Research On Structure Fatigue Strength Evaluation Method Of Heavy Icebreaker

With the global warming,the value of the polar channel has been gradually excavated,and icebreaker,as a special type of ice breaking ship,has gradually attracted people’s attention.Due to the low temperature in the polar region,the steel has changed greatly in the polar environment.And the ice breaking area in the bow of an icebreaker is always in contact with the sea ice during the ice breaking operation of the icebreaker,so that the fatigue problem caused by the ice load is easy to occur there.Therefore,it is necessary to do a research about the fatigue problem caused by the ice load.The following work has been carried out with regard to the heavy icebreaker’s fatigue problem caused by the ice load in this thesis.It is introduced that structural fatigue strength assessment process of heavy icebreaker,which includes numerical simulation method of ship ice collision and fatigue data processing method of heavy icebreaker.The numerical simulation method of ship ice collision includes the introduction of nonlinear finite element theory,the method of eliminating the hourglass phenomenon in the finite element calculation,the penalty function method to deal with ship ice collision simulation contact,and the display center method for solving the collision motion equation.It is also elaborated that the periodic medium analysis method,which is the core algorithm for long-term ship ice collision.Meanwhile,the sea ice material is verified.And fatigue processing method of finite element simulation data includes rain flow counting method,average stress correction,fatigue S-N curve and Palmgren-Miner linear cumulative damage theory.According to the analysis of the navigation field of heavy icebreaker,starting from the distribution of polar ice situation,the ice breaking conditions are set and combined into fatigue conditions based on the design characteristics of heavy icebreaker.The finite element model of heavy icebreaker and sea ice is established,and the finite element simulation calculation of long-term continuous ice breaking and impact ice breaking based on periodic medium analysis method is carried out.The locations of fatigue hot spots are screened out,and these locations are divided into several areas for numbering.The local coordinate system is established.The grid of fatigue hot spots is refined by t*t grid,and the stress concentration factor is calculated.According to the finite element calculation results of ship ice collision,the low temperature fatigue test of heavy icebreaker steel FH40 is designed,including the design of tensile standard parts and fatigue standard parts,the selection of test machine,the measurement method of hot spot stress,the design of fatigue test scheme,and the measurement of low-temperature fatigue life.The median S-N curve and P-S-N curve of toe end welded joint and cross type welded joint’s standard parts at-55 ℃ are calculated by maximum likelihood method and unilateral tolerance coefficient method.According to the finite element simulation results of ship ice collision,the stress time history curves of each fatigue hot spot under different ice breaking conditions are given.The mean value and amplitude of each stress cycle are obtained by using the rain-flow counting method,and the influence of the average stress on the stress amplitude is corrected.The probability density statistical analysis of the stress range of each hot spot in the rain-flow counting period is carried out,and the Weibull distribution is satisfied.Finally,combined with Palmgren-Miner linear cumulative damage theory and the low temperature S-N curve obtained from the test,the cumulative damage degree of each hot spot in the rain-flow counting period is obtained,and the total damage degree in each working condition unit nautical mile is deduced,and the total damage in the design life cycle is given.