Very Low Cycle Fatigue Behaviour of Welded and Deliberately Notched I-Girder

Abstract

Recently, architectural structure is occasionally designed to resist the large earthquake load in such a way that yielding of material would be permitted to occur locally in structural members. In such case, structural members and joints are required sufficient load carrying. capacity and ductility, especially to resist against cyclic loads.
In this report, basic experiments have been carried out to investigate deformation and fracture behaviours of welded I-girder with notches in very low cycle regime.
Specimens used are simple beam type that is a model of column-beam welded joints in framed steel steel structure. The butt welded joint of flange in the specimen contains notches and loacates near the center of beam span. The weld metal has high fracture toughness at room temperature (Critical COD δc=1.25 mm at 0°C).
Tests were carried out under conditions that cyclic load were applied on the center of beam span, and reversed deflection at the loading point was controlled to be constant during fatigue.
Under cyclic loading, ductile crack initiates from the tip of initial notches and propagates stably because of high-toughness weld metal. After ductile crack propagated, the load-deflection curve had negative slope in tension load range. This phenomenon was called failure. As deflection amplitude was small and initial notch length was short, number of cycles to failure increased.
Crack propagation area per one cycle (ΔA) versus COD(δ) relation was given by
ΔA=50δ3.
δis crack opening displacement at the crack tip of previous cycle. This empirical equation gave the upper limit for experiment results. The application would be limited under experimental conditions.
Total crack length (initial notch length+propagated crack length) at failure was decreasing linearly, as controlled deflection amplitude was increased.