Composites Science and Technology 58(1998)409-418 PIl:S0266-3538(97)00139-5 0266353898s1900 CARBON-FIBER-REINFORCED YMAS GLASS-CERAMIC. MATRIX COMPOSITES-IV THERMAL RESIDUAL STRESSES AND FIBER/MATRIX INTERFACES Valerie Bianchi. a Paul Goursat* erik menessier @LMCTS, ESA CNRS 6015, Faculte des sciences, 123 avenue Albert Thomas, 87060 Limoges Cedex, france bCeramigues et Composites, BP 7, 65460 Bazet, france ( Received 21 November 1996; revised 14 July 1997; accepted 17 July 1997) Abstract mation of an interphase which is likely to control the matrix reinforced with continuous carbon fiber uted by hot-pressing exhibit different fructure Se apri deflection and propagation of cracks and the frictional sliding of fibers. Physico-chemical reactions can also lead to wider interfacial separation and changes in frac- according to the sintering thermal cycle. The thermal ture strength The purpose of this paper is to determine the role mismatch in coefficients of thermal expansion and the played by physico-chemical reactions and thermal thermomechanical characteristics of both fibers and residual stresses in determining the strength of the fihe matrix. An ultrasonic technique is used to determine the matrix interface and to predict the composite behavior. An microcracking in the matrix. The infuence of the materials is to follow changes in Young s modulus with ature. First, an ultrasonic technique has beer between the fibers and the matrix on the nature and the used to determine the temperatures at which thermal strength of the fiber/matrix interface is discussed. -C 1998 residual stresses are higher than the matrix strength and Elsevier Science Lid. All rights reserved nduce microcracking in the matrix, which is expected to occur on cooling during the fabrication of composites Keywords: A. ccramic-matrix composites, A. carbon Sccond, thermal residual stresses have bccn cstimat fibres, B. fibre/matrix bond, C. residual stress, D. non- by a model which takes into account the thermoelastic destructive testing anisotropy of carbon fibers. To achieve this goal, the coefficients of thermal expansion(CTE) of carbon fibers were calculated from those of the matrices and the 1 INTRODUCTION composites Thermal conditions in the fabrication process of cat bon- fiber-reinforced YMAS-matrix composites have 2 EXPERIMENTAL PROCEDURES been shown to play a dominant role in the fracture behavior of these composites, which depends strongly 2.1 Young s modulus on the nature and strength of the interfacial fiber matrix The knowledge of material clastic constants allows the bond. 2 The interface must be weak enough to allow calculation of the elastic moduli, which express the energy dissipation by frictional sliding of the fibers in macroscopic proportionality between stress and strain the matrix blocks. The strength of the interface is cor- Elastic constants, which are related to interatomic related with the thermomechanical properties of both potential and have an intrinsic character, can allow the constituents and to the physico-chemical reactions monitoring of certain structural changes in materials, between them. Indeed, residual stresses, caused by the such as glass crystallization and phase transitions thermal expansion mismatch between the fibers and the Moreover, the Youngs modulus of a heterogeneous matrix, can lead to cohesion or debonding of the inter- material is related to the volume fraction and the mor face, which raises or lowers the ultimate strength and phology of each constituent, to the porosity fraction fluences the fracture type. Physico-chemical reactions and geometry, 6 to the cracking direction of the material between the fibers and the matrix can induce the for- and its volume fraction Measurement of the propagation time of an ultra- *To whom correspondence should be addressed sonic wave in a material offers a means of evaluating theELSEVIER Composites Science and Technology 58 (1998) 409418 9 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain PII: SO266-3538(97)00139-5 0266-3538/98 $19.00 CARBON-FIBER-REINFORCED YMAS GLASS-CERAMIC￾MATRIX COMPOSITES-IV. THERMAL RESIDUAL STRESSES AND FIBER/MATRIX INTERFACES Valirie Bianchi,” Paul GoursaF* & Erik MCnessierb “LMCTS, ESA CNRS 6015, Faculte des Sciences, 123 avenue Albert Thomas, 87060 Limoges Cedex, France bCPramiques et Composites, BP 7, 65460 Bazet, France (Received 21 November 1996; revised 14 July 1997; accepted 17 July 1997) Abstract Unidirectional composites consisting of a glass-ceramic matrix reinforced with continuous carbon fibers, fabri￾cated by hot-pressing, exhibit d@erent fracture behavior according to the sintering thermal cycle. The thermal residual stresses in composites are determined from the mismatch in coeficients of thermal expansion and the thermomechanical characteristics of both fibers and matrix. An ultrasonic technique is used to determine the temperature at which, on cooling, residual stresses induce microcracking in the matrix. The influence of the mechanical stresses and physico-chemical reactions between the fibers and the matrix on the nature and the strength of thefiberlmatrix interface is discussed. -0 1998 Elsevier Science Ltd. All rights reserved Keywords: A. ceramic-matrix composites, A. carbon fibres, B. fibre/matrix bond, C. residual stress, D. non￾destructive testing 1 INTRODUCTION Thermal conditions in the fabrication process of car￾bon-fiber-reinforced YMAS-matrix composites have been shown to play a dominant role in the fracture behavior of these composites,’ which depends strongly on the nature and strength of the interfacial fiber/matrix bond.2 The interface must be weak enough to allow energy dissipation by frictional sliding of the fibers in the matrix blocks. The strength of the interface is cor￾related with the thermomechanical properties of both constituents and to the physico-chemical reactions between them. Indeed, residual stresses, caused by the thermal expansion mismatch between the fibers and the matrix, can lead to cohesion or debonding of the inter￾face, which raises or lowers the ultimate strength and influences the fracture type. Physico-chemical reactions between the fibers and the matrix can induce the for- *To whom correspondence should be addressed. 409 mation of an interphase which is likely to control the deflection and propagation of cracks and the frictional sliding of fibers. Physico-chemical reactions can also lead to wider interfacial separation and changes in frac￾ture strength. The purpose of this paper is to determine the roles played by physico-chemical reactions and thermal residual stresses in determining the strength of the fiber/ matrix interface and to predict the composite behavior. An appropriate way to detect structural changes in materials is to follow changes in Young’s modulus with temperature.3 First, an ultrasonic technique has been used to determine the temperatures at which thermal residual stresses are higher than the matrix strength and induce microcracking in the matrix, which is expected to occur on cooling during the fabrication of composites.’ Second, thermal residual stresses have been estimated by a model which takes into account the thermoelastic anisotropy of carbon fibers. To achieve this goal, the coefficients of thermal expansion (CTE) of carbon fibers were calculated from those of the matrices and the composites. 2 EXPERIMENTAL PROCEDURES 2.1 Young’s modulus The knowledge of material elastic constants allows the calculation of the elastic moduli, which express the macroscopic proportionality between stress and strain. Elastic constants, which are related to interatomic potential and have an intrinsic character, can allow the monitoring of certain structural changes in materials, such as glass crystallization and phase transitions.4 Moreover, the Young’s modulus of a heterogeneous material is related to the volume fraction and the mor￾phology of each constituent,’ to the porosity fraction and geometry,6 to the cracking direction of the material and its volume fraction.’ Measurement of the propagation time of an ultra￾sonic wave in a material offers a means of evaluating the