Application of high-frequency mechanical impact treatment for fatigue strength improvement of new and existing bridges
Doktorsavhandling, 2023

This thesis investigates the application of High-Frequency Mechanical Impact (HFMI) treatment for fatigue strength improvement of weldments in existing and new bridges. In the former case, the welds have already been subjected to fatigue loading and accumulated damage before treatment. A fatigue testing program is set up, comprising welded specimens subjected to fatigue loading before HFMI treatment to investigate the efficiency of HFMI treatment on existing structures. Moreover, additional fatigue test results are collected from the literature and analyzed. HFMI treatment is found to be very efficient in extending the fatigue lives of existing structures regardless of the accumulated fatigue damage prior to treatment, given that any surface cracks, if exist, have not grown more than 2.25 mm in depth. For practical applications, HFMI treatment is only recommended if the critical details are verified to be free of any surface cracks.

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.










Variableampli- tude


Hassan al-Karawi

Chalmers, Arkitektur och samhällsbyggnadsteknik, Konstruktionsteknik

Fatigue crack repair in welded structures via tungsten inert gas remelting and high frequency mechanical impact

Journal of Constructional Steel Research,; Vol. 172(2020)

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Fatigue life extension of existing welded structures via high frequency mechanical impact (HFMI) treatment

Engineering Structures,; Vol. 239(2021)

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Fatigue life estimation of welded structures enhanced by combined thermo-mechanical treatment methods

Journal of Constructional Steel Research,; Vol. 187(2021)

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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)

Artikel i vetenskaplig tidskrift

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

Fatigue is known to be a progressive localized permanent damage which occurs in metallic structures that are subjected to repetitive loading. This type of damage occurs even when the components are subjected to stresses well below those causing failure in a single load application. Therefore, the fatigue process ultimately causes cracking which leads to structural failure. This makes the phenomenon of fatigue one of the major threats to the durability and safety of metallic structures. In steel bridges, weldments are essential for joining structural members, but they are usually fatigue-prone links. For that reason, providing a solution to this problem would extend the life expectancy of existing bridges, and make the design of new bridges more efficient.

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

Trafikverket (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

Trafikverket (TRV 2020-68167), 2020-07-01 -- 2022-06-30.





Hållbar utveckling


Building Futures (2010-2018)



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5263



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