Experimental And Numerical Study On Dynamic Splitting Of Brazilian Flattened Disc With Prefabricated Cracks

Mortar,as a composite of concrete matrix,is one typical brittle material.Its mechanical properties have directly influence on stability and safety in engineering.Microcracks are always found in mortar because of heterogeneity and multiphase.As a result,cracks could exist in engineering structures under working condition.Cracks of mixing I-II types are commonly met in engineering.However,only a few studies have been conducted on this mixing type till present because of its complexity.As the developing pattern of this mixing type of cracks is difficult to be interpreted,its dynamical and fractural model is at an exploration stage.Consequently,the research on basic fracture characteristics of mortar under dynamic load is meaningful to improve fracture performance of concrete.In this thesis,both experimental and numerical approaches have been adopted to study Brazilian disc with prefabricated cracks.First,impact splitting test has been conducted by using Split Hopkinson Pressure Bar(SHPB).Then,a full-scale numerical simulation has been carried out based on a FEM software named 3D Realistic Failure Process Analysis-Dynamics(RFPA3D-Dynamics),which takes heterogeneity into account by introducing Weibull distribution function.Finally,by comparing the experimental and numerical results,fracture characteristics of mixing I-II type cracks have been scrutinized in this study and following topics have been discussed in this thesis:(1)Effect of strain rate: Results indicate that damage level of specimen increases together with strain rate.Direction of the primary crack remains unchanged and extends from the prefabricated crack tip to the end of the loading platform.Meanwhile,number of secondary cracks increases.Subsequently,the dynamic fracture toughness of Types I and II increases.However,dimensionless composite fracture toughness ratio remains as a constant.Furthermore,The primary crack propagates from the tip of the prefabricated crack to the loading end of the platform,and the number of secondary cracks increases with strain rate.(2)Effect of dip angle of one prefabricated crack: Results show that the failure pattern of specimens changes with increase of dip angle,and dynamic tensile strength increases accordingly.When dip angle is in between 15 and 75 degrees,dynamic fracture toughness of type I cracks increase,while toughness of type II cracks increases first and then decreases versus the dip angle.Dimensionless fracture toughness ratio increases accordingly.The primary cracks are identified as Type II for dip angle of 15 degrees and Type I for dip angle between 30 and 75 degrees.Propagation direction of secondary cracks is consistent with the dip angle and towards the disc edges.(3)Effect of dip angle,distance and length of two parallel prefabricated cracks: Results show that peak value of tensile load is positively correlated with dip angle of the two paralleled cracks.At the same time,peak value of tensile load increases first and decreases subsequently with increase of distance between two parallel cracks.With increase of length of the two paralleled cracks,peak value of tensile load decreases.