Study On The Water Absorption Characteristics Of Buoyancy Material In Full Ocean Depth Manned Submersibles

A solid buoyancy material composed of hollow glass microspheres and epoxy resin,with excellent characteristics of lightweight,high strength,and low water absorption rate,is an important structural system for manned submersibles in full ocean depth.Regrettably,after testing buoyancy material samples from multiple manufacturers,it was found that their crush strength did not meet the safety factor standard of 1.5 times specified by the China Classification Society for submersibles.To address this critical deficiency,two strategies emerge: enhancing the compressive strength of the buoyancy material and prompting designers to recognize the issue,implementing safety measures when using buoyancy materials beyond specifications in submersible operations.Ensuring safety,the scientific and rational formulation of rules for using materials beyond specifications requires in-depth research on the water absorption characteristics of buoyancy materials.The thesis focuses on solid buoyancy materials,analyzing the variations in water absorption rates at different stages of the water absorption process.The compressive strength of hollow glass microspheres is used as a key mechanical performance indicator.By comprehensively applying various methods such as experimental testing,numerical simulation,and theoretical analysis modeling,the thesis systematically explores the failure mechanisms and water absorption characteristics of buoyancy materials from both micro and macro perspectives.The main research contents of this thesis are as follows:The study investigated how factors like the volume fraction and microstructure of hollow glass microspheres in buoyancy materials affect the mechanical properties.Finite element numerical simulations were employed to compute the effective elastic modulus of buoyancy materials with diverse specifications.Detailed analysis was conducted on how different factors impact the elastic modulus of buoyancy materials.Findings indicate that,while maintaining a constant volume fraction of hollow glass microspheres,the effective elastic modulus of buoyancy materials notably decreases as the ratio of inner and outer diameters of the microspheres increases.Additionally,a qualitative assessment of stress distribution under compressive loads was performed to elucidate the relationship between the equivalent density of microspheres and the failure sequence.Results indicate that,under the same equivalent density of microspheres,hollow glass microspheres with larger outer diameters are more prone to failure compared to other microspheres.A thorough investigation into the compressive performance of solid buoyancy materials led to the proposal of a solid buoyancy material ultimate strain energy model based on elastic strain energy.The model can calculate the ultimate strain energy of hollow glass microspheres with varying particle sizes and analyze the critical particle size leading to chain failure in buoyancy materials under diverse pressure conditions.Findings show that as the radius of hollow glass microspheres increases,the compressive strength of buoyancy materials decreases gradually.Additionally,as water pressure rises,the critical radius for fracture of hollow glass microspheres decreases accordingly.The reliability of this compression model was confirmed through a series of experiments.Additionally,the probability distribution function of elastic strain energy in solid buoyancy materials containing hollow glass microspheres was established,offering a means to assess material performance.To enhance buoyancy material performance by leveraging the varied compressive strength of hollow glass microspheres,the "water pressure screening method" was introduced for microspheres selection.By eliminating microspheres with lower compressive strength,the overall compressive strength of buoyancy materials was successfully enhanced,thereby strengthening the safety reserve of solid buoyancy materials for full ocean depth applications.Based on the self-consistent theory,a single-cell concentric spherical shell model was established,and a stress-absorption coupled simplified model for the free water absorption stage of buoyancy materials was constructed.The water absorption process of buoyancy material laminates was simulated using the finite element method,and the results were compared and verified against experimental data to ensure the accuracy of the simplified model.Further analysis was conducted on the variation patterns of key parameters such as water absorption rate,volumetric stress,diffusion coefficient,etc.,with respect to time and thickness,and corresponding curves were plotted,providing theoretical support for the long-term stable operation of buoyancy materials in deep-sea environments.To further explore the water absorption characteristics of buoyancy materials in extreme deep-sea environments,an in-depth study of the fracture behavior of hollow glass beads was conducted.Through systematic analysis,the close relationship between the damage and water absorption behavior of hollow glass microspheres was clarified,especially during the water pressure-driven stage,where the water absorption process of buoyancy materials is mainly dominated by the fracture of hollow glass beads.Based on this,an innovative water absorption rate model for buoyancy materials at this stage was constructed,and the threshold for the occurrence of chain fracture of hollow glass microspheres was calculated.The feasibility and reliability of the compression model and the theoretical prediction model for water absorption rate were validated through the design and execution of multiple experiments.In response to the limitations of current water absorption rate measurement techniques,a real-time monitoring system was developed to track water absorption rate and crush strength.This system,through continuous data collection and analysis during the water absorption process of buoyancy materials,can determine the water absorption rate under varying pressures and accurately assess crush strength.This enables dynamic monitoring and evaluation of the performance of buoyancy materials,providing a new approach for the precise monitoring of water absorption rate and crush strength.Additionally,safety standards for the use of solid buoyancy materials at full ocean depth were discussed,and a static hydrostatic pressure testing method for solid buoyancy materials at full ocean depth was proposed,offering important guidance for the safe application of buoyancy materials in deep-sea environments.