Rate-dependence Of Crack Propagation Process In Concrete

At present,the crack propagation process of concrete subject to static conditions has been extensively investigated by many scholars.However,many concrete structures are subjected to dynamic loading.The rupture failure process is closely correlated with the loading rate.Therefore,the knowledge of the rate dependence of mode I and mixed mode I-II crack propagation process in concrete is of significant application value.Based on this,this paper adopts experimental and numerical methods to evaluate the mode I and mixed mode I-II crack propagation process in concrete subject to different loading rates.The main work and conclusions in this paper are as follows:(1)With 15 three-point bending concrete specimens,the fracture tests of mode I crack in concrete subjected to the loading rate of 0.012,0.12,1.2,12 and 120 mm/min were carried out.The peak load,the load-crack mouth opening displacement curve(P-CMOD curve),mode I initial fracture toughness,and the fracture energy were measured.The initial fracture toughness is used as a parameter of the mode I crack propagation criterion.Based on the crack propagation criterion,the finite element method is used to simulate the whole process of mode I crack propagation associated with each loading rate,and the numerical simulation results are compared with the test results.The results show that at different loading rates,the P-CMOD curves of three-point bending beams reasonably are in good agreement with the test results,and the relative error of peak load is less than 8%.The mode I initial fracture toughness and fracture energy of concrete are linear with the logarithm of loading rate.The peak load and critical crack mouth opening displacement increase with increasing the loading rate.In addition,under the same load,the length of the fracture process zone,the cohesion of the initial crack tip and theK_R resistance increase with increasing the loading rate.(2)With four different initial crack positions including 60 four-point shearing concrete specimens,the mixed mode I-II crack propagation process in concrete subjected to the loading rate of 0.012,0.12,1.2,12 and 120 mm/min were carried out.The maximum load,the load-crack mouth shearing displacement curve(P-CMSD curve),the load-crack mouth opening displacement curve(P-CMOD curve)and crack propagation path in the whole process of crack propagation were measured.Based on the mixed mode I-II crack propagation criterion of concrete using the initial fracture toughness as the parameter,the finite element method is used to simulate the whole process of mixed mode I-II crack propagation associated with each loading rate,and the numerical simulation results are compared with the test results.The results show that the P-CMSD curve,P-CMOD curve and crack propagation path of the four point shear beam specimens are in good agreement with the test results,and the relative error of the peak load is within 10%,but the relative error between the calculated results and the test results increases with increasing the loading rate.The results also show that the peak load and the crack mouth opening displacement corresponding to the peak load increase obviously with the increase of the loading rate,but the crack mouth shearing displacement is basically unchanged.In addition,the mixed mode I-II fracture process zone length of concrete at the same load increases with the loading rate.Therefore,based on the crack propagation criterion(parameter:initial fracture toughness),the mode I and mixed mode I-II crack propagation process of concrete structures subjected to different loading rates can be predicted by numerical simulations.