| With the reduction of the Arctic sea ice,the rich resources in the region and the unique geographical location,the strategic competition for the Arctic region has become increasingly fierce.Ships sailing in the Arctic region are often faced with extreme low temperature environment.Low temperature will change the mechanical properties of steel at normal temperature,which is prone to low temperature embrittlement and even fatigue fracture.The research on fatigue fracture of Marine steel at normal temperature is relatively complete,but the research on mechanical parameters of crack fracture at low temperature is still less.Therefore,based on the Dugdale model of an infinite tensile plate with a central penetration crack and considering the effect of low temperature on the yield strength and strain hardening,the theoretical formulas of CTOD,CMOD and plastic zone size ρ at different temperatures are derived,and the influence of low temperature on the Mechanics Parameters of EH36 Marine steel is considered,as well.The elastoplastic finite element model was established to verify the validity of the proposed theoretical formula.Based on the relationship between CTOD and CMOD,the prediction model of fatigue crack growth rate based on CTOD of crack tip opening displacement was modified and obtained.According to the American ASTM test standard,the fatigue test data of marine EH36 steel under low temperature were transformed to obtain the fatigue crack growth test data of CTOD amplitude,to verify the validity of the prediction model.Therefore,the research results of this paper can provide reference for the fatigue life prediction and fracture evaluation of cracked structures in polar ships under low temperature environment.The main contents of this paper are as follows:1.The mechanical parameters of elastoplastic fracture are briefly described.The definition and criterion of CTOD and plastic zone size ρ are introduced.Based on the Dugdale model of infinite tensile plate with a central penetration crack,the effect of temperature on the yield strength and strain hardening index is considered.The theoretical formulas of CTOD,CMOD and plastic zone size ρ which can consider the effect of low temperature are derived.2.According to the static tensile test of the steel EH36 used for polar icebreakers,the engineering stress-strain relationship curves at different temperatures were converted into the real stress-strain relationship required for calculation,and the temperature dependence analysis of the steel EH36 was conducted.Using the universal finite element software ABAUQS,a tensile plate model with a central penetrating crack was established.The CTOD,the CMOD and the plastic zone size ρ were calculated at different temperatures.The validity of the deduced formulas was verified.3.Starting from the fatigue crack growth rate prediction model based on CMOD amplitude△u,according to the relation between crack tip opening displacement CTOD and crack maximum opening displacement CMOD in the deduced formula,and considering that low temperature has little effect on CMOD,a fatigue crack growth rate prediction model based on CTOD amplitude △δ was obtained.4.According to the a-N curve of EH36 steel used for polar icebreakers at different temperatures,and according to the empirical formula in ASTM E1290,the a-N curve of EH36 steel at different temperatures was transformed into da/d N-△δ curve,and the validity of the proposed fatigue crack growth rate model was verified.Finally,based on the prediction of fatigue crack growth rate of EH36 steel da/d N-△δ curve,the a-N curve was calculated by programming MATLAB and compared with the experimental data,and the validity of the proposed theoretical formula was verified... |
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