Study On Dynamic Performance And Interfacial Bonding Mechanism Of CFRP Reinforced Concrete Beams Under Impact Load

Carbon fiber reinforced polymer（CFRP）is widely used in the reinforcement of concrete structures under different loads and harsh conditions because of its light weight and high strength.In practical engineering,in addition to static loads,CFRP reinforced concrete structures may also be subjected to dynamic loads such as vehicles,rockfalls,earthquakes,and explosions.However,current studies on the mechanical properties of CFRP reinforced concrete structures are mostly based on static loading,and there are few studies on its dynamic loading conditions.In addition,the interface between CFRP and concrete,as an important meso-component of CFRP-concrete reinforced structure,is a bridge to transfer stresses.Its bond strength has a great influence on the reinforcement of the structure.However,there are few simple and reliable bond-slip models under dynamic load,and it is difficult to predict its stripping bearing capacity,which limits the application of the reinforcement technology under dynamic conditions.Therefore,it is of great significance to study the dynamic performance and interfacial bond failure mechanism of reinforced concrete（RC）structures strengthened with CFRP under dynamic load.The static load test device of RC beam strengthened with CFRP is relatively simple,which can provide some reference for its impact performance index.Therefore,the static performance of the RC beam is usually used as the research basis of its impact performance.In this study,based on the static load test of CFRP reinforced RC beams,considering the influence of impact velocity,CFRP length,width and number of layers,the mechanical properties and interfacial bonding properties of CFRP reinforced RC beams under impact loads are tested and numerically simulated.The main contents and conclusions of this work are as follows:（1）Based on the digital image correlation technique（DIC）,three-point bending tests were conducted on six CFRP reinforced RC beams under static loading.The crack formation process,load deflection and ductility coefficients of the beam were obtained.The influence coefficientβof CFRP reinforcement was introduced,and the equation of the maximum crack width of CFRP reinforced RC beams under short-term loading was modified.The results showed that CFRP can improve the bending strength of the beam,change the failure pattern of the beam,and reduce the crack initial strain rate and the horizontal displacement of the beam.Moreover,the calculation results of the maximum crack width correction formula of CFRP reinforced RC beams were in good agreement with the experimental results.（2）Based on the static load test,the drop hammer impact tests were conducted on 15CFRP reinforced RC beams with impact velocity and length,width and number of CFRP layers as variables.The impact process,failure pattern,impact force time history and displacement time history of the beams under impact load were analyzed.The stress and strain fields on the surface of the beam during the impact process were obtained by DIC,and the crack propagation velocity under impact load was calculated.The inertia force time history curve of each specimen was obtained by theoretical derivation,and a dynamic increase factor（DIF）was proposed to reflect the relationship between the dynamic bending strength and static bending strength of the beam.The results showed that the peak impact force,peak displacement,dissipated energy and crack propagation velocity of the beam increased with the increase of impact velocity.After bonding CFRP,the failure pattern of the specimen changed from bending failure to shear failure,the peak displacement decreases,and more cracks appeared.In addition,the peak inertial force of all beams was in the range of1/2-5/6 of the peak impact force,and the ratio of dissipated energy to initial energy was mainly distributed between 0.60 and 0.85.（3）The interface between CFRP and concrete is the weak link of the component,and plays a decisive role in enhancing the impact resistance and mechanical properties of components.In order to investigate the bond failure mechanism of the interface between CFRP and concrete under impact loading,the local bond stress test method of pasting strain gauges on the surface of CFRP was used to obtain the bond strength of specimens under impact load.The bond failure process of the surface,as well as the interfacial debonding bearing capacity and fracture energy were discussed.The results showed that the load transfer area of CFRP gradually increased with the increase of load,and the stress transfer trend generally developed from the loading end to the free end.With the increase of impact velocity,the peak strain of CFRP,the peak interfacial shear stress and the interfacial debonding bearing capacity gradually increased.The peak value of interfacial bond stress increased first and then decreased with the increase of CFRP length.The width and number of CFRP layers had little effect on the interfacial bond stress.（4）Based on the interface bond feature points(S₀,τ_cr,τ_max)of each specimen under impact load,the rate sensitivity of CFRP-concrete interface bond stress under impact load was quantitatively described by the dynamic growth factor of characteristic parameters,and the average bond-slip relationship considering both impact velocity and CFRP dimensions was obtained.Considering the influence of slip at different positions,a dynamic interface bond slip correction model including the influence factors of CFRP length,width,number of layers and impact velocity was proposed.The dynamic interface bond slip constitutive model considering rate sensitivity increased the influence of impact velocity and had stronger applicability.（5）Based on the laboratory tests and theoretical analysis of drop hammer impact,the ANSYS/LS-DYNA finite element program was used to simulate the impact test of CFRP reinforced RC beams firstly,and the validity of the finite element model was verified according to the test data.On this basis,the influence of different parameters on the impact resistance of RC beams was analyzed.Although the failure pattern obtained by LS-DYNA based on the assumption of material uniformity was slightly different from the actual one,the results of impact force time history and displacement time history which were consistent with the laboratory test can be realized.Secondly,in order to reflect the non-uniform characteristics of concrete materials and CFRP-concrete interface,a secondary development was carried out on the basis of the original three-dimensional finite element program Realistic Failure Process Analysis^3D-Dynamic(RFPA^3D-Dynamic).The model test of CFRP reinforced RC beams based on non-uniform interface control was realized,and the failure pattern and acoustic emission changes of the interface under different parameters were obtained.The results showed that the RFPA^3D-Dynamic program considering the microscopic heterogeneity of materials can realize the heterogeneous multi-crack propagation mode,which was closer to the experimental failure pattern.Therefore,the characteristics of the two programs can be combined to study the dynamic performance and interfacial bonding performance of CFRP reinforced RC beams.In this way,the failure pattern closed to the actual material properties can be obtained,and the dynamic response consistent with the actual situation can be obtained.