13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- phase in the vicinity of an inclusion. a) b) c) Figure 4: Phase morphology after the formation of predeformation-induced ’ martensite with a volume fraction of 27% (a) and of 54% (b) produced by different deformation start temperatures and dislocation coalescence around an inclusion (c). a) b) c) d) Figure 5: Crack initiation at an inclusion (a), image quality of the inclusion in the direction perpendicular to the fracture surface (b), phase distribution around the inclusion (c) with red = austenite phase and green = martensite phase and inverse pole figure of the inclusion and its surrounding matrix (d). The VHCF failure for AISI304 with a volume fraction of ’ martensite > 30% is accompanied by a change of the crack initiation site from the surface to the specimen interior irrespective of the fact that inclusions encircled by clusters of martensite can also be found in the surface region. The fracture surface of the specimens failed in the VHCF region show a typical fisheye morphology. The crack initiation site is surrounded by a fine granular area (FGA), which according to Sakai [9] is characteristic for the VHCF failure of martensitic steels. The FGA for its part is encompassed by a smooth fracture surface which gives the typical fish-eye appearance. According to Murakami and Matsunaga [19] the cyclic stress intensity factor at the outer front of the FGA can be estimated by FGA FGA area K 0.5 . An analysis of all specimens’ FGAs, which failed in the VHCF regime (figure 1a), result in an average KFGA = 4 MPam beiing in good correlation with the threshold value for crack growth of the martensitic phase Kth = 5 MPam as investigated by Bowe et al. [18]. Hence, the formation of FGA can be regarded as crack initiation phase provided one assumes that the K-concept can be applied for the given VHCF failure mechanism. This phenomenon strongly depends on the number of loading cycles as depicted in figure 6a. There is a steady increase of FGA diameter with increasing number of loading cycles. After reaching a critical value (corresponding to KFGA) a stacking fault e e failed austenite grain fracture surface
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