Experimental And Numerical Investigations On Mixed-mode ?-? Fracture Of Rock-concrete Interface

Due to the initial defects during construction or complex loading effect,the bond interface between the concrete dam and the rock foundation is always a weak link,promoting crack initiation even under service loads.The crack propagation under hydrostatic pressure will decrease the carrying capacity and result in the buckling failure of the structure.In particular,the various interfacial crack propagation paths determine the different failure behaviors of a concrete dam on a rock foundation.Therefore,it is significant to predict the facture process and potential crack trajectory to ensure safety and durability of a dam under service load conditions.In fact,the crack in the dam heel can be regarded as being between rock and concrete interface.In order to simulate the whole fracture process and predict the potential propagation modes,the paper carries out the research as follows:(1)Experimental study on mode I fracture properties of rock-concrete interfaceIn this study,rock-concrete composite beam specimens with four different degree of roughness are tested under three-point bending(TPB)to study the effect of interfacial roughness on mode I fracture of rock-concrete interface.The initial cracking loads,the complete load vs.crack mouth opening displacement(P-CMOD)curves and load vs.deformation(P-?)curves are also obtained from the tests.And using ANSYS finite element analysis software,the Mode I initial fracture toughnesses are calculated based on the initial cracking load.Experimental results indicate that,both mode I initial fracture toughness of rock-concrete interface and interfacial fracture energy increase with the roughness.(2)Experimental study on mixed mode I-II fracture properties of rock-concrete interfaceIn this study,rock-concrete composite beam specimens with four different degree of roughness and different rock block are tested under four-point shearing(FPS)to study the effect of interfacial roughness and mode mixity on mixed mode I-II fracture of rock-concrete interface.The initial cracking loads,the P-CMOD curves and load vs.crack mouth sliding displacement(P-CMSD)curves are also obtained from the tests.And using ANSYS finite element analysis software,the initial fracture toughnesses(K1ini,K2ini)are calculated based on the initial cracking load.Experimental results indicate that,the initial fracture toughnesses of all the TPB and FPS specimens can approximately form a quarter of ellipse,and the formed curve moves outward as the interfacial roughness increases,which indicates that the crack resistance increases with the roughness.(3)Numerical study on FPZ evolution of rock-concrete interface under mode I loadingBased on cohesive zone model,initial fracture toughness-based mode I interfacial crack propagation criterion is used to simulate the whole fracture process of composite beams with natural and artificial grooving interface under TPB.Fisrtly,the validity of the criterion is verified by comparing experimental P-CMOD curves and FPZ evolutions with corresponding numerical ones.The verified numerical approach is then utilized to simulate the whole fracture process of three series of composite beams with different sizes and/or initial crack length-to-depth ratios under TPB.And based on numerical results,size effect of FPZ and K2/K1 variation are analyzed and discussed.The results indicate that ligament length significantly affects the FPZ evolution of rock-concrete interface under TPB and K2/K1 is influenced by the specimen size during the propagation of the interface crack.(4)Mixed-mode crack propagation criterion and its application in numerical simulationA universal interfacial crack initiation expression without the effect of degree of roughness has been derived from TPB and FPS tests by normalizing initial fracture toughnesses.Based on the interfacial crack initiation expression,a interfacial crack propagation criterion has been proposed by introducing the fictitious crack model.Also,using ANSYS finite element analysis software,the numerical simulation on interfacial fracture process is conducted by introducing the proposed crack propagation criterion to predict load vs.crack mouth opening displacement(P-CMOD)curves,which show a reasonable agreement with the experimental ones.Finally,combining the maximum circumferential stress and proposed crack propagation criteria,the interfacial crack propagation mode is discussed.The results indicate that,once obtaining the initial fracture toughness of rock,concrete and its interface from experiment,the interfacial crack propagation process and fracture mode,which taking into account the nonlinear characteristics of materials and interface,can be predicted by using the method derived in this study.