Steel　structures　have　been　widely　used　in　constructions　due　to　their　advantages　of　lightweight,　high　strength,　short　construction　time　and　high　recycling　and　reuse　potential.　Fracture　failure　in　steel　structures　should　be　prevented　to　avoid　collapse　of　the　whole　structures.　Micromechanical　fracture　models　can　capture　the　fracture　initiation　mechanisms　and　therefore　can　be　used　to　predict　ductile　fractures　in　steel.　Twelve　smooth　round　bars　were　carried　out　to　obtain　the　material　properties　and　36　notched　round　bars　were　tested　to　calibrate　the　parameters　of　stress　modified　critical　strain　(SMCS)　model　and　the　void　growth　model　(VGM)　for　structural　steels　(　Q235B　and　Q345B)　and　the　corresponding　welds.　Specimens　were　extracted　from　the　base　metal,　the　weld　metal　and　the　heat　affected　zone　(HAZ)　to　investigate　fracture　behaviour　in　different　parts　of　the　welded　joint.　Scanning　electron　microscope　(SEM)　measurements　were　taken　and　finite　element　models　were　developed　to　calibrate　the　models.　The　test　results　and　calibrated　parameters　are　reported.　Moreover,　the　calibrated　models　are　applied　to　analyses　the　fracture　behaviour　of　welded　joints　and　their　accuracy　are　validated.　The　calibrated　and　validated　models　can　be　used　for　further　numerical　fracture　analysis　in　welded　steel　structures.　Copyright　(C)　2021　by　The　Hong　Kong　Institute　of　Steel　Construction.　All　rights　reserved.