Application of high-frequency mechanical impact treatment for fatigue strength improvement of new and existing bridges
Doctoral thesis, 2023
Remelting the surface with a tungsten electrode before HFMI treatment is another solution which has rarely been studied on existing structures. Therefore, several experimental investigations are conducted including fatigue testing, measurement of residual stress, hardness testing and scanning the welds topography to study the effect of combining these two post-weld treatment techniques. The combined treatment is found to be efficient as it induces higher and deeper compressive residual stress and local hardening. These aspects are all considered in numerical simulations conducted to investigate the fatigue behaviour of new and existing weldments treated using this combination. The results verify the superiority of the combined treatment to both individual treatments (TIG & HFMI). Nonetheless, because of the complexity associated with TIG remelting, the combination is only suggested for existing structures containing shallow fatigue cracks which can be fused by a tungsten electrode.
One major hindrance to applying HFMI treatment on weldments in steel bridges is the lack of design rules and recommendations such as consideration of stress ratio (mean stress) and overloads. Therefore, a correction factor (λHFMI) to account for the mean stress effect is derived. This factor is used to magnify the design stress range for fatigue verification of HFMI-treated welded details existing in road and railway bridges. λHFMI is calibrated using measured traffic data that includes millions of vehicles and hundreds of trains. In addition, the characteristic load combination associated with the serviceability limit state is found to be the most appropriate for verifying the maximum stresses in road bridges. Based on the work conducted in this thesis, a complete methodology is proposed for the design and assessment of HFMI-treated welded details in new and existing steel bridges.
Finally, the effect of corrosion on the performance of HFMI-treated weldments is studied by analyzing collected test results. Despite the observed reduction in fatigue endurance of HFMI-treated details due to the removal of top layers improved by residual stresses, the obtained fatigue lives are still longer than the design lives assigned for new welded details even in extreme corrosion conditions. However, corrosion protection and removal of sharp HFMI groove edges via light grinding are still necessary to reduce the susceptibility of weldments to corrosion.
Chalmers, Architecture and Civil Engineering, Structural Engineering
Fatigue crack repair in welded structures via tungsten inert gas remelting and high frequency mechanical impact
Journal of Constructional Steel Research,; Vol. 172(2020)
Fatigue life extension of existing welded structures via high frequency mechanical impact (HFMI) treatment
Engineering Structures,; Vol. 239(2021)
Fatigue life estimation of welded structures enhanced by combined thermo-mechanical treatment methods
Journal of Constructional Steel Research,; Vol. 187(2021)
Corrosion Effect on the Efficiency of High-Frequency Mechanical Impact Treatment in Enhancing Fatigue Strength of Welded Steel Structures
Journal of Materials Engineering and Performance,; Vol. In press(2022)
Mean Stress Effect in High-Frequency Mechanical Impact (HFMI)-Treated Steel Road Bridges
Buildings,; Vol. 12(2022)
Al-Karawi, H. Shams-Hakimi, P. Pétursson, H. Al-Emrani, M. Mean stress effect in High-Frequency Mechanical Impact (HFMI) treated welded steel railway bridges
Verification of the Maximum Stresses in Enhanced Welded Details via High-Frequency Mechanical Impact in Road Bridges
Buildings,; Vol. 13(2023)
High-Frequency Mechanical Impact (HFMI) is a relatively new post-weld treatment method that aims to increase weldments' fatigue strength by inducing beneficial compressive residual stress at the weld toe. However, this method has not yet been implemented in the bridge industry on a large scale. This is attributed to several reasons. First, the existing bridges may have accumulated certain damage which makes the efficiency of the treatment in fatigue life extension very ambiguous. Besides, corrosion may also pose a threat to the fatigue strength of HFMI-treated weldments. In other words, the effect of existing damage and corrosion on the fatigue life should be investigated. Moreover, the stability of the beneficial compressive residual stresses in treated weldments needs to be ensured throughout the design life of the bridge. This includes loading conditions that cause relaxation of these stresses such as loading cycles with high-stress ratios or overloads. These need to be incorporated into the design process in a simplified way.
This work focuses on providing rules, recommendations and guidelines that would enable a safe and efficient application of HFMI treatment on existing and new bridges. To be more specific, this thesis is divided into two parts. The first focuses on HFMI treatment in existing structures, which is studied experimentally and numerically. It is found that the fatigue design life assigned for HFMI treatment can be claimed regardless of the number of applied loading cycles before the application of the treatment. However, the weldments should be free of cracks deeper than 2.25 mm, or preferably crack-free. If a crack exists at the surface, HFMI treatment should be preceded by TIG remelting to remove the whole crack. Besides, the effect of corrosion on the fatigue performance of HFMI-treated weldments is proven. Nonetheless, the proposed design curves for HFMI-treated details can be used even if the bridge exists in a corrosive environment. To reduce the corrosion effect, paint should be applied, and sharp groove edges shall be removed by light grinding.
The second part of the thesis focuses on the design rules of new HFMI-treated bridges. This includes the consideration of the mean stress and overload effects. The research is performed through analyses of measured truck/train loads. A design framework is proposed to include the mean stress effect via one variable called λHFMI. The method is characterized by its simplicity which makes the easy-to-use by bridge designers. Besides, statistical evaluations have shown that the characteristic combination of load actions associated with the serviceability limit state is the best-studied option to incorporate the overload effect in road bridges.
LifeExt - Livslängdsförlängning för befintliga stålbroar
Swedish Transport Administration (TRV 2018/27547), 2018-05-15 -- 2020-11-30.
VINNOVA (2017-02670), 2017-06-08 -- 2019-12-31.
VINNOVA (2021-01045), 2021-05-31 -- 2024-05-30.
Rekomendationer för HFMI-behandling av stålbroar - utveckling av dimensioneringsmetoder och tekniska kravspecifikationer
Swedish Transport Administration (TRV 2020-68167), 2020-07-01 -- 2022-06-30.
Areas of Advance
Building Futures (2010-2018)
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5263