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International audience

International audience

International audience

International audience; Predicting the transition from cavity growth to coalescence is important for understanding and simulating ductile failure. Many models have been published and are generally verified by finite element calculations on structures consisting of a periodic network of single defect cells (e. g. [1]). While this approachis appropriate for low porosity, it may underestimate the interaction between defects at higher porosity, in particular by constraining the orientation of the location bands. This study therefore aims to study the transition between growth and coalescence by considering the plasticity behaviour of a cell with a random population of defects. Similar to the numerical homogenization in Fritzen’s boundary analysis ([2]), a population of spherical defects of the same radius is randomly generated within an elasto-plastic matrix (with and without strain hardening). Its behaviour is then studied by finite elements in large deformations for conditions with periodic limits, at different levels of imposed triaxiality. This study is repeated on different defect population draws, in order to be able to statistically analyze the behavior until coalescence. A criterion for the beginning of coalescence may then be proposed.

A reliable determination of the onset of void coalescence is critical to the modelling of ductile fracture. Numerical models have been developed but rely mostly on analyses on single defect cells, thus underestimating the interaction between voids. This study aims to provide the first extensive analysis of the response of microstructures with random distributions of voids to various loading conditions and to characterize the dispersion of the results as a consequence of the randomness of the void distribution. Cells embedding a random distribution of identical spherical voids are generated within an elastoplastic matrix and subjected to a macroscopic loading with constant stress triaxiality and Lode parameter under periodic boundary conditions in finite element simulations. The failure of the cell is determined by a new indicator based on the loss of full rankedness on the average deformation gradient rate. It is shown that the strain field developing in random microstructures and the one in unit cells feature different dependencies on the Lode parameter L owing to different failure modes. Depending on L, the cell may fail in extension (coalescence) or in shear. Moreover the random void populations lead to a significant dispersion of failure strain, which is present even in simulations with high numbers of voids.

International audience; La prévision de la transition entre croissance et coalescence dans le cadre de la rupture ductile reste unproblème ouvert. Ce travail se propose d’étudier cette transition en considérant explicitement les interactions d’une populations de défauts sphériques répartis aléatoirement dans une matrice élastique - parfaitement plastique. Le comportement d’un volume élémentaire représentatif est calculé par éléments finis en grandesdéformations. Le présent article montre que le modèle de VER proposé permet de représenter correctement l’apparition de la coalescence. Cette méthodologie permet d’étendre la stratégie d’homogénéisation numérique proposée par Fritzen [2012] à la rupture ductile en grandes déformations.

International audience; MCrAlY coatings are used as a protection layer against high temperature oxidation and corrosion for components working in an aggressive environment. In order to predict both coating and substrate lifetime, the characterization of thermomechanical properties of the coating is a challenging issue [1-3]. As these coatings are only a few hundreds of micrometers thick, their properties should be investigated with experimental tests adapted at this small scale [1-4]. In the present study, the mechanical properties of a free-standing MCrAlY coating and their evolution were scrutinized when it is aged in air for a large range of time and temperature. Free-standing micro-tensile specimens were prepared [1] and tested after different time/temperature ageings [4]. Based on digital image correlation to derive the strain evolution during loading, the stress-strain behavior has been identified. Thus a crucial effect of intrusive oxidation from deposition process defects and surface oxidation has been evidenced to drive both the ductility and the fracture mechanism of the tested material. For coarse intrusive oxidation, a loss of ductility has been identified, whereas metallic phase transformation (from  to ) increases the ductility of the material from 0.5 up to 3%. This composite effect will be analyzed through systematic microstructure and homogenization analyses. This methodology could be straightforward to determine the coupling between coating cracking and substrate failure under thermo-mechanical fatigue conditions [5].References[1] D. Texier, D. Monceau, J. C. Salabura, R. Mainguy, E. Andrieu, Materials at High Temperatures, 33 (2016) 325–337.[2] D. Texier, D. Monceau, Z. Hervier, E. Andrieu, Surface and Coatings Technology, 307 (2017) 81–90 [3] Alam, M.Z., Satyanarayana, D.V.V., Chatterjee, D., Sarkar, R., Das, D. K., Materials Science and Engineering: A, 641 (2015) 84-95.[4] Straub, T., Baumert, E.K., Eberl, C., Pierron, O.N., Thin Solid Films, 526 (2012) 176–182.[5] Esin, V.A., Maurel, V., Breton, P., Köster, A., Selezneff, S., Acta Materialia, 105 (2016) 505–518.