No abstract

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 paper presents a method to identify material parameters of a Hyperelasto-Visco-Hysteresis (HVH) model and its application for the simulation of a fluoroelastomer behaviour. This 3D-phenomenological model is based on the additive decomposition of three stress components. Each of these constitutive stresses is related to a physical phenomenon that occurs during mechanical loading: a hyperelastic equilibrium stress response, an irreversible pure hysteresis stress contribution and a rate-dependent viscoelastic stress behaviour.In order to independently identify these parts of the model, an experimental campaign, including multi-step relaxation in traction and compression tests and simple relaxation in tension and compression tests, is used. The hysteretic and hyperelastic contributions are identified considering only the state at the end of the relaxation periods of the multi-step relaxation tests. The viscoelastic response is analytically calculated with the simple relaxation test. As an advantage, the developed identification scheme gives the possibility to discriminate all the stress components of the model. Finally, the numerical simulation of a seal in relaxation is carried out to verify the capability of the proposed HVH model by reproducing the mechanical response of the studied material.

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.