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

• 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.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Maskinteknik -- Teknisk mekanik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Mechanical Engineering -- Applied Mechanics (hsv//eng)

TEKNIK OCH TEKNOLOGIER  -- Materialteknik -- Bearbetnings-, yt- och fogningsteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Materials Engineering -- Manufacturing, Surface and Joining Technology (hsv//eng)

TEKNIK OCH TEKNOLOGIER  -- Materialteknik -- Annan materialteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Materials Engineering -- Other Materials Engineering (hsv//eng)

Nyckelord

HFMI

TIG-dressing

Life extension

Post weld treatment

Gain factor

Linear elastic fracture mechanics

Crack retrofiting

Peening

fatigue crack

Crack detection

Microhardness

Concentration factor

LEFM

TIG-remelting

Strain gauge

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