D. Rouby

Fatigue behaviour related to interface modification during load cycling in ceramic-matrix fibre composites

Abstract Ceramics reinforced with continuous fibres exhibit delayed failure under pulsating load. A micromechanical model describing the fatigue effects is proposed. It is based on a decrease in shear stress at the fibre/matrix interfaces, as a result of interfacial wear caused by see-saw sliding. The main features of this model are as follows. During the first load cycle, the material exhibits multiple matrix cracking and some fibre breaks. The system is then a serial set of matrix cracks, each of them bridged by a parallel set of intact or broken fibres. During subsequent cycles the interfacial shear stress decreases, leading to an increase in the failure probability of the bridging fibres. These changes give both a reduction of stiffness and a widening of the hysteresis loops. For a critical fraction of broken bridging fibres, instability occurs and the specimen fails, thus defining the lifetime. The higher the applied load, the higher is the initial damage on the first cycle and the faster the instability condition is reached. For peak stresses that are lower, but still higher than the proportionality limit, the material also changes but no failure occurs (up to 106 cycles), indicating that the interfacial shear stress decreases to a non-zero value; this limiting value controls the fatigue limit in the lifetime diagram.

Elastic moduli of a 2.5d Cf/SiC composite : experimental and theoretical estimates

This study concerns the characterisation of the elastic properties of a long-fibre-reinforced ceramic-matrix composite. Seven of the nine independent elastic constants of a woven 2.5 D carbon-fibre reinforced SiC ceramic matrix have been measured by an ultrasonic technique associated with a numerical optimisation process. The elastic moduli are recovered by minimising the square deviation between measured and theoretical velocities. The ultrasonic measurements are discussed with the approximation of continuum mechanics, in regard to the wavelength and the size of the microstructural details of the material. For comparison, an estimate of the elastic moduli in all directions is performed with an Eshelby-based model, assuming the composite as a two-dimensional tow-reinforced matrix containing voids. The effective moduli for the tow composite structure are estimated from a first homogenisation step. Volume fractions of tow and matrix cracks slightly opened are taken into account in the microstructural description of the composite from experimental data on mean crack orientations, quantities and shapes. The specific effect of neglecting waviness is estimated from finite-element calculations. In the limit of the uncertainties on the phase moduli, the estimates of elastic moduli are in agreement with the available measured ones. As relevant, estimates are thus provided for the missing measurements.

Effects of temperature and of oxidation on the interfacial shear stress between fibres and matrix in ceramic-matrix composites

Abstract Under cyclic loading, the mechanical behavior of ceramic matrix composites (SiC/SiC, SiC/ MAS-L) changes with the number of applied cycles, as shown by life-time diagrams and shape evolutions of stress/strain loops. According to these observations, a shear-lag model has been developed where the cyclic fatigue effect is attributed to an interfacial wear between fibres and matrix. At high temperature under inert atmosphere, since physical and chemical changes are inhibited, the main effect of temperature on cyclic fatigue of ceramic matrix composites is the release of radial thermal residual stresses. But under vacuum when the temperature is high, fibre/matrix interfaces can be removed due to chemical instabilities. Hence, after ageing under vacuum at high temperature, cyclic fatigue at room temperature of SiC/SiC composites exhibits an increase followed by a decrease in mechanical hysteresis. This can be explained by a decrease in the interfacial shear stress due to the previous heating at high temperature under vacuum. For such treated composites, an original stiffening effect is also observed during cyclic fatigue. This original phenomenon is attributed to a contribution of cracks in transversal yarns.

Direct measurement of crack-bridging tractions: A new approach to the fracture behaviour of ceramic/ceramic composites

Abstract By comparison with bulk ceramics, ceramic-matrix composites exhibit large toughening as a consequence, initially, of matrix crack bridging by intact fibres and then by broken fibres undergoing pull-out. A method for the direct measurement in double-edge-notched specimens of the bridging tractions, p, as a function of crack opening displacement, u, is presented. The p(u) curve shows a peak which corresponds to instability of the bundle of bridging fibres. The tail of the curve is linked to the fibre pull-out process and gives access to the average pull-out length and to the interfacial sliding strength. Finally, the crack-growth resistance induced by the bridging phenomenon can be estimated from the integral of the p(u) curve. Two materials are used in this study: a woven 2D SiC SiC composite processed by SEP, France, and a laminated [ 0 90 ] SiC MAS-L composite processed by Aerospatiale, France. Results of p(u) curve analysis are given and discussed.

The frictional interface: a tribological approach of thermal misfit, surface roughness and sliding velocity effects

In ceramic matrix composites, it is now well known that the cyclic fatigue effects are due to a decrease of the stress transfer capability of the fibres that bridge the matrix cracks, because of some wear induced by the seesaw displacements at the interfaces. Therefore, a better knowledge of the cyclic fatigue effects needs a deeper understanding of the microscale mechanisms operating during the friction between fibre and matrix. This is the main subject of the present paper which deals with single filament model composites submitted to push-out tests at room temperature under dry friction condition. Emphasis is placed here on the roughness of the slipping surfaces in relation with thermal misfit, and with sliding velocity.

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.

Analysis of pull-out and failure of unidirectional bundles in a laminated carbon/carbon composite

Abstract The carbon/carbon composites for braking applications, processed by Messier Bugatti, Division Carbone Industrie (Villeurbanne) are laminates of [0/60/−60] n stacking, needled in the through-thickness direction. The unidirectional plies are made of adjacent meshes of square cross-section which exhibit some porosity between them because of the CVI processing route. The goal of the present paper is the study of the fracture behaviour of an individual mesh, and the analysis of the interfacial properties between the mesh and its surroundings. Tensile tests have been performed on special specimens where the mesh, embedded in the composite bulk over a given embedded length, can be loaded. For a long embedded length ( d > 3.6 mm), the mesh is broken and the ultimate stress is 260±80 MPa. For a shorter embedded length, the mesh debonds first with a mean interfacial shear stress of 18±8 MPa. The post-debonding extraction is controlled by friction, starting with an interfacial shear stress of 8±3 MPa. The pull-out curves are analysed and modelled by using the Amontons–Coulomb law.

Analysis of interfacial sliding in brittle-matrix composites during push-out and push-back tests

Abstract The aim of this work was to study the friction mechanisms in single-filament model composites (SiC/Pyrex), as the fibre slides into the matrix, and to analyse the modification of friction conditions as interface degradation proceeds. Results obtained by push-out and push-back tests on thin sheets of a single-filament model composite are presented and discussed. The push-out set-up used was classical in principle, except that the stiffness of the device can be easily changed. When this stiffness is high, the pushing force is steady (friction under steady regime) and exhibits a well pronounced seating drop, during push-back, as the fibre reaches its original position. When the stiffness becomes low, the load–displacement curve exhibits large serrations because the friction is now in the stick-slip regime. The dependence of the serration characteristics on specimen thickness is described and discussed. The bifurcation between steady sliding and stick-slip regimes is discussed with respect to the jerky nature of the slip at the microscopic scale, because slip needs to break contacts between asperities of the rough surfaces. As the number of push-out/push-back cycles increases, these serrations progressively disappear. This phenomenon can be explained by the progressive degradation of the sliding surfaces. A simple model describing the degradation kinetics was proposed.

Apparent stiffening of ceramic-matrix composites induced by cyclic fatigue

Abstract This work carries on four different long-fibre-reinforced ceramic-matrix composites: a cross-weave SiC/SiC, a cross-ply SiC/MAS-L, a cross-weave C/SiC and a [0,+60,−60] n C/C laminate. Experimentally, cyclic fatigue effect has been observed at room temperature, at high temperature under inert atmosphere, and at room temperature after a previous ageing at high temperature under vacuum. For these four materials, the evolutions of the macroscopic mechanical behaviour with the number of cycles applied can be explained by an evolution of interfaces as well, fibre/matrix interfaces as neighbouring ply interfaces, according to the following mechanisms: (i) interfacial wear of interfaces due to to-and-fro sliding of fibres or of plies under cyclic loading, and (ii) dependence of the residual thermal stresses with the temperature of the test. Previous ageing at high temperature under vacuum can also enable in CMC some physical and chemical changes in the constituents leading for example to a slight removing of fibre/matrix interphases by oxidation. Usually, damage induced by cyclic fatigue in long-fibre-reinforced ceramic-matrix composites leads to a reduction of the tensile apparent elastic modulus as cycling proceeds. But an original macroscopic stiffening has been experimentally observed during cyclic fatigue. This phenomenon has been observed on C/C composite at room temperature, on C/SiC and on SiC/MAS-L at high temperature, and on SiC/SiC at room temperature after previous ageing under vacuum at high temperature. This apparent stiffening is not well understood at present time, but appeared in materials with low interfacial shear strength and is seemingly due to incomplete closure during unloading of the cracks present in transverse yarns.

Acoustic emission monitoring of uniaxial pressing of ceramic powders

Abstract Powder compaction is a critical step in a classical method for the production of zero-defect ceramic products. We have studied the development of this step to determine whether acoustic emission can be used to detect the presence of flaws in a compact. In uniaxial pressing at room temperature, the creation and extension of flaws occur principally during the ejection of the compacts. The type and extent of flaws depend on the mechanical properties of the powder used and on the pressing conditions. Acoustic emission was used to monitor the consolidation by uniaxial pressing of Al2O3 powders and a UO2PuO2 powder. It was possible to detect flaws in the compacts, to determine their type for a given powder, and to identify the type of powder used by a direct examination of cumulative counts of acoustic emission events and by a statistical analysis of amplitude distributions. Hence, the introduction of this technique in a fabrication process offers the possibility of detecting the presence of defects in compacts as soon as they are created. This would decrease the difficulties involved in reuse in the process of the powder from defective products, and would limit further flaw development during sintering, which are two major problems in the nuclear industry.