Effects Of Embedment Length,Spacing And Loading Rate On 3D Dynamic Performance Of Steel Fiber Concrete With Twin Fibers Pullout

Steel fiber reinforced concrete（SFRC）is a new type of multiphase composite material composed of cement mortar,aggregate and steel fiber.It has been widely used in pavement,bridge deck,underground tunnel,port wharf and other major practical projects due to its superior mechanical properties.The research has shown that the interface layer between the concrete matrix and steel fiber is a vulnerable spot of SFRC and its properties have great influence on the mechanical properties of SFRC.The fiber pullout test is a simple and efficient experimental method to study the interfacial characteristics of fiber concrete,but most of the research is limited to static load.In fact,SFRC structures also suffer from dynamic loads such as earthquake,explosion and high speed impact.Compared with the static loading test,dynamic loading has a short duration and it is difficult to capture the whole process of interface layer damage and fracture,so there are few related studies.In this study,the dynamic damage and fracture of SFRC are investigated.RFPA^3D-Dynamics was used to establish a three-dimensional numerical model of twin fibers pullout based on interfacial control.The Weibull distribution function was introduced into the numerical model,and the physical mechanical properties of three-phase materials including concrete matrix,steel fiber and interfacial was randomly assigned in a mesoscopic way,which reflected the mesoscopic heterogeneity of each phase material.The influence of fiber embedment length,spacing and dynamic loading rate on dynamic mechanical properties of specimens was systematically studied through the simulation of the whole failure process of twin fibers pullout,and the dynamic fracture mechanism of steel fiber concrete material based on interfacial control was analyzed.The main research contents of this paper are as follows:（1）Taking fiber embedment length as variable and taking stress wave loading as dynamic load input,a three-dimensional numerical analysis model of twin fibers pullout based on interfacial control was established to study the influence of fiber embedment length change on dynamic performance of twin fibers pullout.The results show that with the increase of fiber embedment length,the debonding rate of the interface between fibers gradually presents a significant difference,and the more obvious the stress concentration of the matrix between the fibers,the stronger the coupling effect of the twin fibers.The distribution and transmission of interface shear stress are greatly affected by the change of embedment length of twin fibers,and the shear stress value of the outer interface element is larger than that of the inner one.With the increase of embedment length,the peak value of shear stress decreases,but when the embedment length increases to 25mm,the decreasing trend is not obvious.In this paper,there is an ideal embedding depth ranging from 30mm to 35mm in the twin fibers pullout model,which makes the steel fiber play the most effective strengthening and toughening role in concrete matrix.The first peak point of acoustic emission energy in the process of twin fibers pullout under dynamic load appears after the peak point of acoustic emission counting curve,showing a"relatively lagging"effect.（2）Taking the fiber spacing as a variable,the influence of the fiber spacing on the dynamic performance of the twin-fibers pullout was studied.The results show that the specimens with short spacing of two fibers are both pulled out,while the specimens with long spacing of two fibers are pulled out separately.The stress concentration of the matrix between fibers is obvious for the twin fibers specimens with small spacing.When the fiber spacing is less than 4 times of the interfacial layer thickness,the coupling effect is obvious.When the fiber spacing is larger than 8 times of the interfacial layer thickness,the coupling effect of fibers decreases.With the increase of fiber spacing,the total acoustic emission energy of the specimens decreases first and then increases.In this paper,there is an optimal fiber spacing of 10mm in the twin fibers pullout model,which minimized the total damage degree of the specimen,and the steel fiber plays the most effective strengthening and toughening role in the concrete matrix.（3）Taking the dynamic loading rate as a variable,the dynamic fracture performance of twin fibers pullout specimens under different loading rates was studied.The results show that the dynamic failure modes of twin fibers pullout specimens are sensitive to loading rate.The higher the loading rate is,the shorter the response time of concrete twin fibers pullout specimen to stress wave is,the faster the interface crack growth rate is and the faster the interface shear stress transfer is.The difference of the shear stress peak value between the inner and outer interface of the twin fibers is smaller.From the increase of cumulative acoustic emission number and total acoustic emission energy,it can be concluded that with the increase of loading rate,the damage of twin fibers pullout specimen is more serious.