Material independent crack arrest statistics

The propagation of (planar) cracks in a heterogeneous brittle material characterized by a random field of toughness is considered, taking into account explicitly the effect of the crack front roughness on the local stress intensity factor. In the so-called strong-pinning regime, the onset of crack propagation appears to map onto a second-order phase transition characterized by universal critical exponents which are independent of the local characteristics of the medium. Propagation over large distances can be described by using a simple one-dimensional description, with a correlation length and an effective macroscopic toughness distribution that scale in a non-trivial fashion with the crack front length. As an application of the above concepts, the arrest of indentation cracks is addressed, and the analytical expression for the statistical distribution of the crack radius at arrest is derived. The analysis of indentation crack radii on alumina is shown to obey the predicted algebraic expression for the radius distribution and its dependence on the indentation load

On probabilistic aspects in the dynamic degradation of ductile materials

Dynamic loadings produce high stress waves leading to the spallation of ductile materials such as aluminum, copper, magnesium or tantalum. The main mechanism used herein to explain the change of the number of cavities with the stress rate is nucleation inhibition, as induced by the growth of already nucleated cavities. The dependence of the spall strength and critical time with the loading rate is investigated in the framework of a probabilistic model. The present approach, which explains previous experimental findings on the strain-rate dependence of the spall strength, is applied to analyze experimental data on tantalum.Comment: 28 pages, 13 figures, 3 table

La dynamique fait son cinéma : De l'apport de l'imagerie et des mesures de champs cinématiques pour l'analyse du comportement dynamique des matériaux

National audienceDepuis de nombreuses décennies, l'imagerie rapide a permis d'observer des phénomènes se produisant sur des échelles de temps très petites (de l'ordre de la milliseconde voire de la microseconde). Avec l'avènement plus récent des caméras numériques, de nouvelles applications sont possibles (p.ex. la tomographie rapide). L'utilisation quantitative d'images est également possible, notamment grâce aux techniques de corrélation et de stéréocorrélation d'images. Différentes applications seront présentées afin d'illustrer les apports pour l'analyse du comportement mécanique des matériaux sous sollicitations dynamiques

Full-field measurements and identification in solid mechanics

This timely book presents cutting-edge developments by experts in the field on the rapidly developing and scientifically challenging area of full-field measurement techniques used in solid mechanics - including photoelasticity, grid methods, deflectometry, holography, speckle interferometry and digital image correlation. The evaluation of strains and the use of the measurements in subsequent parameter identification techniques to determine material properties are also presented. Since parametric identification techniques require a close coupling of theoretical models and experimental measurements, the book focuses on specific modeling approaches that include finite element model updating, the equilibrium gap method, constitutive equation gap method, virtual field method and reciprocity gap method. In the latter part of the book, the authors discuss two particular applications of selected methods that are of special interest to many investigators: the analysis of localized phenomenon and connections between microstructure and constitutive laws. The final chapter highlights infrared measurements and their use in the mechanics of materials. Written and edited by knowledgeable scientists, experts in their fields, this book will be a valuable resource for all students, faculties and scientists seeking to expand their understanding of an important, growing research area

Nouveau dispositif expérimental pour la réalisation d'essais de fatigue thermique à grand nombre de cycles

National audienc

Dialogue essai-calcul pour le pilotage en fissuration contrôlée du béton

La description de la zone de préfissuration, ainsi que la prévision du trajet de fissuration sont un enjeu important, à la fois industriel (estimation des fuites d’une enceinte nucléaire en cas d’accident par exemple), et académique (obtention de modèles d’endommagement et/ou de fissuration simples mais pertinents). Afin de valider expérimentalement un de ces modèles, on développe un essai hybride de fissuration du béton piloté en facteur d’intensité des contraintes (FIC) dans le cadre de ce projet. On se base sur les essais de Nooru-Mohamed [1] réalisés sur des éprouvettes de béton à double entaille sollicitées en mode mixte de fissuration. Le chargement mécanique est appliqué par un hexapode [2].L’état de fissuration est analysé par corrélation d’images numériques (CIN) [3]. Le but est de résoudre un problème inverse qui consiste à définir les conditions aux limites qui imposeront le trajet de fissuration voulu. Cette communication présente les résultats numériques permettant de valider le principe de l’essai hybride proposé. L’état « réel » de l’éprouvette est calculé en utilisant un modèle d’endommagement régularisé par gradient de variable interne [4], tandis que les facteurs d’intensité des contraintes (FIC) nécessaires au pilotage sont calculés par la mécanique élastique linéaire de la rupture (MELR). Une fois que la fissure avance, les nouvelles données (issues de la CIN dans l’essai réel) sont introduites dans un modèle MELR et la fissure est réorientée par modification des conditions aux limites. Pour le pilotage d’essais réels on a deux possibilités : (1) l’évaluation de la position de la pointe de fissure par CIN [3], l’évaluation des FIC et des nouvelles conditions aux limites par la MELR sur un maillage avec une fissure insérée ; ou (2) une évaluation directe des FIC par CIN, le pilotage nécessitant alors de seulement connaître la relation FIC/conditions aux limites donnée par la MELR. [1] M.B. Nooru-Mohamed, PhD thesis, Technische Universteit Delft , 1992. [2] M. Nierenberger, et al., M. Exp. Tech., 2012. [3] S. Roux, et al.. J. Phys. D: Appl. Phys., 42, 214004, 2009. [4] E. Lorentz, S. Andrieux, Int. J. Plast. 15, 119-138 , 1999

Study of Frictional Effects of Granite Subjected to Quasi-Static Contact Loading

The rock fragmentation response to drilling, in particular percussive drilling, is important in order to improve the efficiency of such an operation. The resulting problem includes spherical contact between the drill bit and the material and therefore, a numerical analysis of frictional effects in quasi-static spherical indentation of Bohus granite is presented. The frictional coefficient between the indenter and the granite surface is accounted for in numerical simulations. A previously determined constitutive law is used for the purpose of numerical analyses. The latter consists of a Drucker-Prager plasticity model with variable dilation angle coupled with an anisotropic damage model. Since the tensile strength is random, Weibull statistics was considered. Using a frictionless contact model, the stress state of Bohus granite corresponding to the first material failure occurrence, observed in indentation experiments, was numerically determined. However, the frictional effects, which are of interest in this study, may lead to changes in the numerically established stress state and consequently the Weibull parameters should be recalibrated. The so-called Weibull stress decreases from 120 MPa for a frictionless contact to 75 MPa for frictional contact, and the Weibull modulus from 24 to 12. It is numerically observed that the predicted force-penetration response, using the new set of Weibull parameters, is not influenced by friction. Conversely, the predicted fracture pattern, in the case of frictional contact, is similar to the case of frictionless contact, but its size is somewhat larger. Last, a parametric study analyzing the dependence of the friction coefficient is carried out and no significant changes are detected. The novelty of the present findings concerns the fact that both an advanced damage description in combination with an advanced plasticity model, both implemented for finite element analyses, is used to analyze frictional effects at granite indentation

On the Use of Digital Volume Correlation for the Identification of the Crushing Behavior of Plaster

International audienc

Integrated digital image correlation applied to elasto-plastic identification in a biaxial experiment

International audienceThe identification of the parameters of several constitutive laws is performed with the integrated digital image correlation (IDIC) technique in a biaxial experiment for a cruciform specimen made of stainless steel. The sought material parameters are assessed with the contribution of both reaction forces (from load sensors) and displacement fields (measured via digital image correlation). For each constitutive law a global residual quantifying the model error is assessed