Fatigue Analysis of Welded Components

Designer's Guide to the Structural Hot-Spot Stress Approach

E. Niemi, W Fricke, S J Maddox

• Publisher: Woodhead Publishing
• Year: 2006
• Print ISBN: 9781845691240
• eBook ISBN: 9781845696665
• Topic: Technology & Engineering
• Subtopic: Mechanical Engineering
• Index: Technology & Engineering

This report provides background and guidance on the use of the structural hot spot stress approach to the fatigue design of welded components and structures. It complements the IIW recommendations for 'Fatigue Design of Welded Joints and Components' and extends the information provided in the IIW recommendations on 'Stress Determination for Fatigue Analysis of Welded Components'. This approach is applicable to cases of potential fatigue cracking from the weld toe. It has been in use for many years in the context of tubular joints. The present report concentrates on its extension to structures fabricated from plates and non-tubular sections.Following an explanation of the structural hot spot stress, its definition and its relevance to fatigue, the authors describe methods for its determination. Stress determination from both finite element analysis and strain gauge measurements is considered. Parametric formulae for calculating stress increases due to misalignment and structural discontinuities are also presented. Special attention is paid to the use of finite element stress analysis and guidance is given on the choice of element type and size for use with either solid or shell elements. Design S-N curves for use with the structural hot spot stress are presented for a range of weld details. Finally, practical application of the recommendations is illustrated in two case studies involving the fatigue assessment of welded structures using the structural hot spot stress approach. - Provides practical guidance on the application of the structural hot-spot stress approach - Discusses stress determination from both finite element analysis and strain gauge measurements - Practical application of the recommendations is illustrated in two case studies

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 (11), 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.

1.2 Safety aspects

When using this document the general guidelines on fatigue analysis and safety aspects given in the latest version of the IIW Recommendations (1) should be taken into consideration. For example, it may be appropriate to multiply the fatigue loads by a partial safety factor, γ_F, which will normally be specified in the appropriate design code for a particular structure. Similarly, the fatigue strength may need to be divided by γ_M, as required.

It should be noted that the S-N curves presented in this document do not include any allowance for inaccuracies in stress determination. Furthermore, the stress raising effect of misalignment in welded joints and other such imperfections may not have been taken into account. If the stress analyst chooses to simplify the stress determination, for example by omitting the extrapolation of the structural stress to the weld toe or by totally neglecting misalignment, the load factor, γ_F, should be increased accordingly.

The linear extrapolation technique for approximation of the structural stress at the hot spot (to be described later) may sometimes under-estimate the structural hot spot stress. Therefore, more accurate techniques for stress determination are recommendable for critical components that require a high safety margin, see Appendix 2.

2

The Structural Hot Spot Stress Approach to Fatigue Analysis

2.1 Field of application

The structural hot spot stress approach applies to welded joints for which:

• The fluctuating principal stress acts predominantly transverse to the weld toe (or the ends of a discontinuous longitudinal weld);

• the potential fatigue crack will initiate at the weld toe or end.

Referring to Fig.2.1, the hot spot approach applies to cases a to e. For reasons that will be discussed later, it is also necessary to distinguish between joints that are fully load-carrying and non-load-carrying. Cases c) and h) represent load-carrying fillet welds, whereas case b) represents non-load carrying fillet welds. Fillet welds at cover and collar plates (lugs), case d), are also treated as non-load carrying welds. End welds in cases e) and j), representing gussets or brackets welded on plates, are considered as non-load-carrying. If there is any doubt about the choice of category, the joint should be assumed to be load-carrying. The approach is not applicable to cases where the crack will grow from the weld root and propagate through the weld throat, cases f) to i) in Fig.2.1. Good design practice aims to avoid this kind of behaviour because the crack is not visible before it has propagated through the weld. Moreover, the structural hot spot approach does not apply to continuous welds subject to longitudinal loading. The nominal stress approach (1) is sufficient for such cases.

2.1 Examples of fatigue crack initiation sites in welded joints

The weld detail being assessed will often be situated in a bi-axial stress field. Then it is usually sufficient to apply the structural stress approach to that principal stress which is acts approximately perpendicular (between 30° and 90°) to the weld toe, see Section 2.3. If necessary, the other principal stress can be considered using the fatigue class for longitudinally loaded welds in the nominal stress approach according to Ref. (1).

2.2 Types of hot spot

Hot spots can be classified as two types, as shown in Fig.2.2:

2.2 Examples of the two hot spot types, in a welded girder:

Type ‘a’ is located on the surface of the lower flange.

Type ‘b’ is located on the edge of the web plate in the scallop

Type ‘a’	The weld is located on a plate surface, see also Fig.2.1 a) – d).
Type ‘b’	The weld is located on a plate edge, see also Fig.2.3.

Diffe...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright page
• Preface
• Abstract
• 1: Introduction
• 2: The Structural Hot Spot Stress Approach to Fatigue Analysis
• 3: Experimental Determination of the Structural Hot Spot Stress
• 4: Structural Hot Spot Stress Determination Using Finite Element Analysis
• 5: Parametric Formulae
• 6: Structural Hot Spot S-N Curves
• 7: Case Study 1: Box Beam of a Railway Wagon
• 8: Case Study 2: Hatch Corner Design for Container Ships
• 9: References
• Appendix 1: Symbols
• Appendix 2: A2.4 References
• Appendix 3: References
• Appendix 4: Hot Spot S-N Curves for Welded Aluminium Components