| Underwater explosion can damage the submarine hull,the equipment and the staff in it,which will threat the vitality and fighting capacity of warships.How to improve the shock resistance ability of such weapons has become a more and more urgent request for the current and new vessels.The thesis is under the support of the National 973 project “The Shock Mitigation Mechanism of the Cladding Coated on the Wetted Surface of Submarine”.A new cellular shock mitigation cladding coated on the submarine hull is proposed to isolate and absorb shock energy,which can decrease the load transmitted to the submarine hull and the equipment.On this basis,a new type of shock coating is designed and built,and its shock mitigation mechanism,shock mitigation effects and design guides are analyzed by theoretical,numerical and experimental methods.Main research contents and results are given as follows:(1)A one-dimensional analytical model is established to analyze the shock mitigation effects of the proposed compound shock mitigation coating with rubber and plastic foam subjected to combined loads of hydrostatic pressure and underwater explosion shock loading.Analyses are focused on the compression of the cellular core,fluid-structure interaction effects and cavitation phenomenon.The shock mitigation mechanism of the compound shock coating is then obtained.The results reveal that the shock impedance of the coating is small during the shock loading phase.The impedance mismatch between the water and the coating can isolate part of the shock loading.Most of the shock energy transmitted into the cladding can be absorbed by the plastic deformation of the cellular core,which can decrease the shock energy transmitted to the submarine hull.Therefore,the shock isolation mechanism based on the impedance mismatch and the energy absorption mechanism based on the core deformation function together mitigate the shock loading transmitted to the submarine hull.(2)The established one-dimensional analytical model is used to analyze the shock mitigation effects of the proposed compound shock coating when the coating parameters(including density,yield stress and densification strain)and incident shock wave(including the peak pressure and decay constant of the incident wave and hydrostatic pressure)parameters change.Moreover,the effective coating thickness when the shock energy is just completely absorbed is computed.The results demonstrate that the yield stress and the densification strain of the coating,and the incident shock wave influence the shock mitigation effects a lot.The effective coating thickness is an increasing function of the peak pressure and decay constant of the incident wave and hydrostatic pressure while a decreasing function of yield stress and densification strain of the coating.Based on the results of the parametric study,an explicit expression of the effective coating thickness as a function of coating parameter(yield stress and densification strain)and the incident shock wave(peak pressure and decay constant of the shock loading,and the hydrostatic pressure)are derived.(3)Based on the one-dimensional analytical results,a three-dimensional finite element model is built to investigate the transient response of a submersible hull coated with the designed shock coating subjected to combined loads of hydrostatic pressure and underwater explosion shock loading.Additionally,the shock migitation effects of the coating are discussed.The results indicate that the coating is very effective in reducing the hull deformation,the transmitted energy to hull,and the velocity and the acceleration response when its thickness is enough.Morover,the shock mitigation effects will be better for soft coating.If the thickness of the coating is not enough and the coating is densified,its shock migitation effects will be weakened.The total energy for the coated hull system is much larger than the bare hull system.That is,the bare hull allows for a rapid release of energy into the water while the claddings tend to concentrate energy and then absorb it.Moreover,the derived formulation of the effective coating thickness can estimate the coating thickness very well for the three-dimensional problem.(4)To avoid densification of uniform foam,multi-layer shock mitigation coating is proposed from two aspects: one is to shorten the effective coating thickness by functional graded design when the transmitted stress to the protected structure keeps unchanged;the other is to improve the shock resistance ability of the graded shock coating compared with the equivalent uniform coating on the condition of equal mass and thickness.The results for the former scheme show that a double-layer coating with the hard core on the impinged end can shorten the effective coating thickness by increasing the strength of the hard core or the mass of the front face sheet of the soft core while the transmitted stress to the protected structure is controlled by the soft core.The results for the later scheme show that the graded coating can resist stronger shock loading than the equivalent uniform coating,and the results will be better for the larger gradient.However,the two optimization schemes have their intrinsic shortcomings.For instance,the former sacrifices the mass while the stress transmitted to the protected structure will increase for the later scheme.(5)Based on the shock mitigation mechanism of the coating,a compound coating with rubber and rigid polyurethane foam is manufactured to conduct underwater explosion experiment,including a circular plate and a scaled model of a single hull submarine compartment,which can verify the shock mitigation mechanism and effects of the designed shock coating.The experimental results of the circular plate coated with shock coating illustrate that the designed shock coating can mitigate the impulse transmitted to the wetted interface and the plate deformation.Compared with the results of the bare plate,the decreasing rate of the impulse and the mean deformation for the coated plate are 59% and 41%,respectively.The experimental results of the scaled model of the single hull submarine compartment reveal that the shock coating is very effective in decreasing the hull acceleration,velocity and strain.When the hull shock factor is 0.3,the mean decreasing rates of the peak acceleration,velocity and strain for the hull coated with shock coating are 78.16%,67.79% and 39.77% compared with the hull coated with acoustic coating.When the hull shock factor is 0.35,the mean decreasing rates are 82.62%,70.98% and 49.2%.In addition,the finite element model of the scaled model of the single hull submarine compartment is built and compared with the experimental results,which validates the effectiveness of the numerical model.The computed internal energy results transmitted to the hull indicate that the shock coating is more effective in attenuating the shock energy than the acoustic coating.When the hull shock factors are 0.3 and 0.35,the decreasing rates of peak internal energy transmitted to the hull coated with shock coating are 77.6% and 82.8% compared with the hull coated with acoustic coating. |
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