Research On Stress Wave Propagation Of Underwater Structure

With the country's economic construction and people's living needs,there are many complicated engineering structures and equipment at sea such as ships,offshore platforms,and industrial pipelines.Due to the variable sea surface and underwater environment,the engineering structure working in it is subjected to complex alternating loads caused by different water pressure,temperature changes and flow-induced excitations.In addition,the engineering structure will inevitably have structural and material defects during the construction and welding process.The variable load changes the stress field at the structure,especially at the weld defect.The stress concentration at the weld joint leads to fatigue cracks.The initiation and expansion of cracks is the main source of stress wave signals generated by structural welds.Current research shows that stress wave technology has the potential to be applied to remote monitoring of offshore engineering structures.Monitoring stress wave signals generated by engineering structures and equipment can detect failures and defects early in the life of the equipment,thereby providing more time for planning and implementing necessary maintenance and repair procedures to avoid catastrophic failures.Therefore,studying the propagation of stress waves in underwater structures and the propagation law of stress waves after entering the water has great engineering significance for further exploring the applications of underwater structure health monitoring.In this thesis,taking a cylindrical shell as an example,the theoretical method and numerical simulation are used to analyze the generation of stress waves excited by the dynamic load at the structural weld and its propagation characteristics in the structure.Including the typical guided waves formed under the boundary conditions of the shell structure and the transmission law of the stress wave on the fluid-solid interface in the shell structure(the relationship between the energy distribution generated in the transmission process,the conversion of the wave mode,etc.and the different stress wave sources and different medium characteristic impedances).According to the principle of fracture mechanics this article analyze the physical basis of the stress wave source of the underwater structure,and establish a mathematical model that can reflect the characteristics of the stress wave source of the crack.According to the basic theory of guided waves,the characteristics of typical guided waves formed by the body waves in the structure under the boundary conditions of the shell structure are studied.The design algorithm solves the dispersion equation of the guided waves in the cylindrical shell structure to obtain the dispersion curve.Using numerical simulation and theory analyze the combined method to verify the accuracy of the finite element method for the stress wave propagation of the structure under solid-fluid interaction.Based on the established equivalent-stress-wave source and the verified underwater stress-wave-propagation model,the stress wave response characteristics under different loading conditions are analyzed in the time domain and frequency domain respectively,and the propagation modes under different stress wave sources of shell structure is analyzed using Choi-Williams distribution.In this thesis,based on the generalized Snell's law and the finite element power-flow energy analysis method,the effects of different stress wave sources and different media on the energy transmission of the coupled interface stress wave are qualitatively and quantitatively analyzed,and the energy transfer ratio is calculated.