Issue 35

X.C. Arnoult et alii, Frattura ed Integrità Strutturale, 35 (2016) 509-522; DOI: 10.3221/IGF-ESIS.35.57 518 In case of crack divider, formation of delamination crack is mainly driven by stress component zz  . In front of the crack in a finite thickness plate (for example a CT-specimen), zz  reaches its maximum in the middle of the thickness. To evaluate the intensity of zz  over the thickness of specimen, the out-of-plane constraint factor Z T , for a mode I through thickness crack, has been given by [26]       0.58 1 / 2.3 1 1/2 3/2 2 0.94 / 1 1 1 1 1.218 0.395 0.361 2 2 1 n n z p r B r r r r T r B B B z B                                                               (1) For 0 z T  Where B  thickness, n  strain hardening exponent, p r  average size of plastic zone through the thickness of the plate [26]. As was observed in [1-3, 10, 27], a large delamination crack occurs at the middle of specimen. As a result of equation (1), its size increases with increasing thickness of the specimen [1]. Newly created free surface causes local drop in z T (cf. Figure 10) and condition for the formation of smaller delamination cracks at the quarter of the thickness. Figure 10 : The out-of-plane constraint factor T_z in front of a mode I crack, solid line with delamination crack, dashed line without delamination crack [1]. A number of authors [1, 8-10, 18, 19, 22, 28] noticed an enhancement of the fracture toughness properties ( c K , c J ), significantly higher lower-shelf energy and shifting at lower temperature of the DBTT during Charpy impact tests. The same authors observed the development of multi-delamination cracks that were found on the fracture surface of Charpy and CT-specimens as well as a single central delamination crack on tensile notch or smooth specimens [6, 10]. When fracture toughness tests are performed using CT-specimens, the parameters c K and c J are thickness dependent [29]. Due to the stress distribution at the crack tip, c K and c J increase when the CT-specimen with decreasing thickness. Thin specimens are in plane-stress condition whereas thick specimens are in plane-strain condition. The same dependence was also shown for the Charpy impact test [2] . Then as shown on Figure 10, there is a triaxial stress relaxation, (because zz  is reduced to zero), and transformation of the global plane-strain fracture into a serie of local plane-stress failure