PSI - Issue 14

Filin V.Yu. et al. / Procedia Structural Integrity 14 (2019) 758–773 Filin V.Yu, Ilyin A.V. / Structural Integrity Procedia 00 (2018) 000–000

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Supposing a crack arrest temperature corresponds to a certain K Ia , it should also depend on applied stress and crack size. This needs an account of mixed plane-strain to plane stress mode and thickness effect. FEM simulation was performed with steels of 400 MPa and 600 MPa yield strength, here some derived correlations are presented. Formulae below are given for the following units of measurement: MPa m , MPa, mm,  C. NDT criterion is based on two reference curves. One of them is given in ASME BPVC Section XI, Division 1, it is a lower envelope for reactor steels 550 to 890 MPa yield strength:   29.4 15.6exp 0.025( NDT) Ia    T K . (35) Another correlation for medium estimates is derived from ESSO test results for hull steel 480 MPa yield strength, this formula does not contradict with (34) so it is applicable for further reasoning,   ) (NDT) exp 0.0134( ( ) 1a 1a T NDT K T K   . (36) For NDT specimen FEM simulated by Ilyin et al. (2018) it was found that at first steps of crack propagation K I increases, so crack start values may be referred to as K Ia : (37) FEM simulation undertaken by Ilyin et al. (2018) gives an opportunity to conclude that the required K Ia at the minimum design temperature is   Y Y 1a d σ /1000 0.27 σ ( ) 0.00078    S K T . (38) The corresponding temperature shift between NDT and T d (NDT criterion) should amount to     105 0.27 σ 74.6 ln 0.00078 NDT Y d     S T , (39) that is applicable when T d –NDT  0, i.e. for thickness S > 25 mm. TKB criterion is derived from FEM simulation by Ilyin et al. (2018) of crack growth in TKB specimens of different thickness where an attainment of a certain plastic strain level corresponds to shear fracture while an attainment of a certain stress level is ascribed to cleavage fracture. Such an approach allows a simulation of fracture appearance and percentage of shear portion. The following ‘physical’ features of fracture are observed (Fig. 9): – crack start after an attainment of the ultimate bearing ability of a specimen (plastic hinge), – shear fracture criterion is attained first at a crack start from the notch, – cleavage fracture spots appear at higher thickness or lower critical stress level, – crack tunneling and shear lips. Y 1a (NDT) 0.13   K .