基于延性耗竭的蠕变损伤模型预测含缺陷聚乙烯管接头长期性能

Accidents usually occur in butt fusion welded joints for polyethylene (PE) gas pipes due to the existence of welding defects. These affect seriously the city public security. However, there is currently no way to effectively predict the effects of all kinds of welding defects on the long-term performance for butt fusion welded joints of PE pipes. The creep damage model based on the ductility exhaustion is possibly an effective method, but multi-axis creep ductility factor in the model is derived based on the void growth controlled by power-law and does not take into account the craze which appears in the crack of PE materials when creep damage occurs. Therefore, the creep damage model based on the ductility exhaustion cannot be directly used for PE pips. In this project, butt fusion welded joints of PE pipes were researched. The creep constitutive and creep ductility model related to the stress level were established by creep test. Based on the measured uniaxial and multi-axial creep ductility, the multi-axis creep ductility factor was modified to the calculate the multi-axis creep ductility of PE pipes, and then the creep damage model based on the ductility exhaustion was established. The creep damage model was coupled into the finite element software to simulate the creep rupture test. The simulated results were compared with the test results to verify the accuracy of the creep damage model. Based on the established creep damage model, effects of all kinds of welding defects on the long-term performance for butt fusion welded joints of PE pipes can be predicted. Therefore, this project can provide the theoretical model for the safety assessment of the long-term performance for butt fusion welded joints of PE pipes with welding defects.

聚乙烯（PE）燃气管常因热熔对接接头中存在焊接缺陷而发生事故，给城市公共安全带来了严重隐患，但迄今没有方法可以有效预测各种焊接缺陷对PE管热熔对接接头长期性能的影响。基于延性耗竭的蠕变损伤模型可能是较好的方法，可是模型中的多轴蠕变延性因子是基于空洞幂律长大理论建立的，并未考虑PE管蠕变损伤产生银纹的影响，因而不能直接用于PE管。本项目拟以PE管热熔对接接头为研究对象，通过蠕变试验，建立与应力水平相关的蠕变本构和蠕变延性模型，根据测出的单轴蠕变延性和多轴蠕变延性修正多轴蠕变延性因子，进而建立适用于PE管的基于延性耗竭的蠕变损伤模型，把建立的模型耦合到有限元软件模拟含焊接缺陷试样的蠕变断裂试验，与试验结果对比，验证模型的准确性。通过建立的基于延性耗竭的蠕变损伤模型，可以模拟预测各种焊接缺陷对PE管热熔对接接头长期性能的影响，为含焊接缺陷PE管热熔对接接头长期性能的安全评定提供理论模型。

与金属管相比，聚乙烯（PE）管具有环保、耐腐蚀、质量轻、易成型、抗地震和使用寿命长等优点，因而广泛用于输送油、水和天然气等。在一些发达国家和地区，城市中低压燃气管已经全部使用PE管，我国随着经济的快速发展，城市中原有的金属输水管更新时也都使用PE管，而新建的输水管道已经全部使用PE管。因为PE管在核电站输水管道中表现的良好性能，HDPE管也成为世界各国核电站的首选输水管。.为了形成管网系统，PE管常使用热熔焊接和电熔焊接，这使得焊接接头成为PE管网的薄弱环节，由于焊接工艺的复杂性、焊接环境的多变性等因素，使得焊接接头中可能含有气泡、孔洞、夹杂物和未焊透等焊接缺陷，而且即使焊接接头中没有焊接缺陷，但接头中的材料组成也不可避免的比母材差。另外，由于短时的试验，比如拉伸试验、三点弯试验等不能有效地评价PE管焊接接头长期使用的结构完整性。因此本项目主要利用蠕变试验、长期静液压试验等研究PE管焊接接头长期使用的结构完整性。.基于蠕变试验可知，PE管在常温下即可表现出明显的蠕变现象，而且与金属材料一样，蠕变曲线可以分为三个阶段，即蠕变速率快速增加后减少的第一阶段、蠕变速率稳定的第二阶段和蠕变速率快速增加至断裂的第三阶段。基于蠕变试验建立了蠕变本构方程，以及蠕变断裂延性。研究不同焊接缺陷对焊接接头蠕变寿命的影响发现，随着焊接缺陷的增加，蠕变寿命不断减少。这些试验数据和建立的模型，对PE管的安装铺设具有重要的指导意义，并且可用于对在役PE管的安全评价和寿命预测。