(a) N=5.9×102 cycles (b) N=9.1×104 cycles (c) N=4.5×105 cycles Figure 1: AFM images of transgranular cracking (σa =173 MPa, Material A). Figure 2: Change of intrusion geometry in fatigue (σa =173 MPa, Material A). Figure 3: Change of intrusion depth in fatigue (σa =173 MPa, Material A). In this section, the conditions for the crack-initiation in α-brass will be discussed as a function of slip-band angle relative to the stress axis, stress amplitude, mean stress, and grain size. In α-brass, fatigue cracks were initiated either from slip bands or along grain boundaries. Another example of transgranular cracking process is shown in Fig. 1. It is clear from these AFM images that two parallel fatigue cracks were initiated at N = 9.1 ×104 cycles (Fig. 1 (b)), and they were coalesced at N = 4.5 ×105 cycles (Fig. 1 (c)). The change of the geometry of Intrusion I at cross section A, which is indicated in Fig. 1 (a), is shown in Fig. 2. With crack-initiation at N = 9.1 ×104 cycles, the depth of the intrusion increased rapidly, and with coalescence of cracks at N = 4.5 ×105 cycles, the width of the cracks (intrusion) increased rapidly. Change of the depth of Intrusion I is shown in Fig. 3 as a function of the number of the fatigue cycles, N. Even though the measured value was not actual depth after crack initiation, the depth of the intrusion drastically increased with its outgrowth to a crack at N=9.1×104 cycles. From surface observation, cracks much shorter than the grain-size were not found. Even just after the initiation, a crack prevailed whole grain, and Stage I crack growth was not observed. In the thickness direction, however, the depth of intrusion changed continuously before and after the crack initiation, i.e., no jumping in the depth of intrusion was observed. It may indicates that Stage I crack growth, which is a fatigue crack growth along a slip-band, occurred in the thickness direction. Intergranular Cracking An example of the intergranular cracking is shown in Fig. 4. Slip-bands were formed in a grain on the right side of a grain-boundary, and they were parallel to the grain-boundary. Both intrusions and extrusions were observed in the grain. In the grain on the left size of the grain boundary, slip-bands are not observed. A fatigue crack was initiated from the grain-boundary at N=4.6×104. An intergranular crack was considered to have been initiated along grain-boundaries between grains with high Schmidt factor slip system and with low
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