PSI - Issue 2_A

Takashi Sumigawa et al. / Procedia Structural Integrity 2 (2016) 1375–1382 Author name / Structural Integrity Procedia 00 (2016) 000–000

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3.2. Fatigue damage Figure 5(a) shows FE-SEM images of the upper Cu surface after the fatigue test. Although no defects were present before the experiment, distinct traces appeared on the Cu surface upon cyclic loading. Figure 5(b) shows magnified SEM images of the upper Cu surface. Distinct traces were observed with a straight line-like appearance and inclined at an angle of approximately 45° to the applied normal stress. Analysis of the crystallographic orientation suggested that the traces were slip bands generated by the activity of the primary slip system. The slip bands were composed of extrusion and intrusion with a width of approximately 15 nm. The extrusion grew in the lower-right direction, which corresponded to the primary slip system, and had a height of approximately 30 nm. Although this was very similar to the extrusion/intrusion observed in fatigue of the bulk material [Mughrabi (1978), Lukas et al. (1968), Basinski et al. (1983)], the width (ca. 15-30 nm) was significantly different from that of the bulk (ca. 1 μm [Winter (1974), Volkert and Lilleodden (2006), Zhang et al. (2008)]). Moreover, a crack was observed along the grain boundary (see Fig. 5(c)), which appeared to have been initiated at the collision point of slip bands with the grain boundary where the stress was concentrated. 3.3. Formation stress of slip bands The polycrystalline material possesses complex stress distribution due to the deformation constraint [Sumigawa et al. (2004)]. The microscopic stress distribution in the nano-polycrystalline Cu was obtained by an elastic finite element method (FEM) analysis. The exact shape and crystal orientation of grains on the surface of the nano polycrystalline Cu were reproduced. Taking into account the crystalline orientations of the grains, the elastic

Fig. 5. FE-SEM images of the test section after the fatigue experiment.