Research On Crashworthiness Assessment Method Of Double-hull Ship Structure

With the development of shipping industry,water traffic is becoming busier.Thus,ship collision accidents are inevitably to occur.Equipping ship side with a double-hull can increase the safety of the ship when subjected to impact loads.If collision accidents happen,the existence of the inner hull can guarantee the water-tightness of the cabins after the outer hull plate is ruptured.Until now,extensive research was focused on the deformation resistance and fracture prediction of the outer hull plate as well as the deformation resistance of the web girder.However,investigations on the tearing behavior of the outer hull plate after fracture,the coupling effect between the outer hull and the web girder and the damage characteristic of the inner hull for the double-hull ship structure are seldom involved.Current study is performed to investigate the crashworthiness of the double-hull ship structure suffering head-on collision by a bulbous bow through experimental,numerical and simplified analytical methods.The damage mechanisms of each structural component in the double-hull are researched.Besides,a numerical simulation method that can simulate the collapse process of the double-hull precisely and an analytical method that can predict the resistance response of the double-hull accurately are proposed.The main work of this dissertation is as follows:Firstly,model tests are performed to investigate the tearing characteristics of the outer hull plate after fracture.In the model tests,a sphere-cylinder-cone combined indenter is designed to distinguish the large deformation to initial fracture process and the tearing process.Different kinds of stiffened plates are designed to evaluate the effects of the stiffeners when subjected to impact loads.Resistance-penetration curve and damage process for each specimen are obtained through model tests.Moreover,several tearing features of the plate are concluded.Compared with the large deformation to initial fracture process,the amount of energy dissipated in the tearing process is significant and the effect of the stiffener in the tearing process is more obvious.Besides,the crack in the tearing process belongs to opening mode crack(model “I” crack).In addition,the lateral tearing resistance is irrelevant with the crack number produced in the plate.Then,a numerical simulation method is proposed to predict the initial plate fracture and the plate tearing resistance simultaneously.In the numerical simulation,uniaxial tension tests for standard and notched pieces are conducted to calibrate the initial element failure strain and tearing element failure strain,respectively.Numerical results compare well with that of the experiments,which proves the accuracy of the proposed numerical simulation method.Moreover,the influence of triaxility on the failure strain is considered.The relations of the initial failure strain and the tearing failure strain with the element length in biaxial tension state are presented,respectively.Compared with the commonly used failure criteria,the numerical simulation method proposed is more accurate with larger element size.Afterwards,analytical formulae are built to calculate the large deformation resistance,critical penetration depth and tearing resistance for the plate and the attached stiffeners.For the large deformation and tearing resistances,the deformation patterns of the plate and the attached stiffeners as well as the corresponding resistance responses are proposed.For the initial fracture prediction,a formula is proposed to calculate the critical penetration depth of the stiffened plate,which includes the formula to obtain the critical penetration depth of the plate and a reduction formula of the critical penetration depth for the plate by the stiffener.The proposed equations are validated to be accurate by the model test.Finally,the applicability of the proposed equations is discussed with different scantlings of plate and indenter.Based on the above research of the crashworthiness of the outer hull,the crashworthiness of the inner hull and the web girder in the double-hull is investigated.An experiment of a scaled double-hull punched by a cone and the corresponding numerical simulation are performed.Through the model test,the resistance-penetration curve and the damage mechanisms of the inner hull plate and the web girder are obtained.Through the numerical simulation,the deformation and rupture process of the double-hull are obtained.Moreover,the numerical simulation result can provide the amount of energy dissipated by each member of the double-hull,which demonstrates that the amount of energy absorbed by the inner hull is much lower than that of the outer hull and the web girder can absorb an extent of energy in the collapse process.Finally,analytical formulae are derived to calculate the resistance responses for the inner hull and the web girder.The deformation process of the inner stiffened plate contains the local deformation of the stiffener and the global deformation of the side plate.With the superposition of these effects,the deformation resistance of the inner stiffened plate can be obtained.Besides,the coupling effect between the outer hull and the web girder can delay the fracture moment of the outer hull plate.The relation between the resistance and the in-plane deflection of the web girder is obtained.And the maximum deflection of the web girder is calculated,where the deflection is the addition of the critical penetration depth for the outer hull by the web girder.The accuracy of the proposed analytical method is validated by the model test and the numerical simulation.In addition,the proposed analytical formulae are proved to be able to assess the crashworthiness of the full-scale ship structure according to the research of the scaling effect.