Issue 48

S. Kikuchi et alii, Frattura ed Integrità Strutturale, 48 (2019) 545-553; DOI: 10.3221/IGF-ESIS.48.52 547 where a is the crack length on the surface, c is the crack length along the thickness direction, and t is the specimen thickness. K -decreasing tests were also conducted to approach the fatigue threshold. The present study employed disk-shaped compact (DC(T)) specimens (2 mm thick) [28-30] in accordance with the ASTM standard. The tests were conducted in the ambient laboratory atmosphere under a stress ratio R of 0.1. The fatigue threshold,  K th , is defined as the maximum value of  K under a crack growth rate of 10 -10 m/cycle. Crack lengths were monitored by the unloading elastic compliance method [33]. The magnitude of crack closure was also monitored; the closure stress intensity, K cl , was obtained from the closure load, P cl . After the fatigue tests, fracture surfaces and crack profiles were observed using scanning electron microscopy (SEM), and the microstructure around the crack paths was analyzed using electron backscattered diffraction (EBSD) at an acceleration voltage of 20 kV. R ESULTS AND DISCUSSION Microstructural characterization he microstructure of sintered compacts was characterized using EBSD. The image quality (IQ) map and grain boundary map obtained by EBSD analysis for the MM series is shown in Fig. 1. In the present study, the grain boundary is defined as the high-angle grain boundaries greater than 15º. The MM series contained regions of fine equiaxed grains and regions with a coarse microstructure. The regions of fine equiaxed grains formed a continuous connected three-dimensional network that surrounded the coarse-grained structure. This network is referred to as a harmonic structure in the present study. Figure 1 : Image quality (IQ) map and grain boundary map obtained by EBSD analysis for MM series. Figure 2 : Results of four-point bending fatigue tests, showing stress amplitude as a function of cycles to failure. Fatigue properties determined by four-point bending Fig. 2 shows the results of four-point bending fatigue tests for the sintered compacts (untreated and MM series); a stress amplitude,  a , was applied to the specimen surface as a function of the number of cycles to failure, N f . In this figure, those plots with an arrow represent the run-out specimens without failure at N = 10 7 cycles. The fatigue limit, which was defined as the average value of the maximum stress amplitude without fatigue failure and the minimum stress amplitude with fatigue Stress amplitude  a , MPa Number of cycles to failure N f , cycle 200 0 100 300 : Run-out Four-point bending R = 0.1 10 Hz 400 10 4 10 5 10 6 10 7 10 8 MM Untreated T