【作者】 邱保文；

【导师】 袁泽喜；

【作者基本信息】 武汉科技大学 ， 材料学， 2010， 博士

【摘要】 西气东输工程是我国石油天然气工业的里程碑,其管道安全是与社会经济、政治、人民生命安全密切相关的重大问题。西气东输工程的设计思想采用的是夏比冲击功和DWTT断面纤维率控制方法,而作为衡量材料韧性断裂指标的JIC和CTOD因X80钢具有高韧性而导致测试无效,而广泛应用的传统断裂失效评定图存在参数确定问题。本文主要针对TMCP工艺生产的X80钢,采用具有物理意义的细观孔洞扩展模型来描述其韧性断裂特性,寻找表征X80钢韧性断裂的可靠方法。应用不同颈缩程度的拉伸试验,通过金相观察和微细观测量,探明了X80钢中孔洞萌生的机理,第一阶段孔洞的形核是围绕钙处理硫化物,在颈缩初期即开始了;第二阶段属高应变量阶段,此时孔洞是通过MA岛/基体界面脱离形核。测定了钙处理硫化物/基体界面强度和MA岛/基体界面强度。结合裂纹体断口观察,建立了体胞模型,对X80钢孔洞扩张进行了数值分析,对比分析了细观损伤理论中RT模型、Gurson模型、GTN模型和王自强模型的适用范围,获得了不同应力状态下孔洞扩张演化规律。针对先期形核孔洞聚合阶段的特点,分析了MA岛形核的可能性。体胞分析结果说明,在应力三轴度高于2时,完全可以不用考虑MA岛/基体界面脱离形核孔洞对断裂过程的影响。而在低应力三轴度(小于等于1)下,存在MA岛/基体界面脱离的条件,由于MA岛的含量远大于钙处理硫化物的含量,因此充分考虑二次形核对断裂过程的作用是十分必要的。当夹杂物形态趋于球化,在不同应力状态,不同应力三轴度下,对应的体胞承载能力接近一致;而扁平椭球体孔洞则显示了较低的承载能力。微孔洞密度增加,导致孔洞扩张加快,降低材料的韧性。考虑断裂过程区应力状态分布特点,有限元模拟得到与试验极其近似的裂纹扩展形貌,并获得了与试验记录的载荷-位移曲线高度吻合的预测曲线,使微观结构与宏观力学性能的联系有了真实的物理基础,提高了数值分析的可靠度。通过区域积分方法,获得了表征X80韧性断裂的特征值。测定了X80钢疲劳裂纹扩展速率,采用有限元分析了X80钢管道在役运行条件下表面裂纹疲劳扩展特性,得到了不同初始形态的表面裂纹穿透过程的变化规律。运用已获得的细观力学模型对亚临界穿透裂纹疲劳扩展进行了分析,结合BS7910规范构造了先漏后破准则(即LBB)安全评定图。更多还原

【Abstract】 Natural gas transporting project from the West to the East is a milestone in China’s oil and gas industry, pipeline safety is a major issue closely related to its social, economic, political, people’s lives and safety . Fracture control methods used in the project are Charpy impact energy and fiber rate in DWTT cross-section. For high toughness X80 steel , JIC and CTOD as indicators to measure fracture toughness have invalid test results , and determination of basic fracture toughness parameters in widely applied traditional Failure assessment diagram is a exist problem. This dissertation focused on charactering ductile fracture of TMCP X80 steel using micro-porous dilational model to describe the process of ductile fracture properties, looking for reliable method to characterize fracture toughness of X80 steel.Appling a serials of tensile test of different necking degrees, by metallographic observation and micro observations, the mechanism of void nucleation in X80 steel was proved that, the first stage of voids nucleation occurs around the composite calcium sulfide inclusions, in the early necking stage; the second stage of nucleation of voids occurs at the MA island sites, in a deep necking stage, it begins voids nucleation by MA island / matrix interface decohesion. the strength of calcium sulfide/ matrix interface and the MA island / matrix interface are measured. Numerical analysis of voids growth in X80 steel had performed by cell models, Combining observation of crack body fracture surface and identifying microstructure parameters. Comparative analysis of the meso-damage theorys such as RT model, Gurson model, GTN model and Wang Ziqiang model had made, the applied scopes of them were distinguished. The void evolution laws under various stress states were obtained by finite element calculations. Voids nucleation through MA islands was discussed, and found that suffering high stress triaxiality(>2), stress at MA/matrix interfaces is not enough to take them apart from matrix, while in low stress triaxiality(≤1), interface stress is so higher to make voids nucleation. Load carry capacities of micro voids trending to spheric are near to each other, at the same stress triaxiality. But oblate micro voids show low load carry capacities, decreasing fracture toughness of X80 steel.Considering stress distribution in the fracture process zone, prediction of crack growth in X80 specimens by finite element analysis was approximated to experimental results, and the predicted load - displacement curves were consistent to the experimental results. Well predictions of macroscopic mechanical properties using the micromechanical parameters by cell model calculations were shown, which can improve the reliability of the numerical analysis. Through domain integration, we obtained a characterization value of fracture toughness of X80.Fatigue crack growth rates of X80 steel was tested, propagation process through the wall thickness of the surface crack in X80 pipeline under operating conditions had been performed with different initial surface crack shape using finite element analysis. Fatigue growth analysis of sub-critical breakthrough crack in pipeline had also performed using meso-mechanics model. Leak-before-break criteria (LBB) safety assessment diagram was constructed according to BS7910 specification.更多还原