Study On Constitutive Relationship Of Ultra-high Performance Concrete Based On Damage Mechanics

Many experimental studies have shown that the failure of materials is derived from the development of defects in the materials,such as the growth and connections of existing pores and the production of new ones.The relatively compact mesostructure of UHPC(Ultra-High Performance Concrete)leads to its excellent mechanical properties.However,there must be some initial flaws existing in a heterogeneous material like UHPC and the development of these flaws as a result of external actions is bound to cause the degradation of materials.The improvement of modern design methods and construction level force people to understand the failure mechanism of UHPC more clearly.Hence the meso-damage mechanics method is used to study the stress versus strain relationship in the mesostructure of UHPC.Coupled with the experimental study and numerical fitting,the stress-strain curve of UHPC can be accurately determined.A cell model similar to Gurson's model was established for UHPC in a meso-damage mechanics style.The volume fraction of pores f was employed to characterize the damage development.It was incorporated into the perturbation displacement field and the strain/stress fields were deduced.The potential energy was calculated and then minimized considering the boundary conditions.Consequently,the constitutive relations of the cell model and that of the macroscopic material were obtained in sequence.According to the theory of maximum stacking compactness,some UHPC specimens were produced by adjusting the volume fraction of steel fiber.The correlation performance data of materials were measured.Compared with the material test data,it was found that the comprehensive performance of the specimens was better when the mixture of thick fibers and thin ones was selected.The correlation parameters of constitutive equations were obtained by using the method of parameter fitting.The corresponding stress-strain relation curve was given by using the test data in the present work.The stress-strain curves about different valures of the elastic modulus,the Poisson ratio and the void ratio were calculated.Thus an analytic expression of the damage constitutive relation was proposed by using the numerical fitting method.