FATIGUE OF LARGE-SIZED LONGITUDINAL BUTT WELDS WITH PARTIAL PENETRATION

The Kobe-Naruto and Kojima-Sakaide routes of the Honshu-Shikoku bridges are designed to be of the combination railway and highway type. Consequently, fatigue due to live loads will pose a problem of grave importance. The allowable fatigue stresses employed in the designs of Honshu-Shikoku bridges have been determined on the basis of results of tests on various joint specimens as indicated in [1]. However, the configurations and dimensions of specimens in fatigue tests performed in the past were fairly restricted due to limitations in the capacities of testing machines, and considering the scales of the bridges, it is necessary to examine fatigue properties of various joints using larger specimens. Accordingly, the Honshu-Shikoku Bridge Authority has had a fatigue testing machine of the capacity of maximum dynamic load of 400 ton [4 x 106 N] made and is now carrying out fatigue tests of joints and parts of structures. In the present study, fatigue properties of the partially penetrated longitudinal welded members were examined experimentally using specimens of 800-N/mm2 class high tensile strength steel 45 mm in thickness. Plates 45-mm thick will generally be used in Honshu-Shikoku bridges. For railway bridges in Japan1) the allowable fatigue stress for this type of joint is that for the highest strength group, Category A: 15.3 kg/ mm2 (150 N/mm2) under pulsating stress. Adoption of the same value had been considered for the Honshu-Shikoku bridges2. However, in fatigue tests of panel point structures of trusses3>>4, fatigue cracks were initiated at corner welds of box section truss members after only a small number of repetitive stresses which were not more than the above-mentioned allowable stress. Consequently, for the Honshu-Shikoku bridges, this joint is classified today as Category B (12.75 kg/mm2 (125 N/mm2) under pulsating stress)5). Previous fatigue studies by the authors et. al. on common-size single-bevel groove weld specimens have clarified that the reduction in fatigue strength due to existence of welding residual stress is very substantial6>>'> and that fatigue cracks are initiated from bottom surfaces of weld metal at extremely early stages of the lives'. Studies by Ito et al.'s on the same kind of joint suggested that fatigue strength of this joint decreases as the size of the specimen is enlarged, and therefore, it is important for prevention of fatigue failure of steel members that the fatigue properties of this joint indicated by small-size specimens be confirmed with large specimens.