PSI - Issue 22

Maksym Zarazovskii et al. / Procedia Structural Integrity 22 (2019) 305–312 Maksym Zarazovskii and Yaroslav Dubyk / Structural Integrity Procedia 00 (2019) 000 – 000

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The paper presents critical analysis of the currently existing methods of PTS calculations (namely EPFM aspects), CTB prediction and examples of methodological misconceptions with impact demonstration on RPV lifetime. Practical recommendations for further development of normative documents that specifies CTB are formulated (which include the chemical factor). 2. Computational uncertainties 2.1. J-integral assessment within EPFM Since the EPFM is widely recommended as a computational tool for left part of criterion (1) calculation, including nuclear industry (see the IAEA-TECDOC-1627 (2010)) for the NPP structures and components, the main issue is the EPFM validity. Historically, the J -integral appeared as a procedure that facilitates the calculation of the SIF only. In this case, the necessary condition for a reliable calculation is the strict satisfaction of the small-scale yielding condition, which is included in all standards for the FT determination (see for example ASTM E1820). Eventually, with the development of computational technologies, the usage of the J -integral has become very popular. However, practical application of the J -integral has enough limitations. Technically, the J -integral, even for an LEFM region, can lead to incorrect results and its practical application has enough limitations, among which the following should be noted:  The integration contour must be free from internal forces, such as residual (from welding or thermal stresses);  The material should be homogeneous, in case of Bi-metal the convergence could be hardly reached;  The surfaces of the crack must be free of loadings (e.g. internal pressure);  The integration circuit should not pass through the plastic zone around the crack, so it is necessary to choose an integration circuit that is located as far as possible.  Within EPFM the J-integral inherent absence of convergence in case of non-proportional loading, see Fig.1.

Fig.1 J-integral convergency in time for different radius of integration circuit

The J -integral is known as a path-independent integral and can be calculated using an arbitrary integral path. However, in non-proportional loading or elastic-plastic problems assuming large deformation, the J -integral loses its path-independent property. For PTS analysis a modified version of J -integral must be used see Arai et al. (2018) and Lei (2005). It should be noted, that design RPV brittle strength calculations were performed within LEFM i.e. both sides of criterion (1) were determined in the elastic formulation. But, eventually, left side of (1) transformed to the EPFM, at the same time, right part still determined within LEFM. From the authors point of view, it may be justified only by increase of conservatism of assessments by the (1).