繰返し荷重下での構造用鋼の延性き裂発生クライテリオン―大変形繰返し荷重下での溶接鋼構造部材の延性き裂発生限界評価（第１報）―

Abstract

The evaluation of ductile crack initiation in steel welded structures under seismic loading is crucial for structural design or safety assessment to prevent ductile cracking induced brittle fracture. The main purpose of this study is to propose a transferable criterion for evaluation of ductile cracking in welded structures under large scale cyclic loading from small scale tensile test results. On the basis of the observation of ductile crack initiation behavior of round-bar specimen with/without circumferential notches tested in single tension, the main controlling factor for ductile cracking in two-phase steel used was found to be a nucleation of micro-voids in soft phase (Ferrite phase) near Ferrite-Pearlite interface after a large extent of plastic straining. The material damage concept under reverse loading which correlates the material damage for micro-voids nucleation to macro-scale mechanical parameters was proposed by taking into account two aspects of Bauschinger effect : (a)mechanical aspect which influences deformation and stress/strain behaviors, (b) material damage aspect caused by dislocation behavior. A new criterion for ductile cracking of structural members under cyclic loading was proposed on the basis of that proposed effective damage concept and “two-parameter criterion” which can be applied to the steel structures under increasing load in single direction. The validity of the advanced two-parameter criterion was verified by conducting cyclic loading tests in axial direction for round-bar specimen and cyclic 3-point bending tests for cross-shaped specimens. Consequently, the advanced two-parameter ductile cracking criterion was found to be a transferable criterion for the evaluation of critical loading cycle of structural members from small scale tensile test results.