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S. Ackemrann et alii, Frattura ed Integrità Strutturale, 34 (2015) 580-589; DOI: 10.3221/IGF-ESIS.34.64 586 Fig. 5b shows the fatigue lifetimes vs. COD strain amplitude of PM 16-7-6 for uniaxial and biaxial loading cases (-1 ≤  ≤ 1). The COD strain gave a good correlation of uniaxial and all investigated biaxial fatigue lives including those for shear within a scatter band of factor two. The COD criterion proposed by Sakane et al. [3] is based on crack opening displacement and considers the smaller crack opening in shear loading case. Figure 5 : Fatigue lives N f of PM steel 16-7-6 and cast steel 16-6-6 [9] under uniaxial and biaxial cyclic straining (strain ratio  , phase shift  ) vs. a) von Mises equivalent strain amplitude Δ  vM /2 and b) COD strain amplitude Δ  COD /2 according to [3].  Surface cracks Crack directions of the major cracks which caused fatigue failure were studied on the specimen surface by using electron monitoring in an electron beam universal system since specimen dimensions were too large for SEM. Fig. 6 shows the major cracks in the specimen centers after cycling with  = 1, -1 and 0.5. The major crack was often observed at the transition radius of the specimens (indicated by circles in Fig. 6) which is caused by slight notch effects, as also reported in [5]. The major cracks were oriented at an angle of 45° to the loading axes A and B under equibiaxial loading (  = 1, Fig. 6a) and perpendicular to the axis of maximum principal strain for strain ratios  of 0.5, -0.1, -0.5 and -1 at Δ  vM /2 = 0.4 · 10 -2 (Fig. 6b). These surface cracks propagated in planes of maximum principal strain which correspond to stage I (mode II), see [1, 3]. This result is in agreement with the assumption of Brown and Miller [1] who proposed a crack type for equibiaxial loading which grows through the thickness and results in lower fatigue lifetimes. On the other hand, shear straining at Δ  vM /2 = 0.5 · 10 -2 (Fig. 6d) and 0.6 · 10 -2 revealed major cracks in the specimen center which propagated at first under an angle of 45° to the loading axes A and B. This direction indicated crack propagation in the plane of maximum shear strain which correspond to stage I crack growth (mode II). Subsequently, the major crack bifurcated into two pairs of cracks parallel to the loading axes A and B. Thus, a transition from stage I to stage II occurred. These findings are in good agreement to the results of Parsons and Pascoe [2] who found that stage I crack growth is dominant in the shear experiments prior to fatigue failure. Moreover, they also observed crack bifurcation at each crack tip with transition from stage I to stage II crack growth fairly late in fatigue life of AISI 304 stainless steel. Itoh et al. [3] found also cracks propagating in the maximum shear strain directions (mode II or stage I) only in shear tests, whereas other investigated strain ratios showed stage II cracks (mode I). The results of the present study support the assumption presented in literature [2] that the period of stage I (mode II) crack propagation is much longer under shear loading than under other biaxial conditions. For shear loading there is no tensile stress normal to the planes of maximum shear strain unless the crack path is deflected by material inhomogeneities or crack linking. Scanning electron microscopy was used for further investigations of the major cracks as well as cracks between 0.05 and 0.7 mm in length on the specimen surface. Fig. 7 shows major cracks with length greater than 1 mm and minor cracks smaller than 0.1 mm in length after fatigue failure under shear (  = -1) and equibiaxial (  = 1) loading, respectively. The major crack after shear loading (Fig. 7a) was nearly straight over wide distances and had smooth crack surfaces in contrast to the major crack after equibiaxial loading (Fig. 7b) which showed a distinct zig-zag path with several kinks. Crack branching was