It is known that compressive plastic strain deteriorates the ductility and fracture toughness of steels. So when buckling collapse takes place in structural steel members under cyclic bending load, cracks may easily initiate and propagate at the concave side (compressive side) of buckling deflection.
In this paper, an attempt was made to quantify the crack initiation strength of structural steel members subjected to cyclic bending load after undergoing buckling collapse. For this purpose, crack initiation tests using strip specimens under cyclic bending load were performed with JIS SM 490C steel plate. In these tests, specimens were given various initial compressive displacement amplitude and cyclic displacement amplitude after undergoing buckling, and number of cycles to crack initiation and to fracture were examined. Elastoplastic large deformation FEM analyses were also performed for all loading cases to investigate stress and strain at crack initiation sites.
As results of investigations described above, the following conclusions were obtained.
(1) According to the crack initiation test, cracks initiated at the center of width on the compressive side and at the edge on the tensile side. Finally all specimens were fractured due to propagation of these cracks from compressive side.
(2) Stress-strain relationship under cyclic large plastic strain range can be expressed with combined hardening law.
(3) Relationships between longitudinal strain range of crack initiation site at first cycle and number of cycles to crack initiation and to fracture can be expressed by linear functions on semilogarithmic scale.
(4) New equations has been developed in order to predict crack initiation strength and fracture strength of the structural steel member under cyclic bending load after undergoing buckling collapse.