Structural Stress Approach Research Articles

Sequence effects on the life estimation of welded tubular structures made of S355J2H under uniaxial fatigue loading

The use of hollow sections to form lightweight structures is widespread in common steel processing industries such as crane, commercial vehicle, steel bridge and agricultural machinery construction. The hollow sections are mainly designed as truss or frame structures, in some cases using high-strength and higher-strength steels in order to achieve optimum utilization of the component and material. A new collection of fatigue life data covering sequence effects and the accuracy of the linear damage accumulation is presented. Effects of the shape of the applied load spectra and sequence effects of different amplitudes have been investigated. This document covers tubes of 4 to 8 mm thickness made by low-carbon or mild steel S355J2H. In general, it was found that the spectrum shape and the loading sequence have an influence on the service life. Depending on the shape of the spectrum, random tests tended to lead to shorter service lives than tests with block-loading sequences. An influence of overloads was also found for the tests with interspersed overloads. Typical maximum linear damage sums taken from recommendations and codes of 0.2 or sometimes 0.5 are exceeded for all spectra investigated and in some of the cases even significantly above 1.0. Transferability of the recommendations to component-type structures like tubular joints needs revision to lift its lightweight potential. Using stress concentration factors (SCF) from finite element analysis, typical strength values for the structural and effective notch stress concepts are checked. All joints investigated show a significantly higher strength compared to the IIW recommendations using the structural stress approach or compared to the DVS 0905 with the effective notch stress approach.

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

A hybrid distribution characteristics of equivalent structural stress method for fatigue evaluation of welded structures

In the fields of engineering design and fatigue life prediction, handling limited sample data has consistently posed a challenge. A hybrid distribution characteristics of equivalent structural stress (HDC-ESS) method is presented for the design of fatigue life prediction models with small sample sizes. The method integrates the equivalent structural stress (ESS) method with the maximum likelihood estimation (MLE) method based on the idea of sample augmentation, and adds closed-loop reliability verification to ensure its robustness and correctness. By employing three distinct approaches, an analysis and comparison were conducted on the failure data of small specimen obtained from high-frequency fatigue tests of welded joints. The outcomes reveal that the HDC-ESS method, in comparison to the conventional direct method and the MLE-integrated equivalent structural stress approach, exhibited an enhancement of goodness of fit by 0.1866 and 0.1988, respectively. Furthermore, it exhibited remarkable performance in enhancing stability and optimizing sample quantity.

Numerical Approach to Optimize the Dynamic Behaviour of Structures Considering Structural Durability

In the design of lightweight structures, both the dynamics and durability must be taken into account. In this paper, a methodology for the combined optimization of structural dynamics, lightweight design, and lifetime with discrete vibration engineering measures is developed and discussed using a demonstration structure. A two-sided welded bending beam is excited at the centre and optimal parameters for tuned mass dampers (TMD) are searched, satisfying the requirements for the dynamic behaviour, the overall mass, and the lifetime of the weldings. It is shown that the combination of a reduced order model with the implementation of the structural stress approach at critical welds enables an efficient evaluation of certain design concepts in the time domain. Using this approach, multi-criterial optimization methods are used to identify the best set of parameters of the TMD to reduce the structural vibrations and enhance the durability.

Fatigue evaluation method based on equivalent structural stress approach for bolted connections

In this study, Fa–N curves and σn_s–N curves of bolted connections under different geometric and load parameters were obtained through tensile-shear fatigue tests. The correlation of these curves was poor and did not satisfy engineering requirements. To this end, first, an equivalent structural stress calculation model capable of accurately characterizing the stress state of a bolted connection was derived by considering the bolt load as the input condition. Subsequently, a plate-shell finite element model for bolted connections was established, and the load parameters for bolts were extracted by simulation and inputted into the equivalent structural stress calculation model to obtain the equivalent structural stress at the crack initiation position of a bolted connection. Finally, the normalized Ss–N curve of the bolted connections was fitted considering geometric and load parameters, and a fatigue evaluation method based on equivalent structural stress was designed for bolted connections. This evaluation method can act as a reference for checking the design of bolted connections and predicting their fatigue life.

Fatigue strength assessment of welded joints in the marine environment

Welding is a commonly applied joining method in many applications in arctic and marine conditions, e.g., in ship and offshore structures, and energy production equipment. Such applications are usually subjected to fluctuating load conditions, and during a decades-long service, they may experience millions of load cycles. Consequently, fatigue strength design and acceptable flaw sizes in the welded details of these structures are among the most important design criteria. Multiple fatigue strength assessment approaches exist for assessing the fatigue strength of a welded detail. The present study introduces a numerical and analytical fatigue strength assessment, conducted on a non-load-carrying X-joint, which is a representative joint type used in many steel constructions. Fatigue analyses are carried out following the DNVGL-RP-C203 and BS7910:2013 fatigue design guidelines for offshore steel structures. The stress intensity factors (SIFs) for linear elastic fracture mechanics (LEFM) analyses were obtained using three different methods: the weight function approach, the analytical equations provided in the IIW Recommendations, and by conducting numerical crack propagation analysis using the Franc2D software. All three methods had a good agreement particularly for short crack depths, indicating the applicability of the analytical approaches for the fatigue analyses. The results showed that the consideration of degree of bending at the welded detail is crucial due to the distinguishing notch stress factors of membrane and bending loading, and different stress distributions in the through-thickness direction. In addition, it was found that the LEFM-based fatigue life assessments are significantly more conservative than the life predictions obtained using the structural hot-spot and effective notch stress approaches.

Sequence effects on the life estimation of thin-walled welded tubular structures made of HSS + UHSS under bending

Welded hollow sections are typical in different industries. A database of fatigue life data of welded hollow section joints covering sequence effects and the accuracy of the linear damage accumulation is presented. The effects of the shape of the applied load spectra, sequence effects of different amplitudes have been investigated using two high-strength steels. This document covers thin-walled tubes of 2-mm thickness made of low-carbon or mild steel 1.8849 (S460MH) and austenitic TWIP-steel 1.4678 + CP700 (X30MnCrN16-14). Constant amplitude and two-level load spectra are presented to check the linear damage accumulation. Using stress concentration factors from finite element analysis, typical FAT classes for the structural and the effective notch stress concepts are checked as well. Both structures show much higher strength compared IIW recommendations by structural stress approach and DVS 0905 by effective notch stress approach. Typical maximum linear damage sums taken from recommendations and codes of 0.2 or 0.5 are exceeded for all spectra investigated and in some of the cases even significantly above 1.0. Transferability of the recommendations to component type structures like those tubular joints made of high-strength steel needs revision to lift its lightweight potential but this will require additional data.

Fatigue strength assessment of complex welded structures with severe force concentrations along a weld seam

This paper investigates weld root fatigue strength of complex structures including severe force concentrations along a weld seam. Fatigue tests were carried out for a pillar assembly with various weld configurations. A comparative analysis was carried out with local nominal weld stress (LNWS), structural weld stress, and effective notch stress (ENS) approaches. The study shows that the LNWS approach gives the best prediction, with a scatter range index of 1.26 to 1.68 depending on the weld type. The accuracy of the structural weld stress approach and ENS was significantly lower, with a scatter range index of 4.98 and 5.40, respectively.

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 (SCF) of CFRP-reinforced T/Y-joints via ZPSS approach

This research innovatively considers the Zero Point Structural Stress (ZPSS) approach to evaluate the stress concentration factors (SCFs) of the CFRP-reinforced T/Y-joints. To begin, multiple sets of credible experimental data and Finite Element (FE) results were selected to verify the accuracy of the proposed FE model. The comparison results showed that the ZPSS approach accurately estimated the SCF of tubular T/Y-joints reinforced with CFRP. Furthermore, the ZPSS approach, which considers the stress gradient through the thickness, shows more accurate results than the HSS approach, which considers only the stress at the outer surface of the chord. Then, 132 FE models were simulated to analyze the effects of the joint geometric dimensions and the CFRP laminates on the SCFs of the CFRP-reinforced tubular T/Y-joints. Finally, a parametric equation is proposed based on the FE results to predict the SCFs of tubular T/Y-joints reinforced with CFRP under axial compression loads.

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.

A unified evaluation method for fatigue resistance of riveted joints based on structural stress approach

Many riveted steel bridges were constructed worldwide between 1850 and 1950. A number of them are still in operation, and their fatigue damage is a major burden in maintenance. This paper presents a unified structural stress approach to evaluate the fatigue resistance of riveted joints for steel bridges. The presented approach was utilized to establish and calibrate S-N curves for riveted joints based on fatigue data from 114 tested specimens, and the curves were validated by fatigue test data from large-scale riveted structures. A single fatigue category was obtained to evaluate different types and sizes of riveted structures. The investigation results showed that the presented approach is capable of evaluating different types of riveted joints and structural stresses. The structural stress approach is insensitive to the mesh size and the element type in finite element analysis. The presented method is convenient for practical engineers to evaluate the fatigue resistance and fatigue life of riveted steel bridges.

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.

Fatigue assessment of a full-scale composite box-girder with corrugated-steel-webs and concrete-filled-tubular flange

Performance and aesthetic considerations push the bridge industry towards new structural solutions. A new type of composite box girder with corrugated steel webs was proposed, which utilizes concrete-filled wide-flat steel tube replacing concrete slab as bottom flange. This new design may still be susceptible to fatigue due to its complicated configuration and inevitable repeated heavy traffic loads. Therefore, this research presented a full-scale fatigue test and numerical study of this new composite girder through four-point bending. The test results demonstrated that even under a rather low stress level, the composite girder still sustains fatigue damage under a very high cycles of 9.84 million, which leads to failure as multi cracks have initiated and propagated at top and bottom surface of steel tube around mid-span location. Assessed by the structural stress approach fully relying on finite element analysis (FEA) results and modified version combining FEA results with measurement data, the cumulative damages for all fatigue-sensitive details of test girder are respectively lower and higher than the failure standard 1.0. This indicates that the modified method is in consistent with experimental results, while the original one is not. Accordingly, it is advisable to combine FEA results with measurement data to evaluate fatigue-sensitive details of this composite girder.

Stress-based approach for fatigue life calculation of multi-material connections hybrid joined by self-piercing rivets and adhesive

Dissimilar connections with aluminum EN AW-6016 and high strength steel CR330Y590T-DP are affordable and promising regarding lightweight potential. Self-piercing riveting (SPR) and structural adhesive enable an assembly of such dissimilar combinations. This study provides an approach to assess hybrid joints by superposing the contributions of each joining technique on the resulting fatigue life. The fatigue life estimation of SPR is based on a structural stress approach and material SN curves. For the adhesively bonded connections, a stress-based approach and a master SN curve are used. Both of the approaches for fatigue life estimation show a reasonable agreement with test data.

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.

Fatigue Assessment for Deck-Rib Welded Joints of Orthotropic Steel Deck Using a Simplified Nodal Force Structural Stress Approach

Abstract The fatigue cracking of orthotropic steel deck (OSD) has long been a problem of bridge structures. It is meaningful to assess bridge fatigue life using an appropriate method. In this paper...

Influence of Weld Parameters on the Fatigue Life of Deck-Rib Welding Details in Orthotropic Steel Decks Based on the Improved Stress Integration Approach

Fatigue cracks in orthotropic steel decks (OSDs) have been a serious problem of steel bridges for a long time. The structural stress approach is an important approach for fatigue life evaluation of welded structures. Firstly, two parameters and the mesh sensitivity of the stress-based integration equivalent structural stress approach (stress integration approach for short) are analyzed in this paper. Then, the applicability of the master S-N curve is verified based on experimental data of the deck-rib welding details in OSDs. Finally, the multi-scale finite element model (FEM) of Jiangyin Bridge is established, and the bridge fatigue life calculation steps based on the stress integration approach are given. The influence of the slope of the master S-N curve at high cycles on the bridge fatigue life is discussed. Further, the weld parameter influences on the bridge fatigue life are analyzed, as including the following: (1) The determination of the influence of the weld size changes caused by weld manufacturing errors on the bridge fatigue life; (2) the proposal of a new grinding treatment type, and the analysis of influence of the grinding radius on fatigue life; and (3) a comparison of the fatigue life of the deck-rib welding details under 80% partial penetration and 100% full penetration. The results show that the structural stress calculated by the stress integration approach does not change significantly with the parameters of the isolation body width w and the distance δ between the crack propagation surface and the reference surface. To simplify the calculation, δ is set as 0, and w can be set as the mesh size along the weld length direction. The mesh size of the stress integration approach is recommended as 0.25 times the deck thickness. The slope of the master S-N curve at high cycles significantly affects the bridge fatigue life, and a slope of 5 is reasonable. The weld parameter studies for the deck-rib welding details in the OSD of Jiangyin Bridge show that the change of weld size caused by manufacturing errors can obviously affect the bridge fatigue life, and the fatigue life of five different weld types varies from 51 years to 113 years. The new grinding treatment type, without weakening the deck, is beneficial to improving the bridge fatigue life. The fatigue life increases by approximately 5% with an increase of the grinding radius of 2 mm. The fatigue life of 80% partial penetration is slightly higher than that of 100% full penetration.

Automatically Welded Tubular X‐Joints for Jacket Substructures: Prediction of the Technical Fatigue Crack Location

ABSTRACTTo increase the competitiveness of jacket substructures compared to monopiles a changeover from an individual towards a serial jacket production based on automated manufactured tubular joints combined with standardized pipes has to be achieved. Therefore, this paper addresses fatigue tests of automatically welded tubular X‐joints focusing on the location of the technical fatigue crack. The detected location of the technical crack is compared to numerical investigations predicting the most fatigue prone notch considering the structural stress approach as well as the notch stress approach. Besides, the welding process of the automated manufactured tubular X‐joints is presented.