Crack Initiation Strength of Structural Steel Members Having Experienced Large Compressive Strain due to Buckling Deformation (Fifth Report)

Abstract

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 welded structural steel members subjected to cyclic bending load after undergoing buckling collapse. For this purpose, crack initiation tests using fillet welded specimens under cyclic bending load were performed with JIS SM 490 C 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 fracture were examined. Elastoplastic large deformation FEM analyses based on the combined hardening law 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 fillet weld toe on the center of width of the compressive side. And cracks propagated along the fillet weld toe toward the width direction.(2) Relationships between longitudinal strain range of crack initiation site at first cycle and number of cycles to fracture can be expressed by linear functions on semilogarithmic scale.(4) New equations has been developed in order to predict the fracture strength of the welded structural steel member under cyclic bending load after undergoing buckling collapse.