ENGINEERING INTEGRITY, VOLUME 44, MARCH 2018, pp. 15-22.
The ONR Technical Assessment Guide on the Assessment of the Integrity of Metal Components and Structures that are significant for nuclear safety, [11], points out that “design should be based on deterministic engineering, defence in depth and adequate safety margins”. Moreover, whilst experience of actual component reliability is a relevant consideration it also states that “theoretical analyses to yield predictions of component reliability are unlikely to provide a significant contribution to a safety case”.
scale components with an industrial surface finish, and for environment, as well as allowing for scatter in the data. Total factors of 20 on cycles and 2 on strain were originally used. The fatigue design curves generated were validated, or as ASME stated – checked for appropriateness, by simple tests on 12” and 36” diameter pressure vessels, [12]. These tests are briefly discussed in the ASME Criteria documents, [6, 7]. The alternating stress amplitudes during the tests
Another related and significant paragraph from [11] is under the heading of Structural Integrity Philosophy, which states: “The general lack of adequate reliability data for structural components leads to assessment being based primarily on established deterministic engineering practice. Even when there is confidence in assessing reliability based on existing data and a probabilistic safety case is possible, it is unlikely to be acceptable without substantial support from theoretical analyses and engineering judgement”. Hence with the ONR being a goal setting rather than a prescriptive standard based regime, then the onus is on us in the industry to justify any alternative approach as an option. What is clearly Figure 1. Appropriateness of ASME Fatigue Design Curves Based on of most importance is that engineering Pressure Vessel Tests assessments in design demonstrate an adequate margin that is fit-for-purpose. were known from a comprehensive strain gauge survey of The difficulty is in quantifying what is adequate. the peak stresses. The results, Figure 1, indicated that no crack initiation was detected at any stress level before the WHY THE INTEREST NOW allowable number of cycles from the design curve and no crack penetrated the through wall thickness in less than When you understand the origins of the ASME code through three times the allowable number of cycles. This was judged personal reflections like that provided by Bill Cooper (see by the code committee members at that time as being what above) you realise that in the 1960s they did not have they considered to be a fit-for-purpose approach. all the answers and significant assumptions had to be made. Hence the margins applied to address the gaps in However, that still leaves us in uncertainty of whether (to knowledge were unquantified margins. use ONR or IAEA wording) the result is ‘demonstrably conservative’, ‘suitably conservative’ or ‘very unlikely’ so In an age before we even had finite element analysis, we that there is an ‘adequate margin’. can only admire the work that these pioneers of design codes managed to contribute. But, as our understanding of So why is fatigue still obtaining so much attention in those gaps has improved, it is apparent that now is the time the Nuclear Industry? Although the above tests were to better quantify what the margins should be against fatigue undertaken with a water environment to pressurise the failure and that we modernise our assessment methods. vessels, they were also at ambient temperature. Work undertaken in the 1990s with a water environment at much The fatigue design curves that first appeared in the 1960’s higher temperatures from 150º to 300ºC showed that a were from fairly simple tests with data generated from significant deterioration in fatigue life then occurred. The uniaxial small scale polished specimens of about 9mm US NRC lost patience due to the lack of consensus from diameter from tests in air. They then applied design factors, ASME and in 2007 published its own document NuReg/ to allow for the transference of the data generated to larger
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