PSI - Issue 17

Tamás Fekete / Procedia Structural Integrity 17 (2019) 464–471 Tamás Fekete / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 1. Hypergraph representation of the General Conceptual Model of Structural Integrity (Fekete (2018)).

4.2. Towards the new theoretical foundations of Structural Integrity

Using the framework of the above-described General Conceptual Model, the question has been investigated whether there is such a physical theory or discipline where both the methodological as well as the practice-oriented problems of Structural Integrity can be integrated into a single theoretical – more precisely, theoretical physics – framework; and in this sense, a theory that could serve as the background-theory for Structural Integrity. In Fekete (2018), the main focus was put on the role of modern Thermomechanics in solving Structural Integrity problems of accidental emergency events , which are fast, time-dependent thermomechanics + fracture mechanics problems, in the context of safety critical large-scale pressure systems. The main outcome of that study was that generalized classic thermo + fracture mechanics and the nonlinear field theory of thermo-viscoelastic-electromagnetic fracture can be derived from classic thermodynamics of irreversible processes (see Chen and Mai (2013)), while generalized Thermomechanics with internal variables can handle also microstructural effects (see e.g. Berezovski and Ván (2017)). Therefore, modern Thermomechanics can be used as a background theory for these types of problems. Most recently, computation strategies used in the long-time ageing assessments have been put into the focus of interest, where the calculations nowadays follow the prescriptions of standards (e.g. ASME (2019), or PNAE (1989)). It is well known among experts that these calculations rely heavily on the assumption that the system is in (at least thermal and mechanical) equilibrium – for the most part of the normal operation – , and the slow changes can be appropriately treated in the equilibrium approximation (see e.g. Hobbs et. al. (2011)). That is the reason why the role of time and of dissipation respectively is not coherently handled in ageing assessments, even today – e.g. fatigue calculations and other types of ageing – . The fact that the material models are linear, homogeneous, isotropic and time independent (see also Sect. 3.1.) also suggests that these models come from the late 19 th or early 20 th century (see Maugin (2009)), when the theoretical description of material ageing seemed inaccessible. This is an indication that the applied approach to describe ageing – which is used in the safety standards – , contains contradictions. When looking for the solution, we used the idea of Öttinger (2017) and postulated that dissipation occurs naturally and inevitably, starting from the smallest to the largest length-scales of interest . The general presence of dissipation also unavoidably introduces the arrow of time (see e.g. Martyushev (2017)). This means that the behaviour of the system can be described by time evolution equations , as dictated by the laws of Thermodynamics – see Öttinger (2017). Note that the principle described in the form above is formulated quite rarely in technical literature, even nowadays (as exceptions, see e.g. Hobbs et. al. (2011), Berezovski and Ván (2017), Khantuleva and Shalymov (2017)).