Research On Fracture Properties Of Fiber Reinforced Potassium Magnesium Phosphate Cement-based Composites After High Temperature

The inevitable initial defects in the material are the weak links of the components under external loads.Fracture failure often occurs under complex stress conditions such as earthquakes,fires,and explosions.In addition,the phase composition of the material changes at high temperatures such as fires,and the microstructure is not conducive to bearing.In order to improve the high temperature resistance and crack resistance of building facilities,it is necessary to improve the concrete material,carry out the fracture performance test before and after high temperature,and fully understand the fracture mechanism,which is of great significance for determining the fracture bearing capacity and structural safety design.The fiber damage of engineering cement-based composites with ultra-high ductility after high temperature will lead to the loss of pseudo strain hardening characteristics.On this basis,MKPC with good high temperature stability is considered to replace cement,and MSF is used to replace PVA fiber.The aim is to prepare a fiber-reinforced potassium magnesium phosphate cement-based composite material which still have hardening and softening properties after high temperature.he notched semi-circular bending(NSCB)specimens were prepared and the mode I fracture crack propagation behavior was studied.he main contents and results are as follows:(1)The quasi-static NSCB three-point bending test was carried out to explore the effect of different fiber content on fracture performance before and after high temperature.The content of fibers has no significant effect on the fracture toughness of MKPC,and the fracture toughness mainly depends on the MKPC itself.The content of fibers significantly improved the failure mode and fracture toughness of the specimens.The fracture toughness increases first and then decreases with the increase of MSF content,and reaches the maximum at 2%MSF.On the basis of 2% fiber volume fraction,PVA fiber and MSF are mixed,and the effect is the best when the PVA content is 0.5%.The variation of fracture toughness and fracture toughness with temperature is the same,and decreases with the increase of temperature.(2)The dynamic tensile NSCB three-point bending test after high temperature was carried out by SHPB.The effects of different fiber content and different loading rates on dynamic fracture toughness before and after high temperature were studied.The dynamic fracture toughness of the material was improved by adding fibers.The single-doped fiber system and the mixed fiber system achieved the best results in 2% MSF and 0.5% PVA +1.5% MSF,respectively.The dynamic fracture toughness decreases from 25°C to 500°C,and rises after 800°C and 1100°C.The higher the loading rate,the higher the dynamic fracture toughness of the material.(3)Based on the results of NSCB three-point bending test,a dynamic fracture toughness prediction formula related to loading rate,temperature and fiber content was established,which can better predict the change trend of fracture toughness under the influence of various factors in this paper.(4)Combined with SHPB and crack propagation meter,the dynamic initiation toughness,dynamic propagation toughness and fiber arrest dynamic arrest toughness under different loading rates before and after high temperature were studied.The crack propagation behavior and mechanism were analyzed by X-ray diffraction test,energy spectrum analysis and electron microscope scanning test.In the process of crack propagation,there is a phenomenon of fiber crack arrest,and the crack propagation speed is attenuated.The dynamic fracture toughness is negatively correlated with the crack propagation speed.After the incorporation of fibers,the overall ability of the specimen to resist crack propagation is improved,and the dynamic arrest toughness is greater than the dynamic initiation toughness and the dynamic propagation toughness.The dynamic fracture toughness is positively correlated with the loading rate.After high temperature,the fiber is damaged,especially after the melting of PVA fiber,the dynamic expansion toughness is less than the dynamic crack arrest toughness.