Study On Typical Damage Monitoring Method For FRP-Strengthened Steel Structure Based On Antenna Sensor

FRP materials are more and more widely used in steel structure reinforcement or crack repair because of its high strength-to-weight ratio and high stiffness-to-weight ratio.Cracks in steel structures under FRP coverage and the debonding at FRP ends are two typical damage forms of FRP-strengthened steel structures,and they are also the dominant hidden threats to structural safety.Due to the influence of FRP coverage and material properties of FRP(such as the orthotropic properties of FRP and the weak conductivity of CFRP),the monitoring of structural cracks under FRP coverage is very difficult and there is little related research about FRP ends debonding.In this paper,a new dual-substrste antenna sensor is designed to monitor the typical damage of FRP-strengthened steel structures.The main work and research results of this paper are as follows:1)Combined with the research object of this paper,the damage monitoring model of the dual-substrste antenna sensor with four layers of "patch-substrate-FRPsteel structure" is proposed with originality.Based on the conformal mapping theory and the transmission line model,the theoretical formula of the basic resonant frequency of the dual-substrste antenna sensor is derived.The principle of antenna sensor for identifying steel structure cracks under FRP coverage and the monitoring mechanism of debonding at the end of FRP based on the antenna sensor are expounded,which provides theoretical support for the subsequent research on damage identification of FRP-strengthened steel structures.2)The method of crack identification for steel structure covered by FRP is studied by using antenna sensor and the sensitivity of crack monitoring is studied.The influence of the small crack size and the direction of the crack on the antenna sensor is discussed.The results show that the resonant frequency decreases with the increase of the crack length,and the sensitivity increases with the increase of the crack length.The identifiable minimum crack size variation and minimum crack length are less than 0.5mm.The sensitivity of the antenna sensor to monitor the 30 degree oblique crack is 70% to 90% of the sensitivity of the equal length straight crack.3)For debonding damage at the end of FRP,the mathematical relationship of multi-layer interfacial strain transfer of FRP-strengthened steel structures is deduced.The quantitative analysis of the average strain transfer efficiency after debonding at the end of FRP is studied.The linear relationship between the resonant frequency of the antenna sensor and the microstrain is obtained through the experimental method,and the parabolic relation between the sensitivity of the antenna sensor and the length of the debonding is obtained through polynomial fitting.4)The influence of the weak conductivity of CFRP,FRP thickness and the strain on the damage monitoring of FRP-strengthened steel structures are studied.The results indicate that the weak conductivity of CFRP has great interference to the test signal,but the rule obtained is similar to that of GFRP.The thicker the FRP,the smaller the resonant frequency and the lower the sensitivity.In a certain range of monitoring precision,the effect of strain on the monitoring of crack is very small and can be ignored.The research results in this paper provide a new approach for damage monitoring of FRP-strengthened steel structures.It is of great engineering value to study the damage mechanism of composite structure and to ensure the safety of FRP-strengthened steel structures.