PSI - Issue 28

V. Yu. Filin et al. / Procedia Structural Integrity 28 (2020) 3–10 Filin V.Yu., Ilyin A.V.,Mizetsky A.V. / Procedia Structural Integrity 00 (2020) 000 – 000

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Y (5) Formula (5) requires much more temperature reserve for NDT temperature than formula (3), especially for lesser thickness maybe because Ueda and Handa used experimental data for the thickness range 80 to 100 mm. At the same time Ueda and Handa noted that a high scatter was observed in their experimental NDT vs. CAT (ESSO) results. This might take place because steel usually has some degree of heterogeneity, and NDT is not a full thickness test. The direction of fracture in NDT and ESSO tests is different. For TKB test the correlation formula by Filin (2019) shows that TKB temperature is always higher than CAT: The present paper holds a discussion on simple criteria of ductile and cleavage fracture based on fracture mechanics and applicable for FEM simulation of fracture in specimens, ductile portion in fracture surfaces and the load drop. 2. Model In terms of LEFM, formula (2), the crack arrest temperature (CAT) seems to be of no sense as CAT corresponding to a critical K I a value for a given material should depend on the applied stress level and crack size. A crack arrest effect may be connected with a step-like fracturing stress increase this needs to implement an analysis of mixed mode deformation at the crack front (plane strain, plane strain/plane stress, plane stress). It is supported by known facts of dramatic increase of CAT for high specimen thickness and an appearance of CAT dependence on applied stress for higher thickness of metal. A correlation of ductile portion in fracture of full-thick specimens with K I a is also impossible with no account of mixed mode fracture and thickness effect. Ilyin et al (2018) presented FEM simulation results of fracture in wide plates under nominal-elastic loading in presence of an initial crack. Numerical simulation was performed for low-alloyed shipbuilding steel with a simple exponential representation of the loading diagram out of elastic loading (Holloman-Ludwig equation). A key moment for fracture simulation is setting the fracture criteria. An exact following the known local fracture criteria requires a super fine mesh to fit the physical and geometrical non-linearity of the problem and leads to the loss of the problem symmetry at the expense of getting the shear lips. That is why a lot of known research results are limited by a crack start analysis but not its propagation. An interesting but rather complicated local approach-based model of crack propagation in ESSO specimens is suggested by Shibanuma et al (2016). Material tested and FEM simulated was 40 mm thick rolled plate of steel grade F40SW as per GOST R 52927. Chemical composition: 0.08-0.11% carbon, 1.15-1.65% manganese, 0.65-1.05% nickel, 0.15-0.40% silicon,  0.012% sulfur,  0.015% phosphorus. The applied loading conditions and simplified criteria for FEM simulation have been modified as follows: - Holloman-Ludwig equation parameters of the analyzed steel are as follows:  Y = 397 MPa, strain hardening exponent  = 0.108 (i.e., 11% uniform elongation). - 3D quasistatic problem represents one fourth of a V-Charpy three-point bend specimen, uniform FE mesh in the crack area has the dimensions 0.1 by 0.1 by 0.2 mm, the last figure relates to the direction along crack front. - Two competing conditions are considered: a critical stress referred to as “cleavage” fracture condition and a critical strain referred to as a “ductile” fracture condition. - Loading is realized in the way of step-by-step displacement increase in the direction normal to the crack plane. A crack extends at the expense of uncoupling the mesh nodes for which at least one fracture criterion is fulfilled. Displacement increase (continuation of loading) is only carried when none of the fracture conditions are met after sequential nodes uncoupling. - The first principal stress is considered to be normal to the crack plane. So the simulation of fracture in the plane of symmetry is physically grounded. A pair of values, i.e. mesh size and the coefficient  , allows to simulate in a uniform mesh the cleavage fracture condition corresponding to a certain SIF.  =  1 /  i , (7) where  1 is the first principal stress, 5 153 21.74 ln 293.8 NDT CAT = − +   + − S . ( )                 − + − σ 1 16 2.1 0.01 S  − + + 0.17( 14) S = − 50 ln TKB CAT Y(NDT ) 2 Y(NDT ) 0.44 σ 0.0005 S S . (6)