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The development of new generations of propellants with better energetic properties may be hampered by unsatisfactory mechanical behaviors in terms of strength and toughness. A micromechanical approach is adopted to provide a better understanding of the existing links between the constitutive phase behaviors and the local damage, and the macroscopic mechanical behavior of these materials. Three model materials have been made and tested in uniaxial tension. The stress-strain responses were recorded while monitoring their volume changes that quantify the macroscopic damage. A qualitative description of the local damage was obtained thanks to scanning electron microscopy images of samples under loading. The micromechanical approach consists in finite elements analyses on periodic microstructures of non-regular polyhedral particles embedded in a soft matrix. An original microstructure generation tool has been developed specifically in order to obtain highly filled isotropic microstructures. Debonding at the matrix/filler interface was taken into account with a cohesive-zone model (CZM). The impact of the CZM parameters is discussed, in an effort to make the link between the CZM parameters and how the local damage appears and develops, and between the cohesive behavior and the shape of the macroscopic stress-stretch responses of the heterogeneous materials. This work was supported by the ANR (France) under contract num-ber ANR-10-EQPX- 37, and the authors thank the Délégation Générale de l’Armement (DGA) and ArianeGroup, Vert-le-Petit (France), for their financial supports.

The impact of the loading history on the resistance to break of a carbon-black filled styrene butadiene rubber is explored experimentally. Carbon-black filled rubberlike materials soften significantly upon the first loading due to the well known Mullins effect. The impact of this effect on the critical energy release rate at break, G c , of the considered material is quantitatively estimated. For this purpose, the classical notched pure shear geometry is considered and the seminal global analysis from Rivlin and Thomas (1953) is adopted. Moreover, the same analysis is extended to non-elastic materials in order to account for the Mullins softening and define the critical energy release rate, G c ∗ , characterizing the creation of new crack surfaces without including the energy dissipated by Mullins softening. Both global quantities, G c and G c ∗ , appear decreasing with the increase of softening already undergone by the material, stressing the difficulty of proposing a predictive criterion for the material resistance to failure. Finally, thanks to the local measures of the strain fields on the free surface of the pure shear specimen just before the crack propagation, it has been possible to evaluate the amount of Mullins dissipation upon the crack propagation and to explore the possible existence of an intrinsic value, G 0 , characterizing the crack propagation independently of any other source of dissipation.

International audience; A rubberlike matrix highly filled with spherical micrometric glass beads is submitted to uniaxial tension tests until break. X-ray tomography imaging performed on the material while submitted to uniaxial tension reveals early debonding at the matrix/filler interfaces at the poles of the particles followed by void coalescence creating damage localization. The latter causes a downturn of the macroscopic stress-strain response. These phenomena are analyzed further with three-dimensional finite element simulations, where 64 spherical beads are distributed randomly in a periodic cell. A simple version of the Tvergaard-Hutchinson cohesive-zone model allows to reproduce all the experimental trends well. The effects of the three parameters involved are analyzed, and three different types of macroscopic behaviors are observed corresponding to three different microstructure damages. The value of the initial stiffness of the interface, limited by numerical convergence, has little effect on how the local damage evolves but has a significant impact on the overall macroscopic stress values. The local damage is strongly dependent on the critical strength and the separation failure displacement, and the adhesion energy may be considered as a resulting parameter of the two previous ones. The interfacial critical strength appears to have a significant impact on the damage initiation, either spread across the structure for low values, or localized for high values. Increasing the interface separation failure displacement delays the possible loss of adhesion to a higher strain and preserves the integrity of the composite material.

International audience; a b s t r a c t An experimental study of thermally or water-induced crack healing in an inactive borosil-icate glass, chemically analogous to that used in France for the vitrification of nuclear waste was carried out. Partial welding of glass plates was observed after annealing in air at 425 °C (77 °C below T g) when at least 20 MPa compressive stress was applied, while annealing at 450 °C under 20 MPa led to a complete disappearance of the interface. Closure of indenta-tion-induced cracks was observed during annealing at 400 °C in an ESEM as a result of viscous relaxation of residual stresses but it did not constitute a sufficient proof of crack healing. DCDC specimens were thus pre-cracked in an ESEM and then either annealed at various temperatures (350–490 °C) in secondary vacuum or in air, or left in water at 70–90 °C, sometimes under a compressive stress normal to the crack face. The specimens were then reloaded in the ESEM and the crack opening displacements under a given load were compared to those measured during pre-cracking. The cracks were bridged by an alteration layer over a distance from the crack tip which decreased as loading increased. The restraining effect of these bridges on crack opening was assessed via finite element simulations, using interface elements. The tensile strength of the bridging layer was estimated as 27–39 MPa after vacuum annealing at 400 °C, 11–20 MPa after 15 days in water at 90 °C and 44–78 MPa after 11 days in water at 70 °C under 5 MPa normal compression. Partially healed cracks did not resume propagation from their former crack tip, but due to branch cracks re-initiated a few hundred microns behind it which grow avoiding the healed area. This behaviour was explained using finite element simulations.

WOS; International audience; Polymer matrix composites, and especially short glass fibre reinforced polyamides, are widely used in the automotive industry. Their application on structural components requires a confident mechanical design taking into account the sensitivity of the mechanical response to both temperature T and relative humidity H. In this paper, the constitutive model already developed by the authors (Launay et al., 2011) is applied to describe the non-linear time-dependent behaviour of a PA66-GF35 under various hygrothermal conditions. The extensive experimental database involves testing conditions under and above the glass transition temperature Tg. An equivalence principle between temperature and relative humidity is applied and validated, since the non-linear mechanical response is shown to depend only on the temperature gap T-Tg(H).

International audience; Continuum damage mechanics is used to model the damage induced by a thermal shock to the R7T 7 glass, the French borosilicate glass used for nuclear waste vitrification. A finite element model of the thermal shock is developed in which the elastic constitutive equations are coupled with an anisotropic stress-based damage evolution law. The Weibull distributions of strength measured at various temperatures by biaxial flexural tests are used to identify the parameters of this damage evolution law. Vibration tests are conducted to identify the elastic properties of the glass and to determine the effect of thermal shockinduced damage on the glass residual stiffness. The residual stiffness predicted by the damage model agrees with that measured experimentally. The fractured surface in the glass after a thermal shock is estimated, assuming that all the elastic energy associated with the stresses that are higher than a pre-determined threshold is dissipated in the creation of new surfaces. Comparison between the predicted and experimentally-measured fractured surface is performed. The model is shown to capture the saturation of the crack network density for severe thermal shocks; whereas this is not the case if damage is not accounted for in the constitutive equations.

International audience; This paper focuses on a new analytical solution: the one of the hydrostatic-elastic problem for a wetting fluid inside a 3D penny shaped circular crack in an elastic infinite solid loaded in tension at infinity, when the vapor pressure and the variation of liquid mass can be neglected. With the surface tension phenomena, the liquid is near the crack tip and is generally in tension (negative pressure).

Over several decades, many contributions have been made to understand the physics and to develop models of aggregates of metals and composites. Three books `Modelling small deformations of polycrystals' (eds. J. Gittus and J. Zarka) and `Modelling large deformations of solids' (eds. J. Gittus, J. Zarka and S. Nemat-Nasser) and `New approach of inelastic analysis of structures' (eds. J. Zarka et al.) gave already a general view of the state of the art. Nemat-Nasser, particularly in his impressive book `Micromechanics: overall properties of heterogeneous materials', improved and is still greatly improving our knowledge in this fundamental area. In this paper, which is dedicated to his 60th anniversary, we want to give again and for the last time, our own view of the problem based on (may be) not well known works and also some unpublished theses of some students which were done between 1964 and now. In the first part, we shall underline what the physical quantitative description of inelastic behavior of single crystals means for us by answering the following questions: (1) are the various glides on each crystal slip plane, as introduced in the multiple plastic potential theory, potential internal variables? (no); (2) can we build a real physical model for the crystal in correlation with the observations of the evolution of the crystalline defects, such as the dislocations, the vacancies, the precipitates ... and the thermally activated processes? (yes); (3) can we define the physical hardening parameters (yes) and is the latent hardening higher than the self-hardening? (no); (4) is it possible to reach a description which fits the experimental results? (may be!). Then, in the second part, assuming a given behavior for the crystals, for the global inelastic behavior of polycrystals, we shall try to answer the questions: (5) how many different orientations of crystals are necessary to reach the description of an initially isotropic polycrystal? (three well selected ones!); (6) are the simplified models, such as the self-consistent model, sufficient to represent the interactions between crystals? (yes); (7) are the actual numerical simulations based on rate-dependant or rate-independent crystal plasticity, useful for the industrial applications? (no, unhappily!). Finally, in the third part, we shall give our new general framework of `intelligent' modelling of aggregates where we take into account not only the local behaviors of the crystals but also their size, their shape and their relative distribution. In this framework, it is needed: (i) To build a data base i.e. to obtain some experimental, real or simulated, results where the experts identify all variables or descriptors which may relevant to the given problem. This is, at first, done with some primitive descriptors x which are usually in a limited number. Then, the data are transformed into intelligent descriptors XX in a larger number, using the existing knowledge and theories which are still always insufficient. The descriptors may be numbers, boolean, alphanumeric, name of files which gives access to data bases, or treatments of curves, signals and images. The results or conclusions may be classes (good, not good, ...) or numbers (Young modulus, cost, weight, life time, ...). Usually, the data base may contain roughly 30 to 150 examples with 10 to 1000 descriptors and 1 to 20 conclusions. (ii) To generate the rules with any automatic learning tool. The intelligent descriptors help these learning algorithms. Each conclusion is explained as function or set of rules of some among the input intelligent descriptors with a known reliability or accuracy. If this reliability is too low, it implies that either there is not enough data or there are bad, missing descriptors or the problem was not well described. (iii) To optimize at two levels (inverse problems): (1) Considering the intelligent descriptors as independent; it is possible to get the optimal solution satisfying the special required properties and allowing the discovery of new mechanisms; (2) considering the intelligent descriptors linked to primitive descriptors, it is possible to obtain the optimal solution which is technologically realizable. So, not only a practical optimal solution is obtained, but also the experts may learn the missing parts, may build models or theories based only on the retained intelligent descriptors and guided by the structures of the rules or relationships. We shall illustrate our framework by treating step by step the problem of the global elastic behavior of concrete with a cement matrix and soft or hard inclusions. This problem has been the subject of several papers, providing simplified models, simple bounds of the elastic moduli and sophisticated theories ... were produced. Our aim is to give the results to the engineers in such a practical way that they could: (1) estimate the elastic properties within a few percent of error for any concentration, any shape, or distribution of the inclusions; (2) select the preparation of the aggregate to reach any pre-assigned elastic properties (even at the lowest cost or weight).