Phase-field Variable Research Articles

Adaptive numerical integration of exponential finite elements for a phase field fracture model

Phase field models for fracture are energy-based and employ a continuous field variable, the phase field, to indicate cracks. The width of the transition zone of this field variable between damaged and intact regions is controlled by a regularization parameter. Narrow transition zones are required for a good approximation of the fracture energy which involves steep gradients of the phase field. This demands a high mesh density in finite element simulations if 4-node elements with standard bilinear shape functions are used. In order to improve the quality of the results with coarser meshes, exponential shape functions derived from the analytic solution of the 1D model are introduced for the discretization of the phase field variable. Compared to the bilinear shape functions these special shape functions allow for a better approximation of the fracture field. Unfortunately, lower-order Gauss-Legendre quadrature schemes, which are sufficiently accurate for the integration of bilinear shape functions, are not sufficient for an accurate integration of the exponential shape functions. Therefore in this work, the numerical accuracy of higher-order Gauss-Legendre formulas and a double exponential formula for numerical integration is analyzed.

A variant of stabilized-scalar auxiliary variable (S-SAV) approach for a modified phase-field surfactant model

In this article, we develop a new linear, decoupled, second-order accurate, and energy stable numerical method for a modified phase-field surfactant model (Xu et al., 2020). The proposed scheme is a simple and efficient variant of stabilized-scalar auxiliary variable (S-SAV) method. The proposed scheme not only retains all advantages of S-SAV method but also simplifies the solution algorithm. The phase-field, surfactant, and auxiliary variables are totally decoupled in time, thus we can solve the whole system in a step-by-step manner. The phase-field function ϕ and surfactant ψ can be separately updated by solving two linear semi-implicit systems with constant coefficients and then the auxiliary variable is directly updated in an explicit way. We analytically prove the energy stability and the unique solvability of the proposed method. The numerical experiments show the desired temporal accuracy and energy stability. We numerically investigate the proper stabilization coefficients for the present scheme with specific parameters. Furthermore, various two- and three-dimensional benchmark tests are performed to study the dynamics of surfactant-laden phase separation.

Adaptive Orientation of Exponential Finite Elements for a Phase Field Fracture Model

AbstractOne technique to describe the failure of mechanical structures is a phase field model for fracture. Phase field models for fracture consider an independent scalar field variable in addition to the mechanical displacement [1]. The phase field ansatz approximates crack surfaces as a continuous transition zone in which the phase field variable varies from a value that indicates intact material to another value that represents cracks. For a good approximation of cracks, these transition zones are required to be narrow, which leads to steep gradients in the fracture field. As a consequence, the required mesh density in a finite element simulation and thus the computational effort increases. In order to circumvent this efficiency problem, exponential shape functions were introduced in the discretization of the phase field variable, see [2]. Compared to the bilinear shape functions these special shape functions allow for a better approximation of the steep transition with less elements. Unfortunately, the exponential shape functions are not symmetric, which requires a certain orientation of elements relative to the crack surfaces. This adaptation is not uniquely determined and needs to be set up in the correct way in order to improve the approximation of smooth cracks. The issue is solved in this work by reorientating the exponential shape functions according to the nodal value of phase field gradient in a particular element. To be precise, this work discusses an adaptive algorithm that implements such a reorientation for 2d and 3d situations.

Keeping it together: A phase-field version of path-connectedness and its implementation

We describe the implementation of a topological constraint in finite-element simulations of phase-field models, which ensures path-connectedness of preimages of intervals in the phase-field variable. The constraint takes the form of an energetic penalty for a suitable geodesic distance between all pairs of points in the domain. The main application of our method presented here is a discrete steepest descent of a phase-field version of a bending energy with spontaneous curvature and additional surface area penalty. This leads to disconnected surfaces without our topological constraint but connected surfaces with the constraint. Numerically, our constraint is treated by first transforming the double integral over all pairs of points in the domain to a weighted graph structure and then using Dijkstra’s algorithm to calculate the distance between discrete connected components.

Discussion of hardening effects on phase field models for fracture

Phase field models have been successfully applied in recent years to a variety of fracture mechanics problems, such as quasi-brittle materials, dynamic fracture mechanics, fatigue cracks in brittle materials, as well as ductile materials. The basic idea of the method is to introduce an additional term in the energy functional describing the state of material bodies. A new state variable is included in this term, the so-called phase field, and enables to determine the surface energy of the crack. This approach allows to model phenomena such as crack initiation, crack branching and buckling of cracks, as well as the modelling of the crack front in three-dimensional geometries, without further assumptions. There is yet no systematic investigation of the influence of strain hardening on crack development within the phase field method. Thus, the aim of the paper is to provide an analysis of the effect of kinematic and isotropic hardening on the evolution of the phase field variable.

Approximation of the Mumford–Shah functional by phase fields of bounded variation

In this paper, we introduce a new phase field approximation of the Mumford–Shah functional similar to the well-known one from Ambrosio and Tortorelli. However, in our setting the phase field is allowed to be a function of bounded variation, instead of an [Formula: see text]-function. In the context of image segmentation, we also show how this new approximation can be used for numerical computations, which contains a total variation minimization of the phase field variable, as it appears in many problems of image processing. A comparison to the classical Ambrosio–Tortorelli approximation, where the phase field is an [Formula: see text]-function, shows that the new model leads to sharper phase fields.

A phase-field multirate scheme with stabilized iterative coupling for pressure driven fracture propagation in porous media

Phase-field methods have the potential to simulate large scale evolution of networks of fractures in porous media without the need to explicitly track interfaces. Practical field simulations require however that robust and efficient decoupling techniques can be applied for solving these complex systems. In this work, we focus on the mechanics-step that involves the coupling of elasticity and the phase-field variable. We develop a multirate scheme in which a coarser time grid is employed for the mechanics equation (i.e., the displacements) and a finer time grid is taken for the phase-field problem. The performance of this algorithm is demonstrated for two test cases.

Phase-field model of brittle fracture in Reissner–Mindlin plates and shells

In this paper, a phase-field model is presented for the description of brittle fracture in a Reissner–Mindlin plate and shell formulation. The shell kinematics as well as the phase-field variable are described on the midsurface of the structure. Non-Uniform Rational B-Spline basis functions are used for the discretization of both the displacement/rotations and the phase-field. The spectral decomposition for the tension–compression split is applied on the total strain tensor, which varies through the thickness. Thus, the plane stress condition has to be enforced numerically. Various numerical examples are presented in order to verify the accuracy and effectiveness of the method and a detailed comparison to existing formulations is performed.

Phase Field Modeling of Hertzian Cone Cracks Under Spherical Indentation

A phase field model of brittle fracture has been developed to simulate the Hertzian crack induced by penetration of a rigid sphere to an isotropic linear-elastic half-space. The fracture formation is regarded as a diffusive field variable, which is zero for the intact material and unity if there is a crack. Crack growth is assumed to be driven by a strain invariant. The numerical implementation is performed with the finite element method and an implicit time integration scheme. The mechanical equilibrium and the phase field equations are solved in a staggered manner, sequentially updating the displacement field and the phase field variable. Numerical examples demonstrate the capability of the model to reproduce the nucleation and growth of the Hertzian cone crack.

Pfm-cracks: A parallel-adaptive framework for phase-field fracture propagation

Abstract This paper describes the main features of our parallel-adaptive open-source framework for solving phase-field fracture problems called pfm-cracks . Our program allows for dimension-independent programming in two- and three-dimensional settings. A quasi-monolithic formulation for the coupled two-component system of displacements and a phase-field indicator variable is used. The nonlinear problem is solved with a robust, efficient semi-smooth Newton algorithm. A highlight is adaptive predictor–corrector mesh refinement. The code is fully parallelized and scales to 1000 and more MPI ranks. Illustrative tests demonstrate the current capabilities, from which some are parts of benchmark collections.

A combined finite element–finite volume framework for phase-field fracture

Numerical simulations of brittle fracture using phase-field approaches often employ a discrete approximation framework that applies the same order of interpolation for the displacement and phase-field variables. In particular, the use of linear finite elements to discretize both stress equilibrium and phase-field equations is widespread in the literature. However, the use of P1 Lagrange shape functions to model the phase-field is not optimal, as the latter contains cusps for fully developed cracks. These should in turn occur at locations corresponding to Gauss points of the associated FE model for the mechanics. Such a feature is challenging to reproduce accurately with low order elements, and element sizes must consequently be made very small relative to the phase-field regularization parameter in order to achieve convergence of results with respect to the mesh. In this paper, we combine a standard linear FE discretization of stress equilibrium with a cell-centered finite volume approximation of the phase-field evolution equation based on the two-point flux approximation constructed over the same simplex mesh. Compared to a pure FE formulation utilizing linear elements, the proposed framework results in looser restrictions on mesh refinement with respect to the phase-field length scale. This ability to employ coarser meshes relative to the traditional implementation allows for significant reductions on computational cost, as demonstrated in several numerical examples.

Preconditioning nonlocal multi-phase flow

We propose an efficient solver for saddle point problems arising from finite element approximations of nonlocal multi-phase Allen–Cahn variational inequalities. The solver is seen to behave mesh independently and to have only a very mild dependence on the number of phase field variables. In addition we prove convergence, in three GMRES iterations, of the approximation of the two phase problem, regardless of mesh size or interfacial width. Numerical results are presented that illustrate the competitiveness of this approach.

An efficient Pseudo-spectral based phase field method for dendritic solidification

In the current study, Fourier Pseudo-spectral based dealiasing scheme Random Phase Shift Method (RPSM) using the RK4 temporal scheme is implemented for simulating some benchmark problems of Phase Field based pure metal solidification. This high-resolution scheme is capable of resolving the thin solid-liquid interface accurately without using Adaptive Mesh Refinement (AMR) algorithm even using just adequate number of grid points. Both the conservative and non-conservative forms of the Phase Field equations are studied. It is found that the conservative RPSM scheme gives better computational performances as indicated by the lower values of average CPU time and different error norms calculated for the Phase Field variable. The RPSM based Phase Field model is applied for simulating dendritic solidification and it is found that dendrite shape and dimensionless tip velocity agree well with the existing literature. The RK4 based RPSM scheme is also compared with a 10th order exponential Fourier smoothing Filter in the evolution of four arm dendritic structure and it is observed that the conservative Phase Field equations with filtering scheme display numerical instabilities at later simulation time, while both forms of the RPSM scheme show stable and accurate solutions. Subsequently, the capability of the RPSM based Phase Field model in simulating microstructure evolution consisting of multiple dendrites with random location and orientation is demonstrated. The present study shows that the RPSM based Phase Field model computes dendritic growth accurately and efficiently.

Generalized conservative phase field model and its lattice Boltzmann scheme for multicomponent multiphase flows

In this paper a generalized conservative phase field-lattice Boltzmann model is proposed for immiscible incompressible flows with any number of components. In this phase field model, the each phase field variable satisfies a generalized eikonal equation and the volume fraction constraint. By introducing a quadratic functional, the dynamics of phase field variables can be treated as a gradient flow method. The phase field equations are reformulated based on the mass conservative law and the reduction-consistency conditions. Moreover, based on the generalized free energy functional, a family of generalized continuous surface tension force formulations is deduced by using the virtual work method. A lattice Boltzmann model (LBM) is developed to solve the generalized conservative phase field equations and the hydrodynamics equations. In the numerical simulation section, the motion of circular interfaces and Poiseuille flow with four phases are simulated first. The convergence rate and reduction-consistent of present model for the phase field equations are validated. The computational accuracy of present model for the hydrodynamics equations is also validated. Then serval two-dimensional (2D) examples with pairwise surface tension effect, including the droplets immersed in another fluid with four fluid phases, spreading of liquid lenses and spinodal decomposition with four fluid phases, are simulated. The obtained numerical results are in good agreement with the analytical solutions. Four continuous surface tension force formulations are also comparative studied. Finally, as a practical application, a three-dimensional (3D) bubble rising in a three-layer liquid is investigated.

Irreversibility and alternate minimization in phase field fracture: a viscosity approach

This work is devoted to the analysis of convergence of an alternate (staggered) minimization algorithm in the framework of phase field models of fracture. The energy of the system is characterized by a nonlinear splitting of tensile and compressive strains, featuring non-interpenetration of the fracture lips. The alternating scheme is coupled with an L^{2}-penalization in the phase field variable, driven by a viscous parameter delta >0, and with an irreversibility constraint, forcing the monotonicity of the phase field only w.r.t. time, but not along the whole iterative minimization. We show first the convergence of such a scheme to a viscous evolution for delta >0 and then consider the vanishing viscosity limit delta rightarrow 0.

Convergence of a Robin boundary approximation for a Cahn–Hilliard system with dynamic boundary conditions

We prove the existence of unique weak solutions to an extension of a Cahn–Hilliard model proposed recently by C Liu and H Wu (2019 Arch. Ration. Mech. Anal. 233 167–247), in which the new dynamic boundary condition is further generalised with an affine linear relation between the surface and bulk phase field variables. As a first approach to tackle more general and nonlinear relations, we investigate the existence of unique weak solutions to a regularisation by a Robin boundary condition. Included in our analysis is the case where there is no diffusion for the surface phase field, which causes new difficulties for the analysis of the Robin system. Furthermore, for the case of affine linear relations, we show the weak convergence of solutions as the regularisation parameter tends to zero, and derive an error estimate between the two models. This is supported by numerical experiments which also demonstrate some non-trivial dynamics for the extended Liu–Wu model that is not present in the original model.

Error Analysis of a Decoupled, Linear Stabilization Scheme for the Cahn–Hilliard Model of Two-Phase Incompressible Flows

Here, we carry out rigorous error analysis for a first-order in time, linear, fully decoupled and energy stable scheme for solving the Cahn–Hilliard phase-field model of two-phase incompressible flows, namely Cahn–Hilliard–Navier–Stokes problem (Shen and Yang, SIAM J Numer Anal, 2015). The error estimates are for phase field variable, chemical potential, velocity and further the pressure in $$L^2$$ norm and $$L^{\infty }$$ norm. The scheme combines the projection method, the explicit stabilizing decoupling technique, and the linear stabilization approach together. We further derive the boundness of numerical solution in $$L^\infty $$ norm with the mathematical deduction, and deal with the complex splitting error arising from the decoupling technique. Optimal error estimates are derived for the semi-discrete-in-time scheme.

Weak and stationary solutions to a Cahn–Hilliard–Brinkman model with singular potentials and source terms

Abstract We study a phase field model proposed recently in the context of tumour growth. The model couples a Cahn–Hilliard–Brinkman (CHB) system with an elliptic reaction-diffusion equation for a nutrient. The fluid velocity, governed by the Brinkman law, is not solenoidal, as its divergence is a function of the nutrient and the phase field variable, i.e., solution-dependent, and frictionless boundary conditions are prescribed for the velocity to avoid imposing unrealistic constraints on the divergence relation. In this paper we give a first result on the existence of weak and stationary solutions to the CHB model for tumour growth with singular potentials, specifically the double obstacle potential and the logarithmic potential, which ensures that the phase field variable stays in the physically relevant interval. New difficulties arise from the interplay between the singular potentials and the solution-dependent source terms, but can be overcome with several key estimates for the approximations of the singular potentials, which maybe of independent interest. As a consequence, included in our analysis is an existence result for a Darcy variant, and our work serves to generalise recent results on weak and stationary solutions to the Cahn–Hilliard inpainting model with singular potentials.

Finite element implementation of coupled temperature-rate dependent fracture using the phase field model

In this paper, a coupled temperature-rate dependent fracture of ductile materials is investigated using a phase field model. The model is proposed based on the strain energy of materials, in contrast to a stress-based method, to follow paths of possible cracks through adding a scalar field as the phase field variable. In this case, two sets of governing equations account in the displacement and phase field are obtained, then solved using the nonlinear finite element method and an implicit integration algorithm. The model is implemented in ABAQUS commercial finite element code through a user element subroutine (UEL). The effects of temperature and loading rate on the initiation and propagation of cracks for ductile solids are examined based on the von Mises criterion. Besides, the threshold parameter as a function of temperature is presented to control the effect of plastic deformation on cracks initiation and propagation for different temperature levels. Moreover, new equations are suggested for the initiation and propagation of cracks as a function of temperature when temperature levels are lower and higher than the ductile-brittle transition temperature. Finally, the influence of loading rates and temperatures on the responses of different specimens is evaluated using the proposed model. The comparison of obtained results with other experimental and numerical data confirms the accuracy of the suggested model. The results also verify that the new model can simulate the influence of lower temperatures on cracks’ paths in which the change of the fracture phase from ductile to brittle occurs.

A multi phase-field fracture model for long fiber reinforced composites based on the Puck theory of failure

Phase-Field (PF) methods of fracture have emerged as powerful modeling tools for triggering fracture events in solids. These numerical techniques efficiently alleviate mesh dependent pathologies and are very suitable for characterizing brittle as well as quasi-brittle fracture in a wide range of engineering materials and structures including fiber reinforced composites. In this work, a multi phase-field model relying on the Puck’s failure theory is proposed for triggering intra-laminar cracking in long fiber reinforced composites. The current formulation encompasses the differentiation of fiber and inter-fiber (matrix-dominated) failure phenomena via the consideration of two independent phase-field damage-like variables, and the corresponding evolution equations and length scales. Moreover, for matrix-dominated deformation states, the present formulations endow the incorporation of plastic effects via an invariant-based plasticity model. Special attention is also devoted to its finite element implementation, which is conducted using the user-defined capabilities UMAT and UEL of ABAQUS, in conjunction with the thorough assessment of its thermodynamic consistency. Several representative applications pinpoint the applicability of the proposed computational tool.