Study On Impact Failure Process And Mechanism Of Flattened Brazilian Disc

Cement mortar is one important component of steel fiber reinforced concrete,which directly sticks with the fiber.Steel fiber reinforced concrete composites could,therefore,be strengthened and toughened because of combined action in their mechanical properties.It also affects directly pattern of crack initiation,propagation and penetration.In engineering practice,structures are often deformed under dynamic impact loads,resulting in macroscopic cracks and instability failure.Therefore,it is of great significance to study the mechanism of crack propagation and fiber crack resistance in cement mortar.In this paper,physical experiments and numerical simulations have been both conducted.Firstly,cement mortar specimens are made into flattened Brazilian discs,and impact-splitting tests are carried out using Split Hopkinson Pressure Bar(SHPB).Then,the corresponding numerical model has been established and analyzed in the finite element dynamic analysis software RFPA2D-Dynamic(Realistic Failure Process Analysis).The physical test and numerical simulation results have been compared to analyze the crack propagation mechanism and fiber crack resistance mechanism in cement mortar.Specific research contents are as follows:(1)Flattened Brazilian disc specimens were fabricated with varied intersection angles between two prefabricated cracks.Strength value,crack initiation point and penetration path of the specimens were obtained by using SHPB test principles.The splitting strength of specimens reaches the highest when the intersection angle is 0 degree.The cracks of all specimens originate in the middle of the first crack,and the main crack propagates axially,while the secondary crack penetrates near the flattened end,almost parallel to the direction of the crack.In the numerical simulation,Weibull random non-uniform distribution of material mesomechanics parameters has been adopted,and dynamic triangular stress wave is applied for dynamic finite element analysis.The crack initiation location and the main crack propagation mode are then obtained,which are consistent with the physical test.When the angle is 0 degree,the strength of the specimens reaches also the highest value.(2)Brazilian disc specimens of cement mortar platform have been set with the number of cracks as variable.The strength value and crack propagation path of the specimens were obtained by the SHPB test.The physical test results showed that strength of the specimens decreases with the increase of the number of cracks.In numerical simulation,it is also consistent with the physical test results.With the increase of the number of cracks,the stress at the platform end increases,and the strength of the specimen decreases.A large number of secondary cracks are randomly distributed in the specimens with multiple cracks,so the number of acoustic emission hits and accumulated energy are much higher.(3)With the volume fraction of steel fibers as the variable,a numerical model of steel fiber reinforced cement-based materials under dynamic impact compression has been established in the finite element program.The effects of fiber content on the mechanical properties of specimens and the mechanism of fiber reinforced crack resistance have been studied.The numerical simulation results show that with the increase of fiber content,the compressive strength of steel fiber cement-based materials increases to a certain extent,while less crack initiation appear.In the process of crack propagation,there are three different patterns obtained(i.e.,the direction of crack propagation changed;the crack passed through the fiber and the crack propagation terminated),which show the effect of fiber on the enhancement of crack resistance of matrix materials.