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Fatigue Strength Under Spectrum Loading

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Structural Durability: Methods and Concepts

Part of the book series: Structural Integrity ((STIN,volume 17))

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Abstract

In-service loading often is characterized by random variable amplitudes which cannot be represented by sinusoidal type of loading sufficiently. Rare events may introduce microplastic straining and are followed by a huge number of cycles with moderate to medium stresses. In such a way, in-service loading creates its unique history and may lead to an early termination of serviceability . Chapter 4 looks into fundamental contexts of variable amplitude loading and how to deal with stress analysis . Finite element analysis is introduced, because as an engineer nowadays it does not bear thinking about being without. Hence, a major part is devoted to the fatigue analysis of welded joints which is an excellent playing field to look at nominal stress , structural stress and local stress concepts.

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Notes

  1. 1.

    To explain that this is really a submicroscopic level, we may look at the atomic distance of two neighboring ions of a typical steel: Its lattice constant is on the order of about 250–500 pikometer which means that a massive cube having an edge length of 25 mm contains about 1024 number of atoms.

  2. 2.

    Actually, it is not completely accurate to talk about the finite element method, because there are several methods: The direct approach is the simplest method for solving discrete problems in 1 and 2 dimensions, the weighted residuals method—represented by, e.g., the Galerkin method—uses the governing differential equations directly, and the variational approach uses the calculus of variation and minimization of potential energy and is used by the Rayleigh–Ritz method.

  3. 3.

    Almost 15 years of my professional experience, I was heavily involved in math modeling and finite element analysis and, thus, that opinion comes from observing the processes in the industry: Computational engineering primarily has to provide results and a model update often is performed only when unwanted deviations from the actual physical problem noticeably occur. Results are always achieved when running a finite element analysis, so people just neglect the necessity to verify the accuracy of those results and believe in the robustness of the FEA software and their modeling approach. But especially in design-to-durability studies it is so crucial to achieve a high level of results quality, because the lifetime assessment relies so much on the strain and stress numbers at pretty localized areas which often may not converge easily.

  4. 4.

    It cannot be the aim of this book to give an overview about the FEA software codes available or a recommendation for a specific product. The whole story started with spacecraft engineering in the late 1960s and the development of NASTRAN as a finite element analysis program that was originally developed for NASA under US government funding for the aerospace industry. Today, NASTRAN source code is integrated in a number of different software packages, which are distributed by a range of companies. Other major codes are Ansys and Abaqus which are widely used at the universities as well as in the industry. The internet is an excellent resource to look for code details and alternatives. FEA software development was largely driven by the nuclear power industry too: A very interesting FEA software for structures and thermomechanics analysis is Code Aster that was originally developed for the internal use at the French power supply company Électricité de France (EDF), but is now available under general public license (GNU GPL). Code Aster is a comprehensive package having 1.2 million lines of code, a documentation with 13,000 pages and 2000 test examples demonstrating that this is a serious tool for engineering analysis. A huge number of other software can be found and is ready to be used: Proper computational power is available by almost any state-of-the-art computer hardware and not anymore a bottleneck in computational engineering.

  5. 5.

    ISO 6520-1:2007 is about imperfections in metallic fusion welds and introduces the classification and terminology: A detailed list of 110 possible weld imperfections is shown there.

  6. 6.

    IIW is an umbrella organization which was founded in 1948 by the welding institutes or societies of 13 countries and has today members from 55 countries worldwide. IIW arranges conferences and workshops bringing together the leading specialists twice a year for the development and maintenance of a number of guidelines and recommendations.

  7. 7.

    That is IIW structural detail #216, and its classification can be improved to FAT 71 for steel when the root is checked by appropriate non-destructive inspection (NDI).

  8. 8.

    That is IIW structural detail #214 considering root cracking.

  9. 9.

    SIF solutions and crack propagation will be explained more detailed later, when it comes to an application example for which a fracture mechanics approach was the way to go. For the fatigue life assessment of welded joints that is another approach which competes to the methods described previously.

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Authors and Affiliations

  1. Research Division Structural Durability, Fraunhofer-Institute for Structural Durability and System Reliability | LBF, Darmstadt, Germany

    Ruediger Heim

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Heim, R. (2020). Fatigue Strength Under Spectrum Loading. In: Structural Durability: Methods and Concepts. Structural Integrity, vol 17. Springer, Cham. https://doi.org/10.1007/978-3-030-48173-5_4

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  • DOI: https://doi.org/10.1007/978-3-030-48173-5_4

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