Dynamic Fracture Behavior Of Steel Fiber Reinforced Concrete Beams Under Impact Loading

Concrete,as one of building material,has widely used in military,industrial and civil buildings because of its excellent properties.However,the tensile strength is far less than the compressive strength caused by cracks and durability of concrete material and the further development is restricted.In order to improve the cracks in concrete,graphene oxide and steel fiber materials were added separately in cement-based materials,and Brazil disk specimen and steel fiber concrete beam were prepared.The fracture mechanics properties of concrete under different materials were studied.The main work was as follows:The effect grapheme oxide nanolayer on the mechanical behavior of cement-based composite materials was studied by using split tensile test of Brazilian disc.The ordinary cement mortar and 0.03 wt% graphene oxide cement mortar were prepared,and these materials were made into center straight crack and different angle crack forms of the Brazilian disc specimen.The indirect tensile strength and fracture property of pure I mode,pure II mode and mix I-II mode pure of different specimens were measured.The research results show that graphene oxide nanometer slice layer of cement mortar has the obvious enhancement effect.When 0.03 wt% graphene oxide is added to the cement mortar,the cement mortar compressive strength increases by 34.12%,the indirect tensile strength increases by 18.96%,pure mode I fracture toughness increases by 20.09%,and pure type II fracture toughness increases by 12.07%,for I-II mixed fracture toughness increases by 12.27%.A precast steel fiber reinforced concrete beam specimen with a volume fraction of 1% was prepared,and three point bending tests were carried out for concrete beams by DHR-9401 drop hammer impact test machine.The deformation mode and fracture morphology of specimens at five different loading speeds were analyzed with DIC.The results show that under different impact speeds,specimen fracture energy and the impact of peak value are increased with the increase of impact velocity.When the impact height of drop hammer is 10~40 mm,all specimens are not entirely fracture,the fibers of the fracture surface are all pulled out,and there is no single fiber breakage.When the impact height of drop hammer is 80 mm,the cracks rapidly expand along the prefabricated notch direction and split directly into two parts.Under high speed impact,the cracking velocity is the order of hundreds of meters/seconds,and the cracking velocity decreases gradually at the same impact velocity.There is no obvious change in the propagation velocity of the main crack at different impact velocity.Based on the numerical simulation method,the dynamic response of concrete beam specimens with directional steel fiber reinforced concrete beams under different impact velocities was simulated and analyzed.By comparing with the results of the drop hammer impact test,the rationality of the finite element model was verified.On this basis,the strength of the concrete and the influence of the content of steel fiber on the fracture performance of the steel fiber reinforced concrete beams were studied,and the analysis of the fracture performance of the traditional plain concrete was compared.The results show that the peak value of the impact force is related to the strength grade of concrete under the same impact velocity.The higher the strength of the concrete is,the greater the peak impact force.The volume rate of steel fiber(1%,2% and 4% respectively)has little effect on the peak impact force of concrete beams.Compared with the plain concrete,the peak impact force of steel fiber containing steel fiber is greater than that of the plain concrete under the same impact velocity,and the law of fracture energy change is also the same.It shows that added steel fiber can obviously improve the fracture properties of concrete beam specimens,the specimens of crack extension has a certain inhibitory effect.