Christophe Cluzel

Analysis of a Multiaxial Test on a C/C Composite by Using Digital Image Correlation and a Damage Model

The “planar” digital image correlation technique needs a single CCD camera to acquire the surface patterns of a zone of a specimen in the underformed and deformed states. With these two images, one can determine in-plane displacement and strain fields. The digital image correlation technique used herein is based on Fast Fourier Transforms, which are very effective in reducing the computation cost. Its performance is assessed and discussed on artificial signals and in a real experimental situation. The technique is utilized to analyze experimental results of a plane shear experiment and validate a damage meso-model describing different degradations in a C/C composite material.

Damage modeling at two scales for 4D carbon/carbon composites

Abstract Damage mechanisms and inelastic mechanical phenomena are modeled to the macroscopic scale for multiaxial loading. The studied material made by Societe Europeenne de Propulsion (SEP) is a 4D carbon–carbon (C/C) composite comprising four reinforcement directions. A very simple mathematical material model has been first derived for multiaxial loading as a consequence of some remarkable experimentally observed properties and the material geometry. The anisotropic continuum damage theory introduced by Ladeveze is used. To identify the material constants and functions characterizing the studied 4D C/C material is a rather difficult task: fiber debonding near the edges is very important for tensile tests. This is due to an edge effect, which modifies the local distribution of the stresses near a free surface. The study of this phenomenon is made at a mesoscopic scale with three constituents: the fibers, the matrix and the interfaces. A non-linear-damage model is introduced for the interface. Large scale FE calculations are done using special numerical methods adapted to such problems: a domain decomposition approach is associated with the LAnge Time INcrement (LATIN) method. Finally, the identified material model has been checked on various experiments.

Modelling and identification of temperature-dependent mechanical behaviour of the elementary ply in carbon/epoxy laminates

Abstract This paper develops, at the elementary-ply level, a model of the mechanical behaviour of continuous-fibre carbon/epoxy composite laminates subject to in-plane loads at temperatures in the range −120 to +120 °C. Attention is focused on the influence of temperature on the mechanical behaviour for load intensities extending to material failure. Damage mechanisms are introduced via meso-damage variables associated with the softening of ply stiffness. Inelastic strains are accounted for through a plasticity model coupled with damage. A careful analysis of the influence of temperature shows that experiments performed at three suitably chosen temperatures are sufficient to yield a precise identification of the single layer model within the full temperature range.

Damage modelling of a 4D carbon/carbon composite for high temperature application

Abstract Damage mechanisms and inelastic mechanical phenomena are modeled to the macroscopic scale for multiaxial loading. The studied material is a 4D carbon–carbon composite comprising therefore four reinforcement directions. A very simple mathematical material model has been first derived for multiaxial loading as a consequence of some remarkable experimentally observed properties and the material geometry. The anisotropic continuum damage theory introduced by Ladeveze is used. To identify the material constants and functions characterizing the studied 4D C/C material is a rather difficult task: fiber debonding near the edges is very important for tensile tests. To go further in the test analysis, large finite element computations have been done introducing a mesomodelling of the composite specimen, the meso-constituents being the fiber yarns, the matrix blocks and the interfaces. Finally, the identified material model has been checked on various experiments.

Effects of the thermomechanical loading path on the lifetime prediction of self-healing ceramic matrix composites

Abstract The objective of this work is to study the influence of the loading path on the lifetime of self-healing ceramic matrix composites. The loading can be of different types: mechanical, thermal or chemical. The self-healing process consists of filling some cracks with an oxide plug, thus limiting the diffusion of oxygen toward the fibers. The three phenomena with the greatest influence on the lifetime are (a) crack opening, governed by the mechanical part of the loading, (b) volatilization of the oxide plug, and (c) diffusion governed by both the chemical part of the loading and the activation temperature of the chemical reactions. The examples given show the influence of each part of the loading path.

Experiment / Computation Interactions by Using Digital Image Correlation

The validation of the constitutive equations describing the mechanical behavior of structures is obtained through the comparisons between independent experimental results and numerical computations. In the latter, the boundary conditions are very often postulated. It is proposed to use the experimentally measured displacement on the boundary as an input for the simulations. Consequently, digital image correlation technique has been developed. It gives access to displacement maps and strain fields with an accuracy of 5 10-5. Various applications have been considered. Each one is dealing with one particular aspect i.e. (i) effect of the boundary conditions on the homogeneity of the strain field, (ii) inception of cracking and (iii) comparison between experimental results and numerical predictions.

Variability of a Bolted Assembly Through an Experimental Modal Analysis

Industrial structures are mainly assemblies with complex geometries and non-linear characteristics. Friction and joint preload added to fabrication imperfections lead to a substantial gap between numerical models and real structures. In order to develop accurate generic models, it is then necessary to quantify the behavior variability, especially the one related to the joint conditions. The first part of this paper describes the iterative sizing procedure of an academic assembly which characteristics may vary depending on several input variables (e.g. value of the bolt torque, number and position of preloaded bolts, etc.). The properties of the bolted joint were optimized in order to satisfy a set of conditions in terms of tangential slipping, normal displacement and maximum stress level. The second part concerns the experimental modal analysis of the assembly. The main purpose is to characterize the relationship that exists between the input variables and the measured eigenfrequencies and modal damping of the assembly.