Fatigue Capacity Research Articles

Thickness scaled compression tests in unidirectional glass fibre reinforced composites in static and fatigue loading

Thick laminates can be found in different wind turbine blade sections, such as the root or the spar caps. However, static and fatigue design of thick laminate components is based on thin coupons tests (around 4 mm thick) and safety coefficients determined by the guidelines and standards.The aim of this work is to study the size effect in compression for unidirectional glass fibre reinforced composites. Scaled static and fatigue compression tests were performed on glass/epoxy laminates of 4 mm, 10 mm and 20 mm thick. A series of scaled coupons were designed taking into account the influence of the manufacturing process and the final testing conditions. While static tests on laminates with different thicknesses showed no significant change in the static ultimate stresses, fatigue tests showed a decrease in the fatigue life for increasing thicknesses as well as wider intervals of confidence. In addition, fatigue data were compared with the reduced fatigue capacity reported by the design guidelines showing reserve factors larger than 2.0.

A Simplified Method for Quantitative Reliability and Integrity Analysis of Steel Catenary Risers

Quantitative reliability and integrity analysis of steel catenary risers (SCRs) can provide important information about their safety and toward their cost-effective and optimal design. SCRs are one of the commonly used riser systems in offshore production stations. The consequence of an SCR failure is significant; however, the overall safety of the riser is typically not quantified. Especially, because of the uncertainties associated with environmental conditions and structural capacities, quantitative reliability methods can take advantage of available data and developments in computing technology to provide a strong basis for their reliable engineering decision making. This paper presents a simplified approach for assessing the strength and fatigue reliability of SCRs, accounting for the uncertainties with their yield strength and fatigue capacities as well as the environmental conditions. Moreover, the integrity-based optimal design of riser strength limit state for a target annual probability of failure is discussed. The fatigue reliability of the SCR system is also assessed in component and system levels. The proposed method is then applied to a typical SCR attached to a semisubmersible vessel under Gulf of Mexico (GOM) conditions. Results of dynamic (time-domain) analyses under various environmental conditions are used to quantify the SCR safety and integrity and to optimize its design for a target annual probability of strength failure. By estimating the riser system probability of strength and fatigue failure in its lifetime, the strength and fatigue integrity indices, and the optimality factors of the riser sections for the strength limit state, suggestions are provided to improve the riser design. For example, it was found that considering the two main limit states of strength and fatigue failure of the SCR system, a strength failure at the taper stress joint (TSJ) is the likely mode of failure in this riser system, which has a probability of 0.0035 in its 25 year lifetime.

Numerical analysis of concrete filled Buckling Restrained Braces

This paper is on the numerical analysis of concrete filled Buckling Restrained Braces (BRBs), which are typically used as diagonal bracing members of steel braced frames designed for dissipative behavior and lateral load resistance under seismic action. BRBs are typically composed of a slender steel core which is continuously supported by a concrete casing against buckling. The complex behavior of the BRB devices is usually studied by experimental tests. The numerical analysis is rarely applied, it requires advanced tools, such as modeling of cyclic steel plasticity with combined hardening, cyclic concrete behavior, plastic buckling, contact and friction. In the paper a novel Chaboche based cyclic steel material model is presented, which is developed in ANSYS finite element environment. The model is able to describe all the important features of the cyclic behavior of structural steel, including low cyclic fatigue capacity. The William–Warnke model is used for the concrete material with combined kinematic hardening. The computational model has a special focus on the contact/friction phenomena between the steel core and supporting concrete. The structure and approximations of the numerical models, the calculation of model parameters on the basis of its own test results are detailed. The features of the behavior are demonstrated by the application of the developed 3D solid Buckling Restrained Brace model.

Fragility analysis of wind-excited traffic signal structures

Traffic signal structures are prone to fatigue failure due to wind-induced excitation that is often difficult to characterize due to the variable response arising from a combinations of along- and across-wind effects and the uncertainties associated with the fatigue and fracture phenomena. A probabilistic framework is proposed to predict structure component fatigue life. To solve the problem of wind-induced fatigue, the model is introduced to inter-relate wind hazard to structural response and hence fatigue damage. Stress range demand variabilities are considered separately from the randomness associated with the fatigue capacity. To characterize the natural wind response of a representative structure, full-scale structural monitoring is conducted, and the data were incorporated into an example application of the framework. To successfully demonstrate the efficacy of the proposed framework, resulting wind-induced fatigue life distributions are compared against compiled inspection records for a large traffic signal structure population in Wyoming.

Structural topology optimization of the transition piece for an offshore wind turbine with jacket foundation

The transition piece of a fixed offshore wind turbine is a reinforced part of the support structure that is connected to the wind turbine tower. These structural elements present unique features and are critical components of offshore wind turbines; designed to resist strong bending moments, shear forces and axial loads coming from cyclic environmental loads, such as wind and wave loads, acting during their complete design life of 20 years. Well designed and manufactured transition pieces with optimized ultimate and fatigue capacities, contribute to the structural soundness and reliability of offshore wind turbines. In this paper we investigate the benefits of integrating structural topology optimization in the design process of these elements, specially compared to classical design processes that are strongly based in experience and trial-and-error heuristic procedures. We first develop a solution for a 5 MW reference wind turbine with a jacket support using the classical design process and then apply structural topology optimization techniques to the same design problem. After verification of both solutions, it was found that the optimized solution is lighter and presents hot spots with lower stresses that extend its fatigue life significantly; demonstrating that an integrated design cycle that includes topology optimization can be very effective, speeding up the total design cycle and increasing the reliability of the final product.

The effect of shrinkage cracks on the load bearing capacity of steel-fibre-reinforced roller-compacted-concrete pavements

This paper demonstrates the effect of shrinkage cracks on the static load bearing capacity and on the fatigue performance of RCC pavements reinforced with steel fibres recycled from post-consumer tyres, with the aid of a case study. A numerical method is adopted using material properties (moisture transport, free shrinkage, mechanical and fatigue) derived from experiments. It is shown that surface micro-cracks initially develop predominantly due to differential shrinkage (curling). These micro-cracks penetrate down to approximately a quarter of the slab thickness and it is shown that they reduce the ultimate load bearing and fatigue capacity of the pavement by up to 50 %. Even though shrinkage does not initially induce visible cracks, it significantly intensifies the crack opening due to traffic load by up to 500 % for a crack width of 0.5 mm. For the case studied herein, to assure the long-term performance of the pavement the allowable stress ratio should be reduced by half, to account for the effect of shrinkage distress. This study is useful because for the first time the mechanical characteristics are coupled with the moisture transport mechanism, as well as the shrinkage and fatigue characteristics. This methodology can be used for other similar materials and geometries and can lead to general conclusions regarding the importance of shrinkage distress on the design of pavements.

Effect of radial base-plate welds on ULCF capacity of unanchored tank connections

The base of large steel liquid storage tanks can uplift during severe earthquakes, causing large inelastic rotations at the connection between the tank shell and tank base. While recent experimental studies indicate significantly higher connection rotation capacity than what is specified in the current Eurocode standard (set at 0.2rad), additional radial base-plate welds (present in some tank connection details due to fabrication methods) have never been considered in tests. This study experimentally investigates the effects of these radial base-plate welds on the fatigue capacity of tank shell-to-base connections during uplift. Twelve tank shell-to-base connection specimens taken from existing tanks throughout Switzerland are tested at rotation ranges greater than the current Eurocode limit (eight specimens with radial welds and four specimens without radial welds). Testing indicates that tank base-plate sections containing radial welds govern the shell-to-base rotation capacity during uplift. The rotation capacity of connections containing radial welds was nearly 30% lower (on average) than equivalent connections without radial welds. This reduced capacity is directly related to the reduced base-plate ductility created by the radial weld heat affected zone. All connection capacities were far greater than the current Eurocode limit.

Experimental Research of New Tram Track Construction Operating Fatigue Capacity

The results of dynamical fatigue testing of natural tram track fragment are presented. The fibrous concrete (foundation) was used in track superstructure, the rubber pads “PREFArails” were used for noise and vibration reduction. The cycle testing results analysis demonstrated that “PREFArail” pads ensure the overall structure operability over not less than 25 years without repair. The recommendations for pads and fibrous concrete foundation use on tram tracks in Saint-Petersburg were given.

Modeling of the Flexural Fatigue Capacity of RC Beams Strengthened with FRP Sheets Based on Finite-Element Simulation

In this study, a three-dimensional finite-element analysis (FEA) was conducted to study the parameters that affect the maximum flexural fatigue capacity of RC beams strengthened with fiber-reinforced polymer (FRP) sheets. Forty-seven specimens were designed and analyzed by using FEA. Additionally, a fatigue capacity prediction model was developed to reflect the influences of the major parameters, including the fatigue behavior of steel reinforcement, FRP sheets, and FRP-to-concrete bonding; and the influences of minor parameters, such as the yield strength of steel reinforcement, concrete strength, width and thickness of the FRP sheet, and other parameters. The results of experiments on 181 beams reported in the literature were analyzed to verify the accuracy of the proposed model. The mean values of 1.05 and 1.02 and the corresponding coefficients of variation of 17.12 and 16.06% were determined by comparing the calculation results from the proposed model with the experimental data. These results reflect the superior accuracy of the proposed model in predicting the fatigue capacity of RC beams with and without FRP strengthening.

Fatigue cracks in railway bridge hangers due to wind induced vibrations – Failure analysis, measures and remaining service life estimation

Unexpected wind-induced vibrations of the hangers have caused an early fatigue crack on specific steel components and joints of a railway bridge over the Elbe River at Lutherstadt Wittenberg, Germany. During regular periodic inspection a fatigue crack of approximately 240mm length was found near a butt weld of the longest hanger. The hanger was immediately secured by welded butt straps across the crack. Based on experimental investigations of hanger vibrations additional bracings were added between the hangers to avoid wind-induced vibrations. The weld heat influence zone which was affected by high cyclic stresses was replaced by new material.Nevertheless it was impossible to determine sufficient remaining service life for those remaining bridge components that were exposed to extreme high real load cycles. The grinding of the affected steel surfaces was the key element of the remedial actions. Furthermore, additional fracture mechanic calculations were carried out in order to assess the remaining service life of the welded joints. In this respect, the calculation approach used by Deutsche Bahn AG was compared to further procedures from the mechanical engineering field. These investigations showed that the studied, repaired components have both, bearing and fatigue capacities within the validity of standards.

Assessment of the size effect for use in design standards for fatigue analysis

Reduced fatigue capacity of welded structures for larger thicknesses was introduced in design standards approximately 30years ago. A significant amount of research on this topic was performed during the following years. In general the presence of a size effect was agreed upon. The size effect is considered to be dependent on the plate thickness at the considered hot spot in addition to size of the attachment plate and type of dynamic loading. Only simplified recommendations on the size effect are included in most fatigue design standards. One reason for this is normal scatter in fatigue test data and also somewhat different recommendations based on these data in the literature. This has made it difficult to arrive at full agreement on recommended fatigue analysis procedures. In this paper a review of the background for the size effect in literature and design standards are presented together with a calibration of a fracture mechanics analysis method with fatigue test data. The effect of different parameters contributing to the size effect is illustrated. An attempt has been made to use the calibrated analysis model also to quantify the size effect based on crack growth analyses. Finally some recommendations on size effect to be used in fatigue design standards are presented.

Reliability consideration for fatigue design of sign, luminaire, and traffic signal support structures under wind load

Fatigue failure of sign, luminaire, and traffic signal support structures has been observed. One of the causes is attributed to the along-wind loading (i.e., the natural wind gusts). The Canadian Highway Bridge Design Code (CHBDC) is not specific on the fatigue design wind pressure for these structures. The fatigue design wind load for the support structures recommended in American Association of State Highway and Transportation Officials (AASHTO) is developed based on the “infinite-life” approach, and by assuming that the stress range can be estimated considering that the response due to natural wind gusts can be represented as a constant amplitude sinusoid. The adequacy of this assumption and the implied structural reliability by using the recommended fatigue design wind load are unknown. This study assesses the statistics of the stress range caused by natural wind gusts for the support structures that are amenable for simplified structural representation. The statistics are used together with a probabilistic model of hourly-mean wind speed and fatigue capacity to estimate the fatigue reliability. The results indicate that the recommended fatigue design requirement for the support structures in AASHTO results in a widely varying reliability index. This is because the recommendation does not take into account the damping ratio or stress cycles or spatially varying statistics of wind climate. The results are also used as the basis to suggest an alternative requirement for fatigue design. The parameters involved in the requirement are calibrated based on selected target reliability, and a ready to use Canadian wind map for the fatigue design is provided.

Influence of executive imperfections of railway steel bridges construction on operational capacity and fatigue capacity

In recent years, in our country are modernized and rebuilt several sections of railway lines, mainly main lines. The greater part is adapted to increased speeds. However, such an adaptation involve the reconstruction or construction of new bridges including steel ones. The paper concerns the executive imperfections of constructions of steel bridges which in recent years have been built in the modernized main railway lines. These imperfections consist of, among others, the geometric imperfections from the assumed construction project. This also applies to defects in welds of various types that occur in critical elements of bridge structure. The aim of this paper is to demonstrate the influence of imperfections on the load carrying capacity and fatigue of these structures. Author showed that the observed imperfections have a significant impact on the fatigue life - especially for weld defects, in the case of load carrying capacity their importance is much smaller. These analyses are performed by using the theoretical methods developed in previous reports. In the analysis new standards for the design of steel structures from Eurocodes group were used.

Fatigue experiments on steel cold-formed panels under a dynamic load protocol

A dynamic load protocol has been used to experimentally simulate fatigue behavior in cold-formed metal panels with screwed connections under wind loading. The specific protocol adopted is an adaptation of SIDGERS, originally developed for non-metallic membranes, which is composed of levels each under increasing load values. A total of 19 tests were performed on 3.35 m long by 0.91 m wide panels, identified as Type B-wide rib and Type E, both with screw connections at the edge and at the center, thus conforming two-span specimens. In some configurations the panels were fixed at the valleys, whereas crest-fixed connections were also investigated. Reinforcing the connections by means of washers was also investigated to evaluate their efficiency in improving fatigue capacity. The experimental results show maximum load capacities in improved connections with washers of approximately twice of those with classical connections.

Experimental Fatigue Evaluation of Prestressed Concrete-Filled Steel Tube I-Shaped Bridge Girders

This paper investigates experimentally the fatigue capacity of prestressed concrete-filled steel tube (CFT) I-shaped girders. The girder consists of steel plates, PT tendons, concrete, and steel shear keys. Three 10 m long girder specimens were prepared for different fatigue stress ranges and subjected to a three-point bending test producing a positive moment along the entire girder span. Displacement control was used to prevent sudden failure of the specimens. The test girder specimens exhibited fatigue capacities classified as Category B′ or C′ of the AASHTO criteria for positive and negative moment, respectively. The special testing arrangement used to produce the negative moment is presented, as well as the fatigue testing procedures and results. At the end of the paper, a modified equation for the S–N curve is derived for predicting the remaining life of the girder.

Structural mechanics for design of grouted connections in monopile wind turbine structures

Over the last 30 years it has become standard practice to connect offshore oil and gas structures to their foundation piles using cylindrical shaped grouted connections with shear keys or weld beads. Circumferential shear keys, or weld beads, are provided around the outside of the piles and the inside of the pile sleeves in jacket structures to transfer forces through the grouted connection. The same methodology is also now being used by the wind energy industry to connect wind turbine support structures to their foundation piles. These structures are subjected to rather severe dynamic loading, it is therefore important to document the fatigue capacity of these grouted connections. As a direct result of this need, a joint industry project focusing on the capacity of cylindrical shaped grouted connections with shear keys was initiated by DNV in January 2011 and continued through until completion in May 2012. This project has involved fatigue testing of grouted test specimens in the laboratory, finite element analyses and assessment and development of a recommended design methodology. The design methodology includes the Ultimate Limit State and the Fatigue Limit State. Fatigue testing of full-scale specimens would require very large test setup and hydraulic actuators. Therefore special box specimens were designed with a representative radial stiffness similar to that of large diameter connections, with both full size grout thickness and geometry of the shear keys. An analytical approach for design of these specimens and for the design of grouted connections in monopiles is presented in this paper.

Capacity of cylindrical shaped grouted connections with shear keys in offshore structures

Over the last 30 years it has become standard practice to connect offshore oil and gas structures to their foundation piles using cylindrical shaped grouted connections with shear keys or weld beads. Circumferential shear keys, or weld beads, are provided around the outside of the pile and the inside of the sleeve to give the required capacity. The same methodology is also now being used by the wind energy industry to connect wind turbine support structures to their foundation piles. These structures are subjected to rather severe dynamic loading. It is therefore important to document the fatigue capacity of these grouted connections. As a direct result of this need, a joint industry project focusing on the capacity of cylindrical shaped grouted connections with shear keys was initiated by Det Norske Veritas (DNV) in January 2011 and completed in May 2012. This project has involved fatigue testing of grouted test specimens in the laboratory, finite element analyses and assessment and development of a recommended design methodology. The design methodology includes the Ultimate Limit State and the Fatigue Limit State. Fatigue testing of full-scale specimens would require very large test setup and hydraulic actuators. Therefore, special box specimens were designed with a representative radial stiffness similar to that of large diameter connections, with both full-size grout thickness and geometry of the shear keys. The design and testing of these specimens is presented in this paper together with the main results from the project.

On the low-cycle fatigue capacity of unanchored steel liquid storage tank shell-to-base connections

Under strong earthquake motions, the liquid stored within large unanchored steel tanks can cause rocking and uplift of the tank base from the supporting foundation. Repeated rocking can lead to low-cycle fatigue fractures in the tank base-plate, and ultimately spillage of tank contents. Due to limited research on tank connection capacity, current European and New Zealand tank standards (EuroCode8 part 4 and NZSEE) impose unjustified rotation limits on tank shell-to-base connections (current limit being 0.2 rad). These rotation limits often govern the design of new tanks, and many existing tanks require retrofit to meet compliance. This paper experimentally investigates the fatigue capacity of tank shell-to-base connections, with a focus on the effects from material ductility and applied strain range (rotation range). Twenty-seven tank connections representing two steel material grades are fatigue tested under constant range rotation cycles to generate fatigue-life curves. Tensile loads applied to the base-plate simulate membrane action present in tanks during uplift. Results indicate that the strain capacity of the base-plate base material strongly influences connection fatigue capacity as fatigue fractures originate in the base material away from the weld toe and weld heat affected zone. Increasing base-plate ductility drastically increases connection fatigue life. All connection specimens sustained multiple uplift cycles at rotations greater than the existing EuroCode8 and NZSEE limits.

DFR based fatigue reliability assessment of riveted lap joint accounting for correlations

The objective of this paper is to analyse the effect of correlation on the fatigue reliability of a riveted lap joint with multiple rivets using a fast reliability assessment approach based on the detail fatigue rating method. The detail fatigue rating method, DFR method, is based on the nominal stress fatigue approach. The DFR represents the inherent characteristic of fatigue capacity of a structure independent from the applied fatigue load. The fatigue reliability assessment model of a structure is proposed based on the relations of fatigue loading, characteristic fatigue life and DFR. The upper and lower bounds of fatigue reliability of a riveted lap joint structure with multiple rivets subjected to fatigue loading is analysed using the DFR and Ditlevsen approaches accounting for the correlations between rivet holes failures.

Fatigue capacity of a new connection system for a prestressed concrete hybrid truss web girder

To replace a steel box bridge with construction of medium-span bridges in Korea, prestressed concrete box girder bridges with steel truss web members, also known as hybrid truss bridges (HTBs), are being considered as an alternative bridge type. The core technology of HTB is the connection joint that links the concrete slabs and steel truss pipes. Various construction companies in Japan have developed unique connection systems and applied them to real bridge constructions after verifying their performance through experimental evaluation. In this study, static fatigue tests of a hybrid truss girder have been performed to verify static and fatigue capacities of the newly proposed hinge-type connection system with circular ribs. Based on structural finite-element analysis simulations of the joint's fatigue test result, the simulation results proved that the proposed connection joint has sufficient fatigue capacity for structural application. Finally, it is expected that the hinge connection system with circular ribs developed in this study can be easily applied to real HTB bridge construction.