Pascal Reynaud

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.

Cyclic fatigue of ceramic-matrix composites at ambient and elevated temperatures

Abstract Under cyclic loading, the mechanical behaviour of ceramic-matrix composites changes with the number of cycles applied even at room temperature. In order to identify the microstructural mechanisms of damage due to cyclic fatigue, the macroscopic behaviour is experimentally observed from the lifetime diagram and the evolution of the stress/strain loops with the number of cycles applied. From this approach a micromechanical model has been developed where the cyclic fatigue effect is ascribed to interfacial wear between fibres and matrix. At high temperature, since the testing temperature is low enough to inhibit any physical and chemical change in the materials, the main effect of temperature is due to the release of residual thermal stresses. This phenomenon has been experimentally observed on two ceramic-matrix composites: SiC SiC 2D and SiC MAS L [0,90], where the signs of the residual thermal stresses are different.

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.

Mechanisms of crack propagation in dry plaster

Controlled crack propagation tests were performed on single edge notched bend samples to investigate the crack growth behaviour of dry plaster. The influence of the relative notch depth on the crack resistance curve has been studied and appears to be very important. The results are discussed considering a qualitative model based on the specific microstructure of plaster and in situ observations of the crack propagation. Two mechanisms acting at different scales and undergoing complex interactions are involved: crack bridging by small gypsum crystals acting locally behind the crack tip and secondary cracking in a macroscopic frontal process zone. Interaction of the main crack with secondary ones undergoes substantial branching and crack accelerations leading to bridging destruction due to sudden crack opening.

Slow crack growth study of plaster using the double torsion method

Abstract Double torsion tests were performed to study slow crack growth (SCG) behavior of dry plaster. Preliminary, the fracture toughness K Ic was measured, and higher value than that obtained with notched beams in bending test was found. This can be attributed to interactions between the surfaces of the initial crack in the DT samples. For SCG investigation, both load relaxation and constant loading tests were conducted. It is found that the relaxation method is not suitable for quantitative determination of the SCG law. Indeed, the load relaxation is very limited for dry samples and cannot be correlated to the crack propagation observed on the tensile surface. The constant loading results show that subcritical crack propagation occurs at K I values lower than 30% K Ic . This can be explained by the linkage of the main propagating crack with secondary cracks, the nucleation of which occurs at very low applied stresses.

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.