Microsoft Word - numero_39_art_1 J. Eliáš, Frattura ed Integrità Strutturale, 39 (2017) 1-6; DOI: 10.3221/IGF-ESIS.39.01 1 Focussed on Modelling in Mechanics On adaptive refinements in discrete probabilistic fracture models J. Eliáš Brno University of Technology, Faculty of Civil Engineering, Veveří 331/95, Brno, 60200, Czech Republic elias.j@fce.vutbr.cz ABSTRACT. The possibility to adaptively change discretization density is a well acknowledged and used feature of many continuum models. It is employed to save computational time and increase solution accuracy. Recently, adaptivity has been introduced also for discrete particle models. This contribution applies adaptive technique in probabilistic discrete modelling where material properties are varying in space according to a random field. The random field discretization is adaptively refined hand in hand with the model geometry. KEYWORDS. Adaptivity; Discrete model; Probability; Random field. Citation: Eliáš, J., On adaptive refinements in discrete probabilistic fracture models, Frattura ed Integrità Strutturale, 39 (2017) 1-6. Received: 11.07.2016 Accepted: 12.09.2016 Published: 01.01.2017 Copyright: © 2017 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. INTRODUCTION he adaptivity of model geometry has been originally developed for elastic problems [1,2] and later applied also in inelastic problems with localization [3,4]. The classical rigorous approach involves an error estimation, remeshing criterion, mesh re-generation and transfer of variables onto the new mesh. Recently, the adaptive concept was applied also in discrete modelling [5]. The goal of this work is to extend it for probabilistic discrete models. Discrete models represent the material via collection of interconnected rigid bodies organized into a net structure. There are several versions of discrete models developed and used for many purposes. In case of simulating fracture in concrete, the lattice models are often employed [6-8]. These models represent the concrete meso-structure by projecting it onto the independently generated lattice. They are excellent in describing fracture phenomena, but applicable only for small laboratory specimens due to their extreme computational demands. Another group of discrete meso-level modelling approaches, sometimes called particle models, generates the network geometry directly according to the meso-structure of concrete [9,10]; typically one node for each mineral aggregate. We focus here on the latter group with geometry generated via Voronoi tessellation [11-14]. Though some reduction of computational cost in particle models is achieved when compared to the lattice models, further reduction would be desirable. It can be done by adaptive construction of the discrete geometry as described in [5]. Availability of adaptive refinement allows starting simulation with coarse discretization and refining it adaptively during the simulation run only in areas where needed. T http://www.gruppofrattura.it/pdf/rivista/numero39/audio/1.mp3 J. Eliáš, Frattura ed Integrità Strutturale, 39 (2017) 1-6; DOI: 10.3221/IGF-ESIS.39.01 2 In some applications of fracture simulations, it might be important to consider additional material randomness (besides the one covered by the random location of nodes in the discrete model) usually represented by a random field [15-18]. An extension of the discrete model by fluctuation of material parameters according to a random field was developed in [18,19]. In this contribution, the adaptive concept is extended for such probabilistic discrete fracture models. PROBABILISTIC DISCRETE MODEL he model uses random geometry to avoid directional bias that occurs in any regular structure. Domain of the modeled body is filled with nuclei with randomly generated positions. These nuclei are added sequentially with restricted minimal distance lmin. The parameter lmin controls size of the discrete bodies and therefore it should correspond to the size of heterogeneities in the material. In concrete, this is typically a size of the mineral aggregates. Each of the nuclei will serve as one model node with associated six degrees of freedom, three translational and three rotational. The connectivity of the nodes is given by Delaunay triangulation. Dual diagram of Delaunay triangulation called Voronoi tessellation then creates geometry of the rigid bodies. Rigid bodies have common contact facets, which are perpendicular to their connections because of the Voronoi tessellation properties. There is a complex damage-mechanics based constitutive law used at the facets. Its deterministic version has been adapted from [9], where it is also described in detail. The main material parameters for fracture behavior are tensile strength, ft, and tensile fracture energy, GF. The probabilistic extension of the model is elucidated in [19,20]. Here, only brief description of the probabilistic part is given. Both the tensile strength and fracture energy in tension are assumed to be governed by single random field H with mean value 1 and probabilistic distribution with Gaussian core and Weibull left tail. The correlation structure of the random field is given by square exponential function with single parameter, lρ, called the correlation length. The strength and fracture energy of every model contact with centroid c are given by    f f Ht t c c (1)    G G H 2 F F    c c with X being the mean value of the material parameter X . The square in the equation for fracture energy is added to preserve constant material characteristic length [20]. In the adaptive model, new contacts are created after every refinement. Therefore, the random field values at the new contact centers must be generated after every refinement. This is effectively done using kriging. Initially, standard Gaussian random field realizations ( Ĥ ) are generated on points arranged in a regular grid with spacing lρ/4. Random field value at point  c is then estimated using the optimal linear estimation method [21]   K Tk k cg kk H 1 ˆ   c ψ C (2) and finally standard Gaussian field is transformed onto the Weibull-Gauss field ( H Hˆ  ) using isoprobabilistic transformation. Vector ξ collects realizations of K independent standard Gaussian variables, λ and ψ are K eigenvalues and eigenvectors of the grid covariance matrix and cgC is the covariance vector between the grid points and point c . ADAPTIVITY nly brief description of the adaptive concept in deterministic model is given. Deep elucidation is provided in [5]. The refinement criterion is intuitive. It is based on an average stress in the rigid bodies calculated using the fabric stress tensor. For rigid body associated with node i, the average stress components st   are T O J. Eliáš, Frattura ed Integrità Strutturale, 39 (2017) 1-6; DOI: 10.3221/IGF-ESIS.39.01 3    j j st s t j F c V 1   (3) where j runs over all nodes in contact with node i, F is a vector of contact force,  c is the centroid of the contact facet and V is a volume of the i-th rigid body. The Mazar's equivalent stress serves as measure of the stress level, σeq. Figure 1: Adaptive refinement of discretization in steps; a) schematic explanation; b)-g) application to a 2D model. In probabilistic model, the contacts have random strength. 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