Creep And Durability Performances Of Carbon/Glass Hybrid Fibers Reinforced Polymer Composites

Since the beginning of the 21st century,fiber reinforcement polymer(FRP)composites have become an essential component in civil engineering applications,replacing traditional components such as steel for strengthening/ reinforcing/repairing old structures.The hybridization of carbon/glass fiber to a polymer matrix can adopt the advantages from both fibers by utilizing the high mechanical properties of carbon fiber,high failure strain,and low cost of the glass fibers.The hybrid bars are gradually used for reinforcing tendons,prestressed cables,and bridge cables for civil engineering applications.While applying as a load-bearing structural component in civil engineering applications,the creep and durability of hybrid FRP bars under severe environmental conditions and mechanical loads are significant problems for their design and application.Therefore,this thesis studied the influence of hybridization of carbon/glass fiber,and their various mechanical properties have been analyzed and developed a new inter-laminar layer carbon/glass fiber hybrid FRP composite with enhanced mechanical properties.Furthermore,this thesis analyzed the degradation of mechanical,thermal,and viscoelastic properties of carbon/ glass fiber hybrid bars FRP under freeze-thaw conditions,elevated temperature,water immersion and sustained,and cyclic mechanical load.The main research content and findings of this research work are as follows:(1)Fabrication of carbon/glass hybrid FRP composites and analysis on their tensile propertiesThe fabrication of carbon/ glass hybrid FRP composite with a varying volume of carbon and glass fiber.Each composition in three commonly employed fiber configurations to experimentally determine the suitable composition and configurations and achieve the maximum mechanical properties.The results from the tensile analysis revealed that the combination of low elongation carbon fiber and high elongation glass fiber could lead to the increased failure strain or hybrid effect and non-catastrophic failure or pseudo-ductile behavior of the hybrid FRP composites.The tensile strength and modulus of the carbon/glass hybrid FRP composite were satisfactorily predicted adopting bi-linear ROM,and an existing analytical model was satisfactorily adopted to predict the tensile stress-strain curve of these hybrid composites.The hybrid composite with interlaminar fiber layer configurations with35.3% volume content of carbon fiber exhibited enhanced tensile strength from tensile properties analysis.In addition,the failure strain of the carbon fiber layers in this hybrid composite is 65% higher than the failure strain of the carbon fiber reinforced polymer composite fabricated with the volume content of carbon fibers the same as in the hybrid composite.(2)Flexural and inter-laminar shear properties of carbon/glass hybrid FRP compositesThe flexural and shear test were adopted to reveal the enhancement effect in carbon/glass hybrid FRP composites.The hybrid composite was prepared using the filament winding technique with 35% volume content of carbon fiber and combined with glass fiber in 3 different configurations,which was selected based on the excellent tensile properties obtained from the previous chapter.The performance of the selected hybrid composite was further evaluated under flexural and short beam shear tests.The enhancement effect is evaluated based on the properties obtained for CFRP(3-layer carbon fiber wound)and GFRP(5 layer glass fiber wound)composites.It is observed that the damage of the hybrid composite is progressive and layer by layer damage owing to the enhanced ductility in comparison with non-hybrid composites.The research results show that the overall enhancement effect is higher for the hybrid composite with interlaminar fiber layer configurations.The enhancement effect in the flexural strain is 136% higher for the hybrid composite with interlaminar layer configurations concerning carbon fiber reinforced polymer composites.The shear strength results revealed an excellent bonding between carbon/glass fiber interfaces for the hybrid composite with interlaminar layer configurations.The flexural and shear damage mechanism was analyzed using the scanning electron microscopic technique.(3)Effect of sustained load on the creep of carbon/glass hybrid FRP barsThe flexural creep behavior of pultruded carbon/glass hybrid FRP composite bars was studied under sustained load levels.The creep study was conducted in three different environmental conditions: freeze-thaw preconditioning,exposure to room temperature,and exposure to combined freeze-thaw and sustained load levels.The load levels of 50%,60%,and 70% of the ultimate bending load levels were selected for the creep study.The creep mechanism and the long-term service life based on the rate of strain and flexural modulus were predicted using Findley’s power law.Based on the linear approximation of Findley’s power law,reductions in modulus were observed at a higher percentage for the hybrid FRP bars under combined freeze-thaw and mechanical loads.The time-dependent deflection of freeze-thaw pretreated hybrid FRP bars and untreated bars was studied theoretically by coupling Findley’s power-law model with Euler Bernoulli’s beam theory.The creep deflection of freezethaw pretreated hybrid FRP bars intensified ≈ 2.5 times higher than the untreated hybrid FRP bars,respectively,after a service period of 50 years.The mechanism of the long-term viscoelastic creep behavior of hybrid FRP creep under the exposure conditions was analyzed,and behavioral changes in microstructure are explained via microstructural analysis utilizing scanning electron microscopy.(4)Effect of cyclic and sustained load on the durability of carbon/glass hybrid FRP barsThe combined effect of cyclic/sustained bending loads and water immersion at three different temperatures(25℃,40℃,and 55℃)on the interface between glass fiber shell and carbon fiber core of the hybrid FRP bars was experimentally evaluated.The load levels of 40-60% of the ultimate bending load were selected for inducing cyclic and sustained loads on the mid-span of the HFRP bars.The residual interface shear strength of HFRP bars subjected to 40%-60% sustained load and 40% cyclic load at different soaking temperatures was used for long-term service life prediction based on the Arrhenius theory.The results indicated that the degradation of the HFRP bars was accelerated with the increased exposure temperatures and applied load levels.The effect of cyclic load decreased the mechanical and thermal stability of the HFRP bars due to the higher diffusion of water molecules,leading to the plasticization of resin and fiber/matrix debonding,which was further characterized through scanning electron microscopy.The predicted exposure time required to reach70% interface shear strength retention of the HFRP bars at the area with northern latitude 50°(mean annual temperature of 5.7℃)was evaluated at approximately 16.1years,under coupled 40% sustained load and water immersion.In addition,for coupled,40% cyclic load and water immersion were evaluated at approximately 6.2years.The degradation mechanism was understood with the help of morphological analysis using SEM.This thesis studied the effect of hybridization of carbon and glass fiber and put forward a new hybrid FRP composite system with the optimal composition of glass fiber and carbon fiber in the hybrid FRP composite with enhanced mechanical properties.The combined effect of sustained load and exposure to freeze-thaw environmental conditions on the creep of the carbon/glass hybrid FRP bars were experimentally analyzed.Findley’s power-law model was used to predict the timedepended deformation,flexural modulus,and bending deflection.The long-term degradation of interfacial shear strength was predicted using the Arrhenius equation.The exposure to cyclic load levels and immersion at elevated temperature levels accelerated the rate of degradation.In addition,the prediction results show a lower service life for the hybrid FRP bars exposed to a cyclic load.The degradation mechanism of thermal and mechanical properties was revealed from the microstructural analysis.The presence of matrix rich zone in the glass/carbon fiber layer results in the weak interface shear strength and thus affects the overall performance of the carbon/glass hybrid FRP bars.