基于累积塑性的船体结构多轴低周疲劳裂纹扩展机理研究

The fatigue strength of ship’s structure has very important significance on the safety and survivability. Along with the increasing in ship dimensions and more use of high-strength steel in recently years, the stress and deformation of ship structures are so high and large, which result in very prominent problem of low-cycle fatigue damage to large-scale ships. This has become the key issues demanding prompt solution in the development of large-scale ships. The research on fatigue crack propagation behavior under multiaxial irregular high-stress and low-cycle load is the main problem of fracture failure of ship hull structures in extreme sea conditions.Analysis shows that most overall fractures of ship structure tend to be the coupling interaction result of low-cycle fatigue and increasing plasticity. Therefore, the multiaxial low-cycle fatigue crack propagation analysis and estimation considering the effect of accumulated incremental plastic damage can more realistically predict the fatigue strength of ship hull structures in extreme sea conditions. Based on the accumulated incremental plastic damage at crack tip region, this project will carry out theoretical, numerical and experimental research on the multiaxial low-cycle fatigue crack propagation behavior of ship hull structures. The crack tip opening displacement (CTOD)will be applied to the analysis of multiaxial low-cycle fatigue crack propagation, taking the lead in solving the problem in the field. The research finding of the project could more accurately assess the multiaxial low-cycle fatigue fracture failure of ship hull structures under extreme sea conditions, and also can provide a more advanced and reliable theoretic base and analysis method for the prediction of multiaxial low-cycle fatigue crack propagation behavior of ship hull structures.

船舶结构的疲劳强度对船舶的安全性和生命力有着十分重要的意义。随着船舶主尺度 和高强度钢的广泛使用，使得大型船舶结构的低周疲劳问题成为亟待解决的关键性问题之一。多轴非规则高应力、低循环载荷下船体结构疲劳裂纹扩展行为研究是恶劣海况下船体结构断裂破坏的主要问题之一。研究表明，大多船体结构的总体断裂破坏往往是低周疲劳破坏与递增塑性破坏耦合作用的结果。因此，考虑多轴低周疲劳载荷下船体裂纹板裂纹扩展与累计塑性破坏的耦合影响研究能够更为符合实际地评估恶劣海况中船体结构的低周疲劳断裂。本项目基于裂纹尖端累积塑性变形对多轴低周疲劳载荷下船体结构裂纹扩展行为全面开展理论、模型及数值分析研究，将多轴应力状态下的船体结构裂纹尖端张开位移（CTOD）应用于多轴低周疲劳裂纹的扩展分析。研究结果可以更确切地分析评估船体在恶劣海况下的低周疲劳破坏，为进一步正确预报船体结构的低周疲劳提供更为先进的理论依据及可靠的计算方法。

船舶结构的疲劳强度对船舶的安全性和生命力有着十分重要的意义。随着船舶高强度钢的广泛使用，使得大型船舶结构的低周疲劳问题成为亟待解决的关键性问题之一。多轴非规则高应力、低循环载荷下船体结构疲劳裂纹扩展行为研究是恶劣海况下船体结构断裂破坏的主要问题。研究表明，大多船体结构的总体断裂破坏往往是低周疲劳破坏与递增塑性破坏耦合作用的结果。因此，考虑多轴低周疲劳裂纹扩展与累积塑性的耦合影响研究能够更为符合实际地评估恶劣海况中船体结构的低周疲劳断裂。.本项目以十字形船体裂纹板Q235钢试样为研究对象，基于弹塑性理论，通过理论分析、数值仿真与裂纹扩展实验相结合的方法，系统的研究了船体裂纹板在多轴比例/非比例低周疲劳载荷作用下（包括双轴应力比λ、应力幅Δσ、应力比R 以及相位差θ等不同影响因素）考虑累积塑性耦合作用的多轴低周疲劳裂纹扩展失效机理。通过扩展实验和数值模拟计算对比分析了裂纹尖端的塑性应变累计率以及扩展过程中的裂纹尖端张开位移∆CTOD。大型船体构件在低周疲劳载荷作用下率先在缺陷处逐步产生累积塑性破坏和低周疲劳破坏，而且这种累积递增塑性破坏会促进低周疲劳裂纹的萌生，加剧低周疲劳裂纹的扩展。随后，在多轴弹塑性应力应变场及累积塑性分析的基础上，以多轴应力状态下的∆CTOD为控制变量，建立了考虑累积塑性耦合影响的船体裂纹板多轴比例/非比例低周疲劳裂纹扩展评估模型。本项目分别提出了船体裂纹板试样在多轴比例/非比例低周疲劳载荷下累积塑性的理论解，建立了多轴比例/非比例低周疲劳裂纹扩展速率的预测模型；并分别开展了多轴比例/非比例低周疲劳载荷下累积塑性破坏试验和多轴低周疲劳裂纹扩展试验，通过试验数据对上述理论解和预测模型进行了对比分析，验证了公式的可靠性。研究结果为准确评估船体结构在双轴低周疲劳作用下断裂破坏行为提供重要依据，为将来进一步开展真实恶劣海况下船体结构的低周疲劳寿命的分析与评估，提供理论分析基础和计算方法。