Investigations On Fracture Behaviours Of High Strength Steel Materials And Connections Based On Micromechanical Models

High strength steel(HSS) has been successfully applied in practical steelstructures as its prominent superiorities. However, itsapplications are restricted as the issues of brittle fracture at low temperature, high yield ratio, low ductility and welding, etc. Based on the fracture mechanism of steel, a large amount of experimental investigations and numerical analysis were carried out to study the fracture behavior of HSS structures through investigations of beam-to-column connection, local connection details and materials. The main research works and results covered in this dissertation are as follows:(1) A seriesof tests(such as uniaxial tensile test, Charpy impact test and three-point bending test)were carried out at low temperature to investigate the mechanical properties andtoughness of HSS and its butt welded joint, fracture micro-mechanisms wereanalyzed by electron microscope scanning(SEM) as well. Totally 120 specimens were tested, the effect of steel strength on the materials fracture behavior were analyzed, rich experimental data were collected so as to provide reference for thefracture resistant design of HSS structures in cold regions.(2) On the basis of micromechanism,micromechanical models were proposed to predict fracture initiation of HSS and its corresponding weld. 54 circumferentially notched round bars and 12 smooth round bars were tested under monotonic and high-strain low-cycle(HSLC) fatigue loads. Constitutive modelsfor 460 MPa HSS and weld materials under monotonic tensile loading and cyclic loading were established. The toughness parameters of the micromechanical models were calibrated, and the material damage degradation parameters were calibrated by HSLC fatigue tests.The results indicate that the toughness parameters are not related to the yield ratio but associated with material plasticity.(3) 20 typical HSS cruciform welded joints which represent the local weld region of practical beam-to-column connections were tested under monotonic and cyclic loads to investigate the fracture behavior, two weld types(such as complete penetration butt welded joint and fillet welded joint) were adopted. The effects of weld type, load type and steel strength on the fracture behavior of HSS welded connections were analyzed.(4) Four beam-to-column connections of HSS moment resisting frame were tested under cyclic loads to simulate the earthquake induced loads; four different connection details were included. The influences of the welding technology details and welding access holes shape on the HSLC fracture behavior were analyzed; the fracture behavior related indices were obtained as well.(5) Based on the tests of typical HSS cruciform welded joints and beam-to-column connections of HSS moment resisting frames, refine finite element models for these connections were established. Multi-scale models which integrated the micromechanical model was used to predict the tension fracture and HSLC fatigue fracture of welded connectionsin HSS structures subjected to strong earthquakes. Subsequently the applicability and accuracy of the micromechanical model for predicting HSS connections fracture were validated by test results, and a general effective methodology to predict HSLC fatigue fracture of HSS beam-to-column connections under seismic loadings was provided.