A Comparative Study On Mixed-mode ?-? Crack Propagation Criteria Of Concrete

Due to the asymmetries of the structural shapes and the complexities of the stress conditions,the cracks in the concrete structures are mostly in the complex stress states,most commonly to be mixed-mode ?-?.At present,many scholars have studied the mixed-mode?-? crack propagation of normal strength concrete with the corresponding mixed-mode ?-?crack propagation criteria proposed.However,there is no research on the mixed-mode ?-?crack propagation criterion of high strength concrete.Based on this,this paper adopts experimental and numerical methods to study the whole mixed-mode ?-? crack propagation process and performs comparative analysis on the experimental and numerical results.The main research contents and conclusions in this paper are as follows:(1)With the purpose of acquiring the whole mixed-mode ?-? crack propagation process,the fracture tests of four-point shearing beams were carried out.The concrete strength grades are C20,C40 and C60 respectively,and the ratios of initial stress intensity factors K?/K?are 0,1.61,3.49 and 5.32 respectively.The load-crack mouth opening displacement(P-CMOD)curves,the load-crack mouth shearing displacement(P-CMSD)curves and the crack propagation trajectories of four-point shearing beams were derived.The experimental results show that when the initial stress intensity factor ratio K?/K? is constant,the peak load increases with the increase of the concrete compressive strength.But there is no obvious relationship between the critical crack mouth opening displacement CMODc or the critical crack mouth shearing displacement CMSDc and the concrete compressive strength.When the compressive strength is identified,the peak load and CMSDc increase but CMODc decreases as the ratio of K?/K? increases,and in the same condition,the ratio of CMSDc/CMODc is on the rise.(2)Four mixed-mode ?-? crack propagation criteria,the K(?,?)P-K(?,?)?=K(?,?)ini,the K?P-K??=K?cini,the K(?,?)P-K(?,?)?=0 and the K?P-K??=0 criteria,are respectively applied to simulate the whole mixed-mode ?-? crack propagation process of the four-point shearing beams under the conditions in(1),obtaining the P-CMSD curves,P-CMOD curves and crack propagation trajectories.By comparatively analyzing the experimental results,it can be found that the P-CMSD curves,the P-CMOD curves and the peak loads calculated by the K(?,?)P-K(?,?)?=K(?,?)ini criterion are well consistent with the experimental results.While the errors between the results calculated by the K(?,?)P-K(?,?)?=0 criterion and the experimental ones are larger and increase with the increase of the concrete compressive strength.However,the crack propagation trajectories calculated by these two criteria are quite similar and well consistent with the experimental results.For the four-point shearing beam,the mode ?fracture component has little impacts on the calculated results except the first steps.When compared with the experimental results,the initial cracking loads predicted by the K?P-K??=K?cini criterion are overestimated.In conclusion,the K(?,?)P-K(?,?)?=K(?,?)ini criterion is more applicable to simulate the mixed-mode ?-? crack propagation process of concrete beams with different strength grades and different proportions of mode ? fracture component than the other three criteria.