Distortion-induced Fatigue Cracks Research Articles

Digital simulation of distortion-induced fatigue in steel bridges with different geometrical configurations

Distortion-induced fatigue cracks at web gaps often occur in multi-girder steel bridges under vehicle loads. This paper presents full size digital simulations of steel girder bridges with different geometrical configurations, to explore the distortion-induced fatigue cracking performance. In the digital simulations, four geometrics are considered, in addition to two web gap details, namely the vertical stiffener web gap and the horizontal gusset plate web gap. Introducing multi-layer and multi-pass welds residual welding stress fields, digital fatigue simulation results show that the distortion-induced fatigue cracks at both web gaps are Mode I-II-III combined cracks dominated by Mode I. The coupling of the three modes is more prominent at the vertical stiffener web gap compared to the horizontal gusset plate web gap. In the skewed bridge, curved bridge, and skewed-curved bridge, the significant bending-torsional coupling effect, the structural asymmetry, and the stress deformation asymmetry cause an increment in distortion ranges, fatigue crack growth rates, and the proportions of Mode II and III cracks. The fatigue strength categories for vertical stiffener web gap and horizontal gusset plate web gap in straight, skewed, curved and skewed-curved bridges are category 125, 112, 110 and 110, and category 160, 140, 100 and 100 of the Chinese specification and Eurocode, respectively. Therefore, the geometrical configuration is an important factor affecting the distortion-induced fatigue behavior, which should be considered in the fatigue design and maintenance of steel girder bridges.

Distortion-Induced Fatigue Cracking after Crack Arrest Hole Retrofit of Steel Girder Bridges

Distortion-Induced Fatigue Cracking after Crack Arrest Hole Retrofit of Steel Girder Bridges

Effectiveness and durability evaluation of cold reinforcement for Distortion-induced fatigue in steel bridges

Full-scale fatigue tests and numerical damage simulation were performed to evaluate the effectiveness and durability of two cold reinforcement methods (bonding steel angle, hybrid bonding and bolting steel angle), under the action of fatigue loads and environmental effects. Fatigue test results after reinforcement and the further long-term durability test results including both internal and exterior girders under different environmental effects indicate that two cold reinforcement techniques are highly effective and durable to arrest distortion-induced fatigue cracking and decrease out-of-plane distortion. Numerical fracture evaluation considering both durability and fatigue damage of cold reinforcement joints shows stress intensity factor (SIF) at fatigue crack tips reduced below threshold, demonstrating that cracks would not propagate, consistent with test results. The innovative hybrid cold bonding and bolting steel angles reinforcement method is recommended, because it can not only avoid traffic disruption and angle falling, but also arrest fatigue crack growth effectively without drilling hole damage at girder flanges.

Bonding and bolting angle reinforcement for distortion-induced fatigue in steel girder bridges

Physical fatigue tests and numerical simulation were conducted to verify the effectiveness of cold reinforcement methods of distortion-induced fatigue cracks at web gaps in steel girder bridges. Test results indicate that stop-holes could temporarily arrest crack growth. However, bolting and bonding angle could effectively arrest further cracking, and reduce distortions and fatigue stresses. Numerical simulation analysis results show that the equivalent stress intensity factor decreased dramatically below the corresponding threshold after bolting or bonding reinforcement. The innovative bonding angle reinforcement strategy is more recommended for better reinforcement effectiveness, no damage to the original structure, and avoiding traffic disruption.

Evaluation of novel combined CFRP-steel retrofit for repairing distortion-induced fatigue

Distortion-induced fatigue damage at crossframe-to girder connections affects many steel bridges built before the mid-1980s. This type of damage is associated with a prevalent detailing practice at the time which avoided welding connection plates to girder flanges to avoid fatigue cracks. The lack of connectivity and out-of-plane motion of the crossframe produce a flexible gap region in the girder web where fatigue cracks often develop, requiring frequent inspections, costly maintenance, and repairs.This paper presents a study evaluating the use of CFRP in conjunction with steel sections to repair complex patterns of distortion-induced fatigue cracks in steel bridge web gaps, without the need for bolting to the flange or concrete deck removal. A steel girder-to-crossframe subassembly was tested in distortion-induced fatigue to evaluate the performance of the retrofit measure. Realistic crack patterns were introduced by applying fatigue loading, and the efficacy of the repair method was evaluated by monitoring crack growth with the repair in place. Twelve test trials were performed: five in the unretrofitted condition to propagate the cracks, and seven in the retrofitted condition to evaluate performance of the repair. The study evaluated multiple crack patterns, different configurations of the retrofit, presence or absence of mechanical anchorage to the flange, footprint of the CFRP relative to the steel retrofit elements, and different surfaces connected by the CFRP.The results showed that the combined CFRP-steel retrofit measure was effective in preventing distortion-induced fatigue crack initiation and propagation.

Development of a Distortion-Induced Fatigue Crack Characterization Methodology Using Digital Image Correlation

Abstract Distortion-induced fatigue cracking is a primary maintenance and structural safety concern in steel bridges built prior to the 1980s in the United States. Manual, hands-on inspections are ...

Evaluation and upgrading web-gap distortion retrofits in steel girder bridges

Distortion-induced fatigue cracking in the unstiffened web-gap of cross-frame diaphragms is the most prevalent type of cracking in steel girder bridges. Multi-steel girder bridges experience differential deflection between adjacent girders when subject to vertical loads resulting in a driving force in cross-frame diaphragms. The driving force developed in the cross-frame legs leads to out-of-plane distortion of the web-gap which results in high stress concentrations at the area, following by fatigue damages. This paper introduces an innovative method to retrofit web-gap distortion cracking, upgrade existing retrofits, and compare its effectiveness with two common retrofit techniques. The method involves cutting the existing connection plate and using angles to attach the disconnected part of the connection plate to the web. The method intends to eliminate the local high stresses at the connection plate end with considering minimal interference in original connection design and load path. Also, two other conventional repair methods were investigated, slot method and top-angle measure, to use as a basis for comparing the methods effectiveness. Laboratory testing was performed on a small-scale steel bridge bay by applying a vertical displacement to the free end of a cross-frame diaphragm to simulate the differential deflection between two adjacent girders in real bridges. The results from the testing were compared to findings from finite element analyses (FEA). Test results as well as FEA results for all investigated retrofit techniques are presented herein. Results showed that the newly developed slot-angle technique has significant potential for effectively reducing the stress concentrations in the web-gap region by removing the location of initial stress concentrations and redistributing those stresses over a wider area.

Numerical simulation of distortion-induced fatigue crack growth using extended finite element method

Aiming to investigate the propagation behaviour of distortion-induced fatigue cracks in steel bridge web gaps, multi-scale numerical analysis models were built based on fracture mechanics theory and extended finite element method (XFEM), combining with the full-scale fatigue tests data. Propagation behaviours of representative fatigue cracks in vertical stiffener web gaps and horizontal gusset plate web gaps were analysed. Finite element models of welds connecting web, vertical stiffener and horizontal gusset plate were built, and the welding residual stresses of such details were analysed. Significant transverse welding residual tensile stresses exist at stiffener web weld toes for web gap details. Residual stress measurements were conducted, and the crack shape and the propagation direction path were basically the same with that in the numerical simulation, indicating that the numerical simulation results were relatively reliable. Furthermore, the welding residual stress fields were considered in the crack propagation analysis models. Representative fatigue cracks at web gaps are Mode I leading mixed-mode cracks of Modes I, II and III. Crack propagation considering welding residual stress has faster propagation rate and is more consistent with fatigue test results. The welding residual stress cannot be ignored for analysis and assessment of distortion-induced fatigue cracks in steel bridges.

Repair assessment for distortion-induced fatigue cracks in a seismically retrofitted double-deck bridge complex

Many steel bridges in the United States designed before the mid-1980s are highly susceptible to distortion-induced fatigue cracking. This vulnerability is substantially increased if the out-of-plane driving force caused by differential girder displacement is increased for any reason. This research examines one such case where a double-deck bridge complex, originally built in the 1960s, was retrofitted to improve seismic performance. As part of the retrofit, single angle K-type diaphragms were replaced with stiffer double-angle cross-type diaphragms. This seismic retrofit led to an increase in web-gap stresses, and within approximately one year following the retrofit, inspectors identified numerous fatigue cracks in the web of longitudinal girders where connection plates terminate near top flanges. A repair measure was implemented to provide a positive connection between the connection plate and the girder flange. The purpose was to reduce the high stress concentrations in the web-gap region by restricting the out-of-plane distortion in the web-gap region. Field tests were carried out in addition to developing finite element (FE) models to investigate the efficacy of the repair technique. The results confirmed a significant decrease in the web-gap stress after implementation of the repair, and a subsequent FE analysis showed that the new load path through the repair angle section did not introduce a new fatigue sensitive area. In fact, the repair resulted in stresses well below the constant amplitude fatigue threshold (CAFT) for this type of detail.

Evaluation of a Digital Image Correlation Bridge Inspection Methodology on Complex Distortion-Induced Fatigue Cracking

Distortion-induced fatigue cracking in aging steel bridges is a primary concern for many bridge owners and stakeholders, accounting for the majority of fatigue cracks in bridges in the United States. Currently Departments of Transportation primarily use visual inspections to locate and characterize fatigue cracks. However, this approach has drawbacks, as recent studies have shown that visual inspections are not able to consistently identify realistically-sized fatigue cracks in highway infrastructure. Additionally, it requires significant time and cost to perform visual inspections, and both inspectors and the traveling public are placed at risk during the inspection. Bridge owners and stakeholders would benefit from the development of inspection techniques that do not rely on hands-on human visual inspection – to reduce the risks of harm to the inspectors and traveling public, increase reliability, and decrease the time and cost of performing inspections.Vision-based technologies are an active area of research in the field of structural health monitoring, aimed at developing alternatives to manual inspection for identifying damage to transportation infrastructure. The majority of these studies have focused on macro-indicators of damage, including identifying excessive corrosion, concrete deck deterioration, and large displacements due to substructure movement. Digital image correlation (DIC) is one such vision-based technology that shows promise for detecting and characterizing fatigue cracks, but investigations and applications have thus far been limited to simple, in-plane cracking.Three-dimensional DIC measurements have the ability to capture full-field displacements and surface strains, allowing for the potential of identifying and characterizing out-of-plane fatigue cracks, such as those occurring on steel bridges exposed to loading through differential girder displacement. Since the majority of fatigue cracks arise from out-of-plane loading, investigating the efficacy of DIC for detecting distortion-induced fatigue cracks is of clear value.This paper describes an experimental study in which a scaled steel girder-to-cross-frame specimen was cyclically loaded to produce geometrically complex distortion-induced fatigue cracking. To examine the potential usefulness of DIC for future automated bridge inspections the resulting fatigue cracks were characterized using DIC and visual inspection. The methodology applied proved successful at quantifying certain aspects of the complex, bifurcating cracks, but had difficulty characterizing crack segments that are no longer actively loaded. Additional work is needed to improve the accuracy and reliability of the crack detection results for this technology to potentially be used as an automated inspection tool in the future.

Mechanical Treatment of Crack-Arrest Holes Subjected to Distortion-Induced Fatigue

Fatigue cracking is a major concern for many aging steel bridges in the United States. A common form of crack remediation is the drilling of crack-arrest holes, increasing the crack tip radius and thus reducing the applied stress intensity. However, the majority of cracking that occurs in bridge structures is caused by distortion-induced fatigue. Cross-frame forces generated by differential girder displacement produce multiaxial, mixed-mode fatigue loading at the intersection of the girder web, flange, and connection plate, producing cracking along web-to-stiffener welds and flange-to-web welds. Crack-arrest holes are often used as a first line of defense at these cracks. However, crack-arrest holes deform out-of-plane under distortion-induced fatigue loading, and have been found to be of limited utility in this application.Previous research has shown that mechanical treatment of crack-arrest holes has the potential to improve the efficacy of crack-arrest holes for halting crack propagation for in-plane fatigue, but the performance of such treatments is unknown for distortion-induced fatigue cracks. A research program sponsored by the Kansas Department of Transportation is investigating the effectiveness of one such technology for controlling crack propagation in steel bridge girders susceptible to distortion-induced fatigue. This technology produces a cold-worked region around the crack-arrest hole, introducing a field of compressive residual stresses aimed at retarding crack propagation. Evaluation of the mechanically treated crack-arrest holes is evaluated analytically for both in-plane and out-of-plane fatigue loading. This paper presents background on the problem and preliminary findings of the study.

Sensing distortion-induced fatigue cracks in steel bridges with capacitive skin sensor arrays

Distortion-induced fatigue cracks represent the majority of fatigue cracks in steel bridges in the United States. Currently, bridge owners, such as the state departments of transportation, rely on human inspection to detect, monitor, and quantify these cracks so that appropriate repairs can be applied before cracks reach critical sizes. However, visual inspections are costly, labor intensive, and may be prone to error due to inconsistent skills among bridge inspectors. In this study, we represent a novel strain-based approach for sensing distortion-induced fatigue cracks in steel bridges using soft elastomeric capacitor (SEC) arrays. Compared with traditional foil strain gauges, the SEC technology is a large-area and flexible skin-type strain sensor that can measure a wide range of strain over a large surface. Previous investigations have verified the suitability of a single SEC for sensing an in-plane fatigue crack in a small-scale steel specimen. In this paper, we further demonstrate the ability of SECs for sensing distortion-induced fatigue cracks. The proposed strategy consists of deploying an array of SECs to cover a large fatigue-susceptible region and establishing a fatigue sensing algorithm by constructing a crack growth index (CGI) map. The effectiveness of the strategy was experimentally validated through fatigue tests of bridge girder to cross-frame connection models with distortion-induced fatigue cracks. Test results verified that by deploying an SEC array, multiple CGIs can be obtained over the fatigue-susceptible region, offering a more comprehensive picture of fatigue damage. Furthermore, by monitoring a series of CGI maps constructed under different fatigue cycles, the fatigue crack growth can be clearly visualized through the intensity change in the CGI maps.

Detection of fatigue cracking in steel bridge girders: A support vector machine approach

This study presents an artificial intelligence approach for the detection of distortion-induced fatigue cracking of steel bridge girders based on the data provided by self-powered wireless sensors. The sensors have a series of memory gates that can cumulatively record the duration of the applied strain. The gates are activated as soon as the electrical charge generated by piezoelectric strain transducer exceeds pre-defined thresholds. In the present study, the distribution of the sensor output has been characterized by a Gaussian cumulative density function. For the analysis, extensive finite element simulations were carried out to obtain the structural response of an existing highway steel bridge girder (I-96/M-52) in Webberville, Michigan. Different damage states were defined by extending the lengths of the crack at the web gaps from 10mm to 100mm. Damage indicator features were extracted for different data acquisition nodes based on the sensor output distribution. Subsequently, support vector machine (SVM) classifiers were developed to fuse the clustered features and identify multiple damage states. The results indicate that the models have acceptable detection performance, specifically for cracks larger than 10mm. The best classification performance was obtained using the information from a group of sensors located near the damage zone.

Investigation of Distortion-Induced Web-Gap Cracking in a Seismically Retrofitted Steel Bridge: Repair Measures

AbstractThis paper studies distortion-induced fatigue cracks in a 1960s-era welded plate girder bridge that developed cracks in the web-gap region shortly after completion of a comprehensive seismic retrofit. Finite-element analysis (FEA) and field testing were conducted to investigate the cause of cracking and then to recommend appropriate repair measures for the bridge. The field test results validated the high-stress concentrations in the web-gap region of the bridge that led to fatigue cracking after only a limited number of cycles. FEA results indicated that the high-stress concentrations were principally a result of replacing K-type diaphragms with stiffer cross diaphragms as part of the seismic retrofit. Two conventional repair measures in addition to two innovative repair measures were evaluated. The proposed innovative repair measures focused on ease of installation compared with more conventional options and were found to reduce the distortion-induced stresses well below the constant amplitude f...

Fatigue cracking detection in steel bridge girders through a self-powered sensing concept

Development of fatigue cracking is affecting the structural performance of many of welded steel bridges in the United States. One of the main sources of fatigue cracking is out-of-plane distortion occurring at connections of transverse structural members and longitudinal girders. Distortion-induced fatigue cracks mostly occur in older bridges with members prone to fatigue. Prediction of secondary stresses in these members is difficult using conventional design specifications. This limitation suggests the necessity of utilizing new strategies to analyze the damage caused by distortion-related cracking. This study presents a new approach for detection of distortion-induced fatigue cracking of steel bridges based on the interpretation of the data provided by a newly developed self-powered piezo-floating-gate (PFG) sensor. The PFG sensors are empowered using piezoelectric transducers through harvesting energy from the mechanical loading experienced by the structure. In order to assess the performance of the proposed sensing system, three-dimensional finite element models were developed and the structural response of the girder was subsequently obtained. The fatigue life of the girder was determined based on J-integral concept and Paris Law. Several damage states were defined by extending the fatigue crack lengths. Thereafter, features representing the PFG sensor output were extracted from the strain data for different sensing nodes to detect the damage scenarios. Furthermore, a new data fusion concept based on the effect of group of sensors was proposed to improve the damage detection performance. The results indicate that the proposed method is capable of detecting different damage progression states. This is specifically evident for the sensors that are located close to the damage location. The acceptable performance of the proposed sensing system implies its applicability for other modalities of infrastructure/structural health monitoring (I/SHM).

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.

Investigation and Retrofit of Distortion-Induced Fatigue Cracks in a Double-Deck Cantilever-Suspended Steel Truss Bridge

This paper discusses a comprehensive investigation and retrofit for extensive fatigue cracks in the end connections of floor beams on a double-deck, cantilever-suspended steel truss bridge. The investigation involved three-dimensional (3D) finite-element analyses using global and local models, field measurement of strains and displacements due to live load and temperature, laboratory testing of steel samples for material properties and high-stress low-cycle fatigue characteristics, as well as development of an effective retrofit based on the analytical, experimental, and field testing results. It was concluded that the cracks were a result of distortion induced fatigue in the floor beam web due to interactive deformations of the global structural system under live load and temperature variations. The retrofit entailed removing the fatigue susceptible weld terminations and reinforcing local areas of the floor beam web for out-of-plane distortion. Construction of the fatigue retrofit was completed in July 2011. The repaired structure has performed satisfactorily since then.

An investigation of distortion-induced fatigue cracking under variable amplitude loading using 3D crack propagation analysis

The distortion-induced fatigue cracks in the welded details of the Söderström Bridge are analytically and numerically investigated by performing 3D crack propagation analyses with variable amplitude fatigue loading. In the crack propagation analyses, the effects of bridge loading are defined on the basis of the field measurements in order to simulate crack growth and predict the residual fatigue life of the studied detail as accurately as possible. The effect of crack closure and crack direction while considering the most common criteria is also studied. The results are compared with those obtained from the crack propagation analyses with constant amplitude fatigue loading presented in Aygül et al. [1].The results of the crack growth simulations with variable amplitude fatigue loading have generally shown good agreement with the real crack formation and reveal that the crack growth rates are different in different directions. The crack behaviour in the damaged detail is mainly controlled by the loading and geometrical arrangement of the detail components. There is generally a significant difference between constant and variable amplitude fatigue crack growth analyses and the variable amplitude fatigue crack growth analyses yield more conservative results. The main reason for this difference is the bridge loading and the number of stress cycles defined in the analyses. The crack direction criteria studied in this investigation showed basically the same crack formation and crack growth rate.

Repairing Distortion-Induced Fatigue Cracks in Steel Bridge Girders Using Angles-with-Plate Retrofit Technique. I: Physical Simulations

Repairing Distortion-Induced Fatigue Cracks in Steel Bridge Girders Using Angles-with-Plate Retrofit Technique. I: Physical Simulations

Investigation of distortion-induced fatigue cracked welded details using 3D crack propagation analysis

The behaviour of distortion-induced fatigue cracks in welded details in an existing bridge was studied analytically by performing crack propagation analysis based on linear elastic fracture mechanics. The real load history of the bridge was obtained from strain measurements. These loads were utilised to examine the crack growth rate and the residual service life of the cracked detail. Moreover, the effectiveness, accuracy and applicability of the crack propagation analysis on bridge structures were investigated by simulating a complex case of fatigue cracking using several crack propagation analyses.The results of the analyses revealed that the fatigue crack in the studied details had significantly different crack growth characteristics in different directions. In the thickness direction, for instance, the crack was seen to propagate at a certain rate, which increased with the propagated crack from the beginning and, as expected, the crack propagation rate decreased when the crack grew longer. The crack was subsequently arrested half way through the thickness of the plate. In the longitudinal direction, the crack was not, however, arrested in the same way as in the thickness direction and it continued to propagate at a reduced yet constant crack growth rate. The results also revealed that, even though distortion-induced fatigue cracking was usually caused by a mixed-mode condition (i.e. a combination of modes I, II and III), the governing propagation mode is still mode I. Furthermore, it was also observed that the contribution of modes II and III to crack propagation was very little and dependent on the location of the propagated crack front, as well as the geometrical configuration of the cross-beam.