Mohamed R'Mili

Energy toughness parameters for a 2D carbon fibre reinforced carbon composite

Abstract The present work deals with the study and characterisation of the fracture behaviour of a 2D carbon fibre reinforced carbon laminate. The material was processed by chemical vapour infiltration at the Societe Europeenne de Propulsion (SEP) in France. The behaviour of such material is not perfectly brittle and therefore it cannot be described by a unique toughness parameter such K Ic , G Ic or J Ic but rather by a crack growth resistance curve ( R curve), giving the energy needed for crack extension as a function of crack increment. Several methods for determining R curves are described and discussed in the context of linear elastic fracture mechanics and also by considering a non-linear scheme, taking into account non-elastic behaviour, evidenced in practice by residual crack opening after unloading. These methods are mainly based on measurements of specimen compliance by load-unload cycling during controlled crack growth and the energies are obtained by estimating areas under the load-displacement curve during the cycling. The testing machine is driven by a micro-computer which also collects and processes the data. The fracture behaviour is analysed for two orientations: 1. (i) interlaminar cracking in compact tension (CT) specimens. In this situation, the composite exhibits a crack growth energy having the same order of magnitude as that of a carbonaceous product without reinforcement ( R J/m 2 ). 2. (ii) edgewise cracking (the notch plane and tip are perpendicular to the cloth plane) in single CT specimens with different shapes and sizes. For this orientation, the crack growth resistance is quite high ( R ≈ 5000 J/m 2 ) as a result of energy absorbing mechanisms occurring in the process zone ahead of the crack. The results show that the non-linear analysis is more accurate for this type of material and that the measured R curve depends neither on the notch depth nor on specimen dimensions. The shapes of the load-displacement curves and of the R curves are discussed in relation to mechanisms acting in the process zone, which are strongly influenced by fibre and interface effects.

Caractérisation des fibres par amélioration de l'essai sur mèche avec mesure directe de la déformation

The statistical variation in mechanical properties of E-glass fibres has been studied using loose bundles containing N ≥ 1 000 fibres. Particular care has been taken for preparation of these bundle specimens so as to he able to measure the direct sample deformation with a mechanical extensometer. The direct measure of the bundle strain during a tensile test allows us to obtain a reliable determination of the intrinsic ‘load-strain’ behaviour law and of the following characteristics of the bundle: number of initially intact fibres in the bundle; Weibull shape parameter, strength and failure strain of the bundle and mean strength of the fibres.

Static Fatigue of a 2.5D SiC/[Si-B-C] Composite at Intermediate Temperature under Air

Non-oxide ceramic-matrix composites (CMCs) are subjected to be used in aeronautic applications which require very long duration (up to 100 000h) of materials at high temperatures and under air. Recently a self-healing [Si-B-C] matrix has been developped to enhance strongly the lifetime of CMCs under air. The aim of this work is to study the mechanical behaviour of a SiCf/[Si-B-C] composite with a self-healing matrix under static fatigue, and to determine its lifetime. During the mechanical tests, acoustic emission is detected in order to characterize the damage of the composite in addition to the measurement of the longitudinal deformation of the composite. The analysis of acoustic emission follows a non-supervised procedure of classification. Each event of acoustic emission is described by a set of several parameters, and the total activity can be divided in four classes. The assignment of each class to a damage mechanism is required to follow the spread of damage during fatigue and to determine the mechanisms controlling the failure of the composite.

Identification de la signature acoustique des différents mécanismes d’endommagement lors d’essais de fatigue sur CMC. Application de classificateurs supervisé et non supervisé

The intent of this paper is to identify the damage phenomena controlling the life of composites at high temperatures (> 600°C), and to measure the kinetics of these phenomena during mechanical tests. The analysis of AE data provides a classification into groups of signals having similar characteristics that can be afterward associated to one or to several damage mechanisms. A method of unsupervised classification was used to differentiate the signals resulting from various mechanisms of damage and from failures. Then, a method of supervised classification was developed, so as to establish a classification in real-time whatever the temperature, the applied constraint and the mode loading (static or cyclic).