Application Study Of Unified Phase Field Theory For Modeling Localized Failure In Concrete

The failures of concrete structures are generally caused by highly localized non-uniform deformations(i.e.,strain localization).Such localized non-uniform deformations and their propagations are regarded as prognostics of structural collapse.Therefore,in the context of increasingly demanding refined analysis and design of concrete structures,it is of great importance to predict where localized failures occur and how they affect the overall structural responses.Nowadays,as the rapid development of computer and computational mechanics,numerical methods become more efficient,operable and reproducible,playing important roles in engineering structures.However,the existing numerical methods and their theoretical foundations often lag behind the refined analysis and design of concrete structures.Therefore,it is urgent to innovate new theoretical and numerical methods which can accurately reproduce the full-cycle mechanical behavior of concrete structures.In 2017,based on the theory of thermodynamics,Wu proposed a unified phase-field theory by exploring the common points between the phase-field model in the field of fracture mechanics and the gradient damage model in the field of damage mechanics.The unified phasefield theory not only extends the application field of the phase-field model from the brittle material to quasi-brittle material,but also successfully bridges the gap between fracture mechanics(discrete/interface crack model)and damage mechanics(the smeared crack model),widely attracting the attention of domestic and foreign scholars.Aiming to provide a new direction for the simulation analysis of whole process of concrete structure failure,this paper studies the numerical implementation of the unified phase-field theory at first.Under the premise of solving the technical difficulties such as the staggered algorithm and new degree of freedom of phase-field,the unified phase-field theory has been built in some open source finite element platforms,i.e.,JIVE and OOFEM.Then,the unified phase-field theory is successfully applied to simulate the localized failure in concrete structures.Firstly,through the test of the phase-field sub-problem,the rationality of the regularized topology for sharp crack is verified.Secondly,through the analysis of a series of concrete standard benchmark tests,the optimal ratio and the suggested one of internal length scale and element size are obtained.On one hand,these tests illustrate the applicability and superiority of unified phase-field theory for modelling type I and mixed type failure of concrete structures.On the other hand,the staggered algorithm shows good robustness.Finally,two brittle material tests indicate that the unified phase-field theory is applicable for not only quasi-brittle but also brittle material,which verifies the unified phase-field theory has successfully regularized and unified other classical phase-field models.