PSI - Issue 42

Tamás Fekete et al. / Procedia Structural Integrity 42 (2022) 1467–1474 Tamás Fekete, Éva Feketéné-Szakos / Structural Integrity Procedia 00 (2019) 000–000

1468

2

Observations suggest that ageing is inevitable in nature. According to experimental evidence, the maximum load bearing capacity of Structural Materials ( SM s) subjected to continuous loads decreases steadily over time –i.e., they become increasingly brittle–. From a safety point of view, the highest risk of brittle material behavior is that fracture of the SM initiates somewhere abruptly, and then travels onward to a long distance at high speed. This phenomenon is expressed in the language of Catastrophe or Stability Theory –see Thom (1975), Zeeman (1979), Stewart (1981), Arnold (1984) and Gilmore (1993)– by saying that in a brittle SM a local instability can transform very quickly –in a jump– into a global instability. For a LSS subjected to high loads, such loss of stability can lead to catastrophic failure. This phenomenon is referred to as the system’s structural integrity loss . A SM can behave brittle for two main reasons: (1) it is brittle in its post-manufacture state; (2) it becomes brittle over time; essentially, it ages. During the first half of the 20 th century, several LSS failures were caused by SM s that were brittle right after manufacturing. As a result of technological advances, SM s of LSS s are nowadays stable and tough after production. Local instability can occur also in ductile materials, under increasing loads, but it propagates slowly at first. If the load is further increased, the unstable zone becomes narrower and the processes in the localizing zone are gradually accelerating; fast fracture happens at the end, but with a considerable time-delay after the first instability. Hence, from a safety point of view, ductile fracture is considered less dangerous than brittle fracture by safety engineers. Description of ageing processes –with the resulting embrittlement– is still a relevant scientific issue and finding the correct approach to the embrittlement of SM s on ageing LSS s is a scientific-engineering problem with significant practical consequences. At the Centre for Energy Research in Hungary, ongoing research has been carried out for some years to develop solid theoretical foundations for future SI projects of LSS s. The General Conceptual Model ( GCM ) of SI , the relation between Design Safety Calculations ( DSC s) and the fundamentals of SI were explained in Fekete (2019). The foundations of Structural Integrity Calculations ( SIC s) lying at the heart of SI were outlined in Fekete (2022). Two issues will be addressed in this paper: first, some general considerations are explaining the role of learning in the context of SI problems of LSS s with long SL –called Long-Term Operation ( LTO )–; it is followed by an introduction to a theoretical framework based on modern Thermodynamics and a new ageing model. As mentioned above, LSS s have been designed for a limited SL since about the mid-20 th century; nowadays the DSL ranges from ~ 40 – 60 YO . LSS s are dimensioned according to internationally accepted Design Standards and Guidelines ( DSG s) e.g., ASME Code, RCCM Code, DIN, KTA. The purpose of system DSC s is to demonstrate that the LSS is guaranteed to be in service for at least the DSL . The DSC s shall be carried out in accordance with the referred DSG s. Industry experience over the past decades has proven that the TAL of an LSS can be extended up to 1.5 times the DSL . Extended Operation ( EO ) has been achieved by SIC s tailored to the specific characteristics of the corresponding LSS , with a TAL even greater than the specified EO time. The methodology for SIC s is based on the same standards as for DSC s. Today, there are many efforts to increase the SL of suitable LSS s to twice their DSL , which means that the original 40 – 60 YO would be extended to 80 – 120 years. This longer SL is exactly what LTO refers to. Operating over the LTO time horizon poses several challenges. One is to acquire and transfer appropriate knowledge about the operating system at the right time. The other one is to update the methodology of SIC s with recent scientific advances. The problem of acquiring and transmitting knowledge is addressed first, as follows. When an LSS is operating over the LTO time horizon, it is impossible for a single generation of staff and other relevant experts to monitor the operation of the system from the beginning to the end of its life. During SL , different generations replace each other, and they should pass on all knowledge that they gathered about the system. The new generation should also be able to keep the LSS operating as intended, even in unexpected situations, while protecting the Structural Health ( SH ) and SI of the system to the maximum extent possible. It is a real intergenerational learning process –see Kaplan, Sanchez, Hoffmann (2017), Feketéné Szakos (2018)–, in which elderly people teach youngsters and vice versa. The learning processes need to be prepared and documented consciously and carefully for the next generations. Not only measurement data from the system and measurement data from the trainings, but also informal diaries, narratives, blogs, perspectives, and reflections should be preserved for future generations, because formal and informal learning are entangled –see Wilczek (2016)–. This sharing of knowledge is a cross reaction between the lifelong learning processes of different generations living at the same time. 2. General considerations – the role of learning