Microsoft Word - numero_61_art_11_3429.docx H. Mazighi et alii, Frattura ed Integrità Strutturale, 61 (2022) 154-175; DOI: 10.3221/IGF-ESIS.61.11 154 Hybrid phase-field modeling of multi-level concrete gravity dam notched cracks H. Mazighi, M.K. Mihoubi Mobilisation et Valorisation des Ressources en Eau (MVRE), Ecole Nationale Supérieure d’Hydraulique (ENSH), Blida, Algeria. h.mazighi@ensh.dz, https://orcid.org/0000-0001-5983-5473 mihkam@ensh.dz, https://orcid.org/0000-0002-7858-0127 D. Santillán Dept. de Ingeniería Civil: Hidráulica, Energía y Medio Ambiente, Universidad Politécnica de Madrid, Madrid, 28040, Spain david.santillan@upm.es, https://orcid.org/0000-0002-9749-0522 ABSTRACT. Phase-field models have become a powerful tool to simulate crack propagation. They regularize the fracture discontinuity and smooth the transition between the intact and the damaged regions. Based on the thermodynamic function and a diffusive field, they regularize the variational approach to fracture that generalizes Griffith’s theory for brittle fracture. Phase-field models are capable to simulate complex fracture patterns efficiently and straightforwardly. In this paper, we introduce a hybrid phase- field approach to simulate the crack propagation in laboratory-scale and life- scale structures. First, we apply our methodology to the three-point bending test on notched laboratory beams. Second, we simulate the fracture propagation in a life-size structure: the Koyna gravity dam. We account for the pressure load inside the fracture, and we study the effect of the position and number of initial fractures in the upstream face and the value of the Griffith critical energy release, on the fracture propagation under a flood event. The position of the fracture plays an important role in the final fracture pattern and crest displacements, whereas the value of the Griffith critical energy release alters the onset of the fracture propagation. We conclude that phase-field models are a promising computational tool that may be applied to real engineering problems. KEYWORDS. Concrete; Crack; Dam; Fluid pressure; Phase-field model. Citation: Mazighi, H., Mihoubi, M.K., Santillán, D., Hybrid phase-field modeling of multi-level concrete gravity dam notches crack, Frattura ed Integrità Strutturale, 61 (2022) 154-175. Received: 24.01.2022 Accepted: 21.03.2022 Online first: 30.04.2022 Published: 01.07.2022 Copyright: © 2022 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. https://youtu.be/TH9o8Mi3l0M H. Mazighi et alii, Frattura ed Integrità Strutturale, 61 (2022) 154-175; DOI: 10.3221/IGF-ESIS.61.11 155 INTRODUCTION oncrete is one of the most common building materials in the world, and society demands proper safety levels in those infrastructures made of concrete. Safety assessment tasks require predicting the behavior of the structures until their ruin [1,2]. Engineers demand numerical models to simulate the fracture process. In this line, the two most employed approaches aiming at the initiation, localization, and propagation of cracks are the continuous [3] and discontinuous models [4]. The fracture is considered a discontinuity in the material due to internal or external stresses. The two most widely used theories in fracture mechanics are the Linear Elastic Facture Mechanics (LEFM) [1,5] and Nonlinear Fracture Mechanics (NLFM) [6–9]. The latter theory adopts the nonlinear behaviors which take into account micro-cracks located near the crack tip, the so-called fracture process zone [10]. This theory is suitable for large structures i.e. dams, bridges, etc. Contrary to the NLFM, the LEFM approach is simpler and assumes linear elastic behavior of materials [11]. In parallel, Continuum Damage Mechanics (CDM) [12,13] is a robust concept to model the degradation of materials, leading to the localization of fractures [14,15]. Many models have been proposed to simulate damage in concrete based on CDM [16–25]. Significant research efforts on modeling cracks have been conducted using the Finite Element Method (FEM) [26– 34], and the Extended Finite Element Method (XFEM) [35–43]. Recently, a new continuous model has appeared acknowledged as phase-field [44–47]. It models the discontinuities as a diffusive process and interpolates between both the broken and unbroken regions, the crack is determined by a scalar variable that takes two distinct values (0 inside the crack and 1 away) [48,49]. The damaged regions are determined by a coupled system of Partial Differential Equations (PDEs) based on the energy minimization; thus, additional computations as stress intensity factors are not necessary to calculate the crack initiation and propagation [50]. However, the method requires a regularization parameter called length scale [51], recently validated with experiments [52]. The great advantage of the phase-field approach is the ability to simulate complex crack patterns, such as twisting, kriging, joining [53,54]. It provides excellent results for brittle fracture [49,54–57], ductile fracture [58–61], cohesive fracture [62–64], and other complex applications [50]. In this paper, we focus on the ability of phase-field models to simulate the propagation of multi-level notched cracks in large structures, such as dams, and study its influence on the behavior of the structure compared to the load exerted by the fluid pressure inside the fracture. A recent and robust hybrid formulation of continuum damage mechanics is adopted to solve the governing PDEs due to its low computational cost. Our model encompasses all results previously found by other researchers considering the most influential basic parameters and possible crack levels. The paper is organized as follows: first, we present the governing equation of our model; afterward, we validate our model by comparing our numerical results with reported laboratory experiments on the literature; thereafter, we simulate the fracture propagation in a live-size gravity dam. Finally, we draw some overall conclusions. GOVERNING EQUATIONS his section introduces the coupled mathematical equations for fracture propagation in quasi-brittle materials. Once presented with the geometry of the domain, we derive the PDEs that govern our problem. Geometry Fig. 1 shows the proposed geometry, where the domain Ω  ℜδ has dimension δ{1,2,3}. Ω is composed of two subdomains, the elastic ΩE, and the fracture ΩF. The boundary conditions applied on the subdomain ΩE can be time- dependent Dirichlet conditions on ∂DΩE or time-dependent Neumann conditions on ∂NΩE. We denote with f(x,t) the external traction force applied on ∂NΩE, b(x,t) is the body force, x is the position vector, and u(x,t)  ℜδ is the displacement field at time t. Constitutive relations We compute the crack propagation in quasi-brittle material through a quasi-static phase-field formulation, based on Griffith’s theory, an energy-based failure criterion propagation in brittle materials [1]. The crack propagates when the stored energy is higher than the fracture resistance of the material. C T H. Mazighi et alii, Frattura ed Integrità Strutturale, 61 (2022) 154-175; DOI: 10.3221/IGF-ESIS.61.11 156 Figure 1: Scheme of the domains of the problem. The crack propagation states the minimization of the total potential energy stored ψ: d e sψ= ψ + ψ - ψ (1) where ψd is the critical energy release during the fracture process, ψe is the elastic energy, and ψS are the external sources of energy due to body and surface loads. The critical energy release during the fracture propagation equals:   F F d c c lΩ Ω ψ =  G dS=  G γ dΩ (2) where Gc is the Griffith critical energy release rate for mode-I of fracture, and γl is the crack surface density per unit volume of the solid, given by Miehe et al. [48]:      2 20 l 0 l γ , = + 2l 2 (3) where l0 is the length scale parameter, and ϕ is the phase-field which satisfies the following condition:      0,   if the material is intact     x,t =    1,   if the material is cracked (4) Therefore, ψd equals:   F 2 2d 0 c 0Ω l ψ =  G ( + )dΩ 2l 2 (5) Borden et al. and Zhang et al. [65,66] proposed an analytical solution for computing the length scale parameter, l0, which depends on the resistance tensile strength, ft, the Young’s Modulus, E, and the Griffith critical energy release rate, Gc, as follows: c 0 2 t 27EG l = 256f (6) H. Mazighi et alii, Frattura ed Integrità Strutturale, 61 (2022) 154-175; DOI: 10.3221/IGF-ESIS.61.11 157 In Eqn. (5), the critical energy release can be calculated through [67]:       2 21c c 2 1c K 1-ν ,             plane strain EG =   K ,                       plane stress E (7) where K1c is the fracture toughness and υ is the Poisson’s ratio. The strain tensor is decomposed into two parts [68]:  δ ± a± a a a=1 ε = ε n n (8) where ε+ and ε- are the tensile and compressive strain tensors, respectively, εa is the ath principal strain, and na is the principal direction of strain tensor εa. The elastic energies are expressed as:  ± 2 2 ε ± ± λ ψ ε = tr(ε) +μtr(ε ) 2 (9) where λ and μ are Lamé constants, given by:      νE E λ=     ,   μ=            plane strain (1+ν)(1-2ν) 2(1+ν) E(1+2ν)νE λ=     ,   μ=        plane stress (1+ν)(1-ν) 3(1+ν)(1-ν) (10) The strain tensor is related to the displacement field u by:   T1 ε= ( u+ u ) 2 (11) We adopt the anisotropic formulation proposed by Miehe et al. [68], which states that fractures only propagate under tension, i.e., due to the positive part of the elastic energy. The total elastic energy is then expressed as:        ± + - ε ε εψ ε =g  ψ ε +ψ ε (12) where g(ϕ) = (1-k)(1-ϕ2) + k is the degradation function, and k (0 < k << 1) is a parameter that avoids numerical singularities. The external energy functional, ψS accounts for the body forces and the applied loads on the boundaries as follows:    E s Ω Ω ψ =  f.u dΩ+ t.u d   (13) The fluid pressure inside the fracture, pf, exerts a force on the surface of the fracture. We include this load in the term of traction vector force as follows:         E N N F fΩ Ω Ω t.u d = t.u d -  p n.u d (14) We develop the second term in Eqn. (14) using the Divergence Theorem as follows: H. Mazighi et alii, Frattura ed Integrità Strutturale, 61 (2022) 154-175; DOI: 10.3221/IGF-ESIS.61.11 158        N F E N E f f fΩ Ω Ω p n.u d = .(p u) dΩ- p n.u d    (15) The pressure load on the surface of the fracture is then formulated as a body force applied to the entire domain. To consider the crack propagation, we introduce the phase-field in Eqn. (15):            E N E N f f f fΩ Ω Ω Ω .(p u) dΩ- p n.u d = g( ) .(p u) dΩ- g( )p n.u d (16) Thus, the external energy ψS becomes:             N N s f fΩ Ω Ω Ω ψ =  f.u dΩ- g( ) .(p u) dΩ+ g p n.u d + t.u d   (17) By substituting Eqns. (5), (12), and (17) into Eqn. (1), the Fréchet derivative of the total potential energy ψ yields the following Euler equations [65]:               2 2 +c 0 ε f f 0 G -l =2 1-k 1- ψ ε +p .u+u. p l (18) and:    fσ+p g +f=0 (19) where the Cauchy stress tensor is given by:         ++ - σ=g λtr ε I+2με +λtr ε I+2μ (20) The regime flow inside the porous media material is based on Darcy’s law, which is derived from the Navier-Stokes equation. It describes a linear relationship between the velocity v (m/s) and the gradient of pressure pf (Pa). Lomiz [69] and Louis [70] carried out that the penetration of the fluid through a rock fracture follows the Cubic law developed using the parallel- plates approach [71]. fp v    k (21) with  the dynamic viscosity (Pa.s) and k (m2) the permeability of the porous medium of the fluid respectively. Darcy’s law is only valid for low velocities when the regime is laminar [69,70]. We account for the irreversibility of the crack propagation process by adopting the following strain-history field H+(u, pf, t) [72]:        + + f ε f f x 0,t H u,p ,t =max ψ ε +p .u+u. p  (22) Eqn. (18) can then be rewritten as:        2 2 +c 0 0 G -l =2 1-k 1- H l (23) Thereby, the Euler-Lagrangian equation becomes: H. Mazighi et alii, Frattura ed Integrità Strutturale, 61 (2022) 154-175; DOI: 10.3221/IGF-ESIS.61.11 159                     f + + 0 2 2 0 0 c c σ+p g +f=0 2l 1-k H 2l 1-k H +1 -l = G G (24) Eqn. (24) are the strong form of the phase-field problem, subject to the following Neumann boundary conditions:         f N N σ.n= t+g d p                         in  Ω .n=0                                in  Ω (25) Eqn. (24) summarizes the nonlinear phase-field problem. The equations include the decomposition of the Cauchy stress tensor into its compression and tension parts. Since we assume fractures only propagate under tension, and not due to compression, the degradation function only affects the positive elastic energy. From a mathematical point of view, this assumption leads to a highly nonlinear and computationally expensive system of PDEs. Recently, these drawbacks have been sorted out with the so-called hybrid formulation [73]. The formulation linearizes the problem with a third constraint as follows:                          f + + 0 2 2 0 0 c c + - ε ε σ+p g +f=0 2l 1-k H 2l 1-k H +1 -l = G G x:ψ <ψ : 0 (26) In quasi-static calculation, the fractures propagate under a toughness-dominated regime. The energy expended during the fracture process is much higher than the viscous dissipation [74], i.e., the energy dissipated due to the water flow within the fracture. This assumption involves that the water pressure inside the crack is more or less constant and equal to the hydrostatic pressure at the crack level [50]. Consequently, the term u.∇pf in Eqn. (22) is null. We solve the system of PDEs given by Eqn. (26) with the staggered scheme proposed by Miehe et al. [72], successfully used in engineering problems [50,75]. The displacement field, u, the phase-field, ϕ, and the strain-history field, H+, are solved sequentially, as shown in Fig. 2. This approach requires small loading increments, i.e., small-time steps [50]. We adopt an implicit Backward Differentiation Formula [76] for the time integration. At the beginning of the j+1 time step, we take as the initial condition, the solution of the previous time step, j. The displacement field is first computed using H+,j, and ϕj. Afterward, the stain-history field is updated with uj+1 and ϕj. Finally, the phase-field is updated. In each time step, a minimum tolerance convergence is required. MODEL VALIDATION he purpose of the study proposed in this section is to validate the numerical model presented previously. The validation consists to compare the obtained results from the application of our numerical model on a benchmark widely used in fracture mechanics, which is the notched beam problem, where experimental data are available. Notched concrete beam In this example, we simulate the propagation of a fracture in the notched beam problem [77]. The geometry is depicted in Fig. 3(a), according to Meschke et al. [78]. The thickness of the beam is 127 mm, the height is 254 mm, the length is 1118 mm, and the span is 1016 mm. The mechanical properties are: E = 4.36 x104 MPa, υ = 0.2, ft = 4.0 MPa, and Gc=119 N/m. We simulate the propagation of an initial vertical fracture parallel to the side of the beam. The fracture is 78 mm in length and 5 mm in width and is located in the bottom part of the beam. We apply an incremental vertical displacement u on the top central part of the beam, and we neglect the self-weight. We adopt as length scale parameter l0 = 10 mm. T H. Mazighi et alii, Frattura ed Integrità Strutturale, 61 (2022) 154-175; DOI: 10.3221/IGF-ESIS.61.11 160 Figure 2: Staged scheme for the numerical resolution of the PDE system. We simulate the fracture propagation with a 2-dimensional model under plane strain conditions. We discretize the domain around the fracture with 8272 quadrilateral elements and the rest of the beam with 6427 triangular elements. The mesh size around the fracture is h0 = 1.0 mm = l0/10, small enough to accurately capture the transition from damaged to undamaged regions. We plot the fracture pattern in Fig. 3(b), where the red color represents the fracture, and the evolution of the applied force against the Crack Mouth Opening Distance (CMOD) in Fig. 3(c). We compare our simulations with those reported by Meschke et al. and Mandal et al. [78,79], and experimental results. Our simulations are quite similar to both reported results as well as with laboratory experiments. The applied load increases linearly until the critical load is reached. Afterward, the crack grows quickly, and the CMOD increases. 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