Study On Phase Field Fracture Modelling For Multiscale Spalling Of Underground Concrete Structures Under High Temperature

Concrete is a typical porous multiphase material.When exposed to fire,concrete will experience a series of interacting physical and chemical processes within it.The occurance of thermal spalling involves crack propagation under the coupling actions of multiphysics.Besides,concrete is a heterogeneous material,and the spalling process is strongly related to its mesoscopic structures.Therefore,spalling process from the initiation of microcracks to the macro fracture is a multiscale problem.Unlike ground structures,underground structures reside in soil or rock mass.To consider the interaction between structure and surrounding medium,the soil/rock-structure model is more reasonable.From this viewpoint,the numerical model for simulating spalling behavior of underground structures spans a large scale from mesoscale to the stratumstructure level and possesses multiscale characteristics.This thesis aims at devolping numerical models for fire induced spalling in underground structures,which can incorporate multiphysics,multiscale structures and special environment of underground structures.The models are applied to reproducing the spalling behavior of underground structures,with the purpose of investigating the characteristics and mechanism of thermal spalling,which contributes to the risk assessment and disaster prevention of underground structures.The main contents are as follows:(1)The coupled thermo-hydro mathematical model for concrete at high temperature is presented together with the temperature-and moisture contentdependent thermodynamic,hydraulic and physical parameters.With the initial and boundary conditions,the finite element solution framework for the coupled multiphysics model is established.Through a numerical example,the distributions of temperature,vapor pressure,degree of saturation and other related variables are obtained,which reveals the mass and heat transfer process within concrete during fire including enlarging high-temperature zone and drying zone,as well as moving peak vapor pressure and moisture clog.Furthermore,the sensitivity analysis on the model items and parameters are conducted.(2)The phase field fracture method is incorporated to the developed thermohydro model to simulate the thermal spalling.First,the governing equations for the displacement field and phase field are developed.Subsequently,three examples dealing with fracturing under mechanical loads focus on validation of the self-developed code,as well as investigation on the effects of length scale parameter and staggered methods.In addition,staggered numerical scheme to successively solve the coupled hygrothermal fields and displacement field-phase field is elucidated,where the first three fields are solved monolithically and the last two fields are solved in a staggered way.The two numerical examples of fire-induced spalling aimed at demonstrating the performance of the coupled model,offering an insight into spalling mechanism and assessing the impact of facture on mass transfer and heat conduction.It is concluded that spalling is mainly caused by the restrained thermal strain.(3)The s-version method provides a feasible and efficient way for modeling spalling behavior of concrete tunnel lining at stratum-structure level.With the basic principle of the s-version method,the numerical solution to the coupled thermomechanical-phase field model by s-version method is provided.The subsequent three numerical examples are performed to investigate the performance of the s-version method based phase field fracture model.Also,the effect of the length of the local mesh on the results are analyzed,which can serve as evidence for determination of the size and the arrangement of the subdomains.Taking a tunnel segment as an example,domain decomposition for s-version method is first performed according to the spalling depth.Then the effect of the boundary conditions,the suffered mechanical load,and the component’s size on the spalling characteristics is discussed.Finally,the s-version based phase field fracture model is applied to stratum-structure model to simulate the spalling process of underground concrete structures with different shapes,buried depths,fire loadings,sizes,and stratum types.(4)A concurrent multiscale approach for phase field modeling of fracture is developed to numerically investigate the fracture mechanism of heterogeneous materials.After developing the mesocopic numerical model for concrete,taking a tunnel linging segment as the research object,a comparative analysis on the full mesoscale model,the multiscale model with mono mesh,and the multiscale model based on the s-verison method is conducted.The s-version multiscale models for lining segments with different aggregate configurations are developed,and the effect of the mesoscopic structure on the spalling characteristics is investigated.At structural level,the concurrent multiscale model is developed for spalling modeling of underground structures.It is found that the spalling process is strongly related to the mesoscopic structures and to the thermal properties of these two phases.