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ABSTRACT: This paper presents recent results of static (strain measurements) and dynamic investigations on multiplanar K-joints made of square hollow sections. The objective of these studies is to create the fundamentals for the determination of general formulae for strain and stress concentration factors SNCF (SCF) corresponding to the procedures used in the field of offshore constructions. GENERAL Almost all tests on the fatigue behaviour of hollow section joints published the last few years were carried out using uniplanar test specimens. Therefore in practice, multiplanar hollow section joints are calculated like planar joints. First fatigue tests showed, however, that for multiplanar joints other types of failures can occur. Regarding multiplanar joints, only a few experiments have been done in different countries. In the last years, Efthymiou and Durkin [1], Efthymiou [2], Wordsworth [3] and Makino et al. [4] have published the most well-known articles on multiplanar K-joints made of circular hollow section joints. For square section joints almost no publications are available. In nearly all the cases, the tests are carried out under static load. There is no indication to any systematic analysis of the fatigue behaviour of these types of hollow section joints using the hot spot stress method. First fundamental investigations on multiplanar joints made of circular and square hollow sections under fatigue load are being carried out in Delft and Karlsruhe, sponsored by the ECSC. The final aim of these studies is to give a general formula for the determination of strain (SNCF) and stress concentration factors (SCF) corresponding to the formulae of uniplanar joints, or to give a correction factor to the SNC For SCF-formulae for uniplanar joints. This paper thus represents a consequent continuation of the investigations on uniplanar hollow section joints carried out in Delft and Karlsruhe.
- Europe > Germany > Baden-Württemberg > Karlsruhe Region > Karlsruhe (0.46)
- Europe > Netherlands > South Holland > Delft (0.46)
ABSTRACT: The project "Stress Concentration Factors for Simple Tubular Joints" was a UK Department of Energy (DEn) sponsored project, completed in 1990. The primary objectives of this project were to assess current methods for deriving SCFs in simple tubular joints, to discuss the applicability of the more commonly used parametric equations, and to derive a new set of parametric equations that would reduce some of the anomalies in the existing formulae. In this paper the principal conclusions of this project are summarised and the new Lloyd's Register parametric equations estimating SCFs for simple tubular joints are presented. INTRODUCTION In the 1970's with the increasing development of the hot-spot stress S-N approach to nodal joint fatigue life estimation, it became clear that the determination of reliable SCFs for tubular joints was fundamental to this concept. The first parametric SCF equations covering simple tubular joints were derived by Toprac and Beale (1967) using a limited steel joint database. The prohibitive cost of testing scaled steel models led Reber (1972), Visser (1974) and Kuang et al. (1975) to use finite element (FE) analyses based on analytical models of cylindrical shells. Subsequent equations by Wordsworth and Smedley (1978) using acrylic model specimens and by Efthymiou and Durkin (1985) employing 3-D shell FE analyses, have made considerable advances both in the accuracy of parametric equations and in the range of joints covered. Over this period, differences arose between the experimental procedures used to derive stress concentration factors for simple tubular joints. These differences led to inconsistencies both in the measured SCFs themselves, and also in the SCF parametric formulae based on these measured SCF values. These inconsistencies in SCF derivation are reflected in the hot-spot S-N curves used to estimate fatigue lives for simple tubular joints. In the UKOSRP II project (DEn, 1987), a limited programme of work investigated anomalies between the existing simple joint parametric SCF equations. A test programme on unstiffened tubular joints performed by Lloyd's Register, (Smedley and Fisher, 1990), has further emphasised inconsistencies between test results and the more commonly used parametric equations. While current guidance merely states that ‘the appropriate SCF’ should be used, it is the intention of future guidance (Reynolds and Sharp, 1990) to give more specific directions on which parametric SCFs may be employed in fatigue design, and the procedures that should be considered prior to performing either experimental tests, FE analysis, or in deriving parametric equations.
ABSTRACT: A method for choosing allowable stresses for aluminum structures is presented. Stresses ranging from static stresses to beyond the endurance limit at two million cycles are considered. The proposed conversion of the steel design curves for aluminum recognizes the differences in endurance limits as well as the influence of different Young's moduli and tensile ductility. Fatigue tests were performed for simple test specimens and welded aluminum components. The experimental results are compared with the presented design method and some design codes. INTRODUCTION The use of aluminum in vehicle and offshore applications is increasing. The advantages of aluminum are lightness, corrosion resistance and usually good weldability. Lack of knowledge and experience on the fatigue behaviour, however, may reduce the application of aluminum in spite of the economic advantages available. Various codes, such as BS 118, NS 3471, DVS 1608 have been introduced for dimensioning of welded aluminum structures. They are mainly based on the design codes for steel structures and on test data of small scale and simple geometry specimens. The applicability of the test data is limited because high residual stresses often exist in large welded structures, while they are almost relaxed in the test specimens. On the other hand, the elastic modulus, ductility, notch sensitivity and the fatigue crack initiation and propagation behaviour are significantly different for aluminum and steel structures. Therefore, a great demand still exists for a consistent and comprehensive design code development for welded aluminum structures under fatigue loading. In this paper, a method for choosing allowable stresses for aluminum structures is presented. The proposed conversion of the steel design curves for aluminum recognizes the differences in the endurance limits as well as the influence of different Young's modulus and tensile ductilities. Fatigue tests were performed for welded aluminum components and test specimens.
Abstract: MWD-systems (Measuring, While Drilling) are used predominantly for drilling in offshore regions and require nonmagnetic drill collars. The most important and most widely used materials today are Cr-Mn-N-steels which offer a good combination of properties like hardness, strength, fatigue strength, corrosion resistance and stress corrosion resistance in a surface treated condition with compressive stresses present at and beneath the surface. Fatigue is one of the most severe types of loading in drill collars. In the present paper the fatigue behaviour of Cr-Mn-N-steels is described and discussed. The paper presents the results of research and development activities in the latest years which enabled a significant improvement of the fatigue properties. The fatigue properties, characterized by Wohler experiments (S-N behaviour), low cycle fatigue tests, including cyclic softening and/or hardening, were investigated in strain- and load-controlled tests. Special attention is directed to the variation of the fatigue properties over the cross section because cold working, which is applied to increase the yield strength, the ultimate tensile strength and the fatigue strength, is accompanied by deformation gradients and different flow patterns. Cold forging introduces macroscopical and microscopical internal stresses which are varying over the cross section. The most important factors determining the fatigue properties are discussed in detail e.g. solid solution hardening by means of manganese and nitrogen, the cold working performed by forging and the grain size. Furthermore a survey of attainable fatigue properties over the cross section of drill collars is given. Introduction Because of the increasing use of fossil fuel combustibles it is necessary to make accesible deposits at greater depths with the help of either Rotary Drilling- or Turbo Drilling techniques. For the penetration of different kinds of rocks with diverse kinds of applied drill tools it is required to bring pressure upon the bit.
- Well Drilling > Drillstring Design > Drill pipe selection (1.00)
- Well Drilling > Drilling Operations (1.00)
ABSTRACT: The delayed failures of the high tension bolts has been reported since they were introduced into steel structures and bridges. Most of these failures have been thought to be induced by the diffusible hydrogen because these failures often occurred under polluted circumstances like seaside or industrial areas. Although many studies about hydrogen induced cracking (HIC) have been reported, behavior of diffusible hydrogen in high tension bolts which has a geometrical notch has not yet been solved satisfactorily due to the complexity of many problems about HIC. There is the need to take into account not only the density gradient of hydrogen but also the stress distribution in order to solve the diffusional process of hydrogen in metals under stress. Mathematical development including the contribution of the stress field was done, results were applied into the finite element method (FEM), and a numerical method was used to calculate the incubation time required for diffusible hydrogen to accumulate around the root of a thread. Delayed fracture tests with pre-charged specimens were performed and compared with analytical results. INTRODUCTION The present trend for taller buildings and greater spans has led to 590 MPa tensile strength class or higher classes of high strength steels being considered for use in building construction. Unlike in the case of mild steel, a shortage of the ability of a stress redistribution due to a higher yield ratio won't show the sufficient deformability of frames after reaching mechanism. The loss of sectional area of base metal caused bolt holes must be smaller as far as possible to promote a sound stress re-distribution around bolt holes, and it is necessary that the strength of a high tension bolt must be higher to reduce the loss of section by bolt holes, and also to insure the deformability of bolted joints.
ABSTRACT: Interpretation of CTOD weld data is often complicated by the occurrence of pop-in. This paper argues that existing procedures for classifying the severity of pop-in are inadequate. An alternative strategy based on crack arrest toughness is suggested and illustrated by practical example. INTRODUCTION Weld metal toughness is commonly measured by the CTOD (Crack Tip Opening Displacement) test. A serious problem with this test is the interpretation of pop-ins (short arrested brittle cracks). A typical example is shown in Figure 1. Current practice is to designate such events as δc, the critical toughness of the weld. Thus the well in Figure 1 has a δc of 0.029mm. A δc value this low effectively prevent; the weld from being used in any high integrity structural application. However, this condemnation can be questioned given the high overall toughness shown once the pop-in has arrested. The most recently issued CTOD test standard (ASTM EI290–89) contains a recommended procedure for distinguishing between ‘significant’ and "insignificant" pop-ins based on the change in load and displacement during the pop-in event. A pop-in is judged to be "insignificant" if the inferred load drop at constant clip gauge displacement is less than 5%. There are two serious problems with this recommendation. Firstly, the size of the pop-in which can be excluded is very small (the pop-in in Figure 1 is ‘significant’ by a wide margin). Secondly, the procedure fails to address the true structural significance of pop-in. It is implied that once a pop-in has been judged to be "insignificant" its existence may be forgotten in the structural safety justification. This is clearly incorrect, since if a similar pop-in had occurred at a slightly higher load level it would have been subjected to a higher crack driving force and might not have arrested.
ABSTRACT: The problems of adaptation of reliability conception during the elaboration of the brittle fracture calculation by traditional designing methods are considered. The objective simplification of calculation have not to lower of its trustworthiness, which is defined by degree of closeness of a calculated limited condition and real one. That can be provided by taking the calculation assumptions, which describe adequately the behavior of real object and use the reliable statistical information. The testing of large- scale specimens, which model the elements of structures, take a chance to reproduce the real limited conditions and analysis of their results determine the calculation form including the influence of brittle fracture factors and trustworthiness of calculation. INTRODUCTION The designing of offshore structures include two fundamental engineering problems. In the second place, designing must provide reasonable economy of materials and technology of manufacturing. Using in offshore structures of hard statical indeterminable constructional schemes with considerable section elements, which has been manufactured with high strength steels, has worked at the rigid stress-strain state conditions, has been subjected to complicate combination of statical and dynamical loading and has suffered an influence of severe climate, is likely to choose between structure rupture types the brittle fracture as most dangerous one. Technological standards, and standards for material contain various demands, instructions and conditional criteria, which summon to provide a higher resistance to brittle fracture, but which, however, disperse greatly and has a qualititive but often undeterminate character. Such as, in Soviet designing standards (1975, 1988, 1990) their number became 50. The situation arisen bring to irrational competition of the reliability and the cost of constructions. The aproximateness in estimation of significance of separate demands and instructions for an improvement of the reliability of structures against brittle fracture provide only an approximate level of the reliability.
ABSTRACT: A review has been undertaken of the research, funded by the Department of Energy, associated with the inspection and repair of offshore concrete structures. This includes the preparation of a classification of visible defects and the provision of concrete boxes containing simulated defects for diver training. The limitations of visual inspection are considered. Three concrete marine structures have also been surveyed, to gain data both on the performance of different inspection methods and also on the behaviour of a concrete structure after a significant period of exposure to seawater. The types of damage which have occurred to offshore structures are reviewed, as well as the repair methods needed to reinstate the concrete. The selection of different materials for repair is considered and tests showing their performance both under static and fatigue loading are described. The implications of the results for offshore inspection and repair are discussed. 1. INTRODUCTION In-service inspection and repair of the concrete installations in the North Sea is an essential requirement for their continued integrity and safety. The Department of Energy's Guidance requires that such structures are inspected on a regular basis for renewal of their certificates of fitness. In the latest Guidance some information is also given on structural repairs and modifications which includes the choice of materials for repair of concrete. The earliest concrete platform installed in the UK sector was the Ekofisk storage tank, which is now over 15 years old. Several other concrete structures were installed in the late 1970's and are over ten years old. Most of these structures have performed very well in the harsh North Sea environment and there have been few requirements for repairs to date. Damage from dropped objects has occurred, particularly on the tops of the storage caissons and underwater repairs have been necessary.
- Materials > Construction Materials (1.00)
- Energy > Oil & Gas (0.88)
- Government > Regional Government (0.81)
- Government > Energy (0.81)
ABSTRACT: Reported are fatigue crack growth (FCG) investigations on a quenched and tempered high yield steel in air and in sea-water at cathodic protection. Using the FCG curves found, together with an assumed operational profile {o.p.} of loads, reliability computations were performed on three full penetration fillet welded joints. For a lower loaded joint, exposed to sea-water, very small and ever decreasing failure rates were found, even after as many as 200 traverses of the o.p. For two higher loaded joints, not exposed to sea-water, a failure rate of 0.01 per o.p. was found to occur at approximately 1 to 6 traverses of the o.p., also depending on the weld toe radius. INTRODUCTION In an earlier paper (Tichler and De Jong, 1989), the probabilistic approach of fatigue by means of the program FAILURE RATE has been described. This program essentially consists of numerical integration of a fatigue crack growth (FCG) relationship. Starting from the statistical distributions of the input parameters, the fatigue life distribution is computed by means of Monte Carlo simulation. The results were verified by comparison with fatigue life distributions derived from experimental results. In the present paper, the application of the program to three full penetration fillet welded joints will be described. Joint 1 is a frame to hull joint, exposed to sea-water under conditions of cathodic protection (CP). Therefore, FCG curves for the concerned high yield steel have been determined under these conditions. Joint 2 is a bulkhead to hull joint, not exposed to sea-water. FCG TESTS Experimental The test material is the same quenched and tempered high yield steel as considered in the earlier paper, with a yield strength of 720 MPa and an ultimate tensile strength of 790 MPa, the specifications values being 650 MPa and 750 MPa, respectively.
- Europe > Netherlands (0.29)
- North America > Canada (0.28)
ABSTRACT: The reduction in fatigue strength of welded joints in sea-water, as compared to air, found in various National test programmes has been shown to vary quite widely. The results have therefore been examined, and additional tests have been carried out, to try to determine the reasons for the variability. As a result some correction factors have been derived and the results have been re-analysed, on the basis of which revised environmental reduction factors are proposed. INTRODUCTION The current Department of Energy design guidance for offshore structures (Dept. Energy, 1990) indicates that, for unprotected steel joints in sea-water» the fatigue life is reduced by a factor of 2·0, whilst cathodic protection at the optimum potential restores the life to that measured in air. These recommendations are based on extensive fatigue tests conducted on plate specimens, within the United Kingdom Offshore Steels Research Programme UKOSRP 1 (Dept. Energy, 1988). Further tests have been conducted since the Guidance Notes were drafted in the early 1980's, and a Review Panel has been examining these to establish whether the guidance should be modified. A compilation of data obtained in a number of studies, including UKOSRP, has recently been prepared by Sharp and Thorpe (1988) and is shown in Fig. 1. Since the UKOSRP tests were carried out in two separate laboratories, whose results were in broad agreement with each other, it is clear that the differences outlined above are not attributable to testing techniques in a single laboratory. The results are given in a slightly different form to that used by Sharp and Thorpe. They compared the lives corresponding to particular stresses and presented the results in terms of the environmental reduction factor (ERF), which was defined as (life in air/life in sea-water) for a particular stress range.