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13 - Design of Bolted and Threaded Connections
- Inge Lotsberg
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- Fatigue Design of Marine Structures
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Summary
Introduction
The methodology used for fatigue design of bolted connections that are subjected to dynamic loading is rather old. This becomes clear when studying the third edition of the book on design of bolt connections by Wiegand and Illgner (1962); this book was first issued in 1940, and in the 1962 third edition, among the 254 references provided, the oldest one dates back to 1908. However, the main span of references is from 1930 to 1960.
In the Handbook of Bolts and Bolted Connections, edited by Bickford and Nassar (1998), it is stated that: “The bolted joint is a surprisingly complicated affair.” Although many factors affect its behavior and life, the design of most joints is based on “feel,” supported by “past experience” or “custom.” Because of this, most joints are overdesigned. Nevertheless, due to the uncertainties, it may be considered sound practice to make the joints stronger than the members. Many books and standards on bolted connections have been published; compared with some of these weighty books, the quantity of recommendations that can be written into this single chapter in this book is limited. Thus, this chapter should be considered more as providing an overview, and for more detailed studies, the reader should see other literature, such as the Handbook mentioned earlier, as well as Kulak et al. (1987). A number of ISO and ASTM standards can also be consulted regarding notations, production of bolts, tolerances of bolts, washers, and nuts, mechanical properties, testing, and documentation.
The terminology used for fasteners is presented in ISO 1891 (2009). Fasteners are understood to encompass bolts, and include head bolts that have threads for a nut at one end and stud bolts with threads at both ends and an unthreaded shank between them. The shank between the head and the threaded section of a stud bolt is also called a grip length, and the bolt diameter refers to this part of the bolt.
A number of bolt failures occurred during the first years of offshore activity in the North Sea. This resulted in stricter requirements for bolt design, fabrication, installation, maintenance, traceability, and quality assurance. The design of bolts in marine structures was found to require special attention regarding corrosion protection and dynamic loading. Due to the dynamic loading, it is recommended that bolts are pretensioned, as these bolts are most effective with respect to fatigue capacity.
18 - Planning of In-Service Inspection for Fatigue Cracks
- Inge Lotsberg
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- Fatigue Design of Marine Structures
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General
Degradation of offshore structures is mainly caused by corrosion and fatigue crack growth. The effects of corrosion are included in the design of these structures by incorporating a corrosion allowance or by use of a protection system by anodes and/or coating, enabling control of corrosion development also by possible replacement of anodes. In contrast, fatigue crack growth can be more critical, as cracks can result in a sudden rupture under conditions of large storm loads. Moreover, cracks are hard to detect because they are small for a significant part of crack growth time. Therefore the Fatigue Limit State is important for design of marine structures as well as during operation; see also Section I.4.
Defects much larger than those implicit in fatigue design curves are also of concern as some cracks found during inspection can be attributed to such defects. These defects are significantly larger than normal fabrication defects included in a probabilistic fatigue analysis, and are also sometimes denoted as gross errors, as explained in Section I.4. Therefore, the following safety principles should be implemented:
• Design for adequate fatigue life, including Design Fatigue Factors (DFFs) and a sound corrosion protection system.
• Design for robustness in relation to fatigue failure.
• Plan inspection of the structure during fabrication as well as during service life.
When inspection priorities are set, the potential for abnormal fabrication defects should also be considered. Since inspection after fabrication onshore can be performed much more cheaply and with higher reliability than during operation offshore, it is worthwhile emphasizing such inspections, particularly for components that are significant for the integrity of offshore structures.
As offshore structures possess different robustness with respect to fatigue cracking, and because inspection, repair, and failure costs vary significantly, different inspection strategies may be relevant for different types of structures.
Jackets with four or more legs show a larger reserve strength with X-type bracing than with K-type bracing which was frequently used in older jackets, but less so in new structures. The consequences of a fatigue crack will still depend on the position of the crack, type of loading such as amount of local bending stress over the plate thickness versus membrane stress at the hot spot, and the possibility of stress redistribution during crack growth.
8 - Stress Concentration Factor for Joints
- Inge Lotsberg
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- Fatigue Design of Marine Structures
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Summary
General
Many offshore steel structures are designed as truss frameworks in which tubular members are used as the structural elements, such as in the jacket structure in Figure I.5. For a more detailed background, see, for example, Marshall (1992).
Waves and currents generate relatively small loads on tubular members due to their low drag coefficients. However, the intersections between different members that are connected to the same joint may be rather complex, and this may lead to relatively high local stresses at the hot spot areas with correspondingly short fatigue lives. Thus, in order to design structures that meet the required fatigue life, it is necessary to have adequate knowledge about the stress condition at tubular joints.
Tubular joints may be classified into the following groups:
Simple tubular joints
Overlapping joints
Tubular joints with internal ring stiffeners
Heavy stiffened tubular joints
Grout reinforced joints
Cast steel joints.
A simple tubular joint, as shown in Figure 8.1, is understood to mean a joint other than a circumferential girth weld between tubular members, and that is not stiffened by internal or external stiffeners. Furthermore, the braces are welded into the chord without overlapping each other. Overlapping joints are understood to be overlapping braces at the intersection to the chord. Overlapping joints may be used when there is difficulty in placing the tubular members within a specific area, leading to acceptable eccentricities in the joint. These connections can show rather high capacity with respect to the Ultimate Limit State and the Fatigue Limit State. However, they may be more complex to fabricate than simple tubular joints are, and therefore other solutions are often preferred. For example, one solution would be to increase the chord diameter to allow for a larger space for brace intersections, using internal ring stiffeners to achieve the required chord ring stiffness and capacity. In some joints, such as K-joints, where the axial force in one brace is to be transferred to a second brace, it may also be efficient to use longitudinal stiffeners internally in addition to ring stiffeners to reduce the hot spot stress. These joints may be categorized as heavy stiffened joints. Heavy stiffened joints in jacket structures were more frequently designed during the 1980s than today; see, for example, Callan et al. (1981).
Frontmatter
- Inge Lotsberg
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APPENDIX A - Examples of FatigueAnalysis
- Inge Lotsberg
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- Fatigue Design of Marine Structures
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15 - Fatigue Analysis of Floating Platforms
- Inge Lotsberg
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- Fatigue Design of Marine Structures
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General
DNVGL has issued a number of guidelines for fatigue assessment of floating platforms. These documents include information that is related to:
• modeling environmental loading and response;
• modeling principles for analysis models of FPSO (floating production, storage, and offloading vessels);
• hydrodynamic analysis methodology;
• fatigue analysis methodology;
• documentation and verification of analysis methodology.
Semi-Submersibles
Guidelines for fatigue design of semi-submersibles are presented in DNV-OS-C103 (2014).
Guidelines on fatigue analysis for planning in-service inspection for fatigue cracks of semi-submersibles can be found in appendix B of DNVGL-RP-0001 (2015).
Floating Production Vessels (FPSOs)
Guidelines on how to perform a global and local fatigue analysis of a typical FPSO, with both turret-moored and fixed spreader-moored arrangements, can be found in DNV-RP-C206 (2012).
Guidelines on fatigue analysis of FPSOs for planning in-service inspection for fatigue cracks can be found in appendix C of DNVGL-RP-0001 (2015). The referred documents can be downloaded from the DNV GL web page for free. This is one reason for making this section short.
12 - Probability of Fatigue Failure
- Inge Lotsberg
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2 - Fatigue Testing and Assessment of Test Data
- Inge Lotsberg
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Planning of Testing
Fatigue testing may be planned for different purposes, such as testing for documentation of a general design S-N curve for a considered detail or for qualification of a detail in a project. More time is usually available for planning of the testing for the first case than for the second case. The first case also normally involves tests of more specimens than time allows for in the second case. When planning the testing, the purpose of the test must be clearly defined as early as possible. When the purpose is defined, the number of tests and the testing time required can be planned. During this initial planning phase, how the test data will be assessed and transferred into a recommended design methodology for the considered project should also be considered. Some examples of fatigue testing of different details are included in Sections 2.2–2.6 for butt welds in plated structures and piles, small-scale specimens for simulation of fatigue strength in sailing ship structures, large-scale specimens from sailing ships and floating production ships, fillet welded connections, and cover plates or doubling plates. In Section 2.7 approaches to how fatigue test data can be used for assessment of a recommended design procedure, where the principal stress direction during load cycling is not normal or parallel with the weld direction, are provided.
Constant Amplitude versus Variable Amplitude Testing
Constant amplitude testing is normally performed for derivation of test data representative for the left section of the high cycle part of the S-N curve (or the part of the S-N curve between the low cycle region and the fatigue limit). The fatigue limit in air is understood to refer to the position of the transition of slope in the S-N curve at 107 cycles in Figure 1.1. However, in order to obtain relevant test data for the high cycle region to the right of the fatigue limit, a variable amplitude loading, also known as a spectrum loading, must be used. The reason for this is that some load cycles with a stress range larger than the fatigue limit are needed to initiate a crack, such that crack growth will also occur for stress cycles that are below the fatigue limit. This is further illustrated in Sections 3.2.2 and 3.2.3.
5 - Stresses in Plated Structures
- Inge Lotsberg
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- Fatigue Design of Marine Structures
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Summary
Butt Welds in Unstiffened Plates
Butt welds are important connections in most types of welded structures. Examples are plated structures used in ships, floating offshore platforms, frame structures fabricated from tubular sections, tethers connecting tension leg platforms to the sea bottom, shell structures used in semi-submersible platforms for oil and gas production, towers for support structures for wind turbines, foundation piles, pipelines, risers, umbilicals, and so on. Many of these structures are subject to dynamic loading, and a reliable assessment of the hot spot stress at these connections is required for calculation of fatigue life.
A stress concentration factor (SCF) can be defined as a stress magnification at a detail due either to the detail itself or to a fabrication tolerance, with the nominal stress as a reference value. The maximum stress is often referred to as the hot spot stress and is used in conjunction with S-N data for fatigue life calculation. It is also termed geometric stress or structural stress. This hot spot stress is derived as the SCF multiplied by the nominal stress. The effects of misalignment due to eccentricities at plate thickness transitions and fabrication tolerances on the hot spot stress also need to be considered. In scaled plate test specimens, additional stress due to angular misalignment resulting from welding distortion may also be of importance for the resulting hot spot stress, as explained in Section 2.2.1. However, such an effect is normally considered to be of less significance for actual structures than laboratory test specimens due to differences in boundary conditions during fabrication and loading. The angular deviation may be more significant for structures with thinner plates.
SCFs due to misalignment at butt welds in plates were presented by Maddox (1985) and have been included in fatigue design rules for plated structures for many years. A simple butt weld between two plates, as shown in Figure 5.1, is considered as an introduction to the derivation of SCFs for butt welds. It is assumed that the plates are welded together from plates of the same size, with an eccentricity, δ, and without angular misalignment. The plates are subjected to a membrane loading per unit width N = σnominal · t, where σnominal is nominal stress and t = thickness of the plates.
Preface
- Inge Lotsberg
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Summary
This book is intended to act as a guide for students and practicing engineers for fatigue design of dynamically loaded marine structures. Fatigue of structures is a broad and complex area that requires more background than can be included in design standards. Many papers on fatigue of structures are published each year, and different design approaches have also been issued. However, due to the nature of the fatigue phenomena and scatter in test results, it may be difficult for engineers to obtain a good overview of what is found to be a good recommended fatigue assessment methodology.
The purpose of this book is not to give a complete overview of different design approaches, but rather to provide the reader with a sound background for the most common recommendations in design standards for fatigue assessment of marine structures. The content of this book is colored by the experiences by the author, and it may be relevant to consider this textbook in relation to the Standards with which the author has been most heavily involved, including the Recommended Practice DNVGL-RP-C203 Fatigue Design of Offshore Structures and DNV-RP-C206 Fatigue Methodology of Offshore Ships. However, similar content can also be found in a number of other design standards, such as: ISO 19902 (2007), API RP2A (2014), AWS (2010), BS 7608 (2014), Eurocode 3 (EN 1993–1–9, 2009), and IIW (Hobbacher, 2009). Thus, this book might best be considered as providing background for fatigue assessment of welded structures on a broad basis.
Based on the author's main background experience, a number of DNVGL standards are referenced. As these documents can be downloaded for free from the Internet, they are also useful reference documents for students studying fatigue of marine structures.
Much of this book is related to fatigue capacity of steel structures. The book may be seen as complementary to the Naess and Moan's book, Stochastic Dynamics of Marine Structures. Thus mainly the fatigue capacity of marine structures is considered in this book. The dynamic loading may be due to different sources such as waves, wind, rotors on wind turbines, dynamic response, vortex-induced vibrations, pile driving, and loading and unloading of content.
9 - Finite Element Analysis
- Inge Lotsberg
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- Fatigue Design of Marine Structures
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Welded Connections in Plated Structures
General
Assessment of high cycle fatigue in marine structures is based on linear elastic structural finite element analysis (FEA). Assessment of low cycle fatigue can be based on nonlinear FEA; see also Section 3.1.3. Some basic knowledge about finite elements is recommended in order to prepare finite element models that are representative of the physical behavior of different structural details. Before starting to model a detail, it is necessary to have a clear view of what the outcome of the analysis should be and of how the analysis results should be used, together with S-N data, for assessment of calculated fatigue life. In principle, three different types of finite element models can be prepared for fatigue assessment:
Model for calculation of membrane stresses, to be used together with S-N curves for nominal stress for calculation of fatigue damage.
Model for calculation of structural stress or hot spot that represents the stress due to the considered geometric detail, which is entered into a hot spot stress S-N curve for calculation of fatigue damage.
Model that accounts for the considered geometric detail, including the weld toe, where the calculated notch stress is entered into a notch S-N curve for calculation of fatigue damage.
A rather coarse finite element model may be appropriate for analysis of membrane stresses in plated structures. The elements used should represent a linear membrane stress distribution within each element. This is achieved by using eight-node isoparametric shell elements and four-node shell elements with internal degrees of freedom. The ability to represent bending stress over the thickness is less important for calculation of membrane stresses. This means that if only the membrane stress is to be derived from the analysis, this model cannot be used for fatigue assessment of plated structures that are subjected to significant dynamic lateral pressure. This limitation is also considered a drawback to using the nominal stress approach for such loading conditions and is one reason why structural stress methods were introduced into fatigue design of marine structures during the early 1990s. Structural stress methods are also called hot spot stress methods, and the local geometry of a detail is accounted for in the stress calculation, in addition to plate bending due to lateral pressure. The finite element model here needs to represent linear stress behavior through the plate thickness.
7 - Stresses at Welds in Pipelines, Risers, and Storage Tanks
- Inge Lotsberg
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Acknowledgments
- Inge Lotsberg
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4 - S-N Curves
- Inge Lotsberg
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14 - Fatigue Analysis of Jacket Structures
- Inge Lotsberg
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Summary
General
This chapter is written with mainly jacket structures in mind. However, the same principles can also be used for analysis of other types of bottom-fixed structures such as jack-ups and support structures for wind turbines. Fatigue analysis of jacket structures should be performed for:
• transportation of the jacket from the construction yard to the installation site;
• driving of piles;
• the installed condition.
As fatigue damage is accumulated over different phases, the calculated fatigue damage at each hot spot from each of the operations can be added together.
Transportation of the jacket on a barge to the installation site requires special considerations with respect to fatigue. The most significant hot spots during transportation may differ from those in the installed condition. However, transportation can involve significant fatigue damage, and in some cases, repairs have been needed before the jacket could be installed. Therefore, it is important to plan transportation and assess relevant environmental criteria for the transportation period and route; this includes assessment of available sheltered harbors on route should transportation take longer timer than predicted. In fatigue analysis of the transportation phase, the question arises regarding which Design Fatigue Factor (DFF) should be used. This should be considered in conjunction with the criteria used for transportation. If fatigue damage is expected to accumulate at the same hot spot during transportation as during the installed condition, it is reasonable to use a DFF that is sufficiently large so that the allowable calculated fatigue damage during transportation is low. This should be described in a design specification before the detailed design commences.
It is also important to remember that slender members in a jacket lying on a barge during transportation may be subjected to vortex-induced vibrations in wind. Vortex-induced vibrations in water need to be considered in design for the installed condition. However, the driving forces and damping are different in wind than in conditions with wave and current loading.
Fatigue analysis of the piles in jacket structures shows that a significant part of the fatigue damage can be accumulated at thickness transitions during pile driving. The accumulated fatigue damage governs the probability of fatigue failure, as explained in Chapter 12.
Introduction
- Inge Lotsberg
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History of Fatigue
The history of fatigue of metals, components, and structures goes back to the 1830s when failures of chains in mines were reported due to dynamic loading, and fatigue testing of these chains was performed for mitigation (Schütz 1996). In association with this, the first wires were invented to avoid the problems with fatigue of the chains. Since then, up until year 2000, around 100,000 papers related to fatigue were published (Schijve 2003, 2009). With so much published literature available, providing a broad and objective historical overview would be highly challenging. Thus, the historical presentation provided here is limited to those aspects that are of most relevance as background for this book. Reference is made to Schütz (1996), Stephens et al. (2001), and Anderson (2005) for a more detailed historical presentation related to fatigue.
The term “fatigue” is apparently first mentioned in the literature in 1854, by an Englishman called Braithwaite. In his paper, Braithwaite describes many service fatigue failures of brewery equipment, water pumps, propeller shafts, crankshafts, railway axles, levers, cranes, and so on. At about the same time many disastrous railroad accidents occurred, such as one on 5 October 1842 when an axle broke at Versailles due to fatigue and the lives of 60 people were lost. Failures of railway axles became a serious problem and as late as in 1887, an English newspaper reported “the most serious railway accident of the week.” In many cases these accidents were due to fatigue failures of axles, couplings, and rails.
In some publications, the fatigue strength in terms of S-N curves is presented as “Wöhler curves” that are named after the work that Wöhler performed in Germany to determine the fatigue strength of railway axles based on fatigue testing in the period from 1860 to 1870. Already in 1858, Wöhler was measuring the service loads on railway axles using self-developed deflection gauges. He also introduced the concept of safety factors, where two sets were needed: one for maximum stress in service in relation to static strength, and the other for allowable stress amplitude under dynamic loading. The safety factors were provided for ensuring design for infinite life. The factors were valid only for un-notched specimens, and fatigue testing was recommended for other geometries. Wöhler presented his test data in tables.
11 - Fabrication
- Inge Lotsberg
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General
It is important that designers work in close cooperation with fabricators to achieve structures that are suitable for welding. Simple fabrication and the possibility of access for welding and non-destructive testing (NDT) during fabrication should be planned in order to realize structures that fulfill specified requirements. In order to weld braces to chords without an overlap, there should be a gap between the braces not less than 50 mm, and the brace angle should not be below 30° to allow for proper welding at the heel area (see Figure 8.1). Seam welds and girth welds in tubular sections are usually placed outside the main hot spot areas in tubular joints. Recommendations regarding this can be found in design and fabrication standards such as ISO 19902 (2007), API RP 2A (2014), and NORSOK M-101 (2011).
Selection of Material
Material should be selected to meet requirements in material and fabrication standards, in addition to requirements for yield strength with respect to design for the Ultimate Limit State, as presented in Section I.4. Where the Fatigue Limit State governs the design, using material with a very high-yield strength is not recommended, as the fatigue strength of most steels does not particularly depend on the yield strength when it is welded, as explained in Section 4.4. It is important to use materials with documented properties showing good weldability. Where significant stresses from in-service loads or from fabrication are going to be transferred in the thickness direction, it is important to use material with documented through-thickness properties. It is also important to use material with sufficient ductility and fracture toughness for the lowest in-service temperature that can be expected during the lifetime of the structure. Thus, the grade of steel to be used in different structural parts is generally related to service temperature and thickness. Fracture toughness becomes reduced after welding and this should be accounted for when material is selected. For marine structures it is important to consider the actual environment, potential corrosion, and corrosion protection for material selected for a specific project.
Welding
Welding is a process in which notch-like imperfections and deviations in geometry from nominal are difficult to avoid completely.
1 - Fatigue Degradation Mechanism and Failure Modes
- Inge Lotsberg
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General
It is generally accepted that in welded structures subjected to repeated external loads, microcracks may be initiated rather early in the fatigue life. Based on observations on the development of fatigue cracking, it has become common practice to consider fatigue life as consisting of three phases. These are initiation of a crack, propagation of the crack, and final fracture. Final fracture is simply the fracture under the last up-loading tensile load cycle and may be treated by assessment of unstable fractures, as presented in Chapter 16. However, the distinction between the first two phases is not very clear. The question arises of how to determine that the crack is so large that its growth can be properly defined in laboratory testing and in crack growth analysis. Fatigue crack propagation is understood here to mean the growth of cracks that are so large that the continuum mechanics approach can be applied.
The proportion of fatigue life of a structure that is spent in each of the two first phases depends on the material, the geometry of the detail being considered, and the loading. The initiation period for a fatigue crack in the base material without significant notches is relatively long in comparison with the propagation period. In contrast, at weld toes in structures with more severe stress concentrations, the formation of a dominant crack occurs relatively early in the fatigue life, and the propagation phase constitutes the major portion of the total life. Radencovic (1981) and test data on tubular joints reported by Pozzolini (1981) showed that 70–90% of the fatigue life of welded connections is related to fatigue crack growth. However, as indicated in the preceding paragraph, such numbers also depend on the definition of initiation and the type of connection. At start of fatigue crack growth after initiation it is assumed that a sharp crack tip has developed. The size of the crack at this stage is not so easy to define and in literature it ranges from below 0.1 mm up to 1 mm; see also Sections 3.1.3 and 4.7.4.
The basic mechanism of crack nucleation in the base material is cyclic slip and the extrusions and intrusions at the surface of the base material. At weld toes, however, initiation is more usually from defects at undercuts or from other imperfections in welded connections.
16 - Fracture Mechanics for Fatigue Crack Growth Analysis and Assessment of Fracture
- Inge Lotsberg
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Brittle and Ductile Failures
Introduction
The notation used to describe the development of fracture using adjectives as ductile or brittle depends on whether one refers to the micro or the macro scale. Thus, the wording brittleness and ductility may have different meanings depending on whether it is seen from a metallurgist or a designer. A metallurgist is considering how to best achieve ductile material that shows sufficient strength and elongation at fracture and that is robust with respect to fracture when defects are present in the material. Designers most often presume that the material they are using is ductile and that their main task is to avoid a brittle structural behavior through a less favorable design. However, the post-failure behavior of some structures is such that a global brittle development cannot be avoided and for these structures a larger safety factor is normally recommended than in design with development of a ductile failure mechanism. An example of this is shell structures and structural elements where local buckling may occur; see, for example, Eurocode 3 (EN 1993-1-1 2009). The terminology used by metallurgists and designers may also be combined in a matrix as shown in Table 16.1 for illustration of development of different failure mechanisms. However, there is a gradual development from brittle to ductile along both the horizontal and the vertical axes in Table 16.1.
Design of Ductile Structures
Design standards for marine structures recommend design and fabrication of ductile structures. This requires that ductile materials are used and that sound design principles are followed in order to avoid weak sections in structural elements due to locally reduced sectional area such as at bolt-holes. A requirement for material ductility is expressed through requirements for Charpy V values tested at different temperatures for different thicknesses. Crack Tip Opening Displacement (CTOD) testing is also recommended for documentation of sufficient material toughness for larger thicknesses and more complex components. Furthermore, use of overmatch material is required in butt welds. This means that the weld deposit should have a larger yield strength than the base material. This is normally specified in Welding Procedure Specifications. It is further documented during Welding Procedure Qualification Testing, when a welded butt joint is tested by bending to a specified radius without failure in the weld.
3 - Fatigue Design Approaches
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