| Concrete structures,such as concrete pavements and offshore platforms,will be subjected to cyclic loadings such as vehicle loading and wave loading during their service life,which can cause some degree of damage to concrete materials and have a negative impact on the safety of concrete structures.Under the cyclic loadings,concrete internal stresses and strains undergo repeated changes,leading to changes in the microstructure of concrete materials and the promotion of the propagation of small cracks,which exacerbate existing cracks and cause damage to the material and the degradation of mechanical properties.The mechanical properties of concrete are greatly influenced by the loading history,that is,their damage evolution is nonlinear.Therefore,it is important to study the fracture properties and crack propagation mechanisms of concrete under cyclic loadings,which have important practical significance.This thesis conducted three-point bending fracture tests on concrete under cyclic loadings,studied the fracture properties and evolution rules of the fracture process zone under constant amplitude and envelope cyclic loadings,and obtained the distribution law of cohesive force in the fracture process zone through solving the inverse problem based on the displacement distribution law of the fracture process zone.The impact of cyclic loadings on the softening relationship of concrete in the fracture process zone was revealed,and the main work content of this thesis are following:(1)Three-point bending fracture tests was conducted under constant-amplitude and envelope cyclic loadings to study the evolution rules of CMOD,dynamic modulus,dissipated energy,and CMOD recovery rate of concrete under different loadings.The degradation rules of concrete fracture properties under cyclic loadings were analyzed.(2)The evolution rules of concrete crack length a,displacement distribution slope k,and residual strain of concrete material were studied under different loadings,and the development rules of concrete fracture process zone were analyzed.(3)Based on the displacement distribution law of concrete fracture process zone,the Tikhonov regularization method was used to solve the inverse problem to obtain the distribution of cohesive force in the fracture process zone and the softening curve.The physical characteristics of the softening curve of concrete fracture process zone were clarified,and the fracture mechanism of concrete was explained.(4)Using the method of inverse problem solving proposed in this thesis,the softening curves of fracture process zone under cyclic loadings were calculated,and the mechanism of impact of cyclic loadings on concrete fracture behavior was clarified,and the role mode of residual strain under cyclic loadings was studied. |
