Study On Impact Resistance Of Pre-damaged RC Beams Strengthened With CFRP Under Multiple Impact

Carbon fiber reinforced plastics(CFRP),which are known for their excellent properties such as high strength,lightweight,corrosion resistance,and fatigue resistance,have been widely used in the field of structural reinforcement.The mechanical properties of the interface between CFRP and concrete are complicated,and a lot of relevant experimental and theoretical studies under static and dynamic loading conditions have been conducted by domestic and foreign scholars.However,the current research on CFRP reinforcement mainly focuses on intact structures,and the study of the repair and strengthen effects on damaged structures is not comprehensive enough.In addition,considering that the structures in engineering are often subjected to continuous impacts,it is of great significance to study the dynamic performance of damaged CFRP-strengthened structures under multiple impacts and their dynamic failure mechanism.In this thesis,drop tests were conducted on damaged beams strengthened with CFRP,and the failure pattern and dynamic responses of strengthened beams are obtained by comparing them with intact beams and un-strengthened beams.Based on LS-DYNA program,the threedimensional numerical simulation of CFRP strengthened beams under multiple impacts was performed,and the reliability of the numerical model was verified by comparing it with the results of laboratory experiments.On this basis,the influence of CFRP wrapping method and impact velocity on the multiple impact performance of strengthened beams were further discussed,and the variation laws of impact resistant capacity,deflection and energy dissipation were quantitatively characterized.The main work and conclusions are presented as follows:(1)By conducting multiple impact tests on damaged CFRP-strengthened reinforced concrete beams,the repair effects of different wrapping methods were analyzed.The results showed that the performance of CFRP-strengthened beams was less affected by minor initial damage during the first impact,but the influence of the initial damage gradually increased after being subjected to multiple impacts.Different CFRP wrapping methods could change the failure pattern of the reinforced beam,and the failure pattern of the U-wrapping strengthened beams was mainly impacted bending failure,while the failure pattern of the straight strengthened beams altered from bending failure to shear one.(2)The “restart method” was used to simulate multiple impacts on CFRP-strengthened reinforced concrete beams,and the results were compared with experimental results to verify the model.The results showed that the numerical simulation results could reasonably capture the crack propagation pattern and dynamic response of the strengthened beams with a good agreement with the experimental results,but the numerical simulation was more sensitive to stiffness degradation caused by damage.(3)The effects of different CFRP wrapping methods on the multi-impact performance of the strengthened beam were studied,and the failure pattern,impact resistance,energy dissipation,deformation and recovery ability of the strengthened beam were compared and analyzed.The results showed that the composite CFRP strengthened beams had good resistance to multiple impacts.Compared with the straight,anchored and U-wrapping strengthening methods,the composite CFRP strengthening had a more significant effect on enhancing the impact resistance,energy dissipation,deformation recovery ability,and reducing the deflection ability of the beam.(4)The influence of impact velocity on the dynamic performance of CFRP-strengthened beams was studied based on the composite CFRP strengthened method.It was found that the failure pattern of the composite strengthened beam changed from bending failure at low impact velocities to shear failure at high impact velocities.The peak impact force and mid-span displacement showed a linear relationship with the impact velocity.As the impact velocity increased,the global response time of the impact force became longer,the peak value of impact force and mid-span displacement became larger,the energy dissipation level became higher,and the damage to the strengthened beam became more severe.