Fatigue Sensitive Details Research Articles

Generic method of local thickening in shell element FEM models for orthotropic bridge decks

AbstractThis paper presents a method of estimating the hot‐spot stress in orthotropic bridge decks (OBD) using a shell Finite Element (FE) model, in which local thickening is applied around the welded. The dimensions of the area of local thickening are a function of the weld throat thickness and the plate thickness. A comparison of this method with 3D solid element FE models is presented for various fatigue sensitive details. The comparison is carried out for OBD with trapezoidal stiffeners, both with continuous stiffeners through crossbeams and with stiffeners welded in between crossbeams. It is concluded that the accuracy of estimating the hot‐spot stress with a shell element model using local thickening is significantly higher as compared to a conventional shell element model without thickening. The method of local thickening is adopted in the new prTS 1993‐1‐901. This paper functions as the background to this technical specification.

Fatigue behaviour of butt-lap friction stir welded joints used with aluminum bridge decks

This paper presents the results of experimental tests and a numerical investigation of the fatigue behavior of butt-lap friction stir welded (FSW) joints used in highway bridge decks made from aluminum extrusions. A large-scale fatigue experiment was designed to initiate fatigue failure in the butt-lap FS welds joining two full-scale aluminum extrusions used for highway bridge decks. Ten fatigue tests were carried out on an MTS servo-hydraulic press. Test results were used to analyze the fatigue failure mode of this unique joint type and to derive the S-N curve useful for the characterization and design of fatigue-sensitive details. The fatigue data was assessed using the effective notch stress (ENS) approach as recommended by the International Institute of Welding (IIW). Results show that the IIW design ENS IIW FAT-71 curve derived from fatigue tests on fusion-welded joints is conservative in assessing the fatigue behavior of butt-lap friction stir welded (FSW) joints, especially in the high cycle regime.

Mixed-mode fatigue crack growth analysis of I-shaped steel girders with trapezoidal corrugated-webs

In bridge industry, the composite box-girder with corrugated steel webs and concrete flanges is quite a common practice over recent years. However, this innovation encounters fatigue-sensitive details not fully investigated, especially in the research area of mixed-mode fatigue crack growth. This study addresses the mixed-mode fatigue crack growth analysis of an I-shaped corrugated web girder associated with the represented web-to-flange detail. A linear elastic fracture mechanics (LEFM) based numerical framework was established to simulate the mixed-mode crack propagation. For comparison, a semi-analytical weight function framework was also introduced to predict the pure Mode-I crack propagation life of the same detail. Experimental results of a full-scale inclined corrugated web girder were employed to evaluate these two proposed frameworks. A parametric study was presented based on the proposed frameworks to investigate the effect of initial crack depth and crack shape on fatigue crack propagation life. By comparing the simulation results with test results, the applicability of the proposed LEFM-based numerical framework to the simulation of mixed-mode fatigue crack propagation behaviour in corrugated web girders was discussed.

Fatigue reliability assessment of steel bridges considering spatial correlation in system evaluation

Fatigue is among the most critical forms of deterioration damage that occurs to steel bridges. The performance of steel bridges, which may be seriously affected due to fatigue, should be assessed and predicted. However, the code-based reliability assessment only provides a clear criterion for single details. The whole bridge or a splitting system from the bridge may contain many details which brings system impact on the fatigue assessment. In this paper the system reliability of steel bridges is addressed with focus on the influence of spatial correlations between similar fatigue sensitive details. The study is conducted on a typical riveted railway bridge, from which a subsystem is selected and modelled as a weakest link system. It is assumed that all details in the system are equicorrelated. An efficient modeling approach has been developed to quantify the correlation and help estimate the system reliability analytically. A parametric study was performed to determine the system effects on the fatigue reliability assessment and identify the decisive variables.

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.

A holistic approach to risk-based decision on inspection and design of fatigue-sensitive structures

Design and operation of large welded structural systems (e.g. ship and offshore structures) are challenging due to numerous fatigue-sensitive details, limited available budgets, uncertainties in fatigue damages, inspection & maintenance activities, etc. Traditionally, fatigue design and maintenance planning have been almost disconnected, which restricts coherent decision-making and optimum safety management. Structural design optimization, without quantitatively incorporating the effects of operational maintenance, can hardly result in a structural plan that is optimum in terms of life cycle costs. Also, if the design of a structure is not optimum, maintenance optimization alone cannot really yield a optimum maintenance plan. As operational inspections and maintenance are essential, there are merits to utilize their effects on structural design and meanwhile optimize them at the initial design stage when impacts of decisions are greater. This paper proposes a risk-based approach to holistic decision-making enveloping decisions and uncertainties affecting design, inspection and maintenance of fatigue-sensitive components. Decisions variables in structural scantling and operational maintenance are obtained holistically at the structural design stage by risk-based optimization, based on quantitative assessment of the effectiveness of both structural scantling and maintenance interventions. Optimum fatigue reliability level is also obtained, informed by the effects of uncertainties and failure consequences. The method captures combined benefits of structural scantling and operational maintenance to fatigue reliability and risk mitigation and achieves optimum resource utilization and life cycle cost reduction. Advantages of the proposed method have been demonstrated via a numerical example, in comparison to alternative methods.

New framework for calibration of partial safety factors for fatigue design

In Eurocode standards, three verification schemes are proposed for fatigue design under variable amplitude loadings: 1) Based on constant amplitude fatigue limit; 2) Based on constant amplitude equivalent stress range at 2 ⋅ 106 cycles; 3) Based on accumulated damage. Characteristic values of fatigue resistance and load effects as well as partial safety factors are introduced in design equations in order to achieve a target reliability level. In this paper a new framework for calibration of fatigue partial safety factors is presented. Three different fatigue limit state functions are formulated for direct comparison with the three verification schemes proposed in Eurocodes. The variable amplitude S-N curves used in this framework are defined using an original probabilistic approach. The presented framework is then applied to two typical bridge fatigue sensitive welded joints. The comparison of results with partial safety factors values recommended in Eurocodes shows that the Eurocode-based partial safety factors should be revised by considering different fatigue sensitive details and by further differentiating between the three verification schemes.

Fatigue Assessment of Full-Scale Retrofitted Orthotropic Bridge Decks

Full-scale fatigue tests were performed on two retrofitted orthotropic bridge decks (OBDs). The retrofitting systems consist of adding a second steel plate on the top of the existing deck. The aim is to reduce the stresses at the fatigue-sensitive details and therefore extend the fatigue life of the OBD by stiffening the existing deck plate. Two retrofitting systems were studied. The bonded system consists of bonding a second steel plate to the existing deck by vacuum infusing a thin adhesive layer (2 mm) between the two steel plates. The sandwich system consists of bonding the second steel plate through a thick polyurethane core (15 mm). The aim of the study was to assess the fatigue performance of both retrofittings. No fatigue damage was detected in the retrofitting layers during fatigue tests after three million cycles of wheel load. The stresses close to the deck-plate-to-stiffener welds decreased by at least 55% when using the bonded steel plates system and 45% when using the sandwich steel plates system. Both systems proved to have sufficient fatigue life to withstand traffic wheel loads running on orthotropic bridge decks and help extend the fatigue life of the existing OBD.

Fatigue crack growth analysis of steel elements reinforced with shape memory alloy (SMA)/fiber reinforced polymer (FRP) composite patches

This paper describes a numerical framework to simulate the fatigue crack growth of single edge-notched steel elements patched with shape memory alloy (SMA)/fiber reinforced polymer (FRP) composite patches. The framework includes the small crack propagation. The small crack propagation stage was simulated using the analytical model based on the equivalent initial flaw size method. The bridging effects of the SMA/FRP composite patch were simulated by two consecutive finite element simulations considering the interfacial debonding during fatigue loading. Numerical predictions were validated by comparison with experimental results. A parametric study was conducted using the validated model. The considered parameters were the number of SMA wires used in the patch, the applied stress range, and the length of the crack in the steel substrate at the time of installing the patch. The numerical results indicated that the SMA wires increased the critical crack length at which fracture occurred and that the patch was more effective at lower stress ranges. Further, the SMA/FRP patch can substantially delay the failure of a member with very little remaining fatigue life. The results demonstrate the potential of this technique and highlight the importance of early intervention when repairing fatigue sensitive details.

Incorporation of risk and updating in inspection of fatigue-sensitive details of ship structures

Fatigue cracks, as a structural deterioration mechanism, may lead to unanticipated out of service of naval ships. Consequently, risk assessment and effective inspection planning of naval ships under fatigue damage are needed to achieve economic and sustainable performance of these structures. This paper presents a probabilistic framework for fatigue risk and updating assessment and updating through inspection events. The computation associated with fatigue damage is performed using fracture mechanics and uncertainties are considered within this process. Furthermore, a quantitative risk assessment model using rating functions is presented to identify inspection priority among multiple fatigue-sensitive details. Bayesian techniques are adopted for reliability and risk updating of both inspected and uninspected fatigue-sensitive details at component and system levels. Additionally, correlation of fatigue damage among different critical details is considered and incorporated within risk assessment and updating. A number of analyses are performed to investigate the fatigue risk ranking assessment and updating. Applications are presented on an existing tanker with multiple fatigue-sensitive details.

Distortion-Induced Fatigue Cracking in a Seismically Retrofitted Steel Bridge

This study examines the effect of a seismic retrofit on distortion-induced fatigue cracking in a steel girder bridge in the Midwestern United States. The seismic retrofit of the bridge was deemed necessary following a structural review initiated by the Federal Highway Administration (FHWA) in response to the 1989 Loma Prieta Earthquake. Following the review, the bridge was modified using a conventional seismic retrofit strategy to meet the 1995 FHWA standards for survivability and seismic performance. Upon completion of the retrofit, subsequent inspections identified new cracks (at the location of the retrofit) in the webs of longitudinal girders at transverse stiffener locations in an area known as the web gap. This study investigates the influence of the seismic retrofit strategy on crack formation by comparing principal stresses in the web-gap region of the original and retrofitted bridge under dead, live and thermal loading. A three-dimensional linear-elastic finite element analysis of the bridge before and after the retrofit is presented with results obtained from a commercial finite element software package. Comparisons of stresses before and after the retrofit indicate a measurable increase in stress near the web-gap region attributable to thermal and live loads following the retrofit. In addition, several viable repair strategies to limit these web-gap stresses and subsequent distortion-induced fatigue are presented. The results to date clearly demonstrate the importance of considering fatigue-sensitive details in seismic-retrofit strategies.

LEFM Based Fatigue Design for Welded Connections in Orthotropic Steel Bridge Decks

Modern fatigue design in civil constructions is mainly limited to the use of SN-curves and the hypothesis of Palmgren-Miner, as described in design standards and Eurocode. While using the latter, the fatigue evaluation by the designer is limited because of the out-dated SN-curves compared to current construction technologies and weld properties. This shortcoming has an influence on the current design of orthotropic steel bridge decks. These bridge decks consist of a complex network of longitudinal and transverse stiffeners, welded to a deck plate. Although often used for long span steel bridges, these types of bridge decks still have many fatigue sensitive details. Improvements are made through increased weld quality reducing the lack of weld penetration. However, a trend of increasing the dimensions, which reduces the advantage of small structural weight, exists because of fear of fatigue cracks. To increase the understanding of the fatigue behavior an improved analyzing tool using Linear Elastic Fracture Mechanics (LEFM) and eXtended FEM (XFEM) is proposed. In addition, residual stresses are included. As a result, thickness effects are evaluated for both the longitudinal stiffener and the deck plate. These calculations indicated that increasing the thickness of the deck plate increases the fatigue life of the structure. However, this does not apply to the longitudinal stiffener. The stiffness increase attracts higher stresses to the weld root and therefore reduces the fatigue life of the structure. As a final conclusion, a fixed load sequence seems to have a large influence on the crack propagation, but does not influence the crack length at the final stage.

Probabilistic fatigue life updating accounting for inspections of multiple critical locations

Many steel structures contain multiple fatigue sensitive details that have similar geometries and are subjected to similar load fluctuations. Examples are orthotropic (bridge) decks and stiffened (ship) hulls where tens to thousands of similar details are present in one structure. Generally only visual inspections on fatigue cracks are considered for these structures because more accurate techniques are considered to be too expensive and time consuming when so many details need to be inspected. Visual inspections are known to have a low probability of detection. Consequently Bayesian update techniques usually show a marginal effect of the result of visual inspections on the reliability of structures with respect to fatigue failure. On the other hand the inspection result of one location also provides information on similar details at other locations and thus the significance of the inspection result may be larger if multiple potential crack locations occur and cracks are not detected at any of these locations. This paper provides a probabilistic fatigue crack growth (fracture mechanics) model of a system containing a fatigue sensitive detail at multiple locations that accounts for the results of inspections. Spatial correlations of loading, resistance, and uncertainty variables between the different locations are evaluated and estimated through a literature review and are accounted for in the model. The model is demonstrated on a practical example of an orthotropic bridge deck containing a detail at 100 locations. The paper shows that visual inspections may be effective provided that a certain minimum inspection reliability can be guaranteed, that the structure is relatively tolerant to large cracks, and that the geometry and loading conditions are similar for a large number of locations.

Lightweight Reinforcement Systems for Fatigue-Cracked Orthotropic Bridge Decks

Two systems for reinforcing “in-service” orthotropic bridge decks (OBDs) have been researched: the bonded steel plates system and the sandwich steel plates system. The main idea of these types of reinforcements is to stiffen the existing deck plate, thereby reducing the stress at the fatigue-sensitive details, and thus extending the fatigue life of the OBD. Both reinforcement systems consist of adding a second steel plate to the existing steel deck. The behaviour and the effect of the reinforcement systems on full-scale OBD are investigated. Full-scale static tests and finite element analyses were performed on reinforced deck panels, using realistic wheel loads. The results showed at least 40% of stress reduction close to the fatigue-sensitive details after applying both reinforcements. The two suggested reinforcement systems showed a good performance and proved to be efficient lightweight solutions to refurbish orthotropic bridge decks and extend their life span.

Acoustic emission detection of fatigue damage in cruciform welded joints

Weld seams are critical points for the initiation of fatigue cracks in steel structures subjected to cyclic loads. Semi-elliptical surface cracking at the toes of a fillet weld is not easily found when it is partially through the thickness and subcritical. In this study the acoustic emission (AE) method is used to detect crack propagation in cruciform fillet welded joints that are representative of typical fatigue sensitive details in steel bridge superstructures. The effect of geometry and fatigue load on the AE data is investigated by varying the width of the base plate and the stress ratio. AE data filtering based on load pattern, source location, and waveform feature analysis was implemented to minimize noise-induced AE signals and false indications due to wave reflections. AE time domain features such as amplitude (b-value), counts, signal strength, and absolute energy are employed to study the influence of geometry and fatigue load on the AE data.

Probabilistic optimum inspection planning of steel bridges with multiple fatigue sensitive details

Inspection planning of fatigue sensitive bridges is a complex process. This, in part, is due to various uncertainties associated with the damage propagation and the detection technique used to perform inspections. The planning process starts with identifying the most critical locations of fatigue damage occurrence. In this context, non-destructive testing (NDT) techniques, applied at different times along the service life, can be used to monitor the crack growth and support the fatigue life estimation. The next step is to establish an inspection plan which considers the available budget and the target performance level of the investigated bridge. This paper proposes a probabilistic approach for providing effective inspection plans for fatigue critical bridges. The proposed approach considers multiple fatigue critical locations in the bridge and chooses the best inspection option among a set of predefined types of non-destructive inspections for each detail. Uncertainties in the damage propagation are considered in addition to the uncertainties in the damage detection process. A multi-objective optimization problem, which accounts for various critical details in the bridge, is formulated to find the optimum inspection times and the required NDT technique for each inspection. An existing fatigue critical steel bridge is used to illustrate the proposed probabilistic approach.

Life-Cycle Management Strategy on Steel Girders in Bridges

The major problems affecting the service life of bridges are related to various factors such as fatigue-sensitive details, increased service loads, corrosion deterioration, and the lack of proper maintenance. Among them, corrosion deterioration and fatigue damages of structures particularly to steel girder bridges are the most common ones. Bridges of different structural forms, at different locations or under different climates, may suffer from various degrees of deterioration. Steel girders at different positions of a bridge may also suffer from different degrees of damage. How to effectively maintain the bridge asset at a minimal cost and how to predict the time for future works are crucial, particularly when government funding sources become stretched. A comprehensive bridge management framework assisting stakeholders to appropriately and reasonably prioritize their future maintenance-related works in their bridge stocks, such that stakeholders can better allocate the limited resources, is utmost concerned. This paper proposes an integrated life-cycle management (LCM) strategy on steel girders in bridges in which corrosion deterioration and fatigue damage prediction models are mapped with girders’ performance conditions. A practical example to demonstrate the applicability of the proposed LCM strategy is also illustrated.

Fatigue of steel bridge infrastructure

This paper presents an overview of the development of fatigue design provisions and detailing for steel bridge structures. The historical performance of welded steel bridges in the USA is reviewed with focus on fatigue cracking at cover plate and similar attachment details, as well as distortion induced cracking at web gaps. The role of materials including modern high performance steels (HPS) is reviewed. Variable amplitude loading is also examined and compared with current design of steel bridges for fatigue resistance. Also examined are methods to improve and retrofit fatigue sensitive details, including modern post-weld enhancement by ultrasonic impact treatment (UIT). Orthotropic steel decks are reviewed based on the results of two full-scale prototype laboratory fatigue tests which identified the complex behaviour that occurs at fatigue sensitive details and were verified by field measurements on field installations. The merits of thin epoxy concrete wearing surfaces is examined in terms of the critical role the deck plate thickness has on the epoxy concrete fatigue resistance and durability. The orthotropic deck is the only bridge deck system likely to provide a 100 year life when the deck plate thickness equals or exceeds 16 mm.

Real Truck Loading of a Full-Scale Orthotropic Steel Deck Model

A full scale model of orthotropic deck panel was designed and fabricated referring to the common steel box girder bridges used in China. Experiment was conducted to test the behavior and fatigue performance of the orthotropic steel bridge deck under the actual truck wheel load. The loading test was conducted on the platform specially prepared. Measurement of the over-all structure and the fatigue sensitive details was made to evaluate the performance of the panel. For each gauge position, the results are presented as the form of influence lines.

Improved Cross-Frame Connection Details for Steel Bridges with Skewed Supports

Cross frames and diaphragms play an important role in stabilizing straight steel girders. Commonly used connections between these braces and the steel girders can introduce flexibility, which can have detrimental effects on the bracing behavior, particularly on bridges with skewed supports. This paper documents a research investigation sponsored by the Texas Department of Transportation on connection details for stability bracing in steel bridges with skewed supports. One goal of the study is to propose an improved connection method. One detail being investigated is a round half-pipe stiffener connected to the top and bottom flanges. The round stiffener allows perpendicular connections to the cross-frame connection tab, regardless of the skew angle. Additionally, there are substantial structural improvements when the tubular stiffener is used. Fastening the tubular stiffener to the flanges provides a significant increase in the warping stiffness of the cross section at the support. Finite element modeling has predicted increases in buckling capacity as high as 80% because of this warping restraint. As a result, in the vicinity of the half-pipe stiffener, the brace spacing can likely be increased. The increased spacing in these regions will simplify fabrication and reduce the number of fatigue-sensitive details in the congested support region. This paper describes a method to calculate the increase in girder buckling capacity due to the warping restraint provided by the pipe stiffener as well as the size of pipe for a required buckling capacity or unbraced length with design tools already widely used by structural engineers.