Internationa Journal ELSEVIER International Journal of Fatigue 24(2002)241-248 www.elsevier.com/locate/ijfatigue Observations of fatigue damage process in SiC/SiC composites at room and elevated temperatures Y Miyashita a,, K Kanda,S. Zhu,Y. Mutoh a, M. Mizuno A.J. McEvily Department of Mechanical Engineering, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, Niigata 940-2188, Japar niversity of Electro-Communications, Chofu, Tokyo, Japan Japan Fine Ceramics Center, Atsuta-ku, Nagoya, Aichi, Japan IMS, University of Connecticut, Storrs, CT, USA Abstract Scanning Electron Microscope(SEM)in-situ observation of fatigue damage process of SiC/SiC composites was carried out at room and elevated temperatures. Although single edge V-notched specimens were used, fatigue crack initiated not only at the notch root but also at large pores far from the notch. The widespread damage means that the parameter of linear elastic fracture mechanics cannot be used to describe fatigue crack growth behavior of SiC/SiC materials. Instead, modulus of rigidity(MOR)may be used as a parameter to estimate fatigue damage process. The crack behavior was divided into two types of damage modes depending on the orientation of fiber bundles in front of a notch. No matter which type of damage mode, the final failure occurred at about 85%of the maximum value of mOR. c 2002 Elsevier Science Ltd. All rights reserved Keywords: SiC/SiC composite; Fatigue; Crack growth, Damage process; Elevated temperature 1. Introduction fatigue damage was correlated with the change in rigid- ity of the specimens The SiC fiber reinforced SiC (SiC/SiC) composite is one of the most important ceramic matrix composites for high temperature applications. It has been reported that fatigue limit of SiC/SiC composites is about 80% of the 2. Materials ultimate tensile strength at room temperature, but the fatigue limit decreases at 1000oC, a temperature lower The SiC (Nicalon) fibers were approximately 15 um than that for fiber creep [1-5]. One part of the fatigue in diameter. These fibers were assembled in flattened damage mechanism in SiC/SiC is debonding as a result bundles with about 500 fibers per bundle. The bundles of wear of the interface between the matrix and the fibers were cross-woven to form a layer approximately 0.3 mm [1, 2, 4], as observed by in-situ observations [6, 7).A com- thick. This pre-pleg layer was then coated with carbon lete evaluation of the fatigue damage process in by a Cvd( Chemical Vapor Deposition)technique to SiC/SiC has not been established provide a weak interface between fibers and the Sic In this work, fatigue tests of two types of SiC/Sic matrix which in the next step was formed by a chemical composites were carried out in an SEM(Scanning Elec- vapor infiltration(CVI) procedure. This infiltration pro- tron Microscope)chamber at room temperature and at cedure resulted in a significant amount of porosity. Two 800oC. The fatigue crack initiation and growth processes types of matrices were used. One matrix was simply were observed in detail in the sem. the evolution of standard sic. the second matrix consisted of sic to which boron had been added The boron reacts with oxy- gen to form glassy particulates which seal the matrix phase and inhibit oxidation of the carbon-interfacial Corresponding author. Tel. +81-258-47-9750; fax: +81-258-47 layer [8, 9]. This type of matrix is referred to as an enhanced matrix. The individual layers were then sin E-mailaddress: miyayuki@mech. nagaokaut ac jp(Y Miyashita). tered together to form a 3 mm thick panel 0142-1 123/02/. see front matter e 2002 Elsevier Science Ltd. All rights reserved P:S0142-1123(01)00078-0International Journal of Fatigue 24 (2002) 241–248 www.elsevier.com/locate/ijfatigue Observations of fatigue damage process in SiC/SiC composites at room and elevated temperatures Y. Miyashita a,*, K. Kanda a , S. Zhu b , Y. Mutoh a , M. Mizuno c , A.J. McEvily d a Department of Mechanical Engineering, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, Niigata 940-2188, Japan b University of Electro-Communications, Chofu, Tokyo, Japan c Japan Fine Ceramics Center, Atsuta-ku, Nagoya, Aichi, Japan d IMS, University of Connecticut, Storrs, CT, USA Abstract Scanning Electron Microscope (SEM) in-situ observation of fatigue damage process of SiC/SiC composites was carried out at room and elevated temperatures. Although single edge V-notched specimens were used, fatigue crack initiated not only at the notch root but also at large pores far from the notch. The widespread damage means that the parameter of linear elastic fracture mechanics cannot be used to describe fatigue crack growth behavior of SiC/SiC materials. Instead, modulus of rigidity (MOR) may be used as a parameter to estimate fatigue damage process. The crack behavior was divided into two types of damage modes depending on the orientation of fiber bundles in front of a notch. No matter which type of damage mode, the final failure occurred at about 85% of the maximum value of MOR.  2002 Elsevier Science Ltd. All rights reserved. Keywords: SiC/SiC composite; Fatigue; Crack growth; Damage process; Elevated temperature 1. Introduction The SiC fiber reinforced SiC (SiC/SiC) composite is one of the most important ceramic matrix composites for high temperature applications. It has been reported that fatigue limit of SiC/SiC composites is about 80% of the ultimate tensile strength at room temperature, but the fatigue limit decreases at 1000°C, a temperature lower than that for fiber creep [1–5]. One part of the fatigue damage mechanism in SiC/SiC is debonding as a result of wear of the interface between the matrix and the fibers [1,2,4], as observed by in-situ observations [6,7]. A com￾plete evaluation of the fatigue damage process in SiC/SiC has not been established. In this work, fatigue tests of two types of SiC/SiC composites were carried out in an SEM (Scanning Elec￾tron Microscope) chamber at room temperature and at 800°C. The fatigue crack initiation and growth processes were observed in detail in the SEM. The evolution of * Corresponding author. Tel.: +81-258-47-9750; fax: +81-258-47- 9770. E-mail address: miyayuki@mech.nagaokaut.ac.jp (Y. Miyashita). 0142-1123/02/$ - see front matter  2002 Elsevier Science Ltd. All rights reserved. PII: S0142-1123(01)00078-0 fatigue damage was correlated with the change in rigid￾ity of the specimens. 2. Materials The SiC (Nicalon) fibers were approximately 15 µm in diameter. These fibers were assembled in flattened bundles with about 500 fibers per bundle. The bundles were cross-woven to form a layer approximately 0.3 mm thick. This pre-pleg layer was then coated with carbon by a CVD (Chemical Vapor Deposition) technique to provide a weak interface between fibers and the SiC matrix which in the next step was formed by a chemical vapor infiltration (CVI) procedure. This infiltration pro￾cedure resulted in a significant amount of porosity. Two types of matrices were used. One matrix was simply standard SiC. The second matrix consisted of SiC to which boron had been added. The boron reacts with oxy￾gen to form glassy particulates which seal the matrix phase and inhibit oxidation of the carbon-interfacial layer [8,9]. This type of matrix is referred to as an enhanced matrix. The individual layers were then sin￾tered together to form a 3 mm thick panel