Fatigue Cracks In Steel Bridges Research Articles

Analysis of dynamic features on local fatigue cracks in steel bridges

In steel bridges with orthotropic decks, a common and important engineering structure, fatigue cracks can appear under long-term operation. Existing artificial detection is the main approach to detecting fatigue cracks in steel bridges, in addition to some non-destructive testing or image processing methods, and unmanned aerial vehicle are applied to improve accuracy and efficiency. However, the detection of fatigue cracks in steel bridges based on the variation of dynamic responses or dynamic parameters is another attractive idea, which is still rarely mentioned and investigated in the international literature. The reason is that local cracks usually have negligible effect on the global dynamic characteristics of steel bridges, resulting in the insensitivity to dynamic indexes to fatigue cracks. On this occasion, this study numerically explores and establishes two sensitive damage features to local damage, i.e., the crack in the vertical rib, based on the fatigue cracks found on the Türr Istvan bridge over the Danube River in Hungary. The local vibrations of the bridge are excited through the local impact on the structural details, i.e., the vertical rib, and then the high-order frequencies of the entire bridge are examined. The obtained high-order frequencies are proved to be more sensitive to the fatigue cracks on vertical ribs and increased with the crack depths. On the other hand, the correlation of the stress curves on opposite sides of the crack is sensitive to the depth of the crack. If fatigue cracks are found, the development degree of the fatigue cracks can be monitored more accurately by focusing on the correlation of local stresses near the crack. These two damage features are numerically proven to have feasibility and are expected to break through the insensitivity of the existing dynamic detection methods.

Electrical properties of low temperature-sensitive coating sensor for fatigue crack monitoring in steel bridges

Coating sensors are used for high-precision monitoring of fatigue cracks in steel bridges. However, the time-variation temperature has a considerable impact on the monitoring precision of fatigue cracking of steel bridges. This study developed a coating sensor using silver-plated copper powder (conductive filler)/acrylate composites as a sensing layer of a coating sensor to monitor fatigue cracks considering variable ambient temperatures. The synthesis and preparation of the composite were presented, and the microstructures of the coating sensor composites were observed by scanning electron microscopy. The bulk conductivity of the sensing layer at different conductive filler contents was measured by the four-probe method to investigate the effect of the concentration of the conductive filler on the electrical conductivity and temperature sensitivity of the resistivity. Moreover, the fractional change in resistivity (FCR) of the sensing layer was tested over a range of temperatures to investigate the temperature sensitivity of resistivity at different conductive filler contents. Finally, the composite with the lowest temperature sensitivity of resistivity was selected to prepare a coating sensor for fatigue crack monitoring. The composite of the sensing layer with 60 wt% of conductive filler content had the lowest temperature sensitivity of resistivity with a minimum value (5.1%) of FCR. The maximum range of the FCR did not exceed 6.4% under cyclic loading and variation-time ambient temperatures. The temperature variation had minimal effects on the output signal of the coating sensor compared with the output signal variation caused by fatigue cracking. The proposed coating sensor can be used for high-precision monitoring of fatigue cracking in steel bridges under different ambient temperatures.

Strengthening of fatigue cracks in steel bridges by means of adhesively bonded steel patches

ABSTRACT Steel bridges with orthotropic decks are a common construction method for long-span bridges. The type of construction was developed in the 1960s, and the early structures in particular show increasing crack damage. The reasons for this are, on the one hand, design deficiencies and, on the other hand, massively increased traffic loads. Therefore, there is a need for rehabilitation methods that neither weaken the cross-section, like bolted solutions, nor introduce thermal stresses, like welds, but so far there are no reliable and economic alternatives. This paper summarizes the state of the art on possible damage categories of orthotropic bridge slabs and presents a new rehabilitation alternative using bonded steel patches for crack rehabilitation. It is reported from an ongoing research project how to determine numerically the effectiveness of the bonded steel patches. For this purpose, simple analytical calculations are first used to determine the determining factors and to record their influence. Subsequently, nonlinear numerical calculations with the Drucker-Prager material model are used to identify the stress distributions in the bonded joints in order to optimise the patch geometries. To this end a stiffness factor is presented that can be used to classify such patch bonds and categorize them in terms of their effectiveness.

Human factors affecting visual inspection of fatigue cracking in steel bridges

In this study, the performance data from 30 inspectors evaluating 147 specimens with fatigue cracks in representative in-situ conditions were analysed to identify the role of human factors in bridge inspection. Two performance measures, the percentage of correct detections (detection rate) and the number of false calls, were considered. The variability in both performance measures was large, and only a small amount of the variance could be explained by individual characteristics or environmental conditions. Experience, training, temperature, and inspection duration were correlated with detection rate, while no single factor was correlated with false calls. A multivariate analysis found that the number of false calls could be best estimated considering an inspector’s employment sector and training, the maximum wind speed on the day of the inspection, and the use of a tape measure. Based on these results, recommendations for improved training methods, procedures, and equipment were developed.

Benchmark for Evaluating Performance in Visual Inspection of Fatigue Cracking in Steel Bridges

AbstractVisual inspection is the primary means of ensuring the safety and functionality of in-service bridges in the United States and owners spend considerable resources on such inspections. While...

Fatigue crack detection using a piezoelectric ceramic sensor

Fatigue cracks in steel bridges can lead to brittle fracture, so it is important to find them properly. Piezoelectric ceramic sensors have been identified as inexpensive, high-sensitivity sensors for detecting the fatigue crack. Piezoelectric ceramic sensors do not require a power supply for their operation, making them suitable for long-term measurements. In this study, we conducted fundamental tests on the frequency characteristics of a piezoelectric ceramic sensor and clarified that the single integral value of the output voltage of the piezoelectric ceramic sensor has a linear relationship with strain response at a frequency of 1.0 Hz or less. We then attempted to detect the initiation and propagation of fatigue cracks using the response of the single integral value of the output voltage. The results showed that it is possible to detect the initiation and propagation of fatigue cracks using the change of the response of the integral voltage. Finally, a fatigue test using a girder specimen was carried out. The results showed that it is possible to detect the initiation and propagation of fatigue cracks from a welded part using the change in the response of the integral voltage in the same manner.

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.

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.

A large-area strain sensing technology for monitoring fatigue cracks in steel bridges

This paper presents a novel large-area strain sensing technology for monitoring fatigue cracks in steel bridges. The technology is based on a soft elastomeric capacitor (SEC), which serves as a flexible and large-area strain gauge. Previous experiments have verified the SEC’s capability to monitor low-cycle fatigue cracks experiencing large plastic deformation and large crack opening. Here an investigation into further extending the SEC’s capability for long-term monitoring of fatigue cracks in steel bridges subject to traffic loading, which experience smaller crack openings. It is proposed that the peak-to-peak amplitude (pk–pk amplitude) of the sensor’s capacitance measurement as the indicator of crack growth to achieve robustness against capacitance drift during long-term monitoring. Then a robust crack monitoring algorithm is developed to reliably identify the level of pk–pk amplitudes through frequency analysis, from which a crack growth index (CGI) is obtained for monitoring fatigue crack growth under various loading conditions. To generate representative fatigue cracks in a laboratory, loading protocols were designed based on constant ranges of stress intensity to limit plastic deformations at the crack tip. A series of small-scale fatigue tests were performed under the designed loading protocols with various stress intensity ratios. Test results under the realistic fatigue crack conditions demonstrated the proposed crack monitoring algorithm can generate robust CGIs which are positively correlated with crack lengths and independent from loading conditions.

Verification of the repair effect for fatigue cracks in members of steel bridges based on thermoelastic stress measurement

Ageing steel bridges suffer fatigue cracks thereby necessitating immediate inspection, structural integrity evaluation or repair in the life cycle of steel bridges. We propose nondestructive evaluation techniques employing infrared thermography enabling us to remotely inspect the fatigue cracks in steel bridges and evaluate the structural integrity on the basis of thermoelastic stress measurement. This study presents a structural integrity assessment of steel bridges using remote measurement of the stress field around the fatigue cracks. We focus upon experimental results confirming reduction in the severity of the stress distribution around the fatigue cracks after the repair or reinforcement of members in steel bridges.

Nondestructive Evaluation of Fatigue Cracks in Steel Bridges Based on Thermoelastic Stress Measurement

Long-standing steel bridges suffer fatigue cracks, which necessitate immediate inspection, structural integrity evaluation or repair in the life cycle of steel bridges. The authors proposed NDT and NDE techniques employing infrared thermography at certain stage in the life cycle of steel bridges. This paper presents remote measurement of stress field around fatigue cracks in steel bridges for their structural integrity assessments. Further this paper shows experimental results confirming the severity reduction of stress distribution around fatigue cracks after the repair or reinforcement of steel bridges.

Remote nondestructive evaluation technique using infrared thermography for fatigue cracks in steel bridges

AbstractLong‐standing infrastructure is subject to structural deterioration. In this respect, steel bridges suffer fatigue cracks, which necessitate immediate inspection, structural integrity evaluation or repair. However, the inaccessibility of such structures makes inspection time consuming and labour intensive. Therefore, there is an urgent need for developing high‐performance nondestructive evaluation (NDE) methods to assist in effective maintenance of such structures. Recently, use of infrared cameras in nondestructive testing has been attracting increasing interest, as they provide highly efficient remote and wide area measurements. This paper first reviews the current situation of nondestructive inspection techniques used for fatigue crack detection in steel bridges, and then presents remote NDE techniques using infrared thermography developed by the author for fatigue crack detection and structural integrity assessments. Furthermore, results of applying fatigue crack evaluation to a steel bridge using the newly developed NDE techniques are presented.

Dual Mode Sensing with Low-Profile Piezoelectric Thin Wafer Sensors for Steel Bridge Crack Detection and Diagnosis

Monitoring of fatigue cracking in steel bridges is of high interest to many bridge owners and agencies. Due to the variety of deterioration sources and locations of bridge defects, there is currently no single method that can detect and address the potential sources globally. In this paper, we presented a dual mode sensing methodology integrating acoustic emission and ultrasonic wave inspection based on the use of low-profile piezoelectric wafer active sensors (PWAS). After introducing the research background and piezoelectric sensing principles, PWAS crack detection in passive acoustic emission mode is first presented. Their acoustic emission detection capability has been validated through both static and compact tension fatigue tests. With the use of coaxial cable wiring, PWAS AE signal quality has been improved. The active ultrasonic inspection is conducted by the damage index and wave imaging approach. The results in the paper show that such an integration of passive acoustic emission detection with active ultrasonic sensing is a technological leap forward from the current practice of periodic and subjective visual inspection and bridge management based primarily on history of past performance.

Detection of fatigue cracks in steel bridges by self-reference lock-in thermography

A new remote nondestructive inspection technique that is based on thermoelastic temperature measurement by infrared thermography was developed for the evaluation of fatigue cracks propagated from welded joints in steel bridges. Fatigue cracks were detected from localized high thermoelastic temperature changes observed at crack tips induced by stress singularity under variable loading resulting from traffic on the bridge. A self-reference lock-in data processing technique was developed for improving the signal/noise ratio of the thermal images recorded in the crack detection process. In this study, remote and nondestructive detection of fatigue cracks in an actual steel bridge in service was performed by the self-reference lock-in thermography method. The accuracy of this method was improved with a motion compensation technique.

Development of self-reference lock-in thermography and its application to remote nondestructive inspection of fatigue cracks in steel bridges

A new remote nondestructive evaluation technique, based on thermoelastic temperature measurement by the infrared thermography, was developed for evaluation of fatigue cracks propagated from welded joints in steel bridges. Fatigue cracks were detected from localized thermoelastic temperature change at crack tips due to stress singularity under wheel loading from traffics on the bridge. Self-reference lock-in data processing technique was developed for the improvement of signal-to-noise ratio of the thermal images obtained in the crack detection process. In this paper, experimental results of fatigue crack detection by the self-reference lock-in thermography are reviewed.

Detection of Through-Deck Type Fatigue Cracks in Steel Bridges by Self-Reference Lock-in Thermography

A new remote nondestructive inspection technique, based on thermoelastic temperature measurement by infrared thermography, is developed for detection and evaluation of fatigue cracks propagating from welded joints in steel bridges. Fatigue cracks are detected from localized high thermoelastic temperature change at crack tips due to stress singularity under variable loading from traffics on the bridge. Self- reference lock-in data processing technique is developed for the improvement of signal/noise ratio in the crack detection process. The technique makes it possible to perform correlation processing without an external reference signal. It is very difficult to detect through-deck type fatigue cracks in steel decks by the conventional NDT technique, since they are not open to the inspection. In this paper, self-reference lock-in thermography is applied for detection of through-deck type fatigue cracks. Experiments are carried out to steel deck sample, which simulates an actual steel bridge, during crack propagation test. It is found that significant stress concentration zone can be observed near the crack front, which enabled us to detect through-deck type fatigue cracks and to estimate its size.

Identifying Effective and Ineffective Retrofits for Distortion Fatigue Cracking in Steel Bridges Using Field Instrumentation

It is estimated that nearly 90% of all fatigue cracking is the result of out-of-plane distortion or other unanticipated secondary stresses at fatigue-sensitive details. Neither design specifications nor evaluation specifications provide any guidance on how to evaluate the in-service potential for fatigue cracking at these details. Often, as a result, the effectiveness of various retrofit procedures is questionable and ill fated. There are many examples where implemented retrofit procedures did not work and fatigue cracking reinitiated or continued. Implementation of one or two prototype retrofits is an attractive alternative to ensuring effective retrofits are developed where many details have to be retrofitted. Field instrumentation and testing is an effective means to determine the effectiveness and behavior of a given retrofit strategy. Behavior that may not be anticipated can be identified prior to installing retrofits on a large scale, thereby preventing future problems. This paper provides guidance on how to instrument these details and examines one example where initial retrofit strategies did not work, as demonstrated by their performance through field instrumentation.

Evolution of Fatigue-Resistant Steel Bridges

Fatigue cracking was seldom found in welded highway and railroad bridges from the time of their introduction in the 1950s until the late 1960s. The fatigue design specifications used in that era were developed from a limited knowledge base and largely with small-scale specimens that simulated welded details. During the AASHO Road Test in 1960 fatigue cracks were observed to develop in cover-plated steel bridge beams as a result of the heavy loads and high stress ranges. This observation subsequently resulted in a series of experimental studies supported by NCHRP starting in 1967. The laboratory studies with full-scale details were designed to evaluate the significance of many factors thought to influence fatigue resistance, including loading history (and associated stress states including residual stresses), type of steel, design details, and quality of fabrication. These studies indicated that small-scale specimens overestimated fatigue resistance and that only the stress range for a given detail was critical. As a result fatigue resistance design provisions in use since the 1950s were inadequate and overly optimistic, particularly at longer lives, because the assumption of a fatigue limit of 2 million cycles proved to be incorrect. The results of laboratory studies with full-size specimens and their impact on changing the concept of fatigue design and the bridge fatigue design provisions used for highway and railroad bridges today are reviewed. During the 1970s and 1980s fatigue cracking associated with low-fatigue-strength details (Categories E and E′), such as cover plates and lateral gusset plates, increased. Cracks were also found in transverse groove welds, particularly in attachments such as longitudinal stiffeners, gusset plates and even flange splices. These groove weld cracks generally occurred because large defects were inadvertently fabricated into the welded joint. The occurrence of these cracks was found to be predictable and in agreement with the laboratory fatigue resistance results. The 1970s also exposed an unexpected source of cracking due to the distortion of small web gaps that were frequently used in welded bridge structures. Web gap cracking continues to develop in a wide range of bridge types. It is the source of most fatigue cracks in steel bridges. Existing bridges that are susceptible to fatigue cracks or that develop fatigue cracks at primary details or from web gap distortion are easily repaired or retrofitted to ensure long-term performance. Examples of such repairs are reviewed. The future is bright for welded bridges because the knowledge base and current design provisions make it possible to design and build fatigue-resistant bridges.

Expert System with Learning Ability for Retrofitting Steel Bridges

An expert system for selecting the methods for retrofitting fatigue cracking in steel bridges is developed. The expert system involves a knowledge base with new knowledge representations and a new inference engine. The knowledge is based on 90 cases of fatigue cracking in existing steel bridges, and a knowledge base is constructed with knowledge being represented as production relations. All the relations in the knowledge base are assumed to have a certainty factor. The new inference engine was developed in the C language so that it has the ability to learn by using the knowledge‐based network model. The inference engine is able to modify relations with a certainty factor. Therefore, the knowledge‐based network model can grow into an optimum knowledge‐based network. The present system is evaluated for the actual fatigue cracking in three steel bridges. It is found that the system is able to infer reasonable methods for retrofitting cracked members.