Effect of the porosity on the fracture surface roughness of sintered materials: From anisotropic to isotropic self-affine scaling

To unravel how the microstructure affects the fracture surface roughness in heterogeneous brittle solids like rocks or ceramics, we characterized the roughness statistics of post-mortem fracture surfaces in home-made materials of adjustable microstructure length-scale and porosity, obtained by sintering monodisperse polystyrene beads. Beyond the characteristic size of disorder, the roughness profiles are found to exhibit self-affine scaling features evolving with porosity. Starting from a null value and increasing the porosity, we quantitatively modify the self-affine scaling properties from anisotropic (at low porosity) to isotropic (for porosity larger than 10 %).Comment: 10 pages, 10 figures, Physical Review E in Jan 2015, Vol. 91 Issue

Fissures de rétraction en forme d'étoiles: approches théorique par minimisation d'énergie et expérimentale par séchage de suspensions colliïdales

Les colonnes basaltiques, les polygones martiens, les réseaux de fractures dans les peintures, les septarias sont des patterns contenant des fissures en forme d'étoile, formés par la contraction frustrée du milieu due à un processus de séchage ou de refroidissement. Je présenterai des expériences de séchage de suspensions colloïdales permettant d'obtenir des fissures en forme d'étoile avec un nombre variable de branches. Je montrerai que ce nombre est régi par le principe de minimisation d'énergie de la mécanique linéaire élastique de la rupture. Pour les thèmes, je pense à S14 ou C6

Propagation des fronts de fissure plane dans les matériaux fragiles hétérogènes de dimensions finies

Notre approche vise à tester les modèles classiques de piègage de fissure pour des motifs de ténacité contrôlée et des géométries d'éprouvette réalistes, c'est-à-dire de tailles finies. Nous modélisons l'effet de l'épaisseur du matériau. Les prédictions du modèle théorique sont validées par des calculs de type éléments finis qui convergent vers les deux régimes asymptotiques: milieu semi-infini et géométrie plaque. Dans le cas d'une fissure interagissant avec un défaut unique, nous comparons les résultats expérimentaux avec les calculs par éléments finis sans paramètre ajustable

Fracture spacing in tensile brittle layers adhering to a rigid substrate

A natural question arising when observing crack networks in brittle layers such as, e.g., paints, muds, skins, pottery glazes, coatings, ceramics, is what determines the distance between cracks. This apparently simple question received a wealth of more or less complex and appropriate answers, but no consensus has emerged. Here, we show that the cracks interact mutually as soon as the spacing between them is smaller than ten times the thickness of the layer. Then, a simple Griffith-type balance between the elastic deformation energy and the fracture bulk and debonding costs captures a broad number of observations, going from the square-root or linear increase of the spacing with the thickness, to its decrease with loading until saturation. The adhesion strength is identified as playing a key role in these behaviour changes. As illustration, we show how the model can be applied to study the influence of the layer thickness on crack patterns. We believe that the versatility of the approach should permit wide applicability, from geosciences to engineering

Size effects in the toughening of brittle materials by heterogeneities: A non-linear analysis of front deformations

International audienceTraditional computational approaches in simulating crack propagation in perfectly brittle materials rely on the estimate of stress intensity factors along the rupture front. This proves highly challenging in 3D when the crack geometry departs from very specific cases for which analytical solutions are available, like e.g. the penny-shaped crack geometry. Here, we extend the first-order theory of Gao and Rice (1987), and predict the distribution of the mode I stress intensity factor KI along the front of a tensile coplanar crack that is slightly perturbed from a reference penny-shaped configuration, up to second order in the perturbation amplitude. Our theory is validated against analytical solutions available for embedded elliptical cracks, and its range of validity is further assessed using numerical simulations performed on cosine front perturbations of varying mode and amplitude. It is then used to develop a homogenization framework for the toughness of weakly disordered media. The effective toughness and its fluctuations are bridged quantitatively to the intensity of the toughness fluctuations and their spatial structure. Our theoretical predictions are compared to the results of ~1 million simulations of crack propagation building on our second-order theory and Fast Fourier Transforms. We show that the impact of toughness heterogeneities is size-dependent, as they generally weaken the material when the crack size is lower or comparable to the typical heterogeneity size, but reinforces it otherwise. It results in an apparent R-curve behavior of the brittle composite at the macroscale

Phase-field simulation and coupled criterion link echelon cracks to internal length in antiplane shear

International audienceThis paper provides a comprehensive numerical analysis of daughter crack localization in pure antiplane shear. Although antiplane shear fracture is important in various industrial applications, understanding the morphology of the resulting fragmentation remains challenging. The paper develops innovative phase-field models to induce the facets using a small spatial variation in the toughness field and examines the impact of numerical and material parameters on the newly formed daughter cracks’ shape and spacing. Through meticulous comparison to the coupled criterion, the paper reveals a compelling connection between the internal length-scale of damage regularization, Irwin’s length and the facet crack spacing. Furthermore, the effect of Poisson’s ratio on the crack form and spacing is investigated: the results reveal a significant influence and showcase comparable initiation distances between the numerical simulations and experimental measurements in pure antiplane loading

Effect of the deposition direction on fracture propagation in a Duplex Stainless Steel manufactured by Directed Energy Deposition

International audienceDense volumes of duplex stainless steel are manufactured by directed energy deposition. Compact tension specimens are machined from these volumes in order to evaluate the fracture toughness in two directions : parallel or perpendicular to the deposited layers. Different values are measured in the two cases. In order to understand this anisotropy, additional analyzes are performed on the cracked specimens post-mortem. A classical metallography analysis reveals the highly oriented structure of the material, as well as phase localization. The study of the fracture surface reveals several points. At the macroscale, while the crack surfaces are flat in the parallel case, pronounced shear lips cover half of the fracture surface in the perpendicular case. At the microscale, fracture is ruled by microvoid coalescence. The mesoscale, which is inherited from the deposition strategy, is found to pilot the crack growth. The border between the primary solidified melt pools and the heat-affected zones, which corresponds to the interface between the deposited layers, is the preferred area for crack growth. Analyzing the crack surface roughness confirms the dominance of the mesoscale, as its characteristic lengthscale is retrieved. This explains the differences observed for the two tested directions of fracture: in the parallel case, the crack is aligned with the weak interfaces between layers, which channel the crack growth; in the orthogonal one, out-of-plane excursion of the crack becomes possible allowing the crack to follow a tortuous three-dimensional path that results in a higher toughness than in the parallel situation

Path differences between quasistatic and fatigue cracks in anisotropic media

International audienceThe propagation path of quasistatic cracks under monotonic loading is known to be stronglyinfluenced by the anisotropy of the fracture energy in crystalline solids or engineered materials witha regular microstructure. Such cracks generally follow directions close to minima of the fractureenergy. Here we demonstrate both experimentally and computationally that fatigue cracks undercyclic loading follow dramatically different paths that are predominantly dictated by the symmetryof the loading with the microstructure playing a negligible or subdominant role

Fracture spacing in tensile brittle layers adhering to a rigid substrate

A natural question arising when observing crack networks in brittle layers such as, e.g., paints, muds, skins, pottery glazes, coatings, ceramics, is what determines the distance between cracks. This apparently simple question received a wealth of more or less complex and appropriate answers, but no consensus has emerged. Here, we show that the cracks interact mutually as soon as the spacing between them is smaller than ten times the thickness of the layer. Then, a simple Griffith-type balance between the elastic deformation energy and the fracture bulk and debonding costs captures a broad number of observations, going from the square-root or linear increase of the spacing with the thickness, to its decrease with loading until saturation. The adhesion strength is identified as playing a key role in these behaviour changes. As illustration, we show how the model can be applied to study the influence of the layer thickness on crack patterns. We believe that the versatility of the approach should permit wide applicability, from geosciences to engineering