Fatigue improvement of steel bridges with high-frequency mechanical impact treatment
Doktorsavhandling, 2020

This thesis investigates the performance of fatigue-improved welds with high-frequency mechanical impact (HFMI) for application on new bridges. Fatigue strength improvement with HFMI can enable lightweight design of bridges and allow the utilisation of the benefits of high-strength steels. Studies of various bridge types were performed in this thesis showing that 20% material saving is possible in the main load-carrying members through post-weld treatment and the use of increased steel grades (fy > 355 MPa) where necessary. Limitations of the application of HFMI treatment on bridges were also identified, related to the degree of improvement and choice of steel grade.

Experimental work of HFMI-treated joints with thick main plates relevant for bridges is scarce in the literature and comprehensive studies on the thickness effect are few. Therefore, the thickness effect was studied based on an established database of 582 fatigue test results of different types of HFMI-treated joints, collected from 28 studies. It was shown that the thickness effect becomes weaker than what is recommended for as-welded joints as a result of HMFI treatment. Fatigue experiments were conducted on a typical fatigue-prone detail in steel bridges with load-carrying plates of 40 and 60 mm which showed a significant fatigue strength improvement after HFMI treatment, exceeding recommended fatigue strengths given by the International Institute of Welding. Based on the fatigue experiments, a weak thickness effect was derived for non-load-carrying transverse attachment joints where the attachment and weld sizes are kept constant.

The performance of HFMI-treated welds in composite steel and concrete road bridges was studied through a state-of-the-art review and simulations of variable amplitude in-service stresses in four case-study bridges in Sweden. It was shown that, in such bridges, very high and varying stress ratios are present due to a high portion self-weight stresses, which constitute up to 50% of the highest total stresses. Furthermore, it was revealed that the fatigue-critical locations in HFMI-treated bridges remain unchanged compared with conventional bridges and that compressive overloads pose no detrimental effect that requires additional attention in the fatigue assessment. Variable amplitude experiments with a bridge spectrum load from the case studies were conducted, including both low and high mean stress tests. The low mean stress tests performed equally or better than the constant amplitude fatigue strength, confirming that bridge loads do not pose any additional damaging effect for non-load-carrying transverse attachment specimens. The high mean stress tests clearly reflected the detrimental effect of high tensile self-weight stresses and enabled verification and development of approaches to consider these effects in design.


thickness effect



variable amplitude


Opponent: Prof. Alain Nussbaumer, EPFL, Switzerland


Poja Shams Hakimi

Chalmers, Arkitektur och samhällsbyggnadsteknik, Konstruktionsteknik

Fatigue life improvement of welded bridge details using high frequency mechanical impact (HFMI) treatment

Heinisuo, M & Mäkinen, J (eds) 2015, The 13th Nordic Steel Construction Conference (NSCC.2015). Tampere University of Technology, Tampere.,; (2015)p. 201-202

Paper i proceeding

The thickness effect of welded details improved by high-frequency mechanical impact treatment

International Journal of Fatigue,; Vol. 99(2017)p. 111-124

Artikel i vetenskaplig tidskrift

Assessment of in-service stresses in steel bridges for high-frequency mechanical impact applications

Engineering Structures,; Vol. 241(2021)

Artikel i vetenskaplig tidskrift

Fatigue failure is the culmination of a progressive damage process which occurs in metallic structures or components that are subjected to cyclic loading. The most common example of fatigue failure in everyday life is when a metallic paper clip breaks into two pieces after repeated bending. In this case, the applied stresses exceed the elastic limit of the material, causing significant deformations in the paper clip and resulting in fatigue failure after just a few numbers of load cycles.

In bridges, cyclic loading occurs every time a vehicle passes the bridge. The generated stresses are usually far below the elastic limit of the material, so the deformations are invisible to the naked eye. Still, the fatigue process goes on and can ultimately - after millions of load cycles that occur in the course of many years - result in cracks which cause structural failure. In steel bridges, weldments are essential for the manufacturing and joining of the structural members and components, but welds possess unfavourable properties which makes them the weakest links in the structure in terms of fatigue. Therefore, fatigue of welded joints is a fundamental part in the design of steel bridges and decisive for the material consumption and the choice of steel grades.

High-frequency mechanical impact (HFMI) treatment is a fatigue enhancement method which improves the fatigue properties of welds. The improvement comes mainly from the introduction of compressive residual stresses which enhance the resistance against fatigue cracking and dramatically extends the fatigue life (the endurable number of load cycles). In addition, fatigue improvement with HFMI is steel grade dependent, meaning that the fatigue strength increases with increased steel grade. For steel bridges, this entails a great potential for material saving as the required amount of steel is closely related to the fatigue performance of the weldments. In this thesis, it was shown calculation wise that 17-23 % steel could be saved based on three case-study bridges. Hence, HFMI treatment can allow for lighter steel bridges with substantial cost savings and environmental benefits as a result.

Sustainable Design of Light-weight Steel Bridge, application of post-weld treatment techniques for enhancing the fatigue durability of steel and compisite (steel-concrete) bridges

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Hållbarhetsdriven design av utmattningsbelastade stålkonstruktioner

Formas (2014-638), 2015-01-01 -- 2018-12-31.


Hållbar utveckling


Teknisk mekanik

Bearbetnings-, yt- och fogningsteknik


Annan materialteknik



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4718




Opponent: Prof. Alain Nussbaumer, EPFL, Switzerland

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