Issue 35

X.C. Arnoult et alii, Frattura ed Integrità Strutturale, 35 (2016) 509-522; DOI: 10.3221/IGF-ESIS.35.57 519 caused by delamination cracks. Thus the multi-delamination cracks acts as a sum of plane-stress subunits (cf. Figure 11). [1, 2, 19, 28] and the fracture toughness and impact toughness could increase. Figure 11 : Effective thickness B1, nominal thickness B [1]. Yan et al [6] provided an interesting explanation about the development of delamination taking into account the plastic zone near the crack tip. In the case of Charpy impact test, the specimen bends and a plastic zone is generated at the notch and the contraction in the thickness direction is constrained. In this way, a triaxial stress state is created. When the crack grows, the location of this plastic zone will move from the zone close to the notch to extend across the thickness. The magnitude of stresses in this plastic zone is significantly higher than the yield stress that the delamination cracks generated. Both the low strain-hardening capacity for tempered steel and the elongated grains in the triaxial tensile stress zone facilitate the generation of delamination cracks along the weak paths and lead to distinct delamination cracks. Rao et al [13] noticed similar behavior in aluminum-lithium alloys, having small strain hardening and elongated grains, when they performed fracture toughness tests with CT-specimens. No large delamination cracks or only minor local delaminations were observed in steels having medium capacity for elongation and necking. During a Charpy impact test, delamination cracks may appear if the stresses in the thickness direction are high enough to delaminate the anisotropic microstructure along its weak paths. Perhaps, it is one of the reasons why the delamination cracks are more likely generated at the temperature transition where the fracture mode is mixed for non-active material, and around 11 dpa for irradiated austenitic steel with delta-ferrite content [16, 17]. Guo et al [1] defined a criterion to estimate when the delamination crack will occur in the case of fracture toughness test. It is assumed that the in-plane strength of the material is c  , and the strength in the thickness direction is zc  and zc c    . When   yy c c r    , macrocrack will occur, when   zz c zc r    delamination cracks will occur and the delamination cracks will appear before macrocrack will propagate if zc c    . Kalyanam et al [15] show the influence of the stress distribution of a central delamination crack in CT-specimen. Near the tip of the macrocrack, the delamination crack produced a small region of traction free surfaces on the center plane, where the delamination crack is located, which then led to a sharp gradient in stress fields similar to those observed at the outside surface. It can be noticed that the stress component zz  is equal to zero at the delamination crack location, thus locally, the material is in plane-stress condition. At the head of delamination crack, the stress component yy  is about 1.4 ys  in the absence of delamination cracks and increases to about 2.1 ys  in the presence of delamination cracks. For the stress component xx  , the presence of delamination cracks represents a difference in the order of 0.5 ys  near the center plane. Thus at the head of the macrocrack near the delamination crack, there is plane-stress condition whereas at the head of the delamination crack far from the macrocrack, the triaxial state of stress prevails. To estimate simply the “subunit toughness” in assuming that the steel behaves as a laminate, the following equation could be used [32]