PSI - Issue 17

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

467

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

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reliability – of a structure is decisively influenced by the behaviour of its most inhomogeneous zones. When the design takes the effects of the most inhomogeneous zones into account, and pays special attention to the assessment of the behaviour of the maximally inhomogeneous zones, then the design follows the ‘ maximum inhomogeneity principle ’ . Since the early days of fracture mechanics, it is known that the stress-strain fields are most highly inhomogeneous around cracks and crack-like flaws. In this case, when damage tolerance is required, the effects of crack-like flaws is taken into account in the assessments of inhomogeneous zones. At each refinement step of the design – in order to confirm the adequacy of the modifications – , control calculations are carried out with the aim to prove the adequacy of the modified geometry. These computations follow the maximum inhomogeneity principle. Therefore, the primary objective of these calculations is to study the changes in the behaviour of the inhomogeneous zones, if the geometry is changed. The calculations use the classic approach of thermal stresses, together with fracture mechanics calculations – in case of a damage tolerant design. The modified geometry can be accepted when the stress-strain fields and/or the fracture mechanics parameters at critical locations of the model are in compliance with the particular safety criteria, as prescribed in the design standards, e.g. in ASME (2019), or in PNAE (1989). As soon as the detailed System plan – the geometric model, coupled with the specification of structural materials assigned to it – is completed, it is possible to realistically examine its long-term safety, with the design safety calculations. The design safety calculations are carried out in order to prove that the System will keep its required safety/stability features, starting from the moment of commissioning, at least up to its expected service lifetime; these calculations are applied to the expected time series of loadings and environmental conditions. The System is considered safe, if it satisfies – both as a whole as well as its individual parts – the constraints and the stability criteria prescribed in the design standards (see e.g. ASME (2019), or PNAE (1989)) and safety calculation guidelines (see e.g. Kang and Kupča (2010), or VERLIFE (2013)). The design allowed lifetime of a component is the timeframe between the beginning of the operation of the component, and the moment when one part of the given component violates the relevant prescribed safety criteria. The design allowed lifetime of the System is the timeframe between the start of operation of the System, and the moment when one of its safety critical parts or components violates the relevant safety criteria. The System design safety calculations follow the maximum inhomogeneity principle. When computations are carried out – implemented in the spirit of current design standards – , the following types of analyses are performed, roughly in the following order. • Computations that prove the safety of the System for each stationary state (operating states, test states etc.): during these calculations, the temperature and the stress-strain field is determined; in addition, fracture mechanics calculations are carried out in case of damage-tolerant design. • Computations that prove the long-term safety of the System – under normal operating conditions and anticipated emergency events – at least for its expected service lifetime . These calculations are nowadays called ageing assessments , and they cover the following areas: ○ Mechanical fatigue calculations, which consist of (1) calculations for the expected time series of loadings of the System and (2) evaluations of the residual effects of cyclic changes, derived from the load-history data, concerning (a) volumes, or (b) crack-like flaws – in case of damage-tolerant design – . ○ Assessment of the residual effects of: (1) temperature history, (2) the history of the harsh environment (irradiation, corrosion), (3) wear (caused by the flowing working medium) respectively; all of these effects are examined during operation. All these computations rely heavily on empirical correlations, which are derived from the results of measurements that were performed sometime earlier, under special experimental conditions. It is interesting to note that different design standards (see e.g. ASME (2019), or PNAE (1989)) use slightly different approaches in e.g. mechanical fatigue assessments. • For overload-tolerant design, computations that prove the safety of the System – in cases of postulated accidental emergency events – at least up to the end of the expected service lifetime. The accident assessments cover the following relevant areas: ○ Investigations of the possible consequences of various postulated internal events that can be characterized by great and fast temporal changes in the temperature and pressure variables of the technological system (e.g.