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Due to the advantageous wind conditions, many new structures are planned offshore. The construction of offshore structures with ever increasing sizes requires new concepts to cope with the particular requirements thereof. One often overlooked possibility is adhesive bonding as a structural joining method, which upholds significant optimisation potential. This paper, the first in a series of two, illustrates the potential of adhesive bonding on the widely used jacket structure. For that, the structural concept is adapted towards ease of manufacturing, hybrid joints are defined, adhesives are selected according to requirements resulting from structural and climatic constraints, structural verifications performed, and the validation therefor presented in preliminary small-scale tests-which will be upscaled in the second part of this series. This paper thus serves as a roadmap for practitioners, and outlines the various steps to evaluate the potential of adhesive bonding in complex structures.

Laser beam and laser-GMAW-hybrid welded joints made of high-strength fine-grained structural steels – part 2: fatigue strength. Innovative welding processes, such as laser beam and laser-GMAW-hybrid welding, offer advantages over conventional gas metal arc welding (abbr.: GMAW) processes due to a reduced heat input during the welding of high-strength fine-grained structural steels. The materials, with nominal yield strengths of up to 1300 MPa, are already being used in mobile crane construction and related industries, particularly in telescopic booms. So far, both conventionally welded and laser beam welded joints have been avoided in highly-stressed areas of load-bearing structures. For the latter, in particular, this is due to a lack of experience and design rules. To counter this, part 1 presented the basics of laser beam and laser hybrid welding processes, welding tests and a characterisation of butt welds (visual, dye penetration and X-ray tests as well as transverse tensile tests and hardness field measurements). This article (part 2) focuses the fatigue strength of these welded joints made of the high-strength fine-grained structural steels S960MC, S1100M, S1100QL and S1300QL. The properties under cyclic loading are quantified for the details butt joint with and without different sheet thicknesses as well as for longitudinal stiffeners. In particular, the laser beam welded joints show a high fatigue strength with a remarkably low scatter of the individual test results. Based on the fatigue test results, fatigue classifications for the nominal stress concept are derived and design recommendations are presented.

Circular hollow sections (CHS) offer a variety of constructional and architectural advantages. Up to now, hollow section structures have usually been connected by welding, and more rarely by bolting. However, these established connection methods have disadvantages. Despite its great success in the mobility sector, adhesive bonding is not yet used for joining hollow steel sections in steel construction. Adhesive bonding, however, has significant advantages, especially if combined with classical joining processes. To highlight these advantages, this article presents excerpts from the results obtained in two recently completed research projects. First, a hybrid brace connection for hollow section framework structures is presented which combines welding, bolting and adhesive bonding methods. In addition, the loadbearing behaviour of a novel hybrid grouted connection, which advantageously combines adhesive and grout materials, is analysed and discussed in this paper.

Laser beam and laser-GMAW-hybrid welded joints made of high-strength fine-grained structural steels – part 1: procedure and weld characterisation. In the case of fine-grained structural steels with the currently highest nominal yield strengths of up to 1300 MPa, the execution of high-quality welded joints represents a particular challenge. Innovative welding processes, such as laser beam and laser hybrid welding, show high application potential, but have only rarely been used due to a lack of experience so far. This article, therefore, presents a weld characterisation carried out on four different materials (S960MC, S1100M, S1100QL, S1300QL) and two welding process variants (laser beam welding without filler metal, laser-GMAW-hybrid welding). The investigations of butt welds include visual, dye penetration and X-ray tests as well as transverse tensile tests and hardness field measurements. The weld zones show remarkably small dimensions and only moderate softening, which is particularly concentrated in the tempering zone. Exceptions are the joints of the S1100M, where no softening is observed in the tempering zone.

The stress ratio effect on the fatigue life of materials is a topic which have been studied by two different approaches. On the one hand, several experiments, performed under different stress ratios are necessary to estimate the corresponding Wöhler curves. Afterwards, these curves are considered to estimate the fatigue life under a particular stress range. On the other hand, fatigue failure criteria for fluctuating stress like the Goodman–Haigh relationship, are applied to estimate the stress amplitude for a constant fatigue life. Based on the Stüssi function, this paper presents a low cost model to estimate Wöhler curves and constant fatigue Goodman–Haigh diagrams. This procedure requires a set of tests performed under a particular stress ratio from LCF to HCF, and data from minimum two additional stress ranges for each subsequent stress ratio. An application on data from Ti-6Al-4V samples manufactured by selective laser melting (SLM) is presented.

Zusammenfassung In diesem Beitrag wird das Potenzial von CFK-Verstärkungen zur Steigerung der Restlebensdauer ermüdungsgeschädigter Stahlbauteile untersucht. Hierzu werden Versuche an ermüdungsrissgeschädigten Stahlblechen und Großbauteilen durchgeführt, die mit schlaff oder vorgespannt aufgeklebten CFK-Lamellen verstärkt sind. Zur Fertigung der vorgespannt verstärkten Prüfkörper wird zunächst eine geeignete Vorspannvorrichtung konstruiert und gebaut. Einen wesentlichen Schwerpunkt des Forschungsprojektes stellen Dauerschwingversuche an ermüdungsrissgeschädigten zentrisch gekerbten Stahlblechen dar, die wiederum schlaff oder vorgespannt verstärkt werden. Die Ergebnisse zeigen im Vergleich mit den unverstärkten Referenzproben eine deutliche Reduzierung der Rissspitzenbeanspruchung durch die Applikation von CFK-Lamellen. Dies zeigt die signifikant höhere Restlebensdauer. Die Anordnung der Lamellen und deren Vorspannkraft beeinflussen dabei maßgeblich die Wirksamkeit der Verstärkung. Untersuchungen an Großbauteilen zeigen, dass die reduzierte Rissspitzenbeanspruchung auch bei realitätsnahen Probekörpergeometrien erzielt werden kann. Um die Wirksamkeit einer CFK-Verstärkung mit den bisher im Stahlbau eingesetzten Sanierungsmethoden hinsichtlich einer Restlebensdauererhöhung zu vergleichen, werden Dauerschwingversuche an ermüdungsgeschädigten CT-Proben durchgeführt.

Zusammenfassung Aus den von der Bundesanstalt für Straßenwesen veröffentlichten Zustandsnoten lässt sich ein erheblicher Sanierungsbedarf deutscher Straßenbrücken ableiten. Stahlbrücken weisen häufig Schäden in Form von Ermüdungsrissen an Stellen erhöhter Spannungskonzentration auf. Der Neubau von Brücken ist aus wirtschaftlichen und infrastrukturellen Gründen nicht immer möglich. Somit sind effektive und zuverlässige Methoden zur Verstärkung der geschädigten Tragwerke dringend erforderlich. Die derzeit angewandten Reparatur- und Verstärkungsmaßnahmen weisen verschiedene, spezifische Nachteile auf. Durch eine klebtechnische Verstärkung, bei der CFK-Lamellen über den Ermüdungsriss geklebt werden, kann eine Vielzahl dieser Nachteile vermieden werden. Im Rahmen eines Forschungsprojektes wird das Potenzial schlaff und vorgespannt aufgeklebter CFK-Lamellen zur Erhöhung der Restlebensdauer untersucht. Auf Grundlage eines Anforderungskatalogs werden potenzielle Klebstoffe vorausgewählt und anhand von Kurz- und Langzeitversuchen an modifizierten dicken Zugscherproben experimentell charakterisiert. Mithilfe von Ergebnissen quasi-statischer Versuche an anwendungsnahen CFK-Stahl-Überlappklebverbindungen wird ein Berechnungsansatz zur Abschätzung der Tragfähigkeit und der Mindestverbundlänge für zwei ausgewählte Klebstoffe verifiziert. Darauf aufbauend werden Dauerschwingversuche an der CFK-Stahl-Überlappklebverbindung durchgeführt und Wöhlerlinien erstellt, die als Grundlage für eine Ermüdungsfestigkeitsbemessung dienen können.

A correlation of laboratory corrosion methods with real marine conditions is conducted in the present study. Goal was the definition of an appropriate time scale for each investigated laboratory method for the simulation of marine corrosion. The standardized corrosion in a salt spray chamber and in artificial seawater as well as the empirical method of corroding in an electrolytic setup are investigated. The influence of each corrosion method on physical quantities, which affect the fatigue behaviour and are applied in structural engineering, is taken into consideration. Periodical optical inspection of the surface as well as measurements of thickness loss and surface roughness are carried out for long intervals of deposition. The results of these measurements are statistically processed, in order to validate initial assumptions regarding the evolution of the corrosion influence and enable probabilistic prediction of the specific influence of each corrosion method. Additionally fatigue tests are carried out on butt welds, which were pre-corroded in a salt spray chamber and in artificial seawater. These tests are performed sequentially dry and simultaneously, inside artificial seawater, respectively, in order to evaluate the direct influence of these methods on fatigue life. Similar measurements and results from fatigue testing on specimens of parent material, which were corroded for 2 years in real marine environment in the framework of a previous project are used as reference. All investigated and reference specimens were made of structural steel S355. Conclusions regarding the effectiveness of each investigated method and the influence of corrosion on fatigue are drawn and a first-step time correlation of laboratory and real conditions is made.

Circular hollow sections (CHS) represent a class of tubular structural steel elements that enjoy great popularity among architects, and civil engineers. Connections thereof, however, remain complicated, despite significant developments in welding procedures. The first part of this series of two articles summarises research on adhesive bonding as a substitute to traditional joining techniques for tubular sections on a large scale. For that purpose, suitable adhesives have been selected and fully characterised, their adequacy for the required strength in combination with steel verified on lap shear samples, and finally corresponding adhesively bonded tubular joints tested in quasi-static loading with diameters from 42 mm (joint capacities from 55 kN) up to 300 mm (joint capacities up to 1’800 kN). Additionally to various diameters and overlap lengths considered, effects resulting from different types of imperfections, as axial and angular misalignment, on joint strength were experimentally investigated. Presented results clearly indicate that adhesive bonding is a joining technique adapted for civil engineering applications. The second part of this series presents a design methodology for adhesively bonded tubular joints that fits into the conceptual framework of civil engineering practice.

Capacity prediction of adhesively bonded joints remains challenging; this is particularly true for tubular joints composed of circular hollow sections used in civil engineering. This paper extends its companion paper, which presented experimental results of a large research campaign on full-scale tubular joints, with the aim to develop a dimensioning method. Firstly, experiments were designed to quantify the strength of the adhesive-steel connection; the results thereof were cast into the formulation of an adequate failure criterion, for each of the adhesives considered. Secondly, numerical models of all investigated joint configurations were established, from which the stress-state inside the bonded connection could be determined. A series of analyses clearly showed that stress magnitude and joint capacity did not correlate, thus indicating that a direct stress based methodology is destined to fail. Accordingly, a probabilistic method that is based on the concept of size effects was developed, and implemented. It allowed not only determining average values for joint capacities, but also estimated the expected experimental scattering thereof. The validity of the approach was tested by successfully comparing experimental and computed joint capacities.

The present study reports on results of a completed research project, which investigated amongst others the fatigue strength of weldments made of structural steels S355J2+N, S690QL and S960QL and treated with the post-weld treatment process High Frequency Mechanical Impact (HFMI). Reference tests on as-welded (AW) specimens, which were manufactured from the same material batches, were carried out in order to enable a direct comparison. Main goal of the present tests series was, apart from the revalidation of the higher efficiency of HFMI on high strength steels, to investigate possible scale effects, when the post-weld treatment method is applied on real size structural components. Therefore, usual fatigue test specimens and real size structural beams HEA 260 were tested. The welded structural detail of transverse stiffeners was investigated in all cases. All results are evaluated with the nominal stress approach and they are presented in S-N curves along with the respective FAT classes, which are proposed by the design guidelines of Eurocode and the recommendations of International Institute of Welding (IIW). Beach mark tests were carried out on HFMI-treated small scale fatigue test specimens, in order to investigate the influence of HFMI on the fatigue crack growth behaviour (FCG). In all cases, a significant increase in fatigue strength was achieved by the application of HFMI with the fatigue life extension being more significant for the high strength steels. No scaling effect was documented regarding the application of the method on the structural beams having similar plate thicknesses as the small scale specimens but more complex geometries and significantly longer weldments. Finally, the evaluation of the beach mark tests and their comparison with reference curves found in literature did not allow a clear assessment of the HFMI influence on the fatigue crack propagation (FCP).

This study deals with the fatigue strength of high-frequency mechanical impact (HFMI)-treated unprotected structural details made of mild steel S355 considering the influence of corrosive environmental conditions. The investigations are carried out on butt welded specimens with sheet thickness t = 15 mm and on transverse non-load-carrying attachment specimens with sheet thickness t = 25 mm. Two different methods were applied for the simulation of marine corrosive environment in the laboratory. Specimens first were deposited in a salt spray chamber and then tested subsequently dry at laboratory-air conditions considering the influence of corrosion on the crack initiation. Alternatively, and to cover the effects of corrosion on the crack growth, artificial seawater was used for pre-corrosion, and after a defined timespan, fatigue tests were performed simultaneously with the specimen resting in the corrosive medium. The corrosion fatigue tests were performed in as-welded and HFMI-treated conditions at a stress ratio R = 0.1 under axial tensile and 4-point bending cyclic loading. The test results are evaluated to determine the characteristic fatigue strengths for fixed slopes m = 3 and m = 5 according to IIW recommendations for the as-welded and for the HFMI-treated condition respectively. The results of the experimental investigations based on the nominal stress approach show that the fatigue strength of both specimen types could be significantly increased by the application of HFMI treatment compared to the corresponding specimens in the as-welded condition even if exposed to the investigated corrosive conditions. The comparison with the design proposals of IIW shows that for HFMI-treated butt welds, no reduction of the FAT class due to corrosion is required and the recommended FAT class is still valid. The results for the HFMI-treated transverse attachments are slightly below the design curve recommended by IIW and a proposal to consider corrosion is derived for this case. Additional numerical investigations by applying the effective notch stress (ENS) approach are performed to determine notch stress curves. It was found that for the corroded specimens in the as-welded condition, the FAT class according to IIW could not be reached and adjustments of the existing rules are necessary to consider corrosion effects. However, it can be concluded that the effective notch fatigue resistance recommended by IIW is still applicable in the case of corroded HFMI-treated structural details.

Low-cycle fatigue behavior of modern very high strength and ultra-high strength steels with nominal strengths up to 1100 N/mm² in welded condition is described and the application limit of elastic design rules is defined. Design recommendations are given based on the nominal stress approach as well as the notch stress approach. Thus, welded details made of high-strength steels can be designed for low-cycle fatigue applications. Furthermore, a more accurate assessment can be done, allowing for an economical application of high-strength fine-grain structural steels.

The fatigue life can be represented through S-N curves which describe the relationship between load cycles up to failure and stress range applied in a structure. Providing these curves help the engineers to estimate design stress levels with a high reliability. The nonlinear function proposed by Stussi offers a good approach to model the mentioned curves. However, he did not propose any statistical distribution which allows to make estimations with certain probability. Moreover, the Stussi function does not consider the influence of runouts and their subsequent retests. The Weibull model from Castillo et al. is a statistical method which overcomes this limitation. However, this method cannot be applied to model the S-N curves in the Low Cycle Fatigue regime. In this paper, a Weibull distribution is combined with the Stussi function in order to model the S-N curves, and the characteristics of this proposed method are compared with those of the Weibull model from Castillo et al. In order to establish the advantages and usability of both models, one application considering the experimental data of specimens made of steel S355J2+N is presented. Finally, some recommendations regarding the planning of fatigue tests are made.

To promote a straightforward design guideline, a determination model of S-N fatigue curves was established to consider effects of the initial crack and stress ratio (R-ratio) on the fatigue performance of damaged steel plates strengthened with non- and prestressed CFRP (Carbon Fiber-reinforced Polymer) plates. The modified Paris’ law employing the effective stress intensity factor range was first used to model the fatigue crack growth data obtained from an extensive literature review for various R-ratios, including edge- and center-notched reinforced steel plates under fatigue loadings. Based on these analysis, the fatigue S-N curve function including the fatigue strength Δσc and fatigue threshold ΔσL, was derived to consider the effects of the initial crack and R-ratio. The model was validated using fatigue results of approximately two hundred specimens with various notch sizes and stress ratios. Specific handbooks provided distinctly different values of C and m for mild steel, and among them the recommended values are 6.86 × 10−12 (with the unit of MPa and meter) and 3.0 through the validation using existing test data reported in the literatures. The comparison of the experimental and predicted fatigue lives confirms the feasibility of the proposed S-N curve determination. Furthermore, the practical value U of the crack closure effect is recommended as 1.0 in the negative R region (R

In the present work, the influence of taking into consideration the strain rate-dependent nature of steel S355 during a calculation of the welding residual stresses through a finite element simulation is investigated. The Perzyna material model is calibrated using experimental values found in the literature and is introduced to a validated weld simulation model, where the strain rate dependency had not been taken into consideration before this study. The calculated profiles of the welding residual stresses, for strain rate-dependent and independent behavior, are then compared with experimentally measured profiles. The results of this first-step analysis show that taking into consideration strain rate dependency during a welding simulation of the investigated S355 has non-negligible influence on the calculated welding residual stresses.

The wind energy sector is constantly looking for improving the efficiency of their facilities, including on offshore locations. Part of that strategy is the demand for increasingly larger offshore structures. This cannot be met only by simply upscaling existing solutions, as there are several reasons that prevent such a simple approach. Often the connections are decisive for the dimensioning of the load bearing structures. Novel approaches, as the one this paper presents, are necessary. To allow for even larger structures, a novel hybrid joint, which combines adhesive bonding with welding, is presented. The first part of this paper fully addressed the requirements, and allowed to select two suitable adhesives significantly differing in strength by a ratio of 10:1, and stiffness by a ratio 50:1. Based thereupon, this paper first addresses the design, and the corresponding manufacturing concept, which were fully simulated numerically. Time and effort put into these preliminary steps proved crucial, as it allowed implementing changes and adaptations that otherwise would have hampered the project. Due to its nature, it largely reduces complex, labour, and cost intensive welding operations on the legs of the jacket structure, and substitutes them by relative simple, fast, and cost-effective adhesive bonding. The solution devised offers an alternative to the complex welding on the primary structural elements, resulting notch stresses, and associated local weakening. Thus, the innovative approach relocated the critical section of the joint well outside the former welding seams, and capacity of the hybrid joint is now limited by that of the tubular elements under static and fatigue loading. This was validated at medium size, with samples scaled down to 1:10, and large scale, at 1:5, respectively.

Considering the influence of runouts and their subsequent retests helps to determine the S - N curves which are used by engineers to estimate design stress levels with a high realiability. The Weibull model proposed by Castillo et al. is a unified statistical methodology to describe this phenomen. In this article, a procedure to model the S - N curves, subject to the constraints mentioned above is presented. Finally, two applications considering the experimental data of girders from an antique steel bridge built in 1895 and the experimental data of welded specimens made of steel S690QL are presented.