基于粘弹性损伤模型的聚乙烯管材慢速裂纹扩展机理研究

With the rapid development of the economics, as its “lifeline”, pipelines used for transporting oil and nature gas are also increasing with a speed of 10% per year. Polyethylene (PE) pipe has been found widely used in gas and oil distribution. According to the statistics, 90% of newly-built urban gas pipelines are PE pipes. Slow crack growth (SCG) is the main failure mode of PE pipe. Hence the ability to resist SCG is an important index to characterize the property of PE pipe. However, the measurement of this ability both at home and abroad is mainly conducted through experimental methods. The failure mechanism of SCG in PE pipe material, especially the variation of stress-strain relation and the transfer rule of strain energy in local region after loading, is still not well known in the aspect of theory.In this project, finite element numerical simulation, combined with theoretical analysis, will be used to study the mechanics of SCG in PE pipe material. Based on the continuum damage mechanics and cohesive zone model, a PE cohesive constitutive model including time-independent damage factor will be established. Then the extended finite element method (XFEM) will be utilized to investigate the fracture process of SCG. Meanwhile, notched creep tests for PE pipe will be conducted to verify and modify the analyzed model. Furthermore, ultrasonic phased array real-time imaging inspection technique will be used to monitor the hydrostatic test, and then a comparison will be made between the monitored test results and the predicted model results to validate the established model.

随着我国经济的高速发展，作为国民经济血管的油气管道以年均10%以上的速度飞速增长。聚乙烯管已广泛应用于油气输送。据统计，新铺设的城市燃气管道90%以上均使用聚乙烯管。慢速裂纹扩展是聚乙烯管材最主要失效模式，因而聚乙烯管材耐慢速裂纹扩展能力是衡量管材性能的最重要指标。然而，目前国内外对聚乙烯管材耐慢速裂纹扩展能力的评价仍以实验为主。对慢速裂纹扩展的失效机理，尤其是聚乙烯管材在受载后局部应力应变关系变化以及应变能的转移规律，尚无深入理论探讨。本项目拟通过理论分析和有限元数值模拟相结合的方法研究聚乙烯管材慢速裂纹扩展的力学机制。基于连续损伤力学和内聚力理论，建立包含时间相关损伤因子的聚乙烯内聚力本构模型，采用扩展有限元方法（XFEM）研究慢速裂纹扩展断裂过程，通过管材的切口蠕变实验验证并修正模型参数。采用超声相控阵实时成像检测技术观测静液压实验，对比预测结果，进一步验证模型的有效性。

本课题通过显微电镜观测技术、数字图像相关技术、声发射测试技术、宏观力学测试相结合的测试手段，从微观、细观、宏观多尺度分析研究了慢速裂纹扩展过程中的材料损伤特征。首先通过综合考虑聚乙烯黏弹性力学行为和银纹损伤断裂过程，提出了一种含黏弹性损伤的聚乙烯内聚力本构模型，建立银纹区内材料的牵引力与张开位移之间的定量关系，解释了聚乙烯银纹结构在慢速裂纹扩展过程中的能量耗散机制；基于含损伤的内聚力模型及黏弹性本构模型，运用ABAQUS-UMAT实现了提出的材料模型，采用扩展有限元方法研究了裂纹尖端应力-应变时空转化规律及应变能的释放、转移规律，从力学理论和数值模拟的角度解释了聚乙烯慢速裂纹扩展机理。通过声发射检测技术获得了聚乙烯微观变形机理与声信号的对应关系，并获得了银纹断裂的声学特征，实现了实时、无损的对银纹区或材料早期损伤的检测。提出了声学光学相结合的测试方法，采用声发射技术与数字图像相关技术（DIC）相结合的技术手段，同时检测材料内部损伤和表面变形，对聚乙烯的断裂力学行为进行了研究，初步提出了I型裂纹起裂的能量判据。综合上述研究成果，本研究为探讨聚乙烯慢速裂纹扩展失效机制奠定了坚实的工作基础。项目资助发表SCI论文6篇，EI论文7篇，取得授权专利1篇及已申请专利5篇。成果获得教育部技术发明二等奖1项，美国机械工程师协会会刊（ASME JPVT）年度优秀论文（Outstanding Technical Paper）1项（每年仅评选1篇）。培养博士生5名，其中2名已取得博士学位，3名在读；培养硕士生3名，均已取得硕士学位。其中三名硕士和博士研究生分别获得ASME PVP年会硕士论文展报奖第一名、优秀论文奖竞赛第二名以及论文竞赛优胜奖。