Research On Fatigue Strength Of The Polar Ship

The polar region is rich in natural resources,and it is high value of commercial development and scientific research.As the polar channel is being developed and the ice and snow is melting faster because of global warming,human activities are becoming more frequent in the polar region.However,most areas of the polar region are covered by inches of ice layers.And the extreme low temperature in these regions is a long duration.So the structural safety of a ship sailing in this area must be considered in particular.When the polar ship navigates on the polar region,the high-speed bow dashes against inches of ice layers.The impact causes the hotspot in the polar ship to produce the alternating stress with high amplitude and high frequency.So the fatigue problem need to be considered.What' more,the low temperature actually embrittles the steel and weld joint.So it is very important to research the fatigue problem induced by the ice loads.However the research at home and abroad focuses the ice breaking ability of the polar ship,the yield strength of steel instead of the fatigue problem induced by the ice loads.To analyze the fatigue problem of the polar ship induced by the ice loads,the author has studied the fatigue properties of the steel under low temperature,the fatigue damage of the polar ship induced by the ice loads.The following work has been carried out.1.The fatigue test was carried out under the minus 60 degrees Celsius.The fatigue standard specimen was designed according to the fatigue test standard.The fatigue life of the specimen was got under various stress levels.The test data was analyzed based on the maximum likelihood estimator method.And the median S-N curve of the low-temperature steel was obtained under the minus 60 degrees Celsius.2.The finite element model was established about the continuous and crash ice-breaking way of the polar ship.The simulation process was based on the example of the voyage of the polar ship.Three coupling effects of sea water,sea ice and air was considered in the simulation process.Based on the nonlinear dynamic analysis software LS-DYNA,the impact between the ship and ice layers was solved.The time history of the collision force,the structural response of ship and ice layers were obtained.And the result of the simulation process was analyzed and compared.3.According to the guide and the example of the voyage of the polar ship,the fatiguecondition was established about the voyage process of the polar ship.The ice thickness,speed of the ship and ice thickness probability were determined under various conditions.Base on the structural response of ship above and the guide to fatigue strength of ship structure,the typical hotspot were selected.The local fine mesh of the hotspot above was established.And the stress concentration factor of the hotspot was calculated.4.The rain flow counting method was used to count and analyze the time history of the stress of the hotspot.And the amplitude and mean value of each stress cycle was obtained.The Goodman correction method was used to correct the mean value.Based on the linear cumulative damage theory and the median S-N curve of the low-temperature steel,the fatigue damage of the hotspot was calculated in the statistical cycle of rain flow.According to the calculation formula for cumulative fatigue damage of the polar ship under low temperature,the fatigue damage of the hotspot was obtained in the design life period.