Research On Fatigue Performance And Ice-induced Fatigue Analysis Method Of Icebreaker Structure At Low Temperatures

Climate warming,melting of glaciers and acceleration of globalization in the Arctic region have provided convenience and possibilities for polar navigation,resource development,and polar tourism.The polar regions have also attracted increasing interest from many countries.As an indispensable engineering ship for polar scientific research,exploration,and development,icebreakers have received more and more attention from researchers.The harsh geographical and climatic conditions in the polar regions make icebreakers face severe challenges in the process of opening up waterways.Under continuous icebreaking conditions,the hull structure of an icebreaker is subject to long-term cyclic ice loads,which are likely to cause fatigue damage to the hull structural components.In the thesis,the research on iceinduced fatigue of icebreakers is carried out,and a systematic fatigue assessment method of icebreakers under the conditions of polar low temperature is given.The main research contents of this thesis are as follows:(1)The low-temperature tensile test,low-temperature fatigue S-N curve test and lowtemperature fatigue crack growth test are carried out in the thesis.Firstly,the fatigue S-N curve tests on 3 types of typical welded joints of DH36 steel(longitudinal fillet welded joints,transverse fillet welded joints and load-bearing cruciform welded joints)are carried out at low temperature.The hot-spot stress of the structure is monitored by low temperature strain gauges,and the fatigue life of three typical welded joints with different stress levels is measured under low temperature conditions.Then,according to the test results,the data is processed and corresponding low-temperature fatigue S-N curves with different probability levels are given.DH36 steel and its butt weld joints were chosen,and C(T)specimens are prepared in accordance with relevant specifications and standards.The low-temperature fatigue crack growth tests of DH36 steel and its butt weld joints are carried out under appropriate temperature and stress levels.According to the test results and data processing,the corresponding low-temperature crack propagation law(Paris)parameters are given.Scanning electron microscopy(SEM)is employed to observe the microscopic morphology of the fracture surface,which further explained the fatigue failure mechanism at low temperature.The research results in this part are not only the basis for the subsequent ice-induced fatigue analysis in this thesis,but also can make up for the lack of fatigue parameters(S-N curve and Paris parameters at low temperatures)in the existing specifications that are only applicable to room temperature.(2)Based on the elastoplastic sea ice material model,the compression and bending failure of level ice is simulated.The applied sea ice material model is verified through related sea ice compression and bending tests.Then the ship-water-ice coupling model is established,and the ice load on the ship structure is solved by the nonlinear collision principle and the LagrangianEuler coupling method.On this basis,the thesis proposes a long-duration simulation algorithm of ice load based on the physical characteristics of ship-ice collision.Based on the long-duration simulation algorithm of ice load proposed in the thesis,it is now possible to obtain long-duration ice load stress time history,which can better solve the long-duration simulation problem of ice load fatigue stress.(3)On the basis of obtaining the long-term ice load,the Xuelong 2 icebreaker is chosen as the research object to carry out the corresponding ice-induced fatigue analysis.Firstly,structural dynamic response analysis of the hull structure is carried out and potential fatigue failure points in the hull structure are identified.Combine with the stress concentration factor at the structural points,the hot-spot stress time history at the potential fatigue failure points can be obtained.Then the probability density distribution model is employed to perform regression analysis on the fatigue stress time history data,so the probability density function of the fatigue stress at the corresponding fatigue points can be obtained.After that,the S-N curve and the Paris law are used to solve the fatigue initial life and fatigue crack growth life of the fatigue points of the hull structure respectively.In addition,the fatigue reliability of the icebreaker structural points is updated based on the crack inspection,so as to obtain the fatigue reliability under different crack inspection plans.To sum up,the thesis takes the level ice as an example,and proposes an analysis method for structural fatigue damage of an icebreaker under icebreaking conditions.Aiming at the icebreaking conditions of Xuelong 2 icebreaker,the fatigue damage of the icebreaker under continuous icebreaking mode is analyzed.In the case of changing the form of sea ice(floes and ice ridges,etc.)and sea ice parameters(sea ice thickness,sea ice strength,melting state,etc.),this method can still flexibly analyze the ship structure ice-induced fatigue damage.The iceinduced fatigue analysis method of icebreaker proposed in the thesis provides a complete set of scientific calculation methods for the hull structure design and fatigue evaluation of icebreakers in my country,and perfects the research system of icebreaker hull structure design and fatigue evaluation in China.