PSI - Issue 14

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

772

15

a b Figure 9 — Fracture surfaces showing a propagating crack ( a ) FEM simulated distribution of attained plastic strain intensity  pl, horizontally hatched areas correspond to a crack non-propagation. ( b )Real fracture surfaces of TKB specimens: predominantly cleavage fracture (left), altered fracture (right). FEM results are approximated by the formula   0.006 0.04 (TKB) Y 1a    S K . (40) In view of (38) and a negligible thickness effect, TKB criterion is     0.27 σ 19 74.6 ln 0.00078 TKB Y d     T . (41) This equation tells that a temperature shift in TKB test in reference to T d has an opposite sign, so TKB usually appears the critical test at certification of materials. In our above example ( S = 200 mm,  Y (20°C) = 500 MPa, T d =-10°C), an applied base material should have NDT  -88°C, TKB  +23°C to arrest a cleavage crack. 5. System of standard documentation accompanying the design of new structures A scatter of material properties including fracture toughness has a great effect on the strength and reliability of large critical structures, it can change the reserve factor in 3 times and even more. In other words, when there is no stable quality, the median estimates of material and structure performances shall be unrealistically high. So, in any branch like machine building, shipbuilding and so on, the overall quality control system needs to be established. Any operation in the technological route, e.g. heat treatment, welding or assembling results in the change of properties of the product, so the material performances being directly or indirectly influenced by this operation need inspection and destructive testing. It is never possible in the full amount due to technological and economic reasons as well as stochastic nature of technological deviations. In other words, the exact knowledge of all parameters and performances of a material and structure would be too expensive if ever possible. So we come to the thesis about admission of technologically unavoidable flaws and an optimum amount of quality control. In this context the admissibility criteria for flaws and other technological deviations are developed by structural integrity scientists. On the other hand, even the highest modern level of fracture mechanics is helpless