Experimental And Numerical Simulation Study On Characteristics Of Fracture Process Zone Of Quasi-brittle Materials

There is a finite stress area ahead of the crack tip of quasi-brittle materials such as concrete and graphite,also called the fracture process zone(FPZ),which leads to the nonlinear fracture behavior of materials.Therefore,in order to understand the fracture mechanism of quasi-brittle materials,it is necessary to study the characteristics of the fracture process zone.This paper reports an experimental and numerical study on the fracture properties of concrete and nuclear graphite NG-CT-01 produced in China.Three-point bending tests were performed on center-notch beams,electronic speckle pattern interferometry(ESPI)technique was employed to measure the full-field deformation of the beams,and numerical simulation was performed by using extended finite element method to model the fracture behavior of quasi-brittle materials.Numerical results were compared with the experimental results and excellent agreement was obtained,validating the reliability and accuracy of the numerical model.The study of FPZ in quasi-brittle materials usually focuses on its geometry and softening characteristics.For geometric properties,based on the strain field measured by ESPI technique,the formation and evolution of the FPZ in materials were observed.The geometric characteristics of the FPZ of the two materials were compared and analyzed.For softening properties,various tension softening curves(TSCs)were employed to simulate the fracture response of concrete beams by integrating the cohesive crack model and finite element model.By comparing the deformation obtained by finite element simulation and experiments,the TSCs of plain concrete were evaluated and most suitable TSCs of concrete were recommended.The TSC of the graphite was determined by using a hybrid numerical and experimental approach.A tri-linear model was proposed to explain the correlation between the tension softening behaviors and the fracture mechanism of the FPZ of graphite.Double-K fracture model was employed to predict the crack propagation in graphite.Besides,the influences of initial crack length to beam depth ratio on the fracture responses of the concrete beam and graphite beam,including the load-displacement curves,double-K fracture parameters,TSCs and the length of the FPZ,were evaluated.