Study On Failure Of Continuous Fiberglass Composite Tape Reinforced Thermoplastic Pipe

Reinforced Thermoplastic Pipe(RTP)is an emerging reinforced composite pipe in recent years,which has attracted great attention in various countries and has developed rapidly.Reinforced Thermoplastic Pipe is generally made up of a three-layer structure.The inner layer is mostly a wear-resistant and corrosion-resistant polyethylene layer,which is suitable for transporting fluids.Besides,the inner layer material can be modified according to actual needs.The middle layer is a reinforcing layer,which can be made by various materials.There are some commonly used materials,such as,steel strip,steel wire,aramid fiber,glass fiber,ultra-high molecular weight polyethylene fiber,aramid fiber,glass fiber and resin composite tape.Besides,this layer can be multi-layered according to the pressure level.The outer layer mainly plays the role of anti-scratch and anti-static protection.Continuous fiberglass composite tape reinforced thermoplastic pipe(CFT-RTP),which is independently developed by our research group,is made up of three completely-fused layers.Glass fiber tape,which is made of continuous glass fiber bundles,as a reinforcing layer is wound on a polyethylene inner tube.The outer layer is also covered with polyethylene.The main advantages of CFT-RTP are high pressure resistance,corrosion resistance,light weight,few joints,and low cost.It can overcome the pressure problems of plastic pipes and the corrosion of steel pipes.It can withstand higher working pressures and maintain the advantage of the flexibility of polyethylene pipes.Therefore,it can be made into a continuous coil coiled from tens of meters to nearly kilometers.It is mainly used in the fields of petroleum,natural gas extraction,high-pressure long-distance transportation,and pipelines where municipal water conservancy requires higher pressure to transport media.During the installation and use of CFT-RTP,it will inevitably be affected by the environmental load and the corresponding working load.At the same time,it will also face a series of uncertain factors,which will bring great challenges to the safe use of the pipeline.To ensure the reliability and safety of the pipeline during long-term use,it is necessary to carefully evaluate the mechanical properties of CFT-RTP under different load conditions and its related design safety factor.The coefficient relationships of CFT-RTP burst pressure,hydrostatic pressure,fatigue pressure and working pressure are mainly tested,and the long-term service life of CFT-RTP under different stress conditions is also evaluated.The main research contents and conclusions of this article include the following aspects:(1)Long-term failure behavior of glass fiber in tensile stress fieldIn glass fiber reinforced composite pipes,glass fiber plays an important role in bearing pressure as reinforcing materials.To understand the failure form and failure mechanism of CFT-RTP under stress,we must first understand the behavior of glass fibers and glass fiber reinforced tapes in a stress field.At present,there are relatively few studies on the properties and fracture mechanisms of glass fibers during long-term use.And the long-term service life of glass fibers and their composites under stress is unclear.In this paper,long-term tensile tests are performed on glass fibers and glass fiber composite tapes under different constant tensile stress conditions.The break times of glass fibers and glass fiber composite tapes under different tensile stresses are obtained,and the critical fracture stress of glass fibers is deduced.The fracture morphology of glass fiber in the tensile stress field is analyzed,and the fracture mechanism is explained by the theory of fracture mechanics and the theory of chemical bonds.For the 2400TEX glass fiber produced by Shandong Fiberglass Group Co.,Ltd.,the tensile fracture strength is 1280 MPa,the long-term critical value in the tensile stress field is 55.6%of the fracture strength,and the critical fracture stress of the glass fiber is 711.68 MPa.When the tensile stress is lower than the critical fracture stress,the glass fiber will not fracture.When the tensile stress is higher than the critical fracture stress,the fracture time decreases as the tensile stress increases.Glass fiber surface treatment can delay the fracture time,but does not change the critical fracture stress.In order to ensure the long-term service life of the glass fiber,the maximum stress it bears must not exceed the critical fracture stress.(2)Study on failure of CFT-RTP under short-term blasting pressure loadIn order to determine maximum pressure capacity of the developed CFT-RTP products and provide a reliable theoretical basis for the safe use of pipelines,and also for the research and development of new CFT-RTP products,the strength analysis and calculation methods of this new type of pipeline must be explored.According to the characteristics of CFT-RTP,a short-term burst test will be carried out for CFT-RTP on the basis of fully analyzing the properties of glass fiber and plastic.Based on the data of the pipeline test,the stress of the reinforcing layer is analyzed,so as to provide a reliable basis for determining the nominal pressure of the pipeline.Pipeline burst test can provide a reliable pressure classification basis for a series of pipe tests,including short-term hydrostatic test,long-term hydrostatic test and fatigue test.The maximum bearing capacity of CFT-RTP can be obtained from the blasting test results of the pipeline,which can initially verify the accuracy of the calculation method of the blasting strength of the pipeline and initially estimate the reasonable safety factor of the pipeline.Through the explosion test of the pipeline,this paper analyzes the failure mode,the fusion between the CFT-RTP layers and the overall performance of CFT-RTP.The analysis of the pipeline burst test data can provide the necessary theoretical basis for the safe use of the products and provide the structural parameters for the development of new products.The designability of the CFT-RTP material and its structure is reflected in the fact that the inner and outer layers of the substrate can change the type of material according to the actual working conditions.If necessary,the material can be modified,enlarging the designability of the reinforcing layer.According to the pressure level,parameters such as fiber type,volume fraction,winding angle and number of reinforcement layers can be changed.According to external conditions and application requirements,the form and size of the CFT-RTP structure are changed to make the CFT-RTP meet the performance requirements,and to make the weight and economic indicators as reasonable and optimized as possible.For the 110-1200-2,110-2400-2,and 110-3600-2 three types of RTP composite pipes,the average burst pressures are 5.958MPa,8.308MPa,and 10.4MPa,respectively.(3)Study on failure of CFT-RTP under long-term static pressure loadCFT-RTP long-term static pressure research needs to solve two problems:First,what is the maximum pressure that the pipeline can withstand at a given temperature within a limited time?Second,how long is the service life of the pipeline when it is subjected to a certain pressure at a given temperature?The CFT-RTP is tested for its failure time under different internal pressure loads,therefore the long-term failure life of the CFT-RTP will be deduced.Under long-term hydrostatic pressure conditions,the pressure bearing of CFT-RTP is mainly due to continuous glass fibers in the reinforcement layer,and the stress state is similar to that of continuous glass fibers in the tensile stress field.The stress state of CFT-RTP under long-term hydrostatic pressure conditions is different from that of pure polyethylene pipes,so the relationship between the pressure ratings of polyethylene pipes cannot be completely copied.CFT-RTP with an outer diameter of 110mm,a total wall thickness of 10mm,an inner layer thickness of 6mm,a reinforcement layer thickness of 1.2mm,and an outer layer thickness of 2.8mm under normal temperature conditions has a maximum long-term working pressure of 40%(3.3MPa)of burst pressure.This pressure can meet the requirements of long-term service life of CFT-RTP.The short-term hydrostatic pressure of CFT-RTP is 56%of the burst pressure,that is,the burst pressure is equal to twice the short-term hydrostatic pressure or 2.5 times the maximum long-term use pressure.(4)Study on failure of CFT-RTP under fatigue loadAt present,there are relatively few studies on the fatigue performance and fatigue failure mechanism of CFT-RTP during long-term use and the long-term service life of CFT-RTP under fatigue stress is unclear.In this paper,under the conditions of constant stress ratio and different fatigue average stress,the CFT-RTP is subjected to a long-term pressure cycle fatigue test to obtain the number of cyclic failures of the CFT-RTP under different fatigue average stress and deduce the CFT-RTP critical fatigue failure stress.By analyzing the micro-morphology of the fracture,the crack propagation mechanism under the fatigue stress condition is obtained.Therefore,for the CFT-RTP with an outer diameter of 110mm,a total wall thickness of 10mm,and a burst pressure of 8.32MPa,the CFT-RTP critical fatigue stress is 40.43%of burst pressure and 3.364MPa under the conditions of a cycle amplitude of±3%,a frequency of 10 times/min,and a stress ratio of 0.94.When the average fatigue stress is lower than the critical fatigue stress,the glass fiber will not break,and CFT-RTP will no longer have fatigue failure.When the average fatigue stress is higher than the critical fatigue stress,the number of failures will decrease as the average fatigue stress increases.At lower frequencies,changes in frequency have little effect on CFT-RTP fatigue.In order to ensure the long-term service life of CFT-RTP,the maximum average fatigue stress it can bear must not exceed the critical fatigue stress.