Fatigue Analysis of Welded Components Designers Guide to the Structural Hot spot Stre

[david-smith]

Fatigue Analysis of Welded Components Designers Guide to the Structural Hot spot Stress Approach iiw

Author: niemi-fricke-maddox | Size: 4.33 MB | Format: PDF | Quality: Original preprint | Publisher: iiw (international institite of welding)+crc | pages: 55

1. INTRODUCTION
1.1 General
Traditional fatigue analysis of welded components is based on the use of nominal stresses and catalogues of classified details. A particular type of detail is assigned to a particular fatigue class with a given S-N curve. Such a method is used in the IIW fatigue design recommendations (1). This nominal stress approach ignores the actual dimensional variations of a particular structural detail, which is an obvious drawback.
Moreover, the form of a welded component is often so complex that the determination of the nominal stress is difficult or impossible. This is true even if the finite element analysis (FEA) method is used for the stress analysis.
In the context of potential fatigue failure by crack growth from the weld toe or end, the structural hot spot stress approach goes one step forward. Here the calculated stress does take into consideration the dimensions of the detail. The resulting structural stress at the anticipated crack initiation site ('hot spot') is called the structural hot spot stress. Structural stress includes the stress concentrating effects of the detail itself but not the local non-linear stress peak caused by the notch at the weld toe. This notch effect is included in the hot spot S-N curve determined experimentally. This is reasonable because the exact geometry of the weld will not be known at the design stage. The variation in the local geometry of the weld toe is one of the main reasons for scatter in fatigue test results. By using the lower-bound characteristic S-N curve, lower bound quality of the weld toe is incorporated into the analysis. A single S-N curve should suffice for most forms of structural discontinuity, providing the weld toe geometry is always the same.
An obvious reason for introducing the structural hot spot approach is the availability of powerful computers and software, which make detailed FEA possible for most design offices. However, the approach is also a valuable tool for choosing the locations of strain gauges when validating design by field-testing prototype structures. Moreover, finite element analyses make it possible to produce parametric formulae in advance for easy estimation of structural stresses at various hot spots. The hot spot approach was first developed for fatigue analysis of welded tubular joints in offshore structures. Corresponding fatigue design rules were published by the American Petroleum Institute, the American Welding Society, Bureau Veritas, UK Department of Energy, etc. A review of this topic can be found in Ref. (2). There is now an increasing demand for application of the approach to be extended to all kinds of plated structures. Some progress has been made in doing this, but at present there are differences in the methods recommended for estimating the structural hot spot stress. The first general design rule to include the structural stress (referred to at the time as the geometric stress) approach was the European pre-standard ENV 1993- 1 - 1 (3) (Eurocode 3) but only limited guidance was given. Later, the International Institute of Welding (IIW) published new recommendations containing four fatigue design approaches, including the hot spot approach (1). A background document was also published focusing on definitions and the determination of stresses used in the fatigue analysis of welded components (4).
Subsequently, further research has led to improved procedures for determining the structural hot spot stress, particularly using FEA (5,6), and the provision of background fatigue test data from which to derive suitable design S-N curves (7-10). Furthermore, the ability to establish through-thickness stress distributions using FEA has enabled a method to be developed that uses this information to calculate the structural hot spot stress. Previously attention has focussed on use of the surface stress distribution, approaching the weld in question, and determination of the structural hot spot stress using an extrapolation procedure. Use of the through-thickness stress distribution instead should avoid the need for extrapolation.
The goal of the present document is to help design engineers and stress analysts to implement the structural hot spot stress approach in practice. Symbols and terms are defined as they are used but symbols are also defined in Appendix 1.
Practical examples of the application of the methods described are given in the form of Case Studies in Sections 6 and 7. Moreover, the document should serve as a reference when detailed guidelines for design are developed for particular welded products. The recommendations given here are intended for design of general welded structures subjected to fatigue loading. The document is mainly focused on plated structures, such as bridges, cranes, earth moving machinery, ship hulls, etc. Specific rules are already available for certain fields of application, including tubular structures (1 l), ship hulls (12,13) and pressure vessels (14).
In view of the scope of current experience and the availability of relevant fatigue test data, the recommendations presented in this document are only intended for plate thicknesses above 4 mm.

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