A study on development and application of high performance steel for bridge structures

本课题的研究目标是开发和应用桥梁结构用高性能钢材。1)大塑性应变引起的断裂韧性劣化：针对冷弯工艺或大地震，研究了大塑性应变引起的断裂韧性劣化。此外，通过脆性断口的观察和力学试验，讨论了几种钢的拉伸塑性应变和压缩塑性应变的影响差异。2)大输入热焊接的可能性：由于一些负面影响，如裂纹或性能劣化等，一般已禁止应用大输入热焊接。在本研究项目中，针对高性能钢材，通过进行大型焊接梁试件的CTOD试验，验证了大输入热源焊接的适用性。3)高强钢的疲劳性能：为了使钢桥结构合理化，对节点试件和大型梁试件的疲劳性能进行了探讨。本研究涉及的是从未用于实际桥梁的900 Mpa级钢材。4)地震作用下的低周疲劳行为：针对地震作用下钢桥墩的低周疲劳破坏，进行了低周疲劳试验，并研究了损伤积累机理。5)梁柱节点的脆性断裂：为了提高钢梁柱节点的韧性和延性，进行了循环加载试验，并进行了一些加固和改进技术的试验。在该试验中，可以从实验上模拟从焊接拐角开始的脆性断裂。结果表明，降低角筋拐角处的应变集中是防止脆性断裂的最有效措施。

In this research project, the objective is to develop and apply high performance steel for bridge structures. The followings were studied.1) Deterioration of fracture toughness due to large plastic strain: Concerning the cold-worked bending process or huge earthquakes, the deterioration of fracture toughness due to large plastic strain was studied. In addition, the differences of effect between tensile plastic strain and compressive plastic strains were discussed with some kinds of steels by observation of brittle fractured surface and mechanical testing.2) Possibility of large input heat welding: Because of some negative effects, such as cracks or deterioration of properties and so on, generally, it has been prohibited to, apply the large input heat welding. In this research project, concerning the high performance steels, the applicability of large input heat welding was tested with carrying out the CTOD testing of large scale welded girder specimens.3) Fatigue Performance of High Strength Steel: For rationalization of steel bridges, fatigue performance of high strength steels was discussed with joint and large-scale girder specimens. In this study 900 MPa class steel, which has never used for actual bridges, was dealt with. In addition, improvement methods of fatigue strength were also studied.4) Low Cyclic Fatigue Behavior under Earthquake: Concerning the low cycle fatigue failure of steel pier under earthquake, low cyclic fatigue testing was carried out, and damage accumulation mechanism was studied5) Brittle Fracture of Beam-Column Joint: In order to improve the toughness and ductility of steel beam-column joint, cyclic loading testing was carried out and some retrofitting and improving techniques were also tested. In that testing, brittle fracture which initiated from that welded corner could be simulated experimentally. As a result, decreasing the strain concentration at the corner with corner rib was the most effective for prevention of brittle fracture.