Damage concept for evaluating ductile cracking of steel structure subjected to large-scale cyclic straining

Abstract

Evaluation of ductile crack initiation in steel welded structures subjected to seismic loading is crucial for structural design or safety assessment to prevent brittle fracture induced by ductile cracking. Observation of ductile crack initiation behavior of round-bar specimens with/without circumferential notches tested in single tension revealed that the main controlling factor for ductile cracking in the employed two-phase steel is not growth of voids induced by large inclusions, but nucleation of micro-voids in a soft phase (Ferrite phase) near the Ferrite–Pearlite interface after large-scale plastic straining. The material damage concept under reverse loading, which correlates the material damage for micro-void nucleation to macro-scale mechanical parameters, was proposed in consideration of two aspects of the Bauschinger effect: (a) a mechanical aspect which influences deformation and stress/strain behaviors in steel structures, (b) a 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 the proposed effective damage concept and ‘two-parameter criterion,’ which can be applied to the steel structures under increasing load in a single direction. The validity of the advanced two-parameter criterion was verified by subjecting round-bar specimens to cyclic loading tests along the axial direction and cross-shaped specimens to cyclic 3-point bending tests. Consequently, the advanced two-parameter ductile cracking criterion was found to be a transferable criterion for evaluation of critical loading cycle of structural members from small-scale tensile test results.