Fatigue life extension in existing steel bridges. High-Frequency Mechanical Impact treatment and Tungsten Inert Gas remelting in life extension and fatigue crack repair of welded steel structures
Licentiate thesis, 2020

This thesis investigates the performance of improved welds with two post-weld treatment methods for application on existing structures. High-Frequency Mechanical Impact (HFMI) treatment and Tungsten Inert Gas (TIG) remelting were used for fatigue life extension of welded structures. Axial fatigue testing was conducted on transversal non-load-carrying attachment treated via the investigated methods. Furthermore, more than 250 test results on different treated welded details were collected, sorted and analysed. HFMI-treatment was found to give a significant fatigue life extension even with the presence of cracks up to 2.25 mm. On the other hand, the efficiency of TIG-remelting was also proven when the crack was completely eliminated after remelting. Even if a small part of the crack remains after remelting, fair fatigue life could be expected. However, it is recommended to use HFMI-treatment or TIG-remelting only when the crack inspection is negative before and after treatment respectively.

Complimentary studies showed that the investigated methods induced compressive residual stress, increased the smoothness of the weld toe, increased the local hardness and changed the angular distortion status locally. Moreover, TIG-remelting changed the microstructure in both the fusion zone and the heat-affected zone. HFMI-treatment changed the crack orientation, induced compressive plasticity at the crack tip and caused crack narrowing or even closure. However, these effects were less significant for deeper cracks. Moreover, some practical aspects of the treatment application were investigated. Unlike treating new structures, TIG-electrode should be placed at the weld toe to secure that the maximum fusion depth corresponds to the crack plane. On the other hand, HFMI-indentor should be slanted more toward the base metal than the weld to avoid unintentional crack opening. Moreover, the IIW recommendations for both HFMI-treatment inclination and indentation depth could be extended to cracked structures.

The aforementioned investigated parameters (i.e. residual stress, distortions, local hardness and toe's smoothness) were incorporated in fatigue life predictions for both treatment methods. The base metal S-N curve was used to predict the life of specimens treated via TIG-remelting, while Paris law was used to track the crack propagation of HFMI-treated details. The results corresponded well with fatigue test results. Combining TIG-remelting with HFMI-treatment resulted in welds with higher fatigue strength because of the combined effects of crack closure via TIG-remelting and compressive plasticity via HFMI-treatment.



Life extension

Post weld treatment

Gain factor

Linear elastic fracture mechanics

Crack retrofiting


fatigue crack

Crack detection


Concentration factor



Strain gauge

Opponent: Assoc. Prof. Martin Leitner Chair of Mechanical Engineering Montanuniversität Leoben, Austria


Hassan al-Karawi

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)

Journal article

Fatigue life extension of existing welded structures via High Frequency Mechanical Impact treatment, Hassan Al-Karawi, R.U.Franz von Bock und Polach, Mohammad Al-Emrani

The effciency of high frequency mechanical impact treatment and tungsten inert gas remelting in fatigue life extension of existing welded structures, Hassan Al-Karawi, Mohammad Al-Emrani

Crack behaviour after High Frequency Mechanical Impact treatment in welded S355 structural steel, Hassan Al-Karawi, Joakim Hedegård, Mohammad Al-Emrani

LifeExt - Livslängdsförlängning för befintliga stålbroar

VINNOVA (2017-02670), 2017-06-08 -- 2019-12-31.

Swedish Transport Administration (TRV 2018/27547), 2018-05-15 -- 2020-11-30.

Driving Forces

Sustainable development

Areas of Advance


Materials Science

Subject Categories

Applied Mechanics

Manufacturing, Surface and Joining Technology

Other Materials Engineering

Lic / Architecture and Civil Engineering / Chalmers University of Technology





Opponent: Assoc. Prof. Martin Leitner Chair of Mechanical Engineering Montanuniversität Leoben, Austria

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