Issue 35

X.C. Arnoult et alii, Frattura ed Integrità Strutturale, 35 (2016) 509-522; DOI: 10.3221/IGF-ESIS.35.57 516 As seen in Table 3, there are various kinds of steels where the delamination cracks could be one of the fracture modes. All these steels have a common characteristic, which is the presence of ferrite in the microstructure. Different authors performed metallography analyses to understand the origins and the mechanisms of the delamination cracking process. Typically, it has been concluded that the region between delamination cracks was similar to a ductile tensile failure (cf. Figure 8a) or cup-and-cone shape [1, 18, 19, 22]. Figure 8 : SEM image of Charpy specimen tested at -170°C a), orientation map of grains around the delamination cracks b), and orientation map of grains at the top of subunits c) [18]. Furthermore, dimples were often present in the vicinity of the delamination cracks [1, 22].The necking-like shape of the region between delamination cracks indicates a relatively high degree of ductility even at low and very low test temperatures [1, 18, 22]. Moreover, the arrows in Figure 8a show chains of large voids in the specimen under the fracture surface parallel to the delamination cracks. However Baldi and Buzzichelli [23] note that delamination cracks have a propensity to propagate in brittle manner, and usually the delamination cracks were displayed on brittle fracture surface [8-10, 17, 18, 20, 23]. That is a paradox of delamination cracks. When the failure mode is ductile, majority of investigators claim that the delamination cracks are not developed, but around a transition region, where the shear failure mode and the brittle failure mode are in competition, the delamination cracks have chances to be promoted. As seen above, the delamination cracks are consequences of a very ductile behavior It was observed that delamination crack initiation and growth was located along weak interfaces [2], some delamination cracks follow along the path of the grain boundaries of highly elongated grains [22], or occurred between coarse ferrite grains and a region richer in martensite, bainite or pearlite phases, i.e. between softer and harder phase [9]. Figure 8b [18] shows grain orientation around delamination cracks; two colors dominate, indicating that the texture is highly anisotropic inside this ultrafine-grained steel. The red is for the family crystal orientation {100} and the blue for the family crystal orientation {111}. It can be seen that delamination cracks opened the interface of elongated clusters of grains with different crystal orientation components, demonstrating that the delamination cracks spread along the boundaries of grains having high-angle grain boundary misorientation. Hara et al [3, 8] made a relationship between the microstructure, texture and CTOA (crack tip opening angle) results. Table 4 shows the results of CTOA related to the phases present inside different steels and the texture. According to the authors, for the steel A, there were no delamination cracks on the fracture surface. For the steels B and C, few small delamination cracks were shown on the fracture surface, however, for the steel D, large delamination cracks were present on the fracture surface, and it is for the steel D that the CTOA was the lowest. In this case, delamination cracks can have a negative impact on mechanical properties. The steel D is the one having the highest intensity of crystal plane family {100}. The comparison between the microstructure of steel B (or C) compared with that of steel D clearly shows that deformed ferrite increases sharply the intensity of crystal plane family {100}, and the differences in the