Abstract
High Frequency Mechanical Impact (HFMI) is one of the post-weld treatment methods. In this study, comparative axial fatigue tests were conducted on as-welded and HFMI-treated welded transverse attachment details. The test results demonstrated the efficiency of HFMI-treatment in fatigue life extension of cracked welded structures, providing that the existing crack size is less than 1.2 mm. Cracks were created in some specimens through fatigue testing before HFMI-treatment, while other specimens were not subjected to any fatigue loading prior to treatment. Many of the treated specimens ran-out after 10 million cycles of loading when tested at a stress range of 150 MPa. Therefore, the stress range was increased to 180 MPa or 210 MPa. No remarkable difference was found between the fatigue strength of the crack-free and the cracked treated specimens. It was found that the induced compressive residual stress can exceed the material yield limit, and reach a depth larger than 1.5 mm in most of the cases. The induced compressive residual stress, the local material hardening, the increase in weld toe radius, the change in crack orientation and the shallowness of the crack size were the causatives of the obtained long fatigue lives of the HFMI-treated specimens. Besides, linear elastic fracture mechanics calculations were conducted to predict the fatigue lives of as-welded and HFMI-treated details. The results were in agreement with the experiment. Moreover, the calculations showed that the initial crack size, the clamping stress and the induced compressive residual stress were the main factors behind the scatter in fatigue lives.
Highlights
Many welded bridges in Europe were constructed in the few decades following the second world war
Full penetration butt welded transverse attachment specimens made of S355 structural steel were fatigue tested in three states: As-welded, virgin High Frequency Mechanical Impact (HFMI)-treated and prefatigued HFMI
The fatigue strength of the specimens in as-welded conditions was found to be 125 MPa, which is larger than the assigned value for this detail given by the institute of welding (IIW) recommendations
Summary
Many welded bridges in Europe were constructed in the few decades following the second world war. Thereby, there is a persistent need for maintaining their structural integrity. Fatigue process occurs over time and under loads lower than the ultimate limit state [1]. The degradation of the structure’s integrity with time is -mainly- attributed to the vulnerability of welded joints to fatigue cracks when subjected to cyclic loading. Fatigue of welds gained increasing attention among researchers in structural engineering. Particularity when the design fa tigue life has been reached and a decision is to be made whether the structure should be torn down and replaced or repair action could be sufficient to maintain the structure’s integrity
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