The low-temperature fatigue crack propagation rate of 925A steel, as a rudder steel for polar special ships, has a crucial impact on the evaluation of the fatigue strength of polar ships. The purpose of this article is to study the fatigue crack propagation rate of 925A steel under different low-temperature conditions from room temperature (RT) to −60 °C. The material was subjected to fatigue crack propagation tests and stress intensity factor tests. The experimental tests were conducted according to the Chinese Standard of GB/T6398-2017. The results show that as the temperature decreases, the lifespan of 925A increases. Within a certain stress intensity factor, as the temperature decreases, the fatigue crack propagation rate decreases. At −60 °C, it exhibits ductile fracture; within normal polar temperatures, it can be determined that 925A meets the requirements for low-temperature fatigue crack propagation rates in polar regions. However, in some extreme polar temperatures below −60 °C, preventing brittle failure becomes a key focus of fatigue design. Finally, the fatigue crack propagation behavior at the microscale of 925A steel at low temperatures was described using fracture morphology. The experimental data can provide reference for the design of polar ships to further resist low-temperature fatigue and cold brittle fracture.

For underwater vehicles navigating in Arctic Sea ice-covered waters, fatigue damage is a crucial issue. Many scholars have conducted low-temperature fatigue analysis on low-temperature materials and substrate platforms for ships, and the results show that low temperature is beneficial for improving the mechanical properties of materials. However, they have mainly focused on low-temperature performance experiments and simulation analysis of standard components of materials, and there are very few fatigue life experimental studies on underwater vehicle mechanical structures. This paper conducts experimental investigations on a scaled model of an underwater vehicle rudder mechanism in a polar environment laboratory. Using a comparative analysis method involving simulations and experiments, the low-temperature fatigue test data of the scaled underwater vehicle rudder mechanism are analyzed, and the quantified analysis between fatigue and different low-temperature loading cycles, as well as the influence of icing on the fatigue life, is explored. It is indicated that the Arctic environment can decrease the fatigue life of the underwater vehicle rudder mechanism by deteriorating its material properties. The paper builds the foundation for the fatigue life of an underwater vehicle rudder mechanism in Arctic low-temperature environments.

This study investigates the sealing performance of marine vent valves in low-temperature environments (−30 °C to −40 °C) via thermo-mechanical coupling analysis. Polytetrafluoroethylene (PTFE) was selected as the sealing material for its excellent cryogenic toughness, corrosion resistance, and cost-effectiveness. The total minimum specific sealing pressure ($q_{total}$) of PTFE, corrected for marine vibrations (15–60 Hz), was 3.702 MPa. Using ANSYS Workbench 2022, finite element simulations of a DN200 globe valve showed that low temperatures caused non-uniform thermal contraction, reducing the gasket-poppet contact width (2.5 mm to 1.75 mm) and maximum specific pressure (16.967 MPa to 13.352 MPa), leading to leakage risks. Optimizing the stem preload to 36,000 N restored effective sealing: the maximum specific pressure rebounded to 16.601 MPa, with no pressure below 3.702 MPa. This research provides a method for evaluating low-temperature sealing performance and supports safe vessel operation in cold waters.

To accurately evaluate the load-bearing capacity of Cu-Ni alloy pipelines with double pitting corrosion defects, in this study, the influence of dual-defect morphological parameters on the ultimate load capacity was investigated through finite element simulation. On the basis of the ultimate load capacity model for single pit defects and simulation results, an assessment model was developed for Cu-Ni alloy pipelines containing double pitting defects, and its accuracy was validated through hydrostatic burst tests. The results indicate that the ultimate load capacity increases gradually with increasing interdefect center distance, asymptotically approaching the load-bearing capacity of a single-defect pipeline with an equivalent depth and diameter. The proposed model demonstrates excellent predictive performance, with a maximum error margin within 3%. During failure, Cu-Ni alloy pipelines with double pitting defects develop fine axial cracks at the defect sites while exhibiting significant overall bulging deformation. These findings can be effectively applied to predict the ultimate load capacity of Cu-Ni alloy pipelines with double pitting defects, providing substantial engineering value for an accurate assessment of the load-bearing capacity of pipelines with actual corrosion defects.

Underwater laser detection and communication in polar regions have attracted increasing research interest. The stochastic roughness of the water–ice interface causes the broadening of the incident beam angle and spatial energy uncertainty, posing critical challenges for cross-medium optical transmission. This study presents a theoretical model that couples seawater volumetric scattering attenuation with rough-interface effects in upward laser transmission. The water–ice interface is discretized using Beckmann–Gaussian microfacet geometry. From this, the analytical relationship between local surface normal distributions and Fresnel reflection/transmission coefficients is derived. This enables the quantitative characterization of the process by which interface roughness modulates optical transmission. A full-link energy normalization framework is established. It encompasses normalized seawater-segment power, interface transmission efficiency, and normalized sub-ice received power. Monte Carlo photon tracing is used to simulate scattering in polar seawater and stochastic coupling at the rough interface. The results are experimentally validated. The key findings are as follows: Rising seawater temperature increases absorption but reduces scattering resulting in enhanced overall attenuation and degraded energy transfer efficiency. Interface roughness broadens the local angle-of-incidence distribution, and beyond 30°, energy transfer efficiency drops markedly. As propagation distance increases, normalized received power decreases, the beam spot diameter increases, and collimation quality deteriorates. The simulation results show strong agreement with the experimental measurements. These findings provide theoretical support for sub-ice laser signal acquisition and polarization modulation in polar environments, offering practical guidance for system design in underwater optical communication applications.

A series of temperature impact tests were conducted on submersible rudder materials in a polar low-temperature environment. By using an impact testing machine and a scanning electron microscope, the impact performance of samples at different temperatures was tested, and the fracture morphologies were observed. A systematic study was conducted on the low-temperature impact mechanical behavior of two base materials and their welded joints. The results indicate that within the temperature range of −60 to 20 °C, the rudder base material 925A exhibits excellent low-temperature impact toughness; as the test temperature decreases, the fluctuation range of its impact absorption energy remains relatively small. At −60 °C, the shear lip region of the sample fracture forms a uniform and dense fine dimple structure, endowing the material with the highest crack propagation energy. As the temperature increases, the crack propagation energy tends to decrease. The impact test temperature has a significant zoning effect on the fracture morphology. At room temperature (20 °C), the fiber zone of the fracture presents a fine and deep dimple morphology, corresponding to a higher crack initiation energy. The experimental data obtained from this study provides theoretical and data support for the engineering application of submersible rudder materials under polar low-temperature conditions.