Crack propagation in honeycomb cellular materials: a computational approach

Computational models based on the finite element method and linear or nonlinear fracture mechanics are herein proposed to study the mechanical response of functionally designed cellular components. It is demonstrated that, via a suitable tailoring of the properties of interfaces present in the meso- and micro-structures, the tensile strength can be substantially increased as compared to that of a standard polycrystalline material. Moreover, numerical examples regarding the structural response of these components when subjected to loading conditions typical of cutting operations are provided. As a general trend, the occurrence of tortuous crack paths is highly favorable: stable crack propagation can be achieved in case of critical crack growth, whereas an increased fatigue life can be obtained for a sub-critical crack propagation

From fracture to fragmentation: discrete element modeling -- Complexity of crackling noise and fragmentation phenomena revealed by discrete element simulations

Discrete element modelling (DEM) is one of the most efficient computational approaches to the fracture processes of heterogeneous materials on mesoscopic scales. From the dynamics of single crack propagation through the statistics of crack ensembles to the rapid fragmentation of materials DEM had a substantial contribution to our understanding over the past decades. Recently, the combination of DEM with other simulation techniques like Finite Element Modelling further extended the field of applicability. In this paper we briefly review the motivations and basic idea behind the DEM approach to cohesive particulate matter and then we give an overview of on-going developments and applications of the method focusing on two fields where recent success has been achieved. We discuss current challenges of this rapidly evolving field and outline possible future perspectives and debates

Dynamic fracture of icosahedral model quasicrystals: A molecular dynamics study

Ebert et al. [Phys. Rev. Lett. 77, 3827 (1996)] have fractured icosahedral Al-Mn-Pd single crystals in ultrahigh vacuum and have investigated the cleavage planes in-situ by scanning tunneling microscopy (STM). Globular patterns in the STM-images were interpreted as clusters of atoms. These are significant structural units of quasicrystals. The experiments of Ebert et al. imply that they are also stable physical entities, a property controversially discussed currently. For a clarification we performed the first large scale fracture simulations on three-dimensional complex binary systems. We studied the propagation of mode I cracks in an icosahedral model quasicrystal by molecular dynamics techniques at low temperature. In particular we examined how the shape of the cleavage plane is influenced by the clusters inherent in the model and how it depends on the plane structure. Brittle fracture with no indication of dislocation activity is observed. The crack surfaces are rough on the scale of the clusters, but exhibit constant average heights for orientations perpendicular to high symmetry axes. From detailed analyses of the fractured samples we conclude that both, the plane structure and the clusters, strongly influence dynamic fracture in quasicrystals and that the clusters therefore have to be regarded as physical entities.Comment: 10 pages, 12 figures, for associated avi files, see http://www.itap.physik.uni-stuttgart.de/~frohmut/MOVIES/emitted_soundwaves.avi and http://www.itap.physik.uni-stuttgart.de/~frohmut/MOVIES/dynamic_fracture.av

Fracture of solar-grade anisotropic polycrystalline Silicon: A combined phase field–cohesive zone model approach

Artículo Open Access en el sitio web del editor. Pago por publicar en abierto.This work presents a novel computational framework to simulate fracture events in brittle anisotropic polycrystalline materials at the microscopical level, with application to solar-grade polycrystalline Silicon. Quasi-static failure is modeled by combining the phase field approach of brittle fracture (for transgranular fracture) with the cohesive zone model for the grain boundaries (for intergranular fracture) through the generalization of the recent FE-based technique published in [M. Paggi, J. Reinoso, Comput. Methods Appl. Mech. Engrg., 31 (2017) 145–172] to deal with anisotropic polycrystalline microstructures. The proposed model, which accounts for any anisotropic constitutive tensor for the grains depending on their preferential orientation, as well as an orientation-dependent fracture toughness, allows to simulate intergranular and transgranular crack growths in an efficient manner, with or without initial defects. One of the advantages of the current variational method is the fact that complex crack patterns in such materials are triggered without any user-intervention, being possible to account for the competition between both dissipative phenomena. In addition, further aspects with regard to the model parameters identification are discussed in reference to solar cells images obtained from transmitted light source. A series of representative numerical simulations is carried out to highlight the interplay between the different types of fracture occurring in solar-grade polycrystalline Silicon, and to assess the role of anisotropy on the crack path and on the apparent tensile strength of the material.Unión Europea FP/2007–2013/ERC 306622Ministerio de Economía y Competitividad MAT2015–71036-P y MAT2015–71309-PJunta de Andalucía P11-TEP-7093 y P12-TEP- 105

Crack patterns in heterogenous rocks using a combined phase field-cohesive interface modeling approach: A numerical study

Rock fracture in geo-materials is a complex phenomenon due to its intrinsic characteristics and the potential external loading conditions. As a result, these materials can experience intricate fracture patterns endowing various cracking phenomena such as: Branching, coalescence, shielding, and amplification, among many others. In this article, we present a numerical investigation concerning the applicability of an original bulk-interface fracture simulation technique to trigger such phenomena within the context of the phase field approach for fracture. In particular, the prediction of failure patterns in heterogenous rock masses with brittle response is accomplished through the current methodology by combining the phase field approach for intact rock failure and the cohesive interface-like modeling approach for its application in joint fracture. Predictions from the present technique are first validated against Brazilian test results, which were developed using alternative phase field methods, and with respect to specimens subjected to different loading case and whose corresponding definitions are characterized by the presence of single and multiple flaws. Subsequently, the numerical study is extended to the analysis of heterogeneous rock masses including joints that separate different potential lithologies, leading to tortuous crack paths, which are observed in many practical situations.Ministerio de Economía y Competitividad MAT2015-71036-

A self-adaptive cohesive zone model for interfacial delamination

Interfacial failure in the form of delamination, often results in malfunction or failure of laminated structures. Different numerical approaches have been proposed for the simulation of this process. Due to the appealing feature of predicting both the delamination onset and its growth, cohesive zone models have been widely used to simulate delamination as a result of a gradual degradation of the adhesion between two materials when they become separated. Application of cohesive zone models for the modelling of delamination in brittle interfaces in a quasi-static finite element framework suffers froman intrinsic discretization sensitivity. A large number of interface elements are needed for the discretization of the process zone of a cohesive crack. Otherwise, a sudden release of energy in large cohesive zone elements results in a sequence of snap-through or snap-back points to appear in the global load-displacement response of the system which compromises the numerical efficiency. While computationally expensive path-following techniques can be used to follow the oscillatory path, the efficiency and robustness of brittle cohesive zone models can be significantly increased by reducing the oscillations observed in the global load-displacement behaviour without a further mesh refinement. In line with this purpose, the separation approximation in the process zone is enriched with an adaptive hierarchical extension. The linear separation approximation throughout the cohesive zone element is enriched with a bi-linear function, where the enrichment peak position and the magnitude of the enrichment are regarded as additional degrees of freedom obtained by minimization of the total potential of the global system. The mobility of the peak of the enrichment function within individual cohesive zone elements locally adapts the discretization to the physics governing the problem. Important numerical aspects of the proposed enrichment strategy such as its mobility and uniqueness have been thoroughly investigated while its limitations are addressed. The efficiency and robustness of the enrichment are shown through numerical examples which prove the general applicability of the methodology. In fact, application of the elaborated enrichment eliminates the need for a further mesh refinement while keeping the standard Newton-Raphson approach applicable in the case of a relatively coarse mesh which saves considerable computational costs. Extension of the proposed enrichment scheme to delamination in a threedimensional finite element framework has been carried out as well. Planar interix face elements have been enriched along all edges by bi-linear functions with mobile peaks. The effect of the proposed methodology on reducing discretization-induced oscillations is quantitatively evaluated. To deal with planar crack growth where the crack front is oblique with respect to element edges, a non-hierarchical enrichment strategy is also developed and its performance is compared with its hierarchical counterpart. The self-adaptive finite element formulation is extended to a framework suitable for large deformations and is applied to interfaces in microelectronics under realistic mixed-mode loading conditions. In particular, the material/interface systems used in miniaturized mixed-mode bending tests, which are conducted for a wide range of mode angles, are modelled to make a direct comparison with experimental results. The interface constitutive lawthat is used takes the dependence of fracture toughness on mode-mixity into account. Thereby, the enhanced cohesive zone model can be used for the simulation of the behaviour of brittle interfaces in an accurate, effective, and efficient manner