Search

This study aims to enhance the high-temperature resistance of ultrahigh-performance concrete (UHPC) by optimizing the cement-fly ash-silica fume ternary cement matrix composition and the geometric characteristics of the plastic fibers. The UHPC was designed and prepared with the cement-fly ash-silica fume ternary cement matrix through simplex centroid method and 2% volume fraction steel fiber. Polypropylene fibers with different geometric characteristics were incorporated to enhance the high-temperature resistance. The fire resistance is examined through the high-temperature exposure test. The initial bursting temperature is recorded, and the deterioration of mechanical performance is characterized. It was found that the decrepitation temperature of UHPC first increases and then decreases with the fly ash (FA) content. Meanwhile, the influence of the silica fume (SF) and cement content on the decrepitation temperature is not obvious. The optimum mix ratio of 50%–65% cement, 20%–30% FA, and 10%–20% SF is recommended to prepared the cementitious matrix with 450°C or higher initial burst temperature. The crack mouth opening displacement (CMOD) test assisted with digital image correlation (DIC) examination is conducted to characterize the fracture performance before and after high-temperature exposure. It was found that the fracture resistance of UHPC first increases and then decreases with the silica fume content, which reaches the maximum value of 16.64 N/mm with 27% SF content. Meanwhile the high-temperature resistance of UHPC increases and then decreases with the increase of the length of the doped polypropylene (PP) fibers. The added 1.2% volume fraction PP fiber can resolve the spalling issues of the UHPC materials, and the residual compressive strengths after exposure to 1,000°C of UHPC samples containing 15 mm–18 μm and 15 mm–33 μm fibers can exceed 50 MPa. This study can serve as a solid base for the fire resistance design of UHPC materials in field construction. [ABSTRACT FROM AUTHOR]

By the finite element method, the structural vibration response is calculated under the action of the axial exciting force and the moment with different distribution form, and then the transfer function of the mean square normal velocity is analyzed. The wavenumber spectrum analysis method is used to separate and quantify the shell vibration in the wavenumber domain, and then the relation between the structural vibration characteristics and the structural wavelength is summarized. It is concluded that the structural vibration and radiated noise can be reduced under the symmetric action of axial exciting force and the moment. Based on the above conclusion, a symmetrical thrust bearing supporting system is designed and the stiffness of the supporting structure in the axial direction is controlled by selecting suitable size of structural members, therefore, the structural vibration and radiated noise of the submarine is reduced significantly.

Objectives To improve underwater acoustic communication and sonar detection ability in different seasons in an actual coastal environment, this study focuses on the optimum depth of shallow water acoustic propagation.Methods Using normal mode methods, incoherent transmission loss versus depth and range is calculated with different source locations and frequencies under typical sound speed profiles, and the effects of different sound speed profiles on the optimum depth are studied.Results Optimum depth at low frequencies does not change with sound speed profiles and source locations; it is always located near the middle of the water layer. At high frequencies, when the source is located at the isothermal layer of thermocline shallow water or a water layer with a constant gradient, the optimum depth is equal to the source depth and its conjugate depth within the iso-velocity layer. The optimum depth of positive and negative gradient shallow water is the same as the source depth.Conclusions The results of this study can provide useful references for the selection of receiver depth and passive sonar.

[Objectives] In this paper, the influence of thrust bearing seating on the acoustic radiation of a submarine is studied. [Methods] By utilizing the Finite Element(FE)method and FE coupling fluid interface method, the structure vibration of the pressure hull is calculated with different distributions of longitudinal exciting forces. In this case, the symmetrical distribution of exciting forces and moments are beneficial for reducing vibration. According to the above conclusions, symmetrical thrust bearing seating is designed and the longitudinal stiffness of the system controlled through choosing the size of components. Two submarines are designed, one with conventional thrust bearing seating and the other one with symmetrical thrust bearing seating. The vibration response and radiation noise of the structures of the two submarines are forecasted. [Results] It can been seen that when the propeller longitudinal force is constant, symmetrical thrust bearing seating can significantly decrease the level of vibration and radiation noise. [Conclusions] The results can provide references for the acoustic optimization and design of the thrust bearing seating of submarines.

Statistical Energy Analysis(SEA)is an effective method for solving high frequency structural vibration and acoustic radiation problems. When we use it to analyze submerged structures, it is necessary to consider the actions of fluid as'heavy fluid' relative to structures, which differs from when it is used in the air. The simple model of a submerged cylindrical shell is used to calculate at a higher frequency using FEM/BEM. The SEA and FEM method are then used to calculate the radiation sound pressure level, verifying the accuracy of the SEA prediction for submerged structures. The classified method of subsystems and the effect of the error of the internal loss factor on the accuracy of the results are explored. The calculated results of SEA and FEM/BEM are very different below 400 Hz, and basically the same above 400 Hz. The error caused by the division of different subsystems is about 5 dB. The error in the calculation results caused by the error of the internal loss factor is 2-3 dB. It is possible to use SEA to calculate the radiated noise of an underwater cylindrical shell when the modal density is high enough.For the cylindrical shell, dividing the subsystems along the circumference is not reliable at a low frequency, as it may lead to inaccurate calculation results. At a high frequency, it is more accurate to divide the subsystems along the circumference than the axle. For subsystems with high energy, the internal loss factor has a greater effect on the simulation results, so a more accurate way should be taken to determine the internal loss factor of subsystems with high energy.