Jacques Lamon

A micromechanics-based approach to the mechanical behavior of brittle-matrix composites

Composites are heterogeneous materials and brittle-matrix composites combine brittle constituents. As a consequence, damage and failure exhibit typical features that are discussed in this paper. These phenomena influence the mechanical response under certain conditions. The micromechanics-based models of matrix cracking and fiber failures which are presented are based upon fracture statistics. The resulting model of mechanical behavior requires a limited number of parameters which consist in intrinsic constituent properties including elastic properties and flaw strength parameters. Finally, trends in the influence of constituent properties on the mechanical behavior are predicted.

Damage and failure in ceramic matrix minicomposites: Experimental study and model

Abstract The mechanical behavior of minicomposites under uniaxial tension was investigated. A model based upon probabilistic-statistical approaches to multiple matrix cracking and fiber failures is proposed. Predictions from characteristics of the constituents, including the fibers, the matrix and the interphase, compared satisfactorily with the tensile force-strain curves measured on batches of SiC/C/SiC minicomposites tested under uniaxial tension. The minicomposites have been fabricated by Chemical Vapor Infiltration (CVI) of bundles of 500 SiC fibres by a SiC

The concept of a strong interface applied to SiC/SiC composites with a BN interphase

Abstract Strong interfaces have been shown to allow improvement of the mechanical properties of ceramic matrix composites (CMC). The concept of a strong interface has been established in SiC/SiC composites with pyrocarbon (PyC) or multilayered (PyC/SiC) fiber coatings (also referred to as interphases). The present paper reports an attempt directed at applying the concept of a strong interface to SiC/SiC composites with a BN coating (referred to as SiC/BN/SiC). Fiber bonding and frictional sliding were investigated by means of push-out tests performed on 2D-composites as well as on microcomposite samples, and tensile tests performed on microcomposites. The stress–strain behavior of the SiC/BN/SiC composites and microcomposites is discussed with respect to interface characteristics and location of debonding either in the coating or in the fiber/coating interface.

Tensile creep behaviour of a silicon carbide-based fibre with a low oxygen content

The high-temperature mechanical behaviour and microstructural evolution of experimental SiC fibres (Hi-Nicalon) with a low oxygen content (<0.5 wt%) have been examined up to 1600 °C. Comparisons have been made with a commercial Si-C-O fibre (Nicalon Ceramic Grade). Their initial microstructure consists of β-SiC crystallites averaging 5–10 nm in diameter, with important amounts of graphitic carbon into wrinkled sheet structures of very small sizes between the SiC grains. The fall in strength above 800 °C in air is related to fibre surface degradation involving free carbon. Crystallization of SiC and carbon further develops in both fibres subject to either creep or heat treatment at ∼1300 °C and above for long periods. The fibres are characterized by steady state creep and greater creep resistance (one order of magnitude) compared to the commercial Nicalon fibre. The experimental fibre has been found to creep above 1280 °C under low applied stresses (0.15 GPa) in air. Significant deformations (up to 14%) have been observed, both in air and argon above 1400 °C. The stress exponents and the apparent activation energies for creep have been found to fall in the range 2–3, both in air and argon, and in the range 200–300 kJ mol−1 in argon and 340–420 kJ mol−1 in air. The dewrinkling of carbon layer packets into a position more nearly aligned with the tensile axis, their sliding, and the collapse of pores have been proposed as the mechanisms which control the fibre creep behaviour.

Micro/minicomposites: a useful approach to the design and development of non-oxide CMCs

Abstract Micro (one single filament) and mini (one single fiber tow) non-oxide composites (C/C; C/SiC and SiC/SiC) with simple (PyC or BN) or complex interphases [C (B) or (PyC-SiC) n multilayers] are fabricated in a short time by CVD/CVI. The fiber/matrix interfacial zone is characterized by AES and TEM. Tensile tests are used to assess the mechanical properties and the Weibull statistical parameters of both the fiber and matrix, as well as the fiber–matrix interfacial parameters (τ i ; l d ; G ic ). The tensile stress–strain behavior has been modelled. The tensile curves exhibit the same features as those previously reported for real nD-composites. Lifetime at high temperatures in air is characterized through static/cyclic fatigue tests and modelled. It is improved by replacing conventional pyrocarbon by highly engineered interphases. The micro/mini composite approach is used in the optimization of processing conditions and to derive parameters necessary for the modelling of the thermomechanical and chemical behavior of composites with more complex fiber architectures.

Thermomechanical properties of carbon fibres at high temperatures (up to 2000 °C)

A high-temperature fibre-testing apparatus has been designed. It is dedicated to the determination of various properties at very high temperatures, including electrical conductivity, Youngs modulus, thermal expansion coefficient, strength. Test temperatures as high as 3000 °C can be applied to carbon fibres. Two types of carbon fibres (a PAN-based and a Rayon-based fibre) have been investigated at temperatures up to 2000 °C. The measured properties are discussed with respect to microstructural features.

The influence of the interphase and associated interfaces on the deflection of matrix cracks in ceramic matrix composites

A model is proposed to determine the influence of an interphase on the deflection of a matrix crack in ceramic matrix composites. Then, a finite element analysis is performed for a microcomposite geometry with an annular crack which initiates in the matrix and propagates in the interphase. It is applied to a SiC/C/SiC microcomposite with a pyrocarbon interphase. Criteria for penetration and deflection of the matrix crack are expressed in terms of toughness of the interphase and of the various interfaces (matrix/interphase and interphase/fibre interfaces). The predictions are found to agree with crack deflections observed in practical SiC/SiC composites and with the available interphase toughness data. Results also suggest that the real crack deflection mechanism involves debonding ahead of the propagating matrix crack.

Sharp indentation behavior of carbon/carbon composites and varieties of carbon

Abstract Sharp indentation tests on carbon fiber and carbon matrix composites (C/C composite) were carried out over a wide load range from 0 to 2 N on three different cross sections: normal, parallel and inclined to the fiber axis. For comparison purposes, a variety of carbons including HOPG, glassy C, and pyrocarbon films was also examined. Both the fibers and the matrices displayed first a purely elastic response and second crack-induced damage. A purely elastic behavior was also observed with most of the varieties of carbon considered. Young’s modulus was extracted from the indentation curves either at maximum or at various forces, using the Sneddon equation of elastic response on loading (elastic indentation) or a classical equation based on elastic recovery on unloading (elastoplastic indentation). Results are discussed with respect to features of structure and heterogeneity of material in the stressed volume.

Damage and failure mechanisms of a3-directional carbon/carbon composite under uniaxial tensile and shear loads

Abstract The mechanical behavior of a three-directional carbon/carbon (C/C) composite under tensile and shear loads is investigated in relation with the failure mechanisms and, the fiber architecture. This three-directional C/C composite was produced by Chemical Vapor Infiltration of a needled fiber preform of multiple layers of satin woven tows. The C/C composite exhibited several interesting features including an essentially non-linear stress–strain behavior and permanent deformations. Three families of matrix cracks were identified under tensile and shear loads, including microcracks in the tows, intertow delamination and cracks across the longitudinal tows. It was found that the delamination cracks affect preponderantly the stress–strain behavior and the mechanical properties. Similar features in the mechanical behavior and the failure mechanisms were highlighted under tension and under shear loading.

Ceramic matrix composites : materials, modeling and technology

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