Y. Miyashita et al. /International Journal of Fatigue 24(2002)241-248 o'bundle by using a specimen equipped with a strain gauge at room temperature and developing a strain-actuator dis- rain gauge placement relationship. A tungsten wire heater attached R10 to the specimen gauge section was used to heat the speci mens in the 800%C fatigue tests In order to stabilize the test conditions, the specimen was held at 800oC for 30 min prior to testing The fatigue tests were carried out under axial loading using a sinusoidal wave form at a stress ratio of 0. 1. The test frequency at both temperatures either 1 or 10 Hz, which was reduced to 0. 1 Hz when obtaining stress-strain data. In the fatigue tests the specimens were initially cycled at a maximum stress of 45 MPa, a level which was 15 MPa less than the critical stress for matrix cracking. If cracks were not observed at this level after 5000 cycles at 1.0 Hz or 20,000 cycles at 10 Hz, the Fig. 1. Specimen geometry(in mm) maximum stress was increased by 5 MPa. The specimen was then cycled for an additional 5000 or 20,000 cycles If no cracking occurred the incremental loading pro- 3. Specimens and tests cedure was repeated until a stress range was reached at which cracks were observed. The area under observation The specimen geometry for the fatigue tests is shown was within 2 mm of the crack tip in Fig. 1. A machined notch facilitated the study of the crack initiation and propagation processes. This notch was V-shaped with a notch depth of I mm and a notch 4. Results and discussion root radius of 15 um A servo-hydraulic fatigue machine designed for test- 4. 1. Fatigue life ing of the specimens in the chamber of the SEM was utilized for the in-situ observations and stress-strain The relationship between the maximum stress and hysteresis loops were obtained periodically during number of cycles to failure for both the standard SiC/SiC fatigue tests. In the room temperature tests strains were and enhanced SiC/SiC composites is shown in Fig. 2 measured by using a strain gauge of 1 mm gauge length The large scatter in results can be attributed to wide vari- mounted in front of the notch, as shown in Fig. 1. In ations in both crack initiation and growth behavior the tests at 800C, the displacement of the actuator was Cracks initiated not only in front of a notch tip but also utilized to estimate strains. A calibration was established at large pores remote from the notch tip. Fatigue life at Smooth sp Stress-Number of cycles curve of standard SiC/SiC at RT and 10Hz Stress-Number of cycles curve of standard SiC/SiC at 1000"C and 20Hz 200 The present notched specimen ● Standard SiC/SiC, R A Enhanced SiC/SiC, 800C. Load frequency of data unmarked is 10Hz. 7 Number of cycles to failure Fig. 2. Results of fatigue tests at room and elevated temperatures.242 Y. Miyashita et al. / International Journal of Fatigue 24 (2002) 241–248 Fig. 1. Specimen geometry (in mm). 3. Specimens and tests The specimen geometry for the fatigue tests is shown in Fig. 1. A machined notch facilitated the study of the crack initiation and propagation processes. This notch was V-shaped with a notch depth of 1 mm and a notch root radius of 15 µm. A servo-hydraulic fatigue machine designed for test￾ing of the specimens in the chamber of the SEM was utilized for the in-situ observations, and stress–strain hysteresis loops were obtained periodically during the fatigue tests. In the room temperature tests strains were measured by using a strain gauge of 1 mm gauge length mounted in front of the notch, as shown in Fig. 1. In the tests at 800°C, the displacement of the actuator was utilized to estimate strains. A calibration was established Fig. 2. Results of fatigue tests at room and elevated temperatures. by using a specimen equipped with a strain gauge at room temperature and developing a strain-actuator dis￾placement relationship. A tungsten wire heater attached to the specimen gauge section was used to heat the speci￾mens in the 800°C fatigue tests. In order to stabilize the test conditions, the specimen was held at 800°C for 30 min prior to testing. The fatigue tests were carried out under axial loading using a sinusoidal wave form at a stress ratio of 0.1. The test frequency at both temperatures was either 1 or 10 Hz, which was reduced to 0.1 Hz when obtaining stress–strain data. In the fatigue tests the specimens were initially cycled at a maximum stress of 45 MPa, a level which was 15 MPa less than the critical stress for matrix cracking. If cracks were not observed at this level after 5000 cycles at 1.0 Hz or 20,000 cycles at 10 Hz, the maximum stress was increased by 5 MPa. The specimen was then cycled for an additional 5000 or 20,000 cycles. If no cracking occurred the incremental loading pro￾cedure was repeated until a stress range was reached at which cracks were observed. The area under observation was within 2 mm of the crack tip. 4. Results and discussion 4.1. Fatigue life The relationship between the maximum stress and number of cycles to failure for both the standard SiC/SiC and enhanced SiC/SiC composites is shown in Fig. 2. The large scatter in results can be attributed to wide vari￾ations in both crack initiation and growth behavior. Cracks initiated not only in front of a notch tip but also at large pores remote from the notch tip. Fatigue life at