Issue34

S. Kikuchi et alii, Frattura ed Integrità Strutturale, 34 (2015) 261 - 270; DOI: 10.3221/IGF-ESIS.34.28 263 Fatigue testing The specimen used in the present study was the disk-shaped compact DC(T) specimen (2 mm thick, 25.2 mm wide). The sintered compacts (7.5 mm thickness, 25.2 mm wide) were cut to about 2.5 mm thickness and machined into the specimen. Then, specimen sides were polished with emery papers (#80 to #4000) and mirror-finished using SiO 2 . Fig. 2 shows the schematic illustration showing the specimen preparations. Fatigue crack propagation tests were conducted in an electro-dynamic fatigue testing machine under the condition of five values of the stress ratio, R , ranging from 0.1 to 0.8. To approach the threshold, K -decreasing tests were conducted under the constant- R loading regimen. Specimens were first fatigue pre-cracked for a minimum of 1 mm from the notch tip. The frequency of stress cycling was 30 Hz and the tests were carried out in ambient laboratory atmosphere. The value of the fatigue threshold,  K th , was defined as the maximum value under a crack growth rate of 10 -11 m/cycle. Crack lengths were monitored by unloading elastic compliance method [18]. The magnitude of crack closure was also monitored; closure stress intensity, K cl , was obtained from the closure load, P cl . Based on such measurements, an effective stress intensity range,  K eff = K max - K cl , was estimated. Where K max is the maximum value of stress intensity factor. Microscopic observations After testing, fracture surfaces were observed using scanning electron microscope (SEM) and crack profiles were observed and analyzed using electron backscatter diffraction (EBSD). Figure 1 : Image quality (IQ) maps obtained by EBSD for the (a) IP and (b) Harmonic series [14]. Figure 2 : Schematic illustration showing the specimen preparations. R ESULTS AND DISCUSSION Crack propagation behavior he relation of the crack growth rate, d a /d N , against the stress intensity range,  K , is shown in Fig. 3 for the IP and Harmonic series. In each series, the threshold stress intensity range,  K th , decreased and crack growth rate, d a /d N , increased at a given applied  K value with increasing stress ratio, R . Furthermore, in the Harmonic series, crack growth rates were constantly higher at comparable  K levels and thresholds were lower at comparable stress ratios compared to the IP series. Fig. 4 shows the relationship between  K th and R for each series. The value of  K th tended to be decreased approximately linearly with increasing R and the  K th value of the IP series was higher than that of the Harmonic series at comparable stress ratios. The relation between  K th and R in the IP and Harmonic series are expressed as Eqs. (1) and (2), respectively. T 100μm (a) 100μm (b)