PSI - Issue 59

Eugene Kondryakov et al. / Procedia Structural Integrity 59 (2024) 50–57 Eugene Kondryakov et al. / Structural Integrity Procedia 00 (2023) 000 – 000

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Fig. 4. Engineering and true stress-strain diagrams for steel 15Kh2NMFA.

The equivalent stress distribution fields for all specimens with a stationary crack are shown in Fig. 5. It can be seen that the reduction of crack tip constraint in the miniature specimens compared to the 0.5T C(T) specimens leads to an increase of the plastic zone at the crack tip.

Fig. 5. Distribution of the equivalent stresses in 0.5T C(T) (a), side-grooved 0.5T C(T) (b) and 0.16T C(T) (c) specimens for stationary crack.

Fig. 6 shows the evolution of the normalised values of the opening mode stress component with the applied load in the three types of CT-specimens at two different crack front locations: at the mid-thickness of the specimen (path 1) and near the free surface (path 2), where the normalized distance from the crack tip L norm = L/(W-a). It can be seen that the stress near the crack tip in the miniature specimens (Fig. 6c) is significantly higher than in the 0.5T C(T) specimens (Fig. 6a). This is due to the reduction of constraint at the crack tip (which depends on the specimen's geometric dimensions and decreases with reducing thickness), leading to an increase of the plastic zone around the crack tip. This in turn leads to an increased probability of ductile failure and overestimation of fracture toughness values. Also Fig. 6a,c shows that the stress state in the middle section of the 0.5T C(T) and 0.16T C(T) specimens and at their surface differs significantly. The presence of side grooves reduces the effective specimen thickness and creates conditions close to plane strain in the middle of the specimen, decreasing the influence of surface layers. In Fig. 6b, it can be seen that in specimen with side grooves, the stress across the specimen thickness near the crack tip is almost uniform (Fig. 6b), which should ensure propagation of a uniform crack during specimen fracture. For modeling the fracture and crack propagation process in the XFEM method, a crack initiation and propagation criterions are used. The values of maximum principal stresses σ c or strains ε c are typically used as the parameters for the crack initiation criterion. The crack propagation criterion uses the energy release rate required for crack growth. Many ways of determining these parameter values can be found in the literature, but there is no universal method, so