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| 2. |
- al-Karawi, Hassan, 1993, et al.
(författare)
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Crack behaviour after High Frequency Mechanical Impact treatment in welded S355 structural steel
- 2021
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Ingår i: Bridge Maintenance, Safety, Management, Life-Cycle Sustainability and Innovations Proceedings of the Tenth International Conference on Bridge Maintenance, Safety and Management (IABMAS 2020), June 28-July 2, 2020, Sapporo, Japan. - : CRC Press. - 9780367232788 ; , s. 3113-3119
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Konferensbidrag (refereegranskat)abstract
- High-Frequency Mechanical Impact (HFMI) is a rather new post-weld-treatment method that has shown interesting fatigue life enhancement capabilities. The influence on material behaviour needs, however, to be investigated more to reach a full understanding and fully optimize the treatment, not least when the method is applied to existing welded structures. In this study, an experimental investigation was conducted to examine the effect of HFMI-treatment on crack behaviour in pre-fatigued welded details. A substantial reduction in crack width and a change in crack orientation were observed, both are believed to have beneficial effects in extending the remaining fatigue life. However, these beneficial effects decreased as the crack length increased. In light of the observations, a simplified phenomenological model was proposed to determine the crack opening after HFMI-treatment. In addition to that, finite element (FE) simulations were used to conduct several parametric studies. The conclusions from the FE study could be used to extend some of the IIW recommendations regarding the indenter inclination and the depth of indentation to cracked welds. Moreover, The FE simulations indicated that optimum results could be obtained if the indenter is placed and directed more towards the base metal than towards the weld.
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| 3. |
- al-Karawi, Hassan, 1993, et al.
(författare)
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Crack detection via strain measurements in fatigue testing
- 2021
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Ingår i: Strain. - : Wiley. - 0039-2103 .- 1475-1305. ; 57:4
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Tidskriftsartikel (refereegranskat)abstract
- Fatigue cracks have appeared as a significant issue for joints and connections in existing steel structures in the last decades. Therefore, those are a major inspection and maintenance matter for any steel structure's operator. This emphasises the importance of using a reliable detection method to determine the crack size and assessing the severity of such a crack on the structural integrity of a structure. In this article, the effectiveness of strain measurement in detecting fatigue cracks in transversal non‐load carrying welded attachment subjected to out of plane axial loading is studied. Numerical analysis and experimental investigations allowed to correlate the decrease in strain measured by attached gauges to the crack depth at the weld toe. In addition, different strain evolution patterns were found during fatigue testing, and the fracture surfaces of the specimens were observed to interpret these patterns. Moreover, the crack position with respect to the weld toe surface was predicted via strain measurements.
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| 4. |
- al-Karawi, Hassan, 1993, et al.
(författare)
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Fatigue crack repair by TIG-remelting
- 2021
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Ingår i: Bridge Maintenance, Safety, Management, Life-Cycle Sustainability and Innovations Proceedings of the Tenth International Conference on Bridge Maintenance, Safety and Management (IABMAS 2020), June 28-July 2, 2020, Sapporo, Japan. - : CRC Press. - 9780367232788 - 9780367232788 ; , s. 3120-3127
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Konferensbidrag (refereegranskat)abstract
- Fatigue is one of the most detrimental problems that often limit the service life of steel bridges. Because of that, different post-weld treatment methods have been studied to extend the fatigue life and repair any existing cracks resulted from the traffic loading. The aim of this paper is to study the fatigue life extension of cracked structures by means of Tungsten Inert Gas (TIG) remelting. Fatigue tests were conducted at a stress ratio of 0.29 on a transverse attachment of 16 mm, thick-walled specimens. The specimens were instrumented with multiple strain gauges along the weld to detect the strain drop caused by crack growth. The specimens were pre-fatigued until a crack of around 1 mm- deep was obtained at the weld toe. After crack detection, the weld toes were treated by TIG-remelting to remove any existing cracks and to reduce the local stress concentration. The weld parameters were controlled so that the fusion depth was always larger than the crack depth. A substantial fatigue life extension could be achieved and the testing led to ran-out after 10 million cycles, which was attributed to the improvement in geometry and residual stress. Weld toe radius was measured before and after treatment by a laser scanner. Hole drilling was used to measure the residual stresses in as-welded state and after TIG-remelting. In this paper, a fatigue damage model is also presented. It is shown that the model gives a good prediction of the effect of TIG-treatment in repairing fatigue loaded welded details.
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| 5. |
- al-Karawi, Hassan, 1993, et al.
(författare)
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Fatigue crack repair in welded structures via tungsten inert gas remelting and high frequency mechanical impact
- 2020
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Ingår i: Journal of Constructional Steel Research. - : Elsevier BV. - 0143-974X. ; 172
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Tidskriftsartikel (refereegranskat)abstract
- Rehabilitation of welded structures has gained increasing attention lately. This paper aims at investigating the efficiency of Tungsten Inert Gas (TIG)-remelting and TIG-remelting followed by High Frequency Mechanical Impact treatment (TIG-HFMI) in fatigue life extension. Fatigue tests were carried out on as-welded and cracked specimens after treatment. The lives of the treated specimens increased remarkably by the two methods (TIG and TIG-HFMI). Many of the treated specimens ran-out after 10 million cycles of loading and failed at the clamping location when tested at a higher stress range. The improvement in compressive residual stresses, hardness values and weld toe radii were the reasons behind the life extension. These factors were used for fatigue life estimation in as-welded and TIG-treated specimens using the base metal S[sbnd]N curve. Moreover, the test results together with results from previous tests in the literature demonstrated that these methods can be useful for crack retrofitting as for new structures.
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| 6. |
- al-Karawi, Hassan, 1993, et al.
(författare)
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Fatigue life estimation of treated welded attachments via High Frequency Mechanical Impact treatment (HFMI-treatment)
- 2021
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Ingår i: Modern Trends in Research on Steel, Aluminium and Composite Structures PROCEEDINGS OF THE XIV INTERNATIONAL CONFERENCE ON METAL STRUCTURES (ICMS2021), POZNAŃ, POLAND, 16-18 JUNE 2021. - London : Routledge. - 9781003132134
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- High-Frequency Mechanical Impact (HFMI) is one of the newest post-weld treatment methods that can be used to repair cracks in welded structures. The main effect of this method is to extend the fatigue life by putting the cracks under compression. Linear Elastic Fracture Mechanics (LEFM) is to be used as an analysis tool accompanied by Paris law. Crack growth curves are presented in this work. The effects of the induced residual stress, the initial crack size, the clamping stress, and the stress range are all incorporated in the analysis. Subsequently, the analysis results are compared to experimental results obtained from fatigue testing on transverse non-load-carrying welded attachment. The scatter in fatigue test results is found to be mainly attributed to the scatter in both the induced residual stresses and the variation in the existing crack sizes before treatment. Moreover, the analysis could successfully expect at what conditions the crack would not propagate through incorporating the concept of threshold stress intensity factor.
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| 7. |
- al-Karawi, Hassan, 1993, et al.
(författare)
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Fatigue life extension of existing welded structures via high frequency mechanical impact (HFMI) treatment
- 2021
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Ingår i: Engineering Structures. - : Elsevier BV. - 1873-7323 .- 0141-0296. ; 239
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Tidskriftsartikel (refereegranskat)abstract
- High-Frequency Mechanical Impact (HFMI) is one of the post-weld treatment methods. In this study, comparative axial fatigue tests were conducted on as-welded and HFMI-treated welded transverse attachment details. The test results demonstrated the efficiency of HFMI-treatment in fatigue life extension of cracked welded structures, providing that the existing crack size is less than 1.2 mm. Cracks were created in some specimens through fatigue testing before HFMI-treatment, while other specimens were not subjected to any fatigue loading prior to treatment. Many of the treated specimens ran out after 10 million cycles of loading when tested at a stress range of 150 MPa. Therefore, the stress range was increased to 180 MPa or 210 MPa. No remarkable difference was found between the fatigue strength of the crack-free and the cracked treated specimens. It was found that the induced compressive residual stress can exceed the material yield limit, and reach a depth larger than 1.5 mm in most cases. The induced compressive residual stress, the local material hardening, the increase in weld toe radius, the change in crack orientation and the shallowness of the crack size were the causatives of the obtained long fatigue lives of the HFMI-treated specimens. Besides, linear elastic fracture mechanics calculations were conducted to predict the fatigue lives of as-welded and HFMI-treated details. The results were in agreement with the experiment. Moreover, the calculations showed that the initial crack size, the clamping stress and the induced compressive residual stress were the main factors behind the scatter in fatigue lives.
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| 8. |
- al-Karawi, Hassan, 1993, et al.
(författare)
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Mean Stress Effect in High-Frequency Mechanical Impact (HFMI)-Treated Steel Road Bridges
- 2022
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Ingår i: Buildings. - : MDPI AG. - 2075-5309. ; 12:5
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Tidskriftsartikel (refereegranskat)abstract
- High-frequency mechanical impact (HFMI) is a post-weld treatment method which substantially enhances the fatigue strength of steel weldments. As such, the method enables a more efficient design of bridges, where fatigue is often the governing limit state. Road bridges are typically trafficked by a large variety of lorries which generate load cycles with varying mean stresses and stress ranges. Unlike conventional welded details, the fatigue strength of HFMI-treated welds is known to be dependent on mean stress in addition to the stress range. The possibility of considering the mean stress effect via Eurocode’s fatigue load models (FLM3 and FLM4) was investigated in this paper. Moreover, a design method to take the mean stress effect into account was proposed by the authors in previous work. However, the proposed design method was calibrated using limited traffic measurements in Sweden, and as such, may not be representative of the Swedish or European traffic. In this paper, larger data pools consisting of more than 873,000 and 446,000 lorries from Sweden and the Netherlands, respectively, were used to examine the validity of the previous calibration in both countries. The comparison revealed no significant difference between the data pools with regards to the mean stress effect. Additionally, the previous calibration provided the most conservative mean stress effect and was considered adequately representative for both countries. The proposed design method was further validated using four composite case study bridges. It was also found that the mean stress effect was mainly influenced by the self-weight, while variation in the mean stress due to traffic had a minor influence on the total mean stress effect. Furthermore, it was found that the mean stress effect could not be accurately or conservatively predicted using FLM3 or FLM4.
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| 9. |
- al-Karawi, Hassan, 1993, et al.
(författare)
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Mean stress effect in high-frequency mechanical impact (HFMI)-treated welded steel railway bridges
- 2024
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Ingår i: Steel Construction. - 1867-0520 .- 1867-0539. ; 17:2, s. 81-92
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Tidskriftsartikel (refereegranskat)abstract
- The need for new railway bridges is driven by the growing volume of transportation demands for both passenger and freight traffic on railway networks. In the design of these bridges, the fatigue limit state is a criterion that usually limits the allowable applied load level and thus also the utilization of the high strength of the steel material. Therefore, improving the fatigue performance of welded details by high-frequency mechanical impact (HFMI) treatment leads to a more efficient design. However, the fatigue performance of HFMI-treated welds is known to be affected by the mean stress and this needs to be considered in the design of treated welded details in steel bridges. This is rather straightforward if the bridge is subjected to cycles from one type of train but becomes cumbersome when several different sets of trains (e. g. axle loads, axle distances) cross the bridge. In this article, a factor to take the mean stress effect (including self-weight and traffic load variations) into account is derived from traffic data measured in Sweden. Moreover, the mean stress effect is also predicted using the different fatigue load models in the Eurocode. These models either consist of one-load patterns such as LM71, SW/0, and SW/2 or are composed of different trains with different combinations. It was found that the mean stress effect is underestimated by the first group of models. On the other hand, the mean stress predicted by the light traffic mix is found to be close to that calculated using real traffic data, while other mixes (standard and heavy) underestimate the mean stress effect. Therefore, a correction factor to account for the mean stress effects in real traffic is derived (called here λHFMI). This factor can be used to correct the design stress range for fatigue verification of HFMI-treated welded details in railway bridges.
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| 10. |
- al-Karawi, Hassan, 1993, et al.
(författare)
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The efficiency of HFMI treatment and TIG remelting for extending the fatigue life of existing welded structures
- 2021
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Ingår i: Steel Construction. - : Wiley. - 1867-0520 .- 1867-0539. ; 14:2, s. 95-106
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Tidskriftsartikel (refereegranskat)abstract
- Different post‐weld treatment methods have been developed to enhance the fatigue strength of welded steel structures and extend the service lives of their components. High‐frequency mechanical impact (HFMI) treatment and tungsten inert gas (TIG) remelting are two methods that have attracted considerable interest in recent decades. This paper presents the results of a study of fatigue life extension for pre‐fatigued welded steel details which can be achieved using HFMI treatment and TIG remelting. More than 250 fatigue test results were collected – including different details such as butt welds, longitudinal attachments, transverse attachments and cover plate attachments. HFMI treatment was found to extend the life considerably when the specimens treated were free from cracks or when existing cracks were < 2.25 mm deep. TIG remelting could extend fatigue lives even with cracks > 4 mm deep. In comparison to TIG remelting, HFMI treatment results in a longer fatigue life extension for pre‐fatigued details, provided existing cracks are < 2.25 mm deep. Regarding TIG remelting, the depth of possible remaining cracks was found to be a substantial parameter when assessing the degree of life extension.
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