening the pressure convergence and superposition in the explosion affected cabin, and strengthening the pressure propagation in the adjacent cabins. Without considering the influence of structural deformation, the shock wave presents a peach-shaped and jellyfish-shaped pressure distribution after passing through the pressure relief hole, and the phenomenon of pressure stratification and attenuation with angle occurs.

The results of this paper can provide references for the study of explosion load characteristics in cabins.

To investigate the near-wall rotating cylindrical wake and hydrodynamic characteristics, flow around cylinder at typical gap ratios is investigated.

A numerical simulation of flow around a near-wall rotating cylinder with different gap ratios (G/D = 0.2, 0.8, 1.4) and rotation rates at Reynolds number Re=200 was carried out to compare the cylindrical wake and hydrodynamic characteristics at different gap ratios and rotation rates.

The results show that: For G/D = 0.2, the cylindrical vortex shedding is significantly suppressed and the lift and drag force on the cylindrical surface remain steady. For G/D = 0.8 and G/D = 1.4, at low rotation rates, the "wake vortex" is shed and is similar to the 2S pattern, with sinusoidal periodic fluctuations in the lift and drag coefficients and small amplitude; at higher positive rotation rates, the cylindrical wake pattern is the stable D pattern with no vortex shedding (changing from D⁺ to D⁻ pattern as the rotation rate increases), the "wake vortex layer" is separated from the "wall vortex layer", the "wall vortex" is shed multi-periodically, the lift and drag coefficients are fluctuating multi-periodically and the amplitude is increasing significantly; at higher reverse rotation rates, the cylindrical surface is wrapped by a positive boundary layer, with no vortex shedding and no fluctuations in lift and drag.

The results can provide a reference for the development of high efficient flow control technology.

This paper aims to study the coupling interaction between a truss box floating raft and submarine hull, and proposes a design approach for enhancing the vibration isolation effect of the truss box floating raft-hull coupling system.

First, modal analyses of the aforementioned coupling system and individual floating raft and hull structures are carried out respectively, and comparisons are made of the modal shapes between the floating raft of the coupling system and individual floating raft structure, as well as between the hull structure of the coupling system and individual hull structure. The effects of the thickness of the floating raft panel and isolator stiffness on the isolation of the coupling system in the full-frequency range are then investigated.

The floating raft of the coupling system shows little difference in natural frequency or modal shape compared with the individual floating raft, and the vibration isolation effects are also the same. In addition, the increase in the thickness of the panel and the weakening of the stiffness of the lower vibration isolator will improve the vibration isolation capability of the coupling system.

The results of this study indicate that the mutual influence between the floating raft and hull is small, and can be considered a weak coupling relationship.