Study On Internal Pressure-bearing Capacity And Durability Of CFRP Strengthened Steel Pipe

Pressure steel pipes are the lifeline of national resources,energy transportation,and industrial production.The hygrothermal service environment,complex transport media,and long-term internal pressure may cause corrosion,cracking,and other defects to the pipe,reducing its bearing capacity and service life.Carbon fiber reinforced polymer(CFRP)has the advantages of lightweight,high strength,corrosion resistance,and easy construction.For the defected pipes,the load-bearing capacity can be improved and the service life can be prolonged by wrapping CFRP externally.The small-tow carbon fibers were used commonly due to it mature properties,but their costs are relatively high.Using large-tow carbon fibers can significantly reduce costs and promote the engineering application of CFRP strengthening for pipes.However,the impacts of hygrothermal environments on the durability of large-tow CFRP,CFRP-steel interface,and the strengthened pipes are not clear.The researches on the mechanical properties of CFRP strengthened corroded/cracked pipes are insufficient,and there is a lack of quantitative models to guide strengthening design and performance evaluation.In response to the above issues,the research content and achievements of this article are as follows:The performance evolutions of large and small-tow(48K and 12K)carbon fibers and their CFRPs under deionized water immersion at 25℃,40℃,and 60℃were investigated,and the evolution mechanism was studied through microscopic analysis.The results indicated that immersion led to the debonding or hydrolysis of the sizing agent on the surface of carbon fibers,reduced the tensile strength of single filament,and increased dispersion.At the same time,immersion caused debonding of the fiber-matrix interface and a decrease in the elastic modulus of the matrix.The deterioration of fiber,interface,and matrix led to a degradation of the tensile strength and interlaminar bonding performance of CFRP.The dispersion of 48 K carbon fiber was greater after immersing for 6 months,causing a lower tensile strength of its CFRP than 12 K CFRP.The tensile strength retention rate of 48 K CFRP was about 89.7%.For the 48 K CFRP,the tow size was larger and there were fewer weak tow-matrix interfaces,resulting in higher interlaminar shear strength and mode I interlaminar fracture toughness retention rates compared with 12 K CFRP.The performance evolution of the epoxy resin Twi and its bonded CFRP-steel interface under deionized water immersion at 25℃,40℃,and 60℃ was studied,and the mechanism of performance evolution was revealed through failure mode,micro-morphology,and chemical composition analysis.The results showed that the glass transition temperature of Twi decreased in the initial stage of immersion and then stabilized,with no significant effect of immersion temperature on its retention rate.The tensile strength decreased in the first 6 months of immersion,followed by a recovery with a retention rate of over 73% after 12 months of immersion.The degradation was accelerated by a higher temperature.Due to the lower degree of hydrolysis of Twi’s main chain,plasticization was the main reason for the decrease in glass transition temperature and tensile strength degradation.The strengthening of hydrogen bonds between water molecules and Twi molecular chains was the main reason for the recovery of tensile strength.After 6 months of immersion at 60℃,the single-lap shear strength of the CFRP-steel interface was higher than that of the 25℃ immersion sample,which was attributed to increasing toughness of Twi caused by the high temperature.For the CFRP-strengthened pipe with an external uniform corrosion,hydrostatic burst tests,and finite element simulations were conducted to investigate the effect of the circumferential size of the corroded area and mechanical properties of the putty on the bearing capacity and failure mechanism.For CFRP-strengthened pipes where circumferential stress plays a controlling role,a sudden increase in CFRP stress was caused by the failure of the putty and the failure of the strengthened pipe was caused by the fracture of CFRP when the circumferential size of the corroded area was ≤0.02πD.When the circumferential size of the corroded area was ≥0.06πD,the failure of the putty caused it impossible for CFRP to continue to bear,which led to premature failure of pipe.At this time,the hydrostatic burst pressure decreased with the reduction of elongation at break of the putty.For the CFRP-strengthened pipe with an external uniform corrosion,an elasto-plastic mechanical model considering the stress changes of CFRP and putty after pipe yield was established,and the effects of relevant parameters on bearing capacity were analyzed.A simplified mechanical model suitable for thin-walled pipes was established,and the quantitative relationship between the allowable stress and elastic modulus of putty was obtained,providing a theoretical basis for the selection of putties.The research results indicate that the hydrostatic yield pressure and hydrostatic burst pressure could be calculated accurately by the elasto-plastic mechanical model,with a relative error of less than 16.1% compared to the experimental results.The stress and hydrostatic yield/burst pressure could also be calculated accurately by the simplified mechanical model.The relative error between the simplified model and the elsto-plastic model of pipe/CFRP stress was less than 20%,and that of hydrostatic yield/burst pressure was less than 10%.For the CFRP-strengthened thin-walled pipe with a through-wall crack,the effects of pipe geometric sizes,the angle between the through-walled crack and the pipe axis,the mechanical properties of CFRP,and the prestress level of CFRP on the stress intensity factor(SIF)at the through crack tip were studied based on dimensional and finite element analysis.The research results indicated that the effect of pipe geometric size,CFRP stiffness,and crack angle on SIF was independent of each other.The SIF at the crack tip was negatively correlated with the stiffness of CFRP and linearly decreased with the increase of CFRP prestress level.The SIF decreased in the form of a cosine function as the crack angle increased.The calculation results of the SIF empirical formula at the crack tip were accurate,and the relative error between the formula and the verified finite element model was less than 20%.For the CFRP-strengthened pipe with an external uniform corrosion,tensile test method for CFRP-steel ring circular specimens was established to simulate the circumferential stress state under internal pressure of the CFRP-strengthened pipe.The evolution of hydrostatic yield pressure and hydrostatic burst pressure of the circular specimens under immersion or internal pressure-immersion coupling environments were studied.The evolution mechanism was revealed through failure modes and microscopic analysis.The research results indicated that the hydrostatic yield/burst pressure deteriorated significantly within one month of initial immersion,and then tended to stabilize.After the longest exposure time(6months),the retention rates of hydrostatic yield and burst pressure were higher than 67% and 88%.The main reasons for degradation were the decrease in CFRP stiffness and CFRP tensile strength,respectively.Compared with immersion,the free volume the water absorption rate of the CFRP matrix decreased under the coupling effect of immersion and a stress of 30% CFRP tensile strength,which caused the hydrostatic yield/burst pressure retention rates under coupling effect were higher.Based on the Arrhenius theory,a long-term service reduction coefficient of 66.8% was obtained for the yield pressure of CFRP-strengthened pipes immersed in water at four typical environmental temperatures.The research will provide experimental and theoretical basis for the design and evaluation of the mechanical and durability performance for CFRP strengthened corroded or cracked pipes.