PSI - Issue 68

Akito Tabata et al. / Procedia Structural Integrity 68 (2025) 415–419 A. Tabata et al. / Structural Integrity Procedia 00 (2025) 000–000

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(crack through weld bead) have been reported in the rib-to-deck welded joints in OSDs. Crack through deck plate thickness may cause road surface cave-in by penetrating the deck plate. Crack through weld bead penetrates the weld bead and propagates to the deck plate and rib, which may damage the performance of the longitudinal rib. In either case, there is an urgent need to take measures to ensure the safe passage of moving vehicles [M.hattori et al.(2021)]. As countermeasures in Japan, the specifications for highway bridges [Japan Road Association(2012)] raised the minimum plate thickness of deck plate with U-rib from 12 mm to 16 mm, and the specifications for highway bridges and the Guideline for Fatigue Design of Steel Road Bridges [Japan Road Association(2002)] stipulated that the penetration depth in the through-thickness direction of a closed section rib should be 75% of a closed section rib thickness or more. On the other hand, there is a possibility that the occurrence of crack through deck plate thickness cannot be prevented by increasing the thickness of the deck plate [Japan Society of Civil Engineers (2010)]. In addition, since a crack is initiated from an invisible root part, it is difficult to specify and detect its position, and it is one of the cracks that is difficult to repair in terms of management. Therefore, it is desirable to suppress the crack initiation itself. In this situation, to control the metallographic structure, steel materials have been developed to resist fatigue crack initiation by strengthening the ferritic matrix through solid solution strengthening elements such as Si and Cu to the steel components. It is reported that the developed steel is effective even in the Heat Affected Zone (HAZ) of the joint [T.ito et al.(2023)], and it is expected to suppress the crack initiation of the rib-to-deck welded joints. From the above, in this study, we report the effect of improving the fatigue crack initiation life in the rib-to-deck welded joints using developed steel which shows high resistance to crack initiation. 2. Confirmation of crack initiation site 2.1. Experimental procedure For the purpose of confirming the crack initiation position, through-deck plate crack reproduction experiments were conducted using OSDs partial models specimen assuming the vicinity of the transverse rib section. Figure 1 shows a sketch of the test specimen and loading point. The test specimen is composed of deck, U-rib, transverse rib, and longitudinal rib simulating OSDs. The deck plate is 16 mm thick and, the U-rib is 6 mm thick. The 490 MPa class steel plates with the chemical composition shown in Table 1 were prepared for the deck plate. The developed steel contains of a component to which an appropriate amount of solid solution strengthening elements such as Si and Cu is added. The joints of rib-to-deck were welded so that the penetration depth becomes 75% or more of the rib thickness. Fatigue tests were carried out at a load range was set as 15kN or 16kN, in which the maximum loads are set as 20kN.

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Table 1. Chemical composition of deck plate (mass%) C Mn Cu+Si +Ni+Cr Others Conventional 0.16 0.98 0.20 P, S Developed 0.05 1.56 0.97 P, S, Nb, Ti

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2.2. Results In this study, shown in Figure 2, the number of cycles when the strain fluctuation range measured near the U-rib decreased by 5% from the initial value was defined as crack initiation life N 5%. Figure 3 shows magnified photographs of the crack initiation site and fatigue crack initiation life of conventional steel and developed steel. The fatigue crack