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

本研究课题的目标是开发和应用高性能桥梁结构用钢。主要研究内容如下：1）大塑性应变引起的断裂韧性劣化：针对冷加工弯曲过程或大地震，研究了大塑性应变引起的断裂韧性劣化。此外，通过脆性断口观察和力学性能测试，讨论了拉伸塑性应变和压缩塑性应变对几种钢的影响差异。2）大线能量焊接的可能性：由于大线能量焊接会产生裂纹或性能劣化等负面影响，一般情况下，大线能量焊接已被禁止。本研究针对高性能钢，通过对大型焊接梁的CTOD试验，验证了大线能量焊接的适用性。3）高强度钢的疲劳性能：为了合理设计钢桥，通过节点试件和大型梁试件，探讨了高强度钢的疲劳性能。在这项研究中，900 MPa级钢，从来没有使用的实际桥梁，处理。4）地震作用下的低周疲劳性能：针对钢桥墩在地震作用下的低周疲劳破坏，进行了低周疲劳试验，研究了损伤累积机理。5）梁柱节点的脆性断裂：为了提高钢框架梁柱节点的韧性和延性，进行了低周反复荷载试验，并对节点的加固和改进技术进行了试验研究。在该试验中，可以通过实验模拟从该焊接角部开始的脆性断裂。结果表明，采用角肋降低角部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.