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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( ) ( 2 1 2 σ σ σ σ σ σ − + − + − is a stress intensity. An earlier criterion (  =  1 /  Y ) led to excessive cleavage portion appearance in simulated fracture surfaces so it has been replaced by  1 /  i that makes it even more expedient as it actually refers to the local yield stress. - All the nodes in the crack plane ahead the crack tip are checked for critical  (an attempt of taking into consideration other nodes in planes parallel to the plane of symmetry appeared to change the ideology of the problem solution because a crack comes out of the plane of symmetry). The point ahead the crack tip where the maximum stress is attained may not coincide with the first node (the expected distance from the crack tip is about 3  CTOD). When the maximum stress is attained in some node, nodes closer to the crack tip are also uncoupled but not ascribed to the “cleavage” until the analysis of the full attained data set. If these nodes were kept uncoupled, material in their vicinity is unloaded at the next steps of loading so these points would not be ever “fractured”. - A crack extension to a single mesh size related to the attainment of critical  just after continuation of loading (displacement increase) is not ascribed to “cleavage” fracture. This decision may be explained by an appearance of the simulated load-displacement curve, where even none significant load drops may be seen while this curve contains a lot of single mesh-sized crack extensions on attainment of critical  , see Figure 2. Physically it may be explained by the necessity to speed up a crack before talking about its arrest. ) ( 2 ) 2 3 1 2 3 2 σ = i

a b Figure 2 – Examples of a load-displacement curve of V-notch Charpy (TKDS) specimen: (a) simulated, (b) real test. - The ductile fracture criterion corresponds a critical strain intensity (  cr ) attainment in any node ahead a crack tip in the plane of symmetry. The value of  cr is found based on classic papers on ductile fracture by McClintock (1968), Rice and Tracey (1969), Hankock and Mackenzie (1976) who dealt with the ultimate strain ability referred to an appearance and growth of pores from nonmetallic inclusions. This mechanism dominates for modern steel grades even at the minimum content of nonmetallic inclusions attainable by the metallurgical industry. The strain limit in this case relates to the loss of plastic stability: strain localization between pores and shearing the ligament. Simulation of this process shows that the fracturing strain depends on the stress state, namely on the ratio  m /  i . Strain  0 corresponding to the pore formation stage does not depend on  m , but the pores growing rate increases at a higher  m while the strain corresponding to the loss of stability decreases. Rice and Tracey (1969) suggested a model with initially spherical pores. Hankock and Mackenzie (1976) supposed that the fracturing strain is inversely proportional to the pores growth rate and developed a ductile fracture criterion in terms of global parameters (8). As per a later Handbook, Gorynin ed. (2009), the critical strain intensity  cr may be given as