| After the economic crisis in 2009, many countries in the world exserted industrial revitalization programme, such as "Made in China 2025", "Industry 4.0" and so on, to accelerate economic growth and promote industrial innovation. These industrial programmes indicate that the global energy demand will be further expanded, especially in oil and gas resources. As one of the five modes of transportation, the pipeline is the main transportator of key energy media such as oil and natural gas at home and abroad, due to its low operation cost, high efficiency, excellent safety, limited environmental impact and suitability of long distance transportation. In the smooth progress of the gigantic West-East Gas Pipeline Project and the South-to-North Water Diversion Project, and in the rapid development of urbanization, the pressure pipeline is to become a tie connecting water, oil and gas resources with people’s lives closely. Pressure pipeline has covered most aspects of industry and civil supply, including oil and gas, urban water supply, heating power, electricity, sewage and other fields, and there is a sharp increase in urban gas pipelines and long-distance pipelines in particular.Polyethylene pipe reinforced by winding steel wires (PSP) is a composite pipe independently innovated in China, and it has been in successful application for more than a decade. The principal components of a PSP include a thermoplastic core pipe, an inner composite layer, an outer composite layer, and an outer cover layer. The major function of the core pipe is to prevent leakage and provide corrosion resistance to the transported medium. The unique structure enables PSP to have the advantages of both plastic and metal. Excellent mechanical performances, such as cost-effectiveness, long performance life, and prominent wear-resisting property, have resulted in the wide use of the PSP. PSP become a principal alternative to steel pipelines in China.Although the PSP has a broad application and outstanding prospects for development, the safety of the whole composite pipe system still need more work. Compared with the improvement of the composite pipeline, the problem of the composite pipe joint begins to expose, which becomes the barrier of PSP for more advanced application. The main failure mode of the PSP joint is bulge failure. The higher the inner pressure and the service temperature, the more possibility on failure of PSP would be. Not only does the bulge failure seriously affect the security of the existing composite piping system applications, but also restricts the further application of related new product. Unfortunately, the reason of bulging failure and the failure process have not been well studied or understood, and thus there is no effective way to prevent the bulge failure of PSP right now.In this thesis, research on the cause of bulge failure, failure process, failure mechanism, and long-term performance of PSP joint is carried out, and a finally a failure criterion is proposed based on experiemental investigations and numerical simulation. The main tasks of this work are listed below:(1) Dynamic mechanical analysis test was carried out to obtain the master curve of relaxation modulus according to the time-temperature equivalence principle. An expression describing the variety of the HDPE relaxation modulus with time was proposed using a good quantitative fit of the experimental data. A composite layup finite element model was built, combined with the HDPE relaxation modulus-time relationship, long term mechanical response of PSP could be solved. Experimental investigation of PSP under 40℃ was carried out, and different inner pressure led to differernt results. Interfacial debonding of steel/polymer had been confirmed existed, and it occurred prior to the tension failure of steel wires. Taking account of the experiemental results and numerical simulation, it can be concluded that the bulge failure is composed of interfacial debonding and tension failure, and the interfacial debonding is the key factor to cause the bulge failure.(2) Steel/adhesive resin pull-out tests were carried out under different conditions, and related axisymmetric models were established to simulate pull-out tests. Cohesive elements based on cohesive model were employed to represent the interface between the steel and the polymer resin. Numerical results were verified with the experimental investigation, pull out load-displacement numerical curves were consistent with experimental ones, particularly the peak pullout loads and the corresponding displacement values. Shear stress distribution along the interface was analyzed, and the interfacial failure process was reproduced in the FEM software. After the interfacial shear stress reach its peak value, the mechanical properties of the interfacial material was to go into softening stage, and then the load bearing capability of pull out specimen decreased. In the end. the variety of interfacial mechanical properties with time had been summarized through pull out tests under different temperatures in diverse loading rate and corresponding numerical simulations.(3)According to the actual structure of PSP, a PSP joint finite element model was established based on winding steel wires structure, and the proposed interfacial debonding failure criterion was contained. In the model steel wires were modeled separately with HDPE. The interaction between the two phase components was modeled using cohesive element, in which parameters were consistent with those in pull out model. The newly established model was occupied to analyze mechanical response of PSP joint undertaking short-term inner pressure under 40 ℃. Simulation results indicated that the interfacial debonding criterion index MAXSCRT varied from 0 to 0.77 before the steel wire stress reached its strength limit. So when the pipe reached its strength failure, interface between steels and polymer didn’t start to become damage. During the analysis it was found that the steel stress gradient was the main reason for the formation of interfacial shear stress. Then the joint model was used to analyze the variety of interfacial long term performance of PSP joint under 40℃. Differernt inner pressure resulted in similar varying trends of interfacial MAXSCRT, but the interfacial failure time were distinct. Under 1.5MPa inner pressure, the interface in the joint would not failure in the span of 50h. Under 2.0MPa the interface started to debond at 20h, while 2.5MPa made the interface begin to debond at 10h. So it could be concluded that in the same temperature environment, the internal pressure loads increase would accelerate the debonding of interface. |
PDF Full Text Request