1332 J C. McNulty et al. /Composites Science and Technolog y 61(2001)1331-1338 power-generation industries. Experiments are performed to assess the fatigue performance at a moderately high temperature(815oC). This temperature falls in the regime in which embrittlement has been reported for other SiC based CFCCs, yet is also relevant to the service environ- ments in which CFCC components are expected to operate in turbine engine applications. The sensitivity of fatigue life to stress concentrations is probed through tests on specimens with circular holes and notches as well as straight(unnotched)specimens. The study of the effects of stress concentrations is motivated by the need, in some instances, to use through-holes for either attachment or cooling in engine components. In a broader context, it might also be used to infer the extent of degradation due to foreign impact damage, assuming at the simplest level ( that the damaged region is unable to sustain load and hence acts similarly to a hole or notch. Some additional insights into the local conditions required for embrittle- ment and fracture in the notched geometries are obtained through calculations of the stresses in the notched geo metries, by the use of a non-linear constitutive law appropriate to CFCS 2. Materials and experimental measurements All experiments were performed on a composite com prising SylramicTM SiC! fibers in a plain-weave archi tecture and a Sic matrix produced by a proprietary hybrid process involving chemical vapor infiltration(CVI) and reactive melt infiltration(MD). The microstructure of the (b) composite is shown in Fig. 1. The notable features include the presence of a uniform Bn coating on the fibers, an overcoat'of SiC around the fiber tows(produced by CVn) and a Sic-based two-phase matrix (produced by reactive MI). The matrix is essentially fully dense, with out the large pores that are invariably present in CFCCs made by CVI alone. The absence of pores in the matrix has two consequences. First, it increases the stress at the onset of matrix cracking relative to that of the cvi CFCCs containing pores. Secondly, it is expected to improve the thermal conductivity, especially in the dire tion transverse to the fibers The fiber volume fraction To assess the effects of stress concentrators on low cycle fatigue(LCF), three types of specimens were used: (i) standard dog-bone(unnotched) specimens; (ii) straight tensile specimens of width 2W=31.8 mm, with a center hole of diameter 2a=6.35 mm (a/w=0.2); and (ii) straight tensile specimens of width 2W=31. 8 mm, with a Fig. 1. Scanning electron micrographs of a cross-section through the omposite:(a) at low magnification, showing the longitudinal and center notch of length 2a=6.35 mm (a/W=0.2)and transverse fiber tows; (b)at higher magnifications, showing the melt notch root radius p A0.2 mm. The specimens were tow. The micrographs were taken in backscatter electron imaging infiltrated matrix surrounding a fiber tow, and (c) the interior of the prepared by electrodischarge machining. Four spe mens of each geometry were tested at a temperature of (BED mode, thereby revealing atomic number contrast. The coatings on the fibers are bN. The Sic within the tows was produced by chemical vapor infiltration process, prior to melt-infiltration of th Produced by Dow Corning. remainder of the matrixpower-generation industries. Experiments are performed to assess the fatigue performance at a moderately high temperature (815
C). This temperature falls in the regime in which embrittlement has been reported for other SiC￾based CFCCs, yet is also relevant to the service environ￾ments in which CFCC components are expected to operate in turbine engine applications. The sensitivity of fatigue life to stress concentrations is probed through tests on specimens with circular holes and notches as well as straight (unnotched) specimens. The study of the effects of stress concentrations is motivated by the need, in some instances, to use through-holes for either attachment or cooling in engine components. In a broader context, it might also be used to infer the extent of degradation due to foreign impact damage, assuming at the simplest level that the damaged region is unable to sustain load and hence acts similarly to a hole or notch. Some additional insights into the local conditions required for embrittle￾ment and fracture in the notched geometries are obtained through calculations of the stresses in the notched geo￾metries, by the use of a non-linear constitutive law appropriate to CFCCs. 2. Materials and experimental measurements All experiments were performed on a composite com￾prising SylramicTM SiC1 fibers in a plain-weave archi￾tecture and a SiC matrix produced by a proprietary hybrid process involving chemical vapor infiltration (CVI) and reactive melt infiltration (MI). The microstructure of the composite is shown in Fig. 1. The notable features include the presence of a uniform BN coating on the fibers, an ‘overcoat’ of SiC around the fiber tows (produced by CVI) and a SiC-based two-phase matrix (produced by reactive MI). The matrix is essentially fully dense, with￾out the large pores that are invariably present in CFCCs made by CVI alone. The absence of pores in the matrix has two consequences. First, it increases the stress at the onset of matrix cracking relative to that of the CVI CFCCs containing pores. Secondly, it is expected to improve the thermal conductivity, especially in the direc￾tion transverse to the fibers. The fiber volume fraction is 35%. To assess the effects of stress concentrators on low cycle fatigue (LCF), three types of specimens were used: (i) standard dog-bone (unnotched) specimens; (ii) straight tensile specimens of width 2W=31.8 mm, with a center hole of diameter 2a=6.35 mm ð Þ a=W ¼ 0:2 ; and (iii) straight tensile specimens of width 2W=31.8 mm, with a center notch of length 2a=6.35 mm ð Þ a=W ¼ 0:2 and notch root radius  0:2 mm. The specimens were prepared by electrodischarge machining. Four speci￾mens of each geometry were tested at a temperature of 1 Produced by Dow Corning. Fig. 1. Scanning electron micrographs of a cross-section through the composite: (a) at low magnification, showing the longitudinal and transverse fiber tows; (b) at higher magnifications, showing the melt￾infiltrated matrix surrounding a fiber tow, and (c) the interior of the tow. The micrographs were taken in backscatter electron imaging (BEI) mode, thereby revealing atomic number contrast. The coatings on the fibers are BN. The SiC within the tows was produced by a chemical vapor infiltration process, prior to melt-infiltration of the remainder of the matrix. 1332 J.C. McNulty et al. / Composites Science andTechnology 61 (2001) 1331–1338