Fatigue performance improvement of patch-plate welding via PWHT with induction heating

Steel bridges are susceptible to damages and degradation as a result of the corrosion and fatigue effects. While both preventive and reactive maintenance and repair measures have been adopted in restoring the stiffness and strength of deteriorated bridge members, numerous challenges are faced. For instance, in the widely used patch-plate joining method, both welding and bolting joining methods are used depending on the situations and conditions of the members. Despite being reliable, bolting is disadvantageous in many ways: decrease in the sectional area, increase in weight and poor adhesive property. On the other hand, welding which has been seen as a promising alternative joining method is no fully effective. Welding and welding processes may lead to defects, fatigue cracks and residual stress. As such, these concerns should be thoroughly examined in welding to patch-plate joining to produce high-performance joints.

To this note, Professor Mikihito Hirohata from Osaka University together with May Phyo Aung and Hirohito Katsuda from Nagoya University examined the basic features of residual stress generated by patch-plate welding for the repair of steel structural members through numerical and experimental simulations. Specifically, they proposed a fatigue performance improvement method comprising of post-weld heat treatment and induction heating. Their work is published in the Journal of Constructional Steel Research.

The research team observed that welding residual stresses in the patch and base plates comprised of tension and compression respectively. This was attributed to the differences in the rise of temperature during welding due to the difference in the size of the base plate and patch plate. Additionally, high tensile residual stress almost similar to yield stress of the materials was generated along the weld line direction. Thermal elastic-plastic analysis taking into account the creep properties was used to simulate the welding and post-weld heat treatment processes. The welding residual stress and corresponding reduction by annealing were reproduced at high accuracy.

For the four-point bending fatigue experiment performed on the as-welded specimens using different loading patterns, the fatigue life of the specimens was significantly improved. For instance, the fatigue life nearly doubled when the tensile stress due to bending load was applied to the weld toe. The same trend was reported when compressive stress due to bending load was applied. However, the improvement in fatigue life affected by the residual stress was limited to the magnitude and stress pattern.

A close comparison was conducted to determine the possible difference between the experimental specimen and its simulation model with actual structural members. Even though the simulation is a promising one for investigating the post-weld heat treatment in actual structural members, the following needs to be taken into consideration: size differences between the experimental specimen and actual members, and the differences and effects of the boundary conditions which forms the basis of the future work.

Using the post-weld heat treatment in combination with induction heating, an improvement of the fatigue life of the patch-plate joints was reported. Interestingly, this approach is not only applicable to patch-plate welded joints but also other types of joints involving stress conditions. It is these good results that made Professor Mikihito Hirohata, the lead author, express his confidence in the role of the study in ensuring good health, functionality, and reliability of future steel bridges.