【作者】 施建峰；

【导师】 郑津洋；

【作者基本信息】 浙江大学 ， 化工过程机械， 2011， 博士

【摘要】 当今世界的交通运输方式正朝着高速化、大型化、专业化的方向发展。管道作为五大运输方式之一,是输送石油、天然气等关键能源介质的主要手段,对国民经济的发展和稳定起着至关重要的作用,是国民经济的“生命线”。聚乙烯管具有使用寿命长、运行能耗和维护成本低等优势,符合国家倡导的“低碳”、“绿色”的发展方向,是国家中长期发展规划中主推的化学建材之一。开展聚乙烯管道接头焊接性能研究,突破聚乙烯管道系统安全应用的瓶颈,规范聚乙烯管道行业的设计、制造、安装、运行、维护以及安全状态监测体系,对推动整个聚乙烯管道行业的发展具有重要意义。管道接头是管道系统安全的薄弱环节。电熔焊接是目前最广泛使用的塑料管道和塑料复合管道的连接方式之一。据美国Plastic Pipe Data Collection (PPDC)统计,除第三方破坏以外,大部分聚乙烯管道失效事故发生在电熔接头。由于聚乙烯管道与金属管道性能存在较大差异,因此金属管道中已较成熟的检测和质量评价方法并不适用聚乙烯管道。目前国内外电熔接头的质量控制主要通过设计合理的焊接工艺,制定严格的施工程序来实现,而对焊接后的电熔接头进行无损检测与安全评定方法研究基本为空白。本文在国家“十一五”科技支撑计划课题“生命线工程安全保障关键技术研究及工程示范”(项目编号：2006BAK02B01)、国家“十二五”支撑计划项目“基于风险的特种设备事故预防关键技术研究”(项目编号：2011BAK06B01)和国家高技术研究发展计划(863计划)重点项目“极端条件下重大承压设备风险评价与寿命预测关键技术研究”(项目编号：2009AA044801)的支持下,主要以聚乙烯管道电熔接头为研究对象,系统地研究了聚乙烯管道电熔接头的缺陷分类及其相应的失效模式,针对工程中最常见的冷焊缺陷,本文从形成原因、检测方法与安全评定三方面开展系统研究,主要完成的工作为：(1)采用理论分析与试验研究相结合的方法,将聚乙烯管道电熔接头内部缺陷分为熔合面缺陷、孔洞、结构畸变以及过焊四种基本类型；利用超声相控阵超声成像方法对这四种缺陷进行无损检测,证明该方法可用于聚乙烯管道电熔接头的无损检测并具有足够的精度；对含缺陷聚乙烯管道电熔接头进行拉伸、剥离、爆破和高温静液压试验,检验了各种力学试验方法对研究电熔接头焊接界面性能的有效性；通过高温静液压试验发现含上述缺陷的电熔接头具有沿熔合面的贯穿裂纹失效、沿电熔套筒壁的贯穿裂纹失效和沿金属丝所在圆柱面的贯穿裂纹失效这三种失效模式；(2)得到聚乙烯材料的导热系数、比热容和密度等传热参数随温度的非线性变化规律；在全面考虑聚乙烯材料参数、电熔焊接过程中输入功率以及电阻丝与聚乙烯之间的接触热阻等因素随温度变化规律基础上,建立聚乙烯管电熔焊接过程一维非稳态轴对称传热模型；通过温度场测量和焊接功率及焊接输入热量测量试验,验证所提出的模型的合理性和求解结果的准确性,并进一步分析了电熔焊接冷焊缺陷产生的原因和主要影响因素；(3)发明基于特征线的冷焊超声检测方法;通过电熔焊接过程进行实时超声录像,证明在焊接过程中超声检测特征线是固态聚乙烯和液态聚乙烯之间界面的超声反射信号；系统地分析电熔焊接后电熔接头中仍然残留有特征线的可能成因,包括金属丝周围的气泡,熔融区与未熔融区声阻抗差异以及熔融区边界的微晶；通过超声多次焊接、差式扫描热分析和显微观测试验多角度支持熔融区边界的微晶是形成特征线超声反射信号最主要原因的观点：结合温度场模型,预测了特征线随焊接时间和焊接功率的变化规律。(4)分别基于材料力学的变形能分析和唯象理论方法提出两种通过拉伸剥离试验获得焊接界面剥离能的分析计算方法,解决了以往采用拉伸剥离试验无法直接得到焊接界面粘结性能指标的问题；在此基础上,系统、定量地研究了不同焊接时间对焊接界面粘结性能的影响。通过对试验结果进行理论分析,提出并分析电熔接头焊接过程依次经过强度孕育区、强度形成区、平台区和裂解区的本质原因；应用统计学分析方法,计算得到在给定置信水平下获得合格电熔接头所需的最小焊接时间；结合超声特征线检测方法,提出物理概念清晰、实施方便的电熔接头冷焊缺陷的安全评定方法,并已成功进行工程验证。更多还原

【Abstract】 The modes of transportation in the world are steping forward in the direction of high speed, large-scale and professionality. As one of the five modes of transportation, the pipeline is the main transportator of key energy media such as oil and natural gas. These pipelines usually play a very important role in the development and stability of national economy. As a result, they are regarded as "lifeline" of national economy.Polyethylene (PE) pipe has been found wide application in gas and water distribution since it was initially introduced to the transport of natural gas in the 1960s. It has satisfactory performances including corrosion resistance, resistance to the effects of gas constituents, ease of installation and cost-effectiveness. It is one of the most recommended products used in the field of building materials in National Long-term Development Planning. The joints between pipes are weak links related to the safety of the whole pipeline system. In order to promote the development of PE pipe industry, it is crucial to study the mechanical performance of the welding joint and then propose a reasonable regulation for the installation, operation and maintenance of PE pipeline system.Electrofusion welding is the most widely used welding method of connecting plastic pipes or plastic composite pipes. In this thesis, the method of defect classification and the corresponding failure mode of electrofusion joints are systematically studied. The cold welding defect, which is the most common defect, is systematically studied in respects of forming reason, inspection method and safety assessment method. The main tasks of this work are listed below:(1) The defects in EF joints were classified into four categories:poor fusion interface, over welding, voids and structural deformity. The forming reasons of these defects were analyzed in detail. The mechanical properties of EF joint containing these defects are investigated by conducting tensile tests, burst tests, peeling tests and sustained hydraulic pressure tests. The results showed that there are three main failure modes of EF joint under inner pressure, i.e., cracking through the fusion interface, cracking through the fitting, and cracking through the copper wire interface. (2) By comprehensively taking the temperature-dependent properties of polyethylene and heating wires into consideration, such as thermal conductivity, heat capacity and density, as well as the power input which changes with the wire temperature, a transient axisymmetric heat transfer model was proposed. Experiments were conducted in order to verify the proposed model. The interface temperature between outer surface of pipe and fitting bore was measured by well-installed thermocouples, and the total input energy and real-time power were recorded by the welding machine. The experimental results were found good match with the numerical results calculated from the proposed model.(3) A method to inspect the cold welding defect in electrofusion joint was proposed for the first time, based on a newly discovered Eigen-line, which exists close to the copper wire in the ultrasonic phased array image. To understand when, where and how the Eigen-line would appear, the forming mechanism was investigated in detail. Three factors may cause the appearance of the Eigen-line, i.e., micro-air-bubbles, difference of acoustic impendence of PE in the melted and un-melted region and small-crowded crystallites in the region of the interface of melted and un-melted region. It was found that the number of Eigen-lines was the same as that of welding times when re-welded with decreasing welding time, but only one Eigen-line could be observed when re-welded with increasing welding time. The result showed that small-crowded crystals in the region of the interface of melted and un-melted region may be the dominating factor. This was then verified by the microscope and differential scanning calorimeter (DSC) tests, and the forming process of the small-crowded crystals was discussed in detail. By applying the proposed temperature model, the moving regulation of Eigen-line was predicted and then verified by experimental measurement.(4) Based on the deformation process and phenomenological theory, two methods to calculate the peeling energy in the tensile-peeling tests were proposed. The electrofusion joints with different degree of cold welding were deliberately made and then used to conduct tensile-peeling tests. On the basis of theoretical analysis and experimental results, the reason of each stage of strength formation during electrofusion process, was studied and discussed in detail. A method to obtain the minimum required welding time of electrofusion process was proposed based on statistical analysis. By combining the method to inspect cold welding defect, a method of evaluating the quality of electrofusion joint was proposed.We would like to acknowledge the financial support from the National Key Technology R&D Program of "Eleventh Five-Year Planning Program" (Project No. 2006BAK02B01), National Key Technology R&D Program of "Twelfth Five-Year Planning Program" (Project No.:2011BAK06B01) and key project of national High Technology research and Development Program (Project No.:2009AA044801).更多还原