基于循环孔洞扩张模型的X型圆管节点超低周疲劳寿命预测

海洋工程钢结构在服役过程中，受风、浪、流或地震等极端循环载荷的影响，易发生超低周疲劳断裂破坏，造成人员伤亡及财产损失，因此超低周疲劳断裂分析及寿命预测对于海工结构安全性评估至关重要。然而，现阶段基于累积损伤理论提出的多种超低周疲劳寿命预测模型无法对多尺度节点实现统一预测，造成了实际工程应用的不便。因此文中基于循环孔洞扩张模型开展X型圆管节点超低周疲劳寿命预测。首先，开发了基于循环孔洞扩张模型的VUSDFLD程序，实现ABAQUS与FORTRAN子程序联合应用，利用有限元分析验证循环孔洞扩张模型在X型圆管节点超低周疲劳断裂分析中的有效性；其次，根据多组X型圆管节点超低周疲劳断裂有限元分析结果，在宏观层面提出了一种基于Manson-Coffin公式的超低周疲劳寿命预测公式；最后，依据Miner理论，将适用于等幅加载的超低周疲劳寿命公式扩展至变幅加载情况，验证了多种节点尺寸下超低周疲劳公式的适用性，为工程应用提供了理论依据。

Prediction of ultra-low-cycle fatigue life of X-shaped circular tube joints based on cyclic pore expansion model

In the service process of marine engineering steel structures， under the influence of extreme cyclic loads such as wind， waves， currents， or earthquakes， ultra-low-cycle fatigue fracture failures are prone to occur， causing casualties and property losses. Therefore， the analysis and prediction of ultra-low-cycle fatigue fracture are crucial for the safety assessment of marine structures. However， at present， various ultra-low-cycle fatigue life prediction models based on cumulative damage theory cannot achieve unified predictions for multiscale joints， resulting in inconvenience in practical engineering applications. Prediction of ultra-low-cycle fatigue life of X-shaped circular tube joints based on cyclic pore expansion model is conducted. Firstly， a VUSDFLD program based on the cyclic pore expansion model is developed， which realizes the joint application of ABAQUS and FORTRAN subroutines. The effectiveness of the cyclic pore expansion model in the analysis of ultra-low-cycle fatigue fracture of X-shaped circular tube joints is verified by finite element analysis. Secondly， based on multiple sets of finite element analysis results of ultra-low-cycle fatigue fracture of X-shaped circular tube joints， a prediction formula for ultra-low-cycle fatigue life based on the Manson-Coffin equation is proposed at the macroscopic level. Finally， according to the Miner theory， the prediction formula for ultra-low-cycle fatigue life applicable to constant amplitude loading is extended to variable amplitude loading conditions， and the applicability of various ultra-low-cycle fatigue formulas under different joint sizes is verified， providing a theoretical basis for engineering applications.