Issue 53

R.R. Yarullin et alii, Frattura ed Integrità Strutturale, 53 (2020) 210-222; DOI: 10.3221/IGF-ESIS.53.18 213 Equivalent stress intensity factor Mixed-mode crack growth behaviors often occur in real structures with complex geometry and loading conditions that might additionally change during crack growth. An existing crack under Mode I-loading conditions will propagate within the original crack plane. Mode II-loading generally leads to a kinking of the crack, while Mode III causes a twisting of the crack front. To study the influence of the mixed-mode loading conditions on material fracture resistance parameters, it is necessary to calculate three fracture parameters characterized by the SIFs K I , K II , and K III . They are assigned to the basic fracture modes of a crack and defined by Eqn. (2). The SIF depends on the stress ( σ y , τ xy or τ yz ), the crack length a, and on the boundary correction factors (Y I , Y II or Y III ). If the loading of a structure creates a non- symmetrical and singular stress field near the crack front, then the crack front deforms in a way that not only an opening, but also a planar and non-planar displacement of both crack flanks can be found [20]: I y I K a Y     , II xy II K a Y     , III yz III K a Y     (2) Shlyannikov [21] generalized the numerical method to calculate the geometry dependent correction factors Y I , Y II, and Y III for the SIFs K I , K II , and K III under mixed mode fracture. The present study explores the direct use of FE solution results for calculating the SIFs K I , K II , K III, ahead of the crack tip ( θ =0º): 2 FEM I K r     , 2 FEM II r K r     , 2 FEM III K r     (3) where r, θ , and ω are polar coordinates centered at the crack tip, and FEM i  are the stresses obtained from the FE solution. Chang et al. [22] proposed a concept of effective SIF for three mixed modes. One can simply calculate the effective SIF and evaluate the fracture just by comparing it with the toughness. A new expression for the elastic equivalent SIF in which the in-plane angle of crack deviation  * is included in an explicit form has been proposed in [22]: * * 2 2 * * 2 * 2 1 (1 ) ( ) (1 )cos (1 cos ) 4 sin (5 3cos ) 2 2 eqv I II II III K K K K K K                               (4) In turn, the branching fracture angle is determined from the equation: * * * 2 * * 2 * 2 (1 ) 3 3 3 sin sin 4 cos 3sin 5sin sin 0 2 2 2 2 2 2 2 I I II II III K K K K K                                              (5) Plastic stress intensity factor The expediency of using the plastic SIF to characterize the fracture resistance as the self-dependent unified parameter through examples of using a cruciform specimens, compact tension–shear specimens, real components, a wide range of steels, and titanium and aluminum alloys at different temperatures is discussed in [13-15, 18, 23]. The plastic SIF P M K in mixed-mode small-scale yielding can be expressed directly in terms of the corresponding elastic SIFs using Rice’s J-integral. The J -integral formulation for general mixed mode conditions I, II and III leads to the following expression given by Rigby and Aliabadi [24]: 2 2 2 1 1 1 ( ) 2 I II II I II III J J J J K K K G E        (6) where E is the Young's modulus for plane stress, E*=E /(1-  2 ) for plane strain, G is the shear modulus and  is the Poisson's ratio. According to Hutchinson [25, 26] and Shih [27] for a crack subjected to mixed-mode loading in nonlinear strain hardening materials described by the Ramberg–Osgood constitutive equation, the Rice's J -integral can be expressed directly through the plastic SIF P M K by the following equation: