Fatigue Life Analyses of Welded Structures

Flaws

Tom Lassen, Naman Récho

• Publisher: Wiley-ISTE
• Year: 2013
• ISBN:
  9781118614709
• Edition: 1
• Topic: Technik & Maschinenbau
• Subtopic: Werkstoffwissenschaft

Avoiding or controlling fatigue damage is a major issue in the design and inspection of welded structures subjected to dynamic loading. Life predictions are usually used for safe life analysis, i.e. for verifying that it is very unlikely that fatigue damage will occur during the target service life of a structure. Damage tolerance analysis is used for predicting the behavior of a fatigue crack and for planning of in-service scheduled inspections. It should be a high probability that any cracks appearing are detected and repaired before they become critical. In both safe life analysis and the damage tolerance analysis there may be large uncertainties involved that have to be treated in a logical and consistent manner by stochastic modeling.
This book focuses on fatigue life predictions and damage tolerance analysis of welded joints and is divided into three parts. The first part outlines the common practice used for safe life and damage tolerance analysis with reference to rules and regulations. The second part emphasises stochastic modeling and decision-making under uncertainty, while the final part is devoted to recent advances within fatigue research on welded joints. Industrial examples that are included are mainly dealing with offshore steel structures. Spreadsheets which accompany the book give the reader the possibility for hands-on experience of fatigue life predictions, crack growth analysis and inspection planning. As such, these different areas will be of use to engineers and researchers.

PART I

Common Practice

Chapter 1

Introduction

1.1. The importance of welded joints and their fatigue behavior

Welding is today the most common joining method for metallic structures. Its industrial application is extremely important and many of the large structures designed and erected in the last decades would not have been possible without modern welding technology. Typical examples are steel bridges, ship structures, and large offshore structures for oil exploitation.

The strength analysis of welded structures does not deviate much from that for other types of structures. Various failure mechanisms have to be avoided through appropriate design, choice of material, and structural dimensions. Design criteria such as yielding, buckling, creep, corrosion, and fatigue must be carefully checked for specific loading conditions and environments. It is, however, a fact that welded joints are particularly vulnerable to fatigue damage when subjected to repetitive loading. Fatigue cracks may initiate and grow in the vicinity of the welds during service life even if the dynamic stresses are modest and well below the yield limit. The problem becomes very pronounced if the structure is optimized by the choice of high strength steel. The very reason for this choice is to allow for higher stresses and reduced dimensions, taking benefits of the high strength material with respect to the yield criterion. However, the fatigue strength of a welded joint is not primarily governed by the strength of the base material of the joining members; the governing parameters are mainly the global and local geometry of the joint. Hence, the yield stress is increased, but the fatigue strength does not improve significantly. This makes the fatigue criterion a major issue. The fatigue strength will alone give the requirements for the final dimensions of the structural members such as plates and stiffeners. To overlook this fact may result in fatigue facture and serious consequences.

1.2. Objectives and scope of the book

This book is confined to steel structures made by fusion welding and most of the examples are taken from the offshore industry. The book is divided into three parts which cover the following subjects:

– Part 1: common practice:

- the basic understanding of the fatigue behavior of welded joints based on theoretical considerations and experimental results (Chapters 2, 3 and 4),

- the S-N approach with reference to current rules and regulations (Chapter 5),

- the fracture mechanics approach with numerical computations (Chapter 6).

– Part 2: uncertainties in crack growth and life predictions:

- reliability modeling and risk assessments,

- the random nature of the fatigue damage process and stochastic modeling (Chapter 7).

– Part 3: recent advances in description of the fatigue behavior:

- recent advances in understanding the fatigue process and estimating the fatigue life (Chapters 8, 9, 10, 11 and 12).

The objective of this book is to disseminate, to practicing engineers, the knowledge possessed by the two authors. The main goal is to teach engineers how to cope with frequently occurring problems related to the fatigue design of welded structures. Hence, the scope of the book is primarily about practical problems in structural design and in-service inspection. For this purpose, industrial cases are included along with spreadsheets for carrying out both S-N calculations and fracture mechanics calculations. Although available models of fatigue behavior may not be perfect, they are very useful tools in engineering assessment if properly understood and used. In most practical situations, the shortcomings of the available fatigue models are less important than the problems related to the uncertainty in the parameters included in the models. Furthermore, fatigue design is experimental, empirical, and theoretical – and in that order. Without testing, fatigue analysis often remains an academic speculation. Hence, our agenda in Part 1 of the book is to put forward rather simple models that fit the experimental facts.

In addition to this strategy, it is important to disseminate knowledge on how to deal with uncertainty in a logical and unified manner. Fatigue life data exhibit considerable scatter even under controlled laboratory conditions and the standard deviation is equally as important as the mean value. Furthermore, typical in-service variable loading may be stochastic in nature and stress calculations may be uncertain. These considerations call for some insight into applied statistics and probability calculations. This is emphasized in Part 2 of the book.

Although the book emphasizes the practical aspects of fatigue life calculations and the assessment of crack growth and criticality, some advances in methods and models are included in Part 3 of the book. Chapter 8 focuses on the statistical background of the S-N curves, whereas Chapter 9 is dedicated to the fatigue process. Chapter 10 suggests a life model where the weld notch stress is replaced by the weld notch stress field as the key parameter for fatigue life. Chapter 11 outlines some recent advances in methods of stress calculation for cracked joints. Finally, Chapter 12 describes and models the effect of an overload. All these chapters present methods and models that deviate from the common practice in current rules and regulations. The proposed models are more in accordance with the realistic fatigue damage behavior of welded joints. The practical impact of the model on fatigue design and inspection planning is important.

The ultimate objective is to achieve optimized structures with respect to fatigue design, dimensions and inspection efforts without compromising reliability and safety.

1.3. The content of the various chapters

Chapter 2 provides basic understanding of the fatigue damage process with reference to some failure cases, and gives an overview of parameters influencing the process. Chapter 3 gives some insight into laboratory fatigue testing, which is the basis for rule-based S-N curves. This chapter also includes a brief overview of common statistical methods to cope with the scatter in fatigue life results. Chapter 4 treats the definition and description of the fatigue load spectrum. Accuracy in applied loading description is crucial for the credibility of fatigue life results. Both the time-series approach and the energy-spectrum approach are presented. After having read Chapter 4, the reader is prepared for an elaborate fatigue life calculation scheme based on the S-N model according to rules and regulations. This is presented in Chapter 5. The basis is the original S-N design rules from the Department of Energy, further developed in the Eurocode 3 design rules. To account for corrosive environments, the Norwegian NORSOK and DNV guidance developed for offshore structures in the North Sea is commented upon. Rules for ship structures are also reviewed. Chapter 5 also gives a qualitative assessment of what is a good detail-design of welded joints and how to obtain improvements in fatigue resistance though post-weld treatment. This chapter may in fact be read directly after Chapters 1 and 2, but we have chosen to present it at the end of the sequence as final practical guidance and to sum up.

The above reading recommendation is given for the practicing engineer mainly involved in detailed design and in choosing dimensions for welded joints. The goal is to achieve sufficiently long predicted fatigue lives compared to the target service life. This is the daily task for many steel structural engineers. If, however, decisions must be made regarding the acceptability of existing flaws or crack-like defects in the joints, then Chapter 6 should be included in the engineer’s reading. Chapter 6 is an outline of applied fracture mechanics. In this chapter important questions, such as how fast a crack will grow during service loading and what is the ...