Elasto-plastic Dynamic Responses Of Ship Structures Under Ice Loading

In the past hundred years,global warming has accelerated the melting of Arctic glaciers,resulting in a gradual increase of ice floes,which poses great difficulties and potential threats to polar ships navigating in the ice region.Polar ships are inevitably subjected to ice floe impacts during navigation.Under some extreme ice loads,the structures will experience large plastic deformations,which will have some serious adverse effects on the safety and operational performance of polar ships.Therefore,the problem of large plastic deformation of structure under ice loading has gradually attracted wide attention in the academic and industrial circles.However,due to the complexities of the mechanical properties of ice material and ice-structure interaction process,the dynamic response mechanism of the structure under ice load has not been thoroughly studied,and the theoretical model of structural plastic dynamic response under ice floe impact loading needs to be developed.Besides,the mechanical mechanism of structural accumulated damage under repeated ice loadings is lack.Therefore,the corresponding research is urgently needed.In this thesis,the non-stiffened and stiffened plate structures are selected as the research object.Combined with experimental,numerical and theoretical methods,the elasto-plastic dynamic response of the structure under ice load is studied,the influence law of ice deformations and damage on structural plastic deformation is revealed,and the energy sharing mechanism of ice-structure collision is discussed.On this basis,for the two load forms(ice impact load and local uniform ice load),the accumulation mechanism of structural plastic deformation under repeated ice loads is investigated,and the evaluation method of structural elastic-plastic dynamic response under repeated ice loads is proposed,which can provide guidance for structural design and safety evaluation of polar ships.The main contents and conclusions are as follows:(1)On the basis of the existing research work on the mechanical properties and failure behaviors of ice material,a numerical ice material model is established according to the constitutive relations of concrete material,taking into account the influences of strain rate and temperature.The specific energy absorption(SEA)value calculated by the proposed ice material model is in accord with the experimental SEA statistics.Moreover,the pressure-area(P-A)curve calculated by the proposed ice material model is in good agreement with the standard P-A curve recommended by ISO.The proposed ice model can be used for numerical simulation of strength assessment of structure under ice floe impact in the accidental limit state.(2)The model tests and finite element(FE)simulations of dynamic responses of the plate structures under ice impact are conducted,the plastic deformation mechanism of plate and ice failure modes are studied,and the energy sharing mechanism in the ice-structure collision is analyzed.Moreover,the experimental and numerical results are compared to verify the accuracy of the FE method for icestructure impact.In addition,based on the research results of dynamic responses of the plate structures impacted by different shapes of ice floes and rigid mass,the influence law of ice floe shape on the structural plastic damage degree is revealed.An energy absorption reduction factor(EARF)of the structure under ice impact load is proposed,which can provide some useful information for structural safety assessment and design of polar ships.(3)Based on the energy approach of ice crushing and the rigid-plastic theory method,a theoretical model for the structural dynamic response of ship plate under ice floe impact is developed.An approximate analytical formula for the structural plastic dynamic response under ice impact loading is proposed,which can quickly and accurately solve the impact force,structural plastic deformation,ice crushing length,and energy dissipation of structure.According to the proposed theoretical analytical method,the structural plastic deformations and energy dissipations during the ship-ice collision are evaluated for five different P-A relationships of ice,and three design principles of the structure against accidental ice floe impact are discussed: strength design,shared-energy design and ductile design.(4)The experiments and FE simulations of the elasto-plastic dynamic response of the plate structures under repeated ice impacts are performed.The plastic deformation accumulation mechanisms and energy absorption characteristics of the structure are revealed.The variation laws of structural plastic deformation,energy dissipation of ice and structural energy absorption with the number of ice floe impacts are studied.The numerical results are compared well with the experimental results.On this basis,the validated FE method is adopted to study the occurrence mechanism of the pseudo-shakedown phenomenon of structure under repeated ice impact loadings.Besides,the energy criterion for reaching the pseudo-shakedown state of the structure under repeated ice impacts is proposed.This research work can provide a reference basis for the structural design of polar ships against repeated ice impacts.(5)Combined with experimental and FE methods,the study on the elasto-plastic dynamic response of ship structure under local quasi-static load is carried out.The variation laws of structural deformation,stress and strain level during the loading and unloading process are analyzed.The FE results have a good agreement with experimental results.Additionally,for the scenario of a ship plate subjected to repeated ice loads at different locations,the FE method is utilized to study the accumulation mechanism of plastic deformation of ship plate under repeated local uniform loads at different locations.Furthermore,combined with numerical and rigid-plastic theory methods,a simple design formula to determine plating thickness under local ice load is proposed based on an acceptable level of permanent set and the plastic design principle,providing some useful references for the structural design of polar ships.