Availableonlineatwww.sciencedirect.com COMPOSITES scⅰ enceDirect CIENCE AND TECHNOLOGY ELSEVIER Composites Science and Technology 68 (2008)1165-l1 www.elsevier.com/locate/compscitech Ceramic fiber composites: Experimental analysis and modeling of mechanical properties Dietmar Koch, Kamen Tushtev, Georg Grathwohl Unicersity of Bremen, Department of Ceramic Materials and Components, D-28359 Bremen, Germany Received 16 May 2007: received in revised form 22 June 2007: accepted 29 June 2007 Available online 19 July 2007 Abstract Ceramic fiber reinforced ceramic matrix composites(CMC)are outstanding ceramics with high fracture toughness. This can be real ized if both brittle components of the composite, i.e., fibers and matrix are interacting with each other in an efficient way. Either a weak interface allowing debonding between fiber and matrix controls the fracture processes (WIC-CMC) or the matrix takes this role of a weak and more compliant component(WMC-CMC). An experimental test data base is presented for a WMC-type composite where the materials data are used to establish a model which describes the materials behavior in a macroscopic way. Inelastic deformation and materials damage processes are defined, measured and interpreted on the base of a continuum damage mechanics concept. The elas ic and inelastic response is then predictable up to failure as being dependent on the angle between fiber and loading directions of the specimen o 2007 Elsevier Ltd. All rights reserved Keywords: A. Ceramic-matrix composites( CMCs); B. Stress/strain curves; B. Mechanical properties; C Modelling: C Finite element analysis(FEA) 1. Introduction tolerance of cmc it has to be assured that the fibers remain intact and effective when cracks propagate. For this Continuous fiber reinforced ceramic matrix composites purpose the balance between the strength of the fibers and are mainly developed for specific applications at high tem- the crack resistance of the other microstructural compo- peratures and in oxidative atmosphere. The fibers as rein- nents, i. e. matrix and fiber matrix interface, has to be con forcing components offer higher strength and stiffness trolled in a way that the survival probability of the fibers is compared to the matrices which are, in contrast, character- not too low. This effect can be reached by the adjustment of ized by inferior properties as in most cases their microstruc- the CMC microstructure in two alternative ways tures exhibit microcracks, residual pores and gradients or inhomogeneities caused by the CMC fabrication process. The crack resistance of the fiber matrix interface is low- Typical processing routes are chemical vapor infiltration ered in order to allow debonding between fiber and (CVI), liquid polymer infiltration(LPI), liquid silicon infil matrix CMCs of this type are characterized by a weak tration (Lsi, directed metal oxidation (DiMOx), and cera interface or interphase. They are termed Weak Interface mic slurry impregnation(CSI) which lead to characteristic Composites, WIc properties of the resulting CMC. The interfacial bonding between fiber and matrix is of When CMCs are stressed crack initiation and propaga minor importance as the matrix is weak enough and tion generally originate from the matrix For enhanced flaw susceptible for multiple cracking while the fibers pro- vide strength and crack tolerance of the CMC. These composites are called Weak Matrix Composites, E-mail address: koch(ceramics. uni-bremen de (D. Koch) WMC 02663538/S. see front matter 2007 Elsevier Ltd. All rights reserved doi:10.1016j.compscitech.2007.06.029Ceramic fiber composites: Experimental analysis and modeling of mechanical properties Dietmar Koch *, Kamen Tushtev, Georg Grathwohl University of Bremen, Department of Ceramic Materials and Components, D-28359 Bremen, Germany Received 16 May 2007; received in revised form 22 June 2007; accepted 29 June 2007 Available online 19 July 2007 Abstract Ceramic fiber reinforced ceramic matrix composites (CMC) are outstanding ceramics with high fracture toughness. This can be real￾ized if both brittle components of the composite, i.e., fibers and matrix are interacting with each other in an efficient way. Either a weak interface allowing debonding between fiber and matrix controls the fracture processes (WIC–CMC) or the matrix takes this role of a weak and more compliant component (WMC–CMC). An experimental test data base is presented for a WMC-type composite where the materials data are used to establish a model which describes the materials behavior in a macroscopic way. Inelastic deformation and materials damage processes are defined, measured and interpreted on the base of a continuum damage mechanics concept. The elas￾tic and inelastic response is then predictable up to failure as being dependent on the angle between fiber and loading directions of the specimens. 2007 Elsevier Ltd. All rights reserved. Keywords: A. Ceramic–matrix composites (CMCs); B. Stress/strain curves; B. Mechanical properties; C. Modelling; C. Finite element analysis (FEA) 1. Introduction Continuous fiber reinforced ceramic matrix composites are mainly developed for specific applications at high tem￾peratures and in oxidative atmosphere. The fibers as rein￾forcing components offer higher strength and stiffness compared to the matrices which are, in contrast, character￾ized by inferior properties as in most cases their microstruc￾tures exhibit microcracks, residual pores and gradients or inhomogeneities caused by the CMC fabrication process. Typical processing routes are chemical vapor infiltration (CVI), liquid polymer infiltration (LPI), liquid silicon infil￾tration (LSI), directed metal oxidation (DiMOx), and cera￾mic slurry impregnation (CSI) which lead to characteristic properties of the resulting CMC. When CMCs are stressed crack initiation and propaga￾tion generally originate from the matrix. For enhanced flaw tolerance of CMC it has to be assured that the fibers remain intact and effective when cracks propagate. For this purpose the balance between the strength of the fibers and the crack resistance of the other microstructural compo￾nents, i.e. matrix and fiber matrix interface, has to be con￾trolled in a way that the survival probability of the fibers is not too low. This effect can be reached by the adjustment of the CMC microstructure in two alternative ways: • The crack resistance of the fiber matrix interface is low￾ered in order to allow debonding between fiber and matrix. CMCs of this type are characterized by a weak interface or interphase. They are termed Weak Interface Composites, WIC. • The interfacial bonding between fiber and matrix is of minor importance as the matrix is weak enough and susceptible for multiple cracking while the fibers pro￾vide strength and crack tolerance of the CMC. These composites are called Weak Matrix Composites, WMC. 0266-3538/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2007.06.029 * Corresponding author. E-mail address: dkoch@ceramics.uni-bremen.de (D. Koch). www.elsevier.com/locate/compscitech Available online at www.sciencedirect.com Composites Science and Technology 68 (2008) 1165–1172 COMPOSITES SCIENCE AND TECHNOLOGY