Deep-sea Pressure Research Articles

The effect of deep-sea pressure during long-term aging on the tribological performance of fabric-reinforced phenolic composites

The influence of hydrostatic pressures up to 30 MPa on the friction and wear behavior of fiber reinforced polymer composites (FRPCs) was investigated together with seawater absorption, interfacial structure and mechanical properties. The results revealed that after aging in the deep-sea environment, the wear rate of the FRPC increased from 1.53 × 10−6 mm3/N∙m to 3.61 × 10−6 mm3/N∙m, and the mechanical strength decreased reversibly. The damage mechanism was assumed to be dominated by the combined effect of crack propagation in the polymer matrix and interfacial debonding at high seawater pressures, characterized by a decreased resin modulus and increased interfacial thickness.

Damage evolution and failure mechanism of 2.5D woven composite tubes under quasi-static lateral compression

2.5D woven composites are ideal candidate materials for deep-sea pressure tubes owing to their excellent out-of-plane properties. This paper presents the damage evolution and failure mechanism of 2.5D woven composite tubes under quasi-static lateral compression. To conduct this study, 2.5D woven composite tubes with different thickness-to-diameter ratios, 0.04, 0.07 and 0.10, were designed and prepared. The quasi-static lateral compression tests were carried out in order to evaluate the progressive damage analysis, combining high speed photographic image with acoustic emission technologies. The results show that the increase of the ratio of thickness-to-diameter, the deformation and shear failure of the sample can be inhibited obviously. Due to the enhanced interlayer interaction, the lateral stiffness of the sample is obviously improved, so the lateral bearing stability of the sample is improved. When the peak load of samples with the thickness-to-diameter ratio of 0.1 reached 11.51 kN, it exceeded that of samples with thickness-to-diameter ratios of 0.04 and 0.07 by 450% and 82%, respectively. Furthermore, the failure mechanisms of samples with the thickness-to-diameter ratio of 0.1 were controlled by delamination fracture, whereas that of 0.04 and 0.07 were mainly influenced by shear failure and delamination failure, respectively.

In Situ Measurement of Deep-Sea Salinity Using Optical Salinometer Based on Michelson Interferometer

Ocean salinity plays an important role in oceanographic research as one of the fundamental parameters. An optical salinometer based on the Michelson interferometer (MI) suitable for in situ measurement in deep-sea environments is proposed in this work, and it features real-time calibration and multichannel multiplexing using the frequency modulated continuous wave (FMCW) technique. The symmetrical sapphire structure used to withstand deep-sea pressure can not only achieve automatic temperature compensation, but also counteract the changes in optical path length under deep-sea pressure. A model formula suitable for optical salinity demodulation is proposed through the nonlinear least squares fitting method. In vertical profile testing, the optical salinometer demonstrated remarkable tracking performance, achieving an error of less than 0.001 psu. The sensor displays a stable salinity demodulation error within ±0.002 psu during a three-month long-term test at a depth of 4000 m. High stability and resolution make this optical salinometer have broad development prospects in ocean observation.

A Highly Sensitive 3D-Printed Flexible Sensor for Sensing Small Pressures in Deep-Sea High-Pressure Environment.

The origin of life on Earth is believed to be from the ocean, which offers abundant resources in its depths. However, deep-sea operations are limited due to the lack of underwater robots and rigid grippers with sensitive force sensors. Therefore, it is crucial for deep-sea pressure sensors to be integrated with mechanical hands for manipulation. Here, a flexible stress sensor is presented that can function effectively under high water pressure in the deep ocean. Inspired by biological structures found in the abyssal zone, our sensor is designed with an internal and external pressure balance structure (hollow interlocking spherical structure). The digital light processing (DLP) three-dimensional (3D) printing technology is utilized to construct this complex structure after obtaining optimized structural parameters using finite element simulation. The sensor exhibits linear sensitivity of 0.114 kPa-1 within the range of 0-15 kPa and has an extremely short response time of 32 ms, good dynamic-static load response capability, and excellent resistance cycling stability. It shows high sensitivity of 1.74 kPa-1 even under 30 MPa static water pressures and the theoretical working pressure can exceed 110 MPa, which provides a new solution for deep-sea robots.

Buckling behavior analysis of corrugation reinforced composite pressure shell: Analysis of the influence of corrugation structural parameters

Based on CFRP(Carbon Fiber Reinforced Plastics) cylindrical pressure shell, the CFRP circumferential corrugation pressure shell is proposed in this study. The structure can further improve the buckling resistance capacity of CFRP pressure shell. Five CFRP circumferential corrugation pressure shells with varying corrugation structure parameters are designed and compared with CFRP cylindrical pressure shell. The buckling behaviors of the shells are analyzed in detail by nonlinear finite element method. The result show that reasonable corrugation structure can significantly improve the resist buckling capacity of CFRP cylindrical pressure shell. However, the increase of corrugation structure parameters will exacerbate the damage to the CFRP layers outside the pressure shell. Ultimately, pressure shell N2 with the best buckling resistance capacity is selected for hydrostatic pressure test, and the experimental results are found to align with the numerical simulation results. This study offers a novel approach for the design of deep-sea pressure shell structures.

A lightweight AuxHex zero Poisson's ratio pressure-resistant sandwich cylindrical shell and its load-bearing and sound insulation behaviors: Design, simulation and experiment

Pressure-resistant hulls are critical components in underwater vehicles and submersible systems. Exploring novel materials and structures has been a growing trend to overcome conventional structural limitations and achieve multifunctionality. Mechanical metamaterials with tunable physical properties can offer promise. Therefore, this study focuses on designing and fabricating a pressure-resistant sandwich cylindrical shell using AuxHex zero Poisson's ratio (ZPR) mechanical metamaterials. Structural optimization is employed to develop a lightweight AuxHex ZPR unit cell to withstand deep-sea pressure. Mechanical and acoustic experiments were then conducted on a 3D-printed Titanium alloy specimen, and the rationality of numerical modeling was validated by the experimental results. Load bearing and sound insulation properties of the designed ZPR sandwich cylindrical shell have been numerically analyzed and compared with its geometrically similar positive Poisson's ratio (PPR) and negative Poisson's ratio (NPR) shells. The designed ZPR structure that yields a mass density ratio of 0.569 g/cm³ can withstand hydrostatic pressure at 1000 m depths without strength or buckling damage, while adequate reserve buoyancy can be maintained. The submerged ZPR shell structure can provide superior protection for inner spaces and its average sound transmission loss (STL) is more than 5 dB higher than the PPR and NPR shells.

Ultimate bearing behavior in the post-buckling stage of composite pressure hulls based on new detection methods

This study employs novel testing methods and analysis techniques to investigate the ultimate load-bearing characteristics and failure mechanisms of composite thin-walled pressure hulls. An analytical model based on the classical laminated plate theory, incorporating the Hashin damage failure criterion, is developed to determine their ultimate bearing capacity. Five carbon fiber cylindrical shells are manufactured to validate the failure mechanism and evaluate the model's predictions. Deep-sea pressure testing and ultrasonic testing are conducted on these shells. The resulting data is analyzed using both macroscopic and microscopic approaches to gain insights into their load-bearing behavior and failure mechanisms. The study demonstrates the significant influence of the thickness (t) to radius (R) ratio on the ultimate load-bearing capacity of these composite thin-walled pressure hulls. Failure primarily arises from compression failure in both the matrix and fibers. The load-bearing process encompasses uniform shrinkage, critical buckling, and post-buckling states. Material failure primarily occurs during the post-buckling stage, while no damage occurs during the initial buckling. On a microscale, failure at the interface between the fibers and matrix is identified as the primary contributor to overall structural failure. Enhancing the fracture energy of the interface is recommended to enhance the ultimate load-bearing capacity of the hulls.

Long-term storage under pressure in deep sea improved the microbiological safety and physical properties of whale meat

This study aimed to clarify the effects of deep-sea pressure storage on the quality of whale meat, especially microbiological safety and physical properties, to examine the effectiveness of deep-sea storage for long-term aging of whale meat. Microbiological safety, physical properties, color and appearance, water content, water activity, and pH of whale meat were examined after storage in the deep sea at depths of 2200–6000 m (22–60 MPa) for 4 months. During storage under high pressure at a depth of >4000 m (40 MPa), the growth of aerobic bacteria was inhibited in whale meat. The toughness of whale meat stored in deep sea at a depth of >4000 m became significantly tender than that before deep-sea storage. Long-term storage of whale meat under high pressure and low-temperature conditions in the deep sea at a depth of >4000 m was clarified to improve the microbiological safety and tenderness of whale meat.

The Initiation Characteristics of Corrosion Fatigue Crack in 18Ni (250) Steel

Maraging steel is the material of deep sea pressure equipment. The characteristics of corrosion fatigue crack initiation in 18Ni (250) were investigated by performing a tensile fatigue test under different loading conditions. The mechanism of corrosion fatigue crack initiation was analyzed considering the influence of welding. The results showed that the crack initiation life decreases with increasing load and increases with the stress ratio. In a corrosive seawater environment, crack initiation is affected by both mechanical damage and electrochemical corrosion. The proportion of the crack initiation life of the total sample life is approximately 71.45–99.55%, where the larger this proportion is, the smaller the fatigue zone is. Crack initiation and microcrack propagation in maraging steel correspond to the microfracture mechanism of dimple fractures. The microfracture mechanism is strongly affected by the presence of a weld but weakly affected by the weld position. The results of this study provide a theoretical basis for predicting the corrosion fatigue life of deep-sea submersible pressure hulls.

Structure design and deformation analysis of double hydraulic cylinder deep-sea pressure simulator

Because of the particularity of the deep-sea environment and the difficulty of on-site observation and research, it is particularly important to establish a deep-sea simulation system for research. Based on the high measurement accuracy requirement of deep-sea simulation equipment, a design scheme of a deep-sea pressure simulation device with a double hydraulic cylinder that can measure small water pressure changes is proposed. Firstly, the working parameters and pressure principle of the design are introduced, and then the deformation of the deep-sea pressure simulation device is analyzed. The mathematical model of piston feed and pressure without considering the deformation of the sealed pressure cylinder and the mathematical model of piston feed and pressure considering the deformation of the sealed pressure cylinder are established respectively. Finally, the relationship curve is analyzed by using origin software. The results show that when the deep-sea pressure simulator is used to simulate the underwater environment, it is necessary to consider not only the influence of liquid compression on the pressure but also the influence of deformation on the pressure.

Mechanical Properties of Spherical Deep-Sea Pressure Structures by Integral Hydrobulge Forming with Triangular Patch Polyhedrons

Mechanical Properties of Spherical Deep-Sea Pressure Structures by Integral Hydrobulge Forming with Triangular Patch Polyhedrons

Fluid-structure interaction of spherical pressure hull implosion in deep-sea pressure: Experimental and numerical investigation

Implosion may occur when a hollow pressure structure with geometric imperfections works in deep-sea environments. Therefore, the implosion phenomenon and failure mechanisms of a titanium alloy spherical pressure hull are investigated by experiments and developed numerical methods in ultra-high-pressure water conditions. Firstly, the experiments were conducted using a full-ocean-depth sea environment simulator. Then the validity of the numerical analysis were demonstrated by comparing the shock wave of fluid and destroyed fragments of structure. Finally, the characteristics of underwater implosion were examined under different hydrostatic pressures, including the propagation of shock waves, high-speed motion of the compressible flow, nonlinear deformation of the spherical pressure hull, and energy balance and evolution. The results showed that the vertical impact effect occurs during the underwater implosion of a metallic sphere. Moreover, the shock wave emerges earlier and the cracks break into smaller fragments with the increase of hydrostatic pressure. Besides, the smaller volume of the air cavity is compressed and the larger amplitude of potential energy is dropped when the hydrostatic pressure is larger. Meanwhile, the internal energy of air and structure increases, while the internal and kinetic energy of air oscillates slightly due to the pulsation characteristics of the air cavity.

Optimized Design of the Carrier Structure of an Autonomous Glide Marine Seismometer

Mobile Earthquake Recording in Marine Areas by Independent Divers (MERMAID) provides a possibility for long-term and large-scale observation of natural seismic P waves, but it does not have mobility and can only drift with ocean currents, resulting in observation equipment locations that are too sparse or too dense, both of which are not suitable for network observation. Therefore, this paper developed a new type of Autonomous Glide Marine Seismometer (AGMS) with mobility and the ability to adjust the observation position. The AGMS adopts a flying saucer shape, which has better hydrodynamic characteristics and better motion stability. This paper focused on the material, shape, and structure of the pressure-resistant shell for the selection of design and strength checking research. Using the finite element analysis method and introducing the initial defect, the results showed that the yield strength of the pressure-resistant shell decreases with the initial defect value. The calculation results were compared and analyzed with the relevant theoretical formulas and specification calculation results, and all the results met the design requirements. The results of this design could also provide reference for the design of related deep-sea pressure chambers.

Creep behavior of titanium alloy used in deep-sea pressure shell considering tensile/compressive asymmetry: Experiments and numerical modeling

Titanium alloy pressure shell of deep-sea submersible suffers from room-temperature creep problem, which can affect the safety of the structure. There is currently a lack of research on the tensile/compressive asymmetry of the creep response of titanium alloy structures at room temperature. In this paper, room-temperature creep behavior and mechanism of titanium alloy were investigated by creep test of twisting of titanium alloy square plate. Taking cylindrical pressure shell with geometrical imperfection and multi-sphere pressure shell as examples, the creep mechanical behavior of titanium alloy pressure shells under complex stress states were investigated by numerical simulation. It is found that only under tensile stress can slip bands move across phase boundaries, resulting in tensile/compressive asymmetry in creep behavior. The proposed DTC model can accurately describe the room-temperature creep behavior of titanium alloy structures. For cylindrical pressure shell with geometrical imperfection, creep deformation results in a 121 % increase in the magnitude of geometrical imperfection. The results obtained with the tensile model towards the conservative side. For multi-sphere pressure shell, the evolution of physical quantities cannot be accurately described without considering the tensile/compressive asymmetry in creep properties. The range of relative error when using traditional models is between 5% and 33%.

Room temperature creep mechanisms of Ti–6Al–4V ELI alloy with equiaxed microstructure under different applied stresses

Titanium alloys are often used to make deep-sea pressure hulls and the creep strain produced during service is nonnegligible. The variation of dislocation structures with room temperature creep stresses in equiaxed α grains of Ti–6Al–4V ELI alloy was examined using transmission electron microscope. The plastic deformation mechanism varied with the applied stress, mainly manifested by the change of dislocation patterns and the sequential activation of slip systems. At the stress below the creep threshold, even though almost no macroscopic creep was generated, a small number of immobile dislocations and dislocation networks occurred in the sample. The networks were composed of dislocations with different Burgers vectors, and part of them would evolve into low angle grain boundaries (LAGBs). At the stresses above the threshold, the density of dislocations in α grains increased with the stress level, and the basal <a>, pyramidal <a> and <c+a> slips were activated successively which also contributed to creep and participated in the formation of LAGBs. Prismatic <a> slip was always the predominant creep deformation mechanism for the studied stress range. The activation of difficult slip systems was attributed to the stress concentration and stress redistribution between soft and hard phases/grains. In general, the amount of LAGBs increased with the applied creep stress, but the LAGBs formed by dislocation networks decreased.

Editorial: Design and mechanical failure of deep-sea pressure structures

Editorial: Design and mechanical failure of deep-sea pressure structures

Analysis of Sealing Characteristics of Lip Seal Rings for Deep-Sea Separable Pressure Vessels

Deep-sea pressure vessels are specialized pressure vessels designed for automatic deployment from underwater to the surface. These vessels find extensive applications in underwater life-saving and transportation. Their interiors are furnished with a pair of sealing rings, one of which is lip-shaped, and the other is a convex shape, to ensure a dependable seal. With increasing water depth, the sealing rings experience augmented pressure, resulting in a gradual pressing of the rings into the sealing groove. Using ANSYS workbench finite element software, a two-dimensional axisymmetric lip seal finite element model using forces for overall constraint was established, the complete process of progressive pressing into the sealing groove was simulated, and the deformation, contact stress, maximum shear stress, and von Mises stress distribution was also simulated. We also conducted a comparative analysis of lip seals under low and high-water pressure sealing conditions. The findings of the study indicate that when subjected to a combined effect of the installation pre-tightening force and the working water pressure, the lip seal experiences complete compression into the sealing groove at a specific water depth. When subjected to the simultaneous influence of water pressure on the sealing ring material and friction force on the contact surface, two extremes of contact stress manifest in the primary sealing zone of the lip seal. These extremes have the capacity to elevate the contact stress and the effective sealing width, ultimately leading to an improvement in the sealing performance. Concurrently, as the water pressure gradually increases, the inner concave circle of the sealing ring experiences stretching, leading to a reduction in stress concentration, equivalent stress, and shear stress to a considerable extent. This mechanism ensures that the lip-shaped sealing ring retains sufficient strength. This study offers a viable solution for conducting sealing studies on deep-sea separable pressure vessels.

Effect of hydrostatic pressure on multilevel structure and bending progressive damage of 3D braided composites under simulated deep-sea environment

Compared to metal materials and laminated composites, three-dimensional (3D) braided composites are showing innovatively great potential in deep-sea pressure shell applications owing to their light-weight, excellent out-of-plane mechanical properties and near-net forming capacity. In this paper, three-dimensional six-directional (3D6D) braided composites with different epoxy resins are designed and fabricated. The samples are put in a deep-sea pressure simulator at 10 MPa pressure for 80 h and experimentally test via the three-point bending method. A multi-level evaluating approach, namely nano-scale, micro-scale, meso-scale and macro-scale, is proposed. The effect of the hydrostatic pressure on multilevel structure and progressive failure behaviours are analyzed. Results indicate that, although the moisture absorption does not lead to the generation of new chemical bonds on the nanoscale in the carbon/epoxy composites, the moisture absorption has significant effects on the bending mechanical properties and damage initiation of 3D6D braided composites. Moreover, after deep-sea hydrostatic pressure, the strength of 3D6D braided composites decrease by 15.64–30.93 %, and the damage volumes increase by 0.039–0.405 %. Furthermore, the final failure mechanism of 3D6D braided composites after deep-sea hydrostatic pressure are largely governed by the fiber-matrix debonding and matrix cracking.

Three-Dimension Crack Propagation Behavior of Conical-Cylindrical Shell

The conical-cylindrical shell is prone to stress concentration in the convex cone position under the action of deep-sea pressures. This results in unidirectional or bidirectional positive tensile stresses on the surfaces of the shell. The conical-cylindrical shell is a large, welded structure. Welding residual stress was generated at the cone-column joint position, resulting in high-stress concentration at this location. Under both the residual stress of welding and seawater pressure, cracks easily form and propagate on the shell weld toe, leading to fatigue damage and even structural failure. In this paper, based on the seawater’s alternating load and the residual stress of welding, the three-dimensional crack propagation process was studied for the submarine conical-cylindrical shell. The effects of crack depth and shape ratio on crack propagation trend and fatigue life were analyzed. The results can provide references for predicting the crack propagation trend, assessing the remaining life and evaluating the structural safety of the submarine conical-cylindrical shell.

A transient magneto-mechanical numerical model of a tank based on improved jiles-atherton theory

Magnetic field generated by the ferromagnetic material of underwater detectors varies with seawater pressure, which may affect the accuracy of deep-sea resource exploration. The effect of stress on magnetization is cumulative. In order to accurately simulate the influence of deep-sea pressure on the magnetic field around underwater detectors, it is very important to develop a transient magneto-mechanical numerical model. In this paper, a transient magneto-mechanical coupling model of a tank was developed to obtain the magnetic signal deviation caused by the magneto-mechanical coupling effect. By improving the Jiles-Atherton theory for complex stress and calculating the variation in magnetic field with pressure, the hysteresis effect between magnetization and stress was verified. Under different background magnetic fields, the effect of stress on magnetic field was determined by calculating the magnetic field distribution under different pressures.