Study On Rate Sensitivity Of Mode ? Crack Propagation Process In Concrete

At present,domestic and foreign scholars have obtained a series of results on the research of the crack propagation process of concrete under dynamic load,but the existing results have not yet explored the influence of initial crack height ratio and concrete strength on the rate sensitivity of concrete crack propagation process under dynamic load.In order to solve the above problems,this paper uses a combination of experiment and numerical simulation to study the influence of initial crack height ratio and concrete strength on the rate sensitivity of mode?crack propagation process of concrete.The main work and conclusions are as follows:Four types of C30 concrete three-point bending beams with initial crack height ratio(0.2,0.3,0.4,0.5)were used to conduct concrete mode I fracture tests at five loading rates(0.012,0.12,1.2,12,120 mm/min),Initiation fracture toughness,unstable fracture toughness,fracture energy,P-CMOD curve,P-?curve and crack propagation velocity were obtained.The finite element method was used to simulate and calculate the above-mentioned crack propagation process,K_R resistance curve and the length of the fracture process zone with the mode I crack propagation criterion of concrete using the initiation toughness as the parameter,and the calculation results are compared with the test results.The results show that the initial height ratio has no effect on the rate sensitivity of initiation fracture toughness,unstable fracture toughness and fracture energy.The crack propagation velocity increases first and then decreases.The position of the peak load and the position where the load drops to about 30%of the peak load are the two critical points where the crack propagation velocity starts to increase and falls to a stable propagation velocity,and crack propagation velocity decreases with the increase of the initial crack height ratio.The P-CMOD curve simulated by the criterion is in good agreement with the test,and the calculated peak load error does not exceed 10%.The larger the initial crack height ratio,the greater the K_Rresistance of the specimen,and the greater the rate sensitivity of the K_Rresistance curve.Using the same method,the three-point bending concrete beams with four strength grades(C20,C40,C60,C80)and initial joint height ratio of 0.3 were tested and numerically simulated.The results show that the rate sensitivity of initiation fracture toughness,unstable fracture toughness and fracture energy decreases with the increase of concrete strength.The crack propagation velocity increases with the increase of concrete strength grade,the difference is more significant at high loading rate.The P-CMOD curve simulated by the criterion is in good agreement with the test,and the calculated peak load error does not exceed 10%.The higher the concrete strength,the greater the K_Rresistance,but the lower the rate sensitivity of the K_R resistance and the change in length of fracture process zone.The specimen with the lower strength grade has a longer length of fracture process zone,and as the loading rate increases,the difference in length of fracture process zone becomes more obvious.The research in this paper shows that both the initial crack height ratio and the concrete strength will affect the mode I crack propagation process of concrete under dynamic loading.And the mode I crack propagation criterion of concrete with initiation toughness as a parameter can simulate the crack propagation process under different loading rates.