Hot Spot Stress Approach Research Articles

Foundation Types of Fixed Offshore Wind Turbine

Offshore wind turbines are supported by various foundations, each with its considerations in design and construction. Gravity, monopile, and suction bucket foundations encounter geotechnical issues, while jacket and tripod foundations face fatigue problems. Considering this, a gravity foundation based on a steel skirt was developed, and a monopile foundation was analyzed for Pile-Soil Interaction using the p–y curve and 3D finite element method (3D FEM). In addition, for suction bucket foundations, the effects of lateral and vertical loads were analyzed using 3D FEM and centrifuge tests. Fatigue analysis for jacket and tripod foundations was conducted using a hotspot stress approach. Some hybrid foundations and shape optimization techniques that change the shape to complement the problems of each foundation described above were assessed. Hybrid foundations could increase lateral resistance compared to existing foundations because of the combined appendages, and optimization techniques could reduce costs by maximizing the efficiency of the structure or by reducing costs and weight. This paper presents the characteristics and research directions of the foundation through various studies on the foundation. In addition, the optimal design method is presented by explaining the problems of the foundation and suggesting ways to supplement them.

Weld Fatigue Damage Assessment of Rail Track Maintenance Equipment: Regulatory Compliance and Practical Insights

<div>The use of appropriate loads and regulations is of great importance in weld fatigue assessment of rail on-track maintenance equipment and similar vehicles for optimized design. The regulations and available loads, however, are often generalized for several categories, which proves to be overly conservative for some specific categories of machines. EN (European Norm) and AAR (Association of American Railroads) regulations play a pivotal role in determining the applicable loads and acceptance criteria within this study. The availability of track-induced fatigue load data for the cumulative damage approach in track maintenance machines is often limited. Consequently, the FEA-based validation of rail track maintenance equipment often resorts to the infinite life approach rather than cumulative damage approach for track-induced travel loads, resulting in overly conservative designs. The work presented in this article evaluates and compares the weld fatigue damage of track maintenance equipment for EN loads using infinite life approach and AAR loads using cumulative damage approach and highlights the need of having distinct category for track maintenance machines in regulations based on its usage and application to have close representative loads for the endurance limit approach or suitable loads for the cumulative damage approach. Additionally, the study utilizes the BS7608 regulation to determine weld class and predict weld fatigue damage. Both nominal and hot-spot stress approaches are employed to thoroughly investigate and calculate fatigue damage, providing a comprehensive analysis of weld fatigue in rail maintenance equipment.</div>

Investigations on the influence of angular and linear misalignment on the fatigue strength of HFMI-treated structural steel butt joints

The effect of High-Frequency Mechanical Impact (HFMI) treatment on the fatigue resistance of structural steel S355 butt welds with macro geometric imperfections outside of the usual fatigue design limits was investigated. Thin-walled specimens (t = 8 mm) were manufactured with different degrees of angular and linear misalignment and fatigue tested in the as welded and HFMI treated condition. The fatigue test results show that HFMI treatment can compensate the fatigue strength-reducing effects of macro geometric imperfections (ΔσC,HFMI = 127 N/mm2, m = 5), but also that linear and angular misalignment does not reduce the fatigue strength of butt welds as severely as rules and guidelines suggest (ΔσC,AW = 107 N/mm2, m = 3). Underestimating the fatigue strength determined by testing by more than 40%, numerical analysis and strain gauge measurements show that the application of the effective notch stress approach and the structural hot-spot stress approach delivers conservative assessment results, which might not always be representative for the actual fatigue strength benefit that can be gained by HFMI treatment.

Structural hot-spot stress approach for toe cracking from plate edge of load-carrying welded attachment

Structural hot-spot stress approach for toe cracking from plate edge of load-carrying welded attachment

Fatigue assessment of CFST joints using the effective notch stress approach

Compared to tubular joints formed by hollow steel sections, concrete-filled steel tubular (CFST) joints exhibit more complicated failure modes and mechanism under fatigue load due to the nonlinear material confinement and the associated stress redistribution. For a reliable fatigue design of such composite tubular joints, these complexities need to be fully addressed and reasonably quantified, which necessitates an accurate approach for assessing their fatigue performance. The effective notch stress approach is widely recognized in fatigue design for its applicability and efficiency in evaluating the fatigue strength since it can rationally account for the effects of geometric configurations and welding defects on the fatigue performance. However, it has not been widely adopted in the assessment of CFST joints due to the high costs involved in modelling and computing. In this paper, a refined nonlinear finite element model is established, which facilitates the acquisition of reliable notch stress values. Subsequently, a statistical analysis is conducted on the available fatigue data of CFST joints, employing the computed notch stress to yield an S–N curve. A comparative evaluation on the results obtained from the nominal stress approach, the hot-spot stress approach and the effective notch stress approach testifies the superior accuracy of the last in assessing the fatigue strength of CFST joints.

Empirical modeling of stress concentration factors using finite element analysis and artificial neural networks for the fatigue design of tubular KT‐joints under combined loading

AbstractThe hotspot stress (HSS) approach for the fatigue design of tubular joints requires that peak HSS be known. Peak HSS in tubular joints is usually determined based on the stress concentration factor (SCF) estimated from empirical models developed through extensive experimental investigations and finite element analysis. While peak HSS usually occurs at a KT‐joint's crown and saddle points, its location may change if the tubular joint is subjected to a combination of axial, in‐plane bending, or out‐of‐plane bending loads. This study investigated the peak HSS and its location in a typical KT‐joint subjected to the combined loading. Specifically, empirical models to determine the SCF around the brace axis have been developed using extensive finite element analysis and artificial neural networks (ANN) simulations. Less than 3% error was noticed between peak HSS determined through developed models and FEA. Hence, the ANN‐based SCF equations and principle of superposition can be used to calculate peak HSS rapidly for fatigue design of tubular joints. This methodology is applicable for developing empirical models for SCF in other tubular joints and boundary conditions.

Obliquely Loaded Welded Attachments Fatigue Categorization in Steel Bridges

The detail category for base metal at the toe of transverse stiffener-to-flange and transverse stiffener-to-web fillet welds is defined as Category C' in the current AASHTO LRFD Bridge Design Specifications (2020, 9th Ed.) and as Category C in the AREMA (2020) Manual for Railway Engineering and the AISC Steel Construction Manual (2017, 15th Ed.). These are often referred to as short attachments due to their very short length (< 2 in.) in the direction of the primary stress range. Sometimes it is necessary to place a stiffener or a connection plate at an angle different than perpendicular to the web, such as in skewed bridges. Increases in the effective length of the stiffener along the flange in the longitudinal direction are seen as the plate is rotated away from being perpendicular to the web. The other extreme occurs when the stiffener is rotated completely 90° and is perfectly parallel to the web and the longitudinal stress range. In this instance, this is identical to the long attachment and classified as Category E (length > 4 in.). The current specifications and manuals, on the other hand, do not have classification on how to address the potential effects on fatigue performance of angles in between these two extremes. This paper summarizes finite element analysis studies based on local stress and structural hot-spot stress approaches that were conducted to investigate and classify welded attachments placed at angles other than 0° (transverse) or 90° (longitudinal) for a variety of stiffener geometries and thicknesses. This study includes new classification for incorporating the findings into the AASHTO LRFD Bridge Design Specifications, AREMA Manual for Railway Engineering, and the AISC Steel Construction Manual.

Stress concentration factors of CFRP-reinforced tubular K-joints via Zero Point Structural Stress Approach

The SCFs of the tubular K-joints reinforced with carbon fiber reinforced polymer (CFRP) were computed using the zero-point structural stress (ZPSS) method. After employing several sets of data to verify the proposed finite element (FE) model, a comparison between the ZPSS and hot spot stress (HSS) approaches was established. The results indicate that ZPSS approach is superior to HSS approach in calculating the SCFs. Then 319 models were established to parametrically assess the efficiency of the CFRP strengthening on the fatigue performance of the tubular K-joints. The findings show that the CFRP strengthening dramatically minimizes SCFs at the weld toe's heel, saddle, and crown. In addition, the orientation of the first layer of the CFRP laminate and the layups had significant effects on the SCFs along the weld toe. Moreover, three precise equations were proposed to determine the SCFs of CFRP-reinforced K-joints at the heel, saddle, and crown, respectively.

Comparison between PSM and IBESS approaches for the fatigue life estimation of weldments

In the framework of the fatigue assessment of welded components, several methods are available in design standards such as the nominal stress, hot-spot stress, notch stress and linear elastic fracture mechanics approaches. The present paper aims at comparing two advanced local approaches for the fatigue strength assessment of different welded joints made of steel. The first one is IBESS which is based on short crack fracture mechanics. The second one is the PSM which instead involves the strain energy density approach. Both methods will be briefly presented, and the fatigue life prediction results discussed. The results obtained for the joint geometries considered in this work show advantages and drawbacks of the approaches, which are thoroughly analysed as well.

Numerical analysis and discussion on the hot-spot stress concept applied to welded tubular KT joints

Nominal stresses have been used for a long time for the assessment of fatigue resistance of welded joints, however, this approach has strong limitations since the definition of the nominal stress may be subjective for complex welded details and/or complex loading. On the other hand, the hot-spot stress approach has been proposed to overcome these limitations considering the structural geometrical discontinuities.However, the hot-spot stress methods also present certain limitations, and the present study aims at evaluating the available numerical and analytical hot-spot stress methods proposed by DNVGL (2016) and IIW (2014). The particular case of an offshore tubular KT joint has been considered herein and discretized in two planes. It has been studied numerically using the ABAQUS software coupled with the hot-spot stress extrapolation methods described in IIW (2014) and DNVGL (2016). The influence of the weld geometry has been considered and evaluated. In addition to the numerical method, the present study has also considered the analytical approach proposed in DNVGL (2016) derived from the combination of Efthymiou solutions for the stress concentration factor with the method of superposition of stresses. The numerical models according to IIW (2014) have been found to be more conservative when compared with the mesh-size methods proposed by DNVGL (2016), both in numerical modelling without the weld or with weld. For the numerical models with weld cord, the mean values of normalized difference index obtained for all braces together, as a result of comparing numerical results with analytical solutions, are lower, when compared with results obtained from the numerical models without weld cord.

Fatigue strength assessment of as-welded and HFMI treated welded joints according to structural and local approaches

High-frequency-mechanical-impact (HFMI) is a post-weld treatment for improving the fatigue strength of welded structures, by introducing compressive residual stresses, localized strain hardening and enlarged weld toe radii. The Peak Stress Method (PSM) is extended for the first time to the fatigue strength assessment of HFMI treated steel joints, by means of a dedicated analysis procedure and new fatigue design curves. Experimental data generated from joints tested under uniaxial loading, both in the as-welded and HFMI treated conditions, have been taken from the literature and have been successfully analysed by using the PSM and the structural hot-spot stress (SHSS) approaches.

Simulation study on fatigue behavior of wrap-around weld at rib-to-floorbeam joint in a steel-UHPC composite orthotropic bridge deck

As an innovative cement-based material, the ultra-high performance concrete (UHPC) has been recently utilized to reinforce the orthotropic steel bridges. Based on an in-situ steel-UHPC composite orthotropic deck bridge, both the field testing and multi-scale finite element analysis modelling were conducted to investigate the fatigue behavior of wrap-around weld at rib-to-floorbeam (RF) joint, and factors influencing stress were further studied to achieve an infinite fatigue life. It is found that a primary tensile stress cycle is jointly produced by the axle group of truck, while a small reversal compressive cycle will be generated when the axle is directly located on the floorbeam. The stress behavior of fatigue detail is extremely sensitive to the localized effect of axle loading instead of entire truck configuration. Since significant stress concentration and high stress gradient exist in the wrap-around weld, it is essential to utilize the hot-spot stress approach rather than nominal stress approach. The wrap-around weld presents an obvious out-of-plane bending deformation, which results from the torsion effect and Poisson’s effect. The Dong’s structural stress reveals that the surface stress is dominated by bending stress. The application of bulkhead can greatly improve the fatigue performance of wrap-around weld. Both the large-scale rib and U-shaped rib are preferable, which are available to achieve an infinite fatigue life without extra fabrication procedure.

Fat classes of welded steel details derived from the master design curve of the peak stress method

In this paper, the peak stress method (PSM) is adopted to analyse the fatigue strength of steel welded joints. According to this method, a single design curve is expressed in terms of a properly defined equivalent peak stress and it is valid for fatigue design of arc-welded steel joints. Private companies often need simple finite element beam models for fatigue strength assessments, because of the large dimensions of the structures. However, beam elements provide nominal stresses (and not local stresses) that must be compared with appropriate fatigue strength values (the FAT classes) available in design standards. Due to the limited number of FAT classes available, finding the appropriate one is frequently troublesome, particularly when complex geometries are considered. The objective of this work is to define FAT classes in terms of nominal stress for a number of geometrically complex structural details, starting from the design curve of the PSM. FAT classes have also been determined using the hot spot stress approach. Then the results obtained with the two methods are compared. The structural details analysed in the present paper are typically adopted in amusement park structures and are not classified in common design standards.

Structural response and fatigue assessment of a small vertical axis wind turbine under stationary and non-stationary excitation

The paper discusses the response and fatigue damage of the supporting tower of a small vertical axis wind turbine subject to stationary and non-stationary excitation due to wind, turbine rotation, emergency stop and start. Using in-field measurements and the Hot Spot Stress Approach, the study analyses a number of aspects that are usually disregarded in design verifications. The results highlight the importance of the detail modeling, the fundamental role played by the non-stationary conditions and the errors committed when using conventional models of the load. The emphasis of this paper is put on two main aspects: the use of full scale data and the detection of isolated events of non-stationary vibrations, the effects of which can be disregarded when using stress time-histories of 10 min - 1 h.

Experimental fatigue assessment of connections between plate-type longitudinal ribs and non-slit transverse ribs in orthotropic steel decks

The present study investigated the fatigue strength of the connection between longitudinal and transverse ribs using a method proposed in previous studies. Fatigue test results combined with finite element analysis and residual stress estimation confirmed that the fatigue performance of the connection can be assessed by the hot-spot stress approach and the fatigue design curve of FAT 80 (≃ JSSC E). Although the fatigue strength of the weld toes at longitudinal rib was higher than that of FAT 225, the compression residual stress, which was measured at the weld toes, was considered to affect the fatigue strength. Since the residual stresses of actual structures are hardly controlled, fatigue assessment with FAT 80 (≃ JSSC E) should be applied to the weld toes at both longitudinal and transverse ribs for safe side evaluation.

Fatigue behavior of low alloy structural steel single-lap bolt-welded joint

The fatigue behavior of Q345B low alloy structural steel single-lap bolt-welded joint is studied compared to bolted and welded joints. A series of static and fatigue experiments of bolt-welded, bolted and welded joints were carried out, and its static finite element analysis were performed. The experiments showed that the fatigue failure modes of bolt-welded and welded joints are fatigue fracture at weld toe. With the increase of stress levels, the location of crack initiation trends to move from weld toe to weld root. Compared to welded and bolted joints, the usage of welds and bolts in bolt-welded joint can enhance fracture resistance of the joint. The welds and bolts of bolt-welded joint can improve the bearing capacity. The nominal stress, hot spot stress (HSS) and local strain energy density (SED) approaches were used to estimate fatigue life of bolt-welded and welded joints. The local SED of bolt-welded joint shows a smaller scatter band in fatigue life estimation compared to the nominal stress and HSS method. The results showed the bolt-welded joint has a higher fatigue strength, and the fatigue assessment of bolt-welded joint under local SED can better estimate the fatigue life. This study provides theoretical and experimental references for the fatigue design of single-lap bolt-welded joint.

An overview and comparative assessment of approaches to multi-axial fatigue of welded components in codes and standards

Three types of commonly used local stress approaches recognized by Codes and Standards for fatigue of welded joints are reviewed and evaluated with respect to a comprehensive collection of multi-axial fatigue test data available in literature. These local stress approaches are surface-extrapolated hot spot stress, effective notch stress and traction structural stress. The hot spot stress and notch stress approaches are based on those stipulated in Eurocode 3 (2005) and IIW Recommendations (2016). Traction structural stress is based mainly on ASME VIII Div 2 Code (2007) and its latest development. The results show traction structural stress method offers a significantly improved data correlation over hot spot stress and notch stress approaches.

A new method of predicting hotspot stresses for longitudinal attachments with reduced element sensitivities

For the complicated structural details in ships and offshore structures, the traditional hotspot stress approaches are known to be sensitive to the element variables of element topologies, sizes, and integration schemes. This motivated to develop a new approach for predicting reasonable hotspot stresses, which is less sensitive to the element variables and easy to be implemented the real marine structures. The three-point bending tests were conducted for the longitudinal attachments with the round and rectangular weld toes. The tests were reproduced in the numerical simulations using the solid and shell element models, and the simulation technique was validated by comparing the experimental stresses with the simulated ones. This paper considered three hotspot stress approaches: the ESM method based on surface stress extrapolation, the Dong's method based on nodal forces along a weld toe, and the proposed method based on nodal forces perpendicular to an imaginary vertical plane at a weld toe. In order to study the element sensitivities of each method, 16 solid element models and 8 shell element models were generated under the bending and tension loads, respectively. The element sensitivity was analyzed in terms of Stress Concentration Factors (SCFs) in viewpoints of two statistical quantities of mean and bias with respect to the reference SCFs. The average SCFs predicted by the proposed method were remarkably in good agreement with the reference SCFs based on the experiments and the ship rules. Negligibly small Coefficients of Variation (CVs) of the SCFs, which is measure of statistical bias, were drawn by the proposed method.

Fatigue strength enhancement by structural modification of transverse to longitudinal rib connections in orthotropic steel decks with open ribs

This paper describes an investigation of fatigue strength enhancement for transverse to longitudinal rib connections in orthotropic steel decks with open ribs by structural modification. A structural hot-spot stress approach was used to evaluate the fatigue strength. The results of a finite-element analysis revealed that the fatigue strength decreased with increasing span length of the longitudinal ribs. The structural modifications to the connection, which reduced the stiffness of transverse ribs due to section loss, absorbed the local deformation of the deck plate and consequently reduced the structural hot-spot stress.

Fractures on a drillship derrick, evaluation and improvement

During the service life, the drillship derrick structure is exposed to wave and wind actions and drilling operations. Due to these actions, the derrick structural members are subjected to the variable amplitude stresses and very high number of load cycles that may generate the fatigue cracks, leading to the total structural fracture eventually. The fractures found on the footing of a drillship derrick are analysed, and based on the structural durability, the improvements are determined for the specific case. To perform the structural durability evaluation and to propose the necessary improvements, at the beginning of the analysis, it is important to define the loadings (corresponding loads due to waves, wind and drilling acting on the derrick structure) taking into account the drillship service. After that, in order to validate the fatigue life of the structure, the stresses caused by these loads must be determined, their representative spectra (cumulative frequency distributions) have to be derived and the data regarding corresponding allowable stresses should be defined. Using the methods defined in the classification societies' rules and the finite element analysis, the procedure for determination of the operational stress spectra and evaluation of the fatigue life utilising the hot-spot stress approach for highly stressed areas of a drillship derrick, cracked at usage, is presented.