Study On DIC Measurament For Main Fatigue Crack Propagation Of RC Beams Strengthened With CFL Under Severe Environments

Fiber reinforced polymer(FRP), widely used in reinforcement of reinforced concrete(RC) structures because of its high strength, corrosion resistance, anti-fatigue, etc. The crack propagation life occupy the vast majority of the total life for RC structures strengthened with FRP under cyclic loading, taking into account the humid salt spray and other harsh environment of RC components in China Southern and coastal areas, the research of crack expansion under the combining function of environment and fatigue load of FRP strengthening RC structures is of important scientific significance.Because of the shortcomings in the observation of crack extensions of traditional methods, the introduction of new experimental methods is imminent. Digital image correlation(DIC) method is nondestructive detection, high precision and real-time measurement, so this paper takes CLF reinforcement RC beam as the experimental object and carry out the research on crack measurement method with DIC method. The main contents and conclusions of this paper are as follows:(1) Built three-dimensional digital image correlation measurement system in the MTS experimental platform, and selected acquisition(Computing) regional production in view of the expansion of the main crack. Then made the speckle field and calibrate the camera accordingly, then discussed the crack propagation experiment error and measurement accuracy through the calibration results.(2) Fatigue experiment was presented on 9 pieces of beams in salt fog environment and 1 beam in high temperature high humidity environment at different temperature, humidity, salinity of 10 different environmental conditions with the MTS system and the salt spray environment, hot and humid environment simulation control system developed by the project team. Then acquire image of the main crack expansion on 7 specimens.(3) On the problems of image acquisition under harsh environment including light source, environment box fog, window attachment drops of water and salt, beam surface water and other interference factors, this paper designed the image acquisition method under high temperature and high humidity environment, under fatigue loads and salt spray environment, and fatigue load interaction respectively, including the design and production of the various experimental tool.(4) Calculated the collected images based on DIC method and the surface displacement field from 3D reconstruction of reinforced beam, and measured the propagation of the main crack morphology combining with the assistance of strain field in the displacement field. To confirm the position of crack tip with the improved design of the shape of the Dual Rectangular Shape Line Method, through the combination of Displacement Difference Method and Reference Point Distance Increment Method to measure different position and time scale of crack width.(5) Experimental results show that the main fatigue crack extension of the reinforcement beam under harsh environment can be divided into four stages of the initiation stage before cyclic loading, rapid expansion, stable propagation and unstable propagation of crack height, the a~N curve and the maximum crack width of b~N curve will better reflect the stage of crack propagation, and the crack height a, width b and the number of cycles N showed a good linear relationship in the logarithmic coordinate system; In addition, the change of the width of the cracks in a single cycle is a sine function with time. In the salt spray experiment, stationary crack height increases with the increase of temperature, the effect of salinity is not obvious; the coefficient A that reflects crack growth rate is maximum at the temperature of 35 centigrade while the coefficient will also increase with higher salt concentrations.