Existing steel bridges are subjected to both increasing traffic loads and natural aging, both are capable of causing severe durability problems. Dependable rehabilitation methods are attracting attention as the promising methods to enhance structural durability and/or structural performance. One possible rehabilitation method, for improving fatigue performance, is the use of residual stress-based post-weld treatments such as peening. A number of studies has been performed and it has been proven that residual stress-based treatments are an effective way of increasing the fatigue lives of newly built steel bridges, and even enhancing the fatigue performance of existing structures. Provisions have been developed to ensure the proper execution of peening and several codes have considered its beneficial effect in the fatigue design of welded structures. Various analytical approaches are used to predict the fatigue performance of welded structures and the beneficial effects of residual stress-based post-weld treatments. In most codes and recommendations, variations of the “S-N curve” approach are employed. Linear elastic fracture mechanics (LEFM) and strain-based fracture mechanics (SBFM) are widely accepted approaches for making more precise predictions of the treatment benefit. Cohesive zone fatigue models are also recently introduced for predicting fatigue crack growth in as-received and peened welds. Despite all the research conducted, there are still two main unanswered questions related to the application of peening treatments. First, it is claimed that peening can be more effective for civil structures where a considerable portion of the total applied stress is due to permanent loads and thus peening is applied under load. However, most of research done so far has studied effects peening prior to the introduction of the structural self weight. Secondly, considering the nature of these treatments, some concerns have been raised regarding their effectiveness under actual in-service loading conditions, as most of the reported test-based studies only demonstrated the fatigue performance improvement under constant amplitude tension-only loading conditions. The current study was undertaken to examine the fatigue performance of welds peened-under load and to determine the effectiveness of peening for improving the fatigue performance of welds subjected to realistic in-service loading conditions. Moreover, a previously developed strain-based fracture mechanics (SBFM) model for predicting fatigue performance of welded details under different loading and treatment conditions, and a previously developed damage-based cohesive zone model for steel specimens were evaluated and calibrated. Fatigue tests were conducted on welded steel specimens, simulating different loading and peening conditions. Dye penetrant was used to stain cracked specimens upon detection of cracks and a crack front marking loading scheme was used to study the crack front shape. The alternating current potential drop (ACPD) method was used for continuous crack growth monitoring for both as-welded and peened specimens under different loading schemes. It was observed that cracks propagated at different rates in specimens treated under load than in the normally peened and as-welded specimens. Material tests were also conducted to determine the mechanical properties of the steel base metal. Secondary effects of peening were investigated by microhardness measurements and weld toe measurements. A number of typical weld toe defects was also detected. Residual stress measurements showed a uniformly distributed tensile residual stress near the surface of the untreated specimen. Needle peening the specimen resulted in a significant change in the residual stress distribution through the specimen thickness. In all cases, peening resulted in a significant increase in the fatigue life. However, greater fatigue life improvements were observed in lower stress ranges. Of the specimens tested under constant amplitude loading, those peened under load experienced the largest fatigue lives. For the variable amplitude loading tests, the untreated specimens had mean fatigue lives slightly less than observed in the constant amplitude tests. A previously developed strain-based fracture mechanics (SBFM) model was used to estimate analytically the effectiveness of peening applied to welded details. The model was able to predict the fatigue lives for both the as-welded and peened specimens for all loading conditions. It correctly estimated the additional benefit of peening when applied under a relatively small prestress level. The model predictions were used to estimate the additional benefit of peening under load. A previously developed cohesive zone model was introduced and applied to predict fatigue crack growth in a weld detail under cyclic loading. Fatigue tests were simulated using the finite element program ABAQUS. The material parameters α and β were chosen by iteration. Other fatigue tests were simulated and the model correctly predicted the effects of varying the applied stress range, R ratio, and residual stress level on the fatigue behaviour.

Existing steel bridges are subjected to both increasing traffic loads and natural aging, both are capable of causing severe durability problems. Dependable rehabilitation methods are attracting attention as the promising methods to enhance structural durability and/or structural performance. One possible rehabilitation method, for improving fatigue performance, is the use of residual stress-based post-weld treatments such as peening. A number of studies has been performed and it has been proven that residual stress-based treatments are an effective way of increasing the fatigue lives of newly built steel bridges, and even enhancing the fatigue performance of existing structures. Provisions have been developed to ensure the proper execution of peening and several codes have considered its beneficial effect in the fatigue design of welded structures. Various analytical approaches are used to predict the fatigue performance of welded structures and the beneficial effects of residual stress-based post-weld treatments. In most codes and recommendations, variations of the “S-N curve” approach are employed. Linear elastic fracture mechanics (LEFM) and strain-based fracture mechanics (SBFM) are widely accepted approaches for making more precise predictions of the treatment benefit. Cohesive zone fatigue models are also recently introduced for predicting fatigue crack growth in as-received and peened welds. Despite all the research conducted, there are still two main unanswered questions related to the application of peening treatments. First, it is claimed that peening can be more effective for civil structures where a considerable portion of the total applied stress is due to permanent loads and thus peening is applied under load. However, most of research done so far has studied effects peening prior to the introduction of the structural self weight. Secondly, considering the nature of these treatments, some concerns have been raised regarding their effectiveness under actual in-service loading conditions, as most of the reported test-based studies only demonstrated the fatigue performance improvement under constant amplitude tension-only loading conditions. The current study was undertaken to examine the fatigue performance of welds peened-under load and to determine the effectiveness of peening for improving the fatigue performance of welds subjected to realistic in-service loading conditions. Moreover, a previously developed strain-based fracture mechanics (SBFM) model for predicting fatigue performance of welded details under different loading and treatment conditions, and a previously developed damage-based cohesive zone model for steel specimens were evaluated and calibrated. Fatigue tests were conducted on welded steel specimens, simulating different loading and peening conditions. Dye penetrant was used to stain cracked specimens upon detection of cracks and a crack front marking loading scheme was used to study the crack front shape. The alternating current potential drop (ACPD) method was used for continuous crack growth monitoring for both as-welded and peened specimens under different loading schemes. It was observed that cracks propagated at different rates in specimens treated under load than in the normally peened and as-welded specimens. Material tests were also conducted to determine the mechanical properties of the steel base metal. Secondary effects of peening were investigated by microhardness measurements and weld toe measurements. A number of typical weld toe defects was also detected. Residual stress measurements showed a uniformly distributed tensile residual stress near the surface of the untreated specimen. Needle peening the specimen resulted in a significant change in the residual stress distribution through the specimen thickness. In all cases, peening resulted in a significant increase in the fatigue life. However, greater fatigue life improvements were observed in lower stress ranges. Of the specimens tested under constant amplitude loading, those peened under load experienced the largest fatigue lives. For the variable amplitude loading tests, the untreated specimens had mean fatigue lives slightly less than observed in the constant amplitude tests. A previously developed strain-based fracture mechanics (SBFM) model was used to estimate analytically the effectiveness of peening applied to welded details. The model was able to predict the fatigue lives for both the as-welded and peened specimens for all loading conditions. It correctly estimated the additional benefit of peening when applied under a relatively small prestress level. The model predictions were used to estimate the additional benefit of peening under load. A previously developed cohesive zone model was introduced and applied to predict fatigue crack growth in a weld detail under cyclic loading. Fatigue tests were simulated using the finite element program ABAQUS. The material parameters α and β were chosen by iteration. Other fatigue tests were simulated and the model correctly predicted the effects of varying the applied stress range, R ratio, and residual stress level on the fatigue behaviour.

The need for rehabilitation of bridges has become a critical challenge due to aging and an increase in traffic loads. Many of these bridges are exceeding their design fatigue life. Since many of these bridges are structurally deficient, they need to be rehabilitated or replaced by a new bridge. The most susceptible and weak parts of steel bridges to cracks and fatigue are the welds, due to the presence of high stress concentrations, tensile residual stresses, and imperfections as a result of the welding process. Inspection and repair of welds are difficult and elimination of welded details is not possible in steel bridge construction. Ultrasonic impact treatment (UIT) is a promising and innovative post-weld treatment (PWT) method for improving the fatigue performance of existing welded steel and steel-concrete composite structures such as highway bridges. The fatigue resistance of treated joints is enhanced by improving the geometry of the weld toe, and introducing compressive residual stresses. However, a lack of tools for quality assurance has slowed UIT’s adoption by bridge authorities. The current study was undertaken to examine the fatigue performance of structural steel welds subjected to UIT at various levels, including intentional under-treatment and over-treatment, and to relate the fatigue performance of the treated welds to geometric and metallurgical properties measured to control the treatment quality. The last objective was to use the laboratory results to develop acceptance criteria for the quality control of UIT in bridge applications. Fatigue tests of non-load carrying fillet welded attachments were conducted on properly treated, under-treated, and over-treated weld toes. Statistical analyses of the fatigue life data were performed and crack growth was monitored using the alternating current potential drop (ACPD) method. Measurement of local properties (such as weld toe geometry, local hardness, and residual stresses) and examination of the weld toe microstructure were also performed on the untreated and treated welds. The effects of weld toe geometry on the local stresses in the untreated and treated welds were also investigated using elastic finite element analysis (FEA) to obtain the stress concentration factor (SCF) for the different treatment cases and to examine the changes in the SCF for the different weld toe geometries. Based on the statistical analysis performed in this research, the results illustrated that UIT significantly improved the fatigue lives of weld details regardless of the investigated level of treatment quality. The fatigue lives of welded details under constant amplitude (CA) loading and constant amplitude loading with under-loads (CA-UL) were increased up to 30 and 27 times respectively. On average, the fatigue life of the treated weld details was slightly lower under CA-UL than under CA loading. Treatment quality had little impact on the mean of the S-N curves. However, it did impact the design (95% survival probability) S-N curves, with the curve associated with a proper treatment slightly higher than the curves for poor or unknown treatment quality. Local near-surface microhardness and compressive residual stresses were greatest for the over-treated welded details, followed by the properly treated and then the under-treated welded details. Increasing the treatment speed resulted in a greater reduction in the surface microhardness and compressive residual stresses than decreasing the treatment intensity. Finite element analyses showed that changes in weld toe geometry due to UIT can cause a decrease in the SCF near the surface of the treated weld toe. The SCF was the lowest for the properly treated steel specimens and slightly higher for the under-treated specimens. For the over-treated specimens, the SCFs were nearly as high as for the untreated weld. The SCF increases as the thickness of the flange increased up to 19 mm. With further flange thickness increase to 38 mm, the SCF did not change substantially. The work presented herein demonstrated that indent depth measurements from the base metal side, commonly used for quality control, may not identify over-treatment on their own. Indent depth measurements from both the weld and the base metal sides, obtained by measurement of weld toe impressions, offer a good alternative means for identifying over-treatment. However, for identifying under-treatment, indent depth measurements should be used in conjunction with visual inspection for traces of the original weld toe.

The need for rehabilitation of bridges has become a critical challenge due to aging and an increase in traffic loads. Many of these bridges are exceeding their design fatigue life. Since many of these bridges are structurally deficient, they need to be rehabilitated or replaced by a new bridge. The most susceptible and weak parts of steel bridges to cracks and fatigue are the welds, due to the presence of high stress concentrations, tensile residual stresses, and imperfections as a result of the welding process. Inspection and repair of welds are difficult and elimination of welded details is not possible in steel bridge construction. Ultrasonic impact treatment (UIT) is a promising and innovative post-weld treatment (PWT) method for improving the fatigue performance of existing welded steel and steel-concrete composite structures such as highway bridges. The fatigue resistance of treated joints is enhanced by improving the geometry of the weld toe, and introducing compressive residual stresses. However, a lack of tools for quality assurance has slowed UIT’s adoption by bridge authorities. The current study was undertaken to examine the fatigue performance of structural steel welds subjected to UIT at various levels, including intentional under-treatment and over-treatment, and to relate the fatigue performance of the treated welds to geometric and metallurgical properties measured to control the treatment quality. The last objective was to use the laboratory results to develop acceptance criteria for the quality control of UIT in bridge applications. Fatigue tests of non-load carrying fillet welded attachments were conducted on properly treated, under-treated, and over-treated weld toes. Statistical analyses of the fatigue life data were performed and crack growth was monitored using the alternating current potential drop (ACPD) method. Measurement of local properties (such as weld toe geometry, local hardness, and residual stresses) and examination of the weld toe microstructure were also performed on the untreated and treated welds. The effects of weld toe geometry on the local stresses in the untreated and treated welds were also investigated using elastic finite element analysis (FEA) to obtain the stress concentration factor (SCF) for the different treatment cases and to examine the changes in the SCF for the different weld toe geometries. Based on the statistical analysis performed in this research, the results illustrated that UIT significantly improved the fatigue lives of weld details regardless of the investigated level of treatment quality. The fatigue lives of welded details under constant amplitude (CA) loading and constant amplitude loading with under-loads (CA-UL) were increased up to 30 and 27 times respectively. On average, the fatigue life of the treated weld details was slightly lower under CA-UL than under CA loading. Treatment quality had little impact on the mean of the S-N curves. However, it did impact the design (95% survival probability) S-N curves, with the curve associated with a proper treatment slightly higher than the curves for poor or unknown treatment quality. Local near-surface microhardness and compressive residual stresses were greatest for the over-treated welded details, followed by the properly treated and then the under-treated welded details. Increasing the treatment speed resulted in a greater reduction in the surface microhardness and compressive residual stresses than decreasing the treatment intensity. Finite element analyses showed that changes in weld toe geometry due to UIT can cause a decrease in the SCF near the surface of the treated weld toe. The SCF was the lowest for the properly treated steel specimens and slightly higher for the under-treated specimens. For the over-treated specimens, the SCFs were nearly as high as for the untreated weld. The SCF increases as the thickness of the flange increased up to 19 mm. With further flange thickness increase to 38 mm, the SCF did not change substantially. The work presented herein demonstrated that indent depth measurements from the base metal side, commonly used for quality control, may not identify over-treatment on their own. Indent depth measurements from both the weld and the base metal sides, obtained by measurement of weld toe impressions, offer a good alternative means for identifying over-treatment. However, for identifying under-treatment, indent depth measurements should be used in conjunction with visual inspection for traces of the original weld toe.

Stålindustrin står för 7 % av världens koldioxidutsläpp. För att minska industrins miljöpåverkan är ett alternativ att introducera konstruktioner i höghållfasta stål eftersom det ger möjlighet till slankare konstruktioner och därmed mindre materialåtgång. Användning av höghållfasta stål i svetsade konstruktioner kan dock vara problematiskt då de utsätts för utmattningslaster vilket kan leda till oanade brott i konstruktionen. För att förbättra svetsade konstruktioner mot utmattning kan PWT tillämpas.I arbetet undersöktes möjligheterna till implementering av PWT i tillverkningsstadiet samt eventuella hinder till varför metoden inte används. För att bilda en uppfattning om möjligheten till alternativa metoder för att öka utmattningshållfastheten för en svets har gällande myndighetskrav samt normer för utförande av vanliga svetsar undersökts. Genom intervjuer har åsikter från olika aktörer inhämtats för att besvara frågeställningarna.Efter genomförd undersökning konstaterades att den bristande förmågan att kontrollera spänningar i en svets är en bidragande faktor till varför PWT i dagsläget inte används i tillverkningsstadiet inom byggbranschen. Intervjun med Trafikverket resulterade i alternativet att ta fram en särskild kravspecifikation enligt Krav Brobyggande, som vid godkännande skapar möjlighet för implementering av PWT. Krav Brobyggande är Trafikverkets dokument men krav och råd för brobyggande i Sverige. / The steel industry stands for 7 % of the worlds emission of carbon dioxide. An alternative to reduce the environmental impact of the industry is to introduce structures of high strength steel since it provides opportunity for slimmer structures and thereby less material consumption. However, the use of high strength steel in welded structures can be problematic when exposed to fatigue loads, which can lead to unsuspected breaks in the structure. To improve welded structures against fatigue PWT can be implemented.The possibilities for implementation of PWT during manufacturing and possible obstacles as to why it should not be used was investigated in this thesis. To form a perception for the possibilities of alternative welding methods to increase fatigue strength the existing regulations and norms of execution for as welded conditions was examined. Opinions from different participants in the construction process were interviewed to answer the questions at issue.The lacking ability to verify stress in a weld was concluded to be a contributing factor as to why PWT is not currently used during the manufacturing process in the construction industry. The interview with Trafikverket resulted in the alternative to create a specific requirement specification according to demands in Krav Brobyggande. Approval of the specific requirement specification could lead to possibility to implement PWT in the construction industry. Krav Brobyggande is Trafikverkets document with demands and advice for bridge construction in Sweden.

Stål är ett material som används i olika byggnadskonstruktioner. I de byggnadskonstruktioner som utsätts för upprepade belastningar kan utmattning ske i materialet. Utmattning leder till permanenta skador i form av sprickbildning och slutligen brott. I svetsade konstruktioner är det ofta svetsar som har den lägsta utmattningsstyrkan. För att förbättra en svets utmattningsstyrka finns efterbehandlingsmetoder som benämns Post Weld Treatment (PWT). Idag används den beräkningsmetod som är föreskriven i Eurokod vid verifiering av svetsar. Beräkningsmetoden är förenklad och kan underskatta objektets livslängd med hänsyn till utmattning. Syftet med arbetet är att studera olika dimensioneringsmetoder som behandlar utmattningsbelastade svetsar samt hur tillämpning av PWT kan förbättra en typsvets livslängd. Målet med arbetet är att studera en typsvets och visa skillnaden i antalet lastcykler mellan beräkningsmetoderna samt hur många lastcykler samma typsvets förväntas öka med PWT. Teorin och resultaten utgår från vetenskapliga artiklar, litteraturstudier och enfallstudie som behandlar både en genomsvetsad stumsvets samt en kälsvets lokaliserade på en I-balk. Resultatet pekar mot att den metod som används idag underskattar livslängden och att PWT kan markant kan förbättra en svetsutmattningsstyrka. / Steel is a material used in various building structures. Fatigue can occur in the material if building structures is exposed for repeated loads. Fatigue leads to permanent damages such as crack initiations and fracture. It is common that welds in welded structures have the lowest fatigue strength. A welds fatigue strength can be improved with treatments termed Post Weld Treatment (PWT). Today a welds fatigue strength is verified with a method described in Eurocode. The calculation method is simplified which can underestimate the objects number of lifecycles regarding fatigue. The purpose with this essay is to study different structural design methods for fatigue exposed welds and how many lifecycles a typeweld will increase when applying PWT. The goal with this essay is to study a typeweld and calculate the number of lifecycles between the calculation methods and to show how many lifecycles the same type weld will increase when applying PWT. The results and theory are based on scientific articles, literature studies and a casestudy which both contains a through welded butt weld and a fillet weld placed on an I-beam. The results points at that the calculation method that is used today underestimates the number of lifecycles and that the number of lifecycles increased significant after PWT.