Master S-N Curve Approach For Low-cycle Fatigue Life Prediction Of Welded Beam-to-column Connection In Steel Moment Resisting Frames

There existed multitudes of post–earthquake brittle fracture cracks of beam–to–column connections(BCCs)in steel moment resisting frames(SMRFs)after Northridge Earthquake in 1994 and Kobe Earthquake in 1995,which resulted from high–level stress fatigue loading born by beam–to–column connections subjected to variable alternating earthquake–excited fatigue loading,then there emerge irreversible plastic strains to bring about brittle fractures on BCCs.Hence,a large number of studies were comducted via academicians at abroad and home for BCCs.However,the present methods studying fatigue resisting capacity have been confined to specified fields corresponding to different research means.Nominal stress method failed to take stress concentration effects of welded components;Structural hotspot stress method begets severe mesh sensitivity based on stress concentration effects,but structural stress proposed by Dong is mesh-insensitive,then it's structural stress and fracture mechnics that were utilized to import generalized equivalent structural stress parameters comprising two–stage crack propagation modes incorporating short cracks and long cracks to get fatigue life evaluation method considering Master S–N Curve that synthetically accounts for the case that stress concentration,loading modes and component thickness have an effect on fatigue properties,S–N data of all welded components comply with the identical Master S–N Curve which confines to welded structures or welded components.Fatigue life analyses of welded BCCs in SMRFs employed Master S–N Curve in combination with multiscale modeling.Firstly,multiscale models of BCCs in SMRFs,efficiently constituted between discriminant element types(Beam-Solid model,Beam-Shell-Solid model,Beam-Solid)get formulated,it's the mesh–insensitivity structural stress approach that eliminates or alleviates mesh–sensitivity on FE solution of the same multiscale model under different mesh–sizes;then,equivalent structural stress and fatigue life of different weld toe lines were analyzed according to mesh–insensitivity structural stress approach of BCCs;the pseudo–static analysis of BCCs was implemented under control of reversal loading of displacement–controlled,so Master S–N Curve approach was employed to discuss high–cycle elastic–stress fatigue and low–cycle plastic–strain fatigue of weld toes on BCC subjected to different loading modes;finally after solid BCCs get embedded into SMRFs,SMRF multiscale model with solid BCCs was deformed to evaluate seismic low–cycle fatigue life referring to Master S–N Curve approach,and fatigue life corresponding to welded residual stress was debated for welded components.The aforementioned analyses manifest multiscale models of BCCs in SMRFs both decrease computational costs and ensure calculation accuracy of crucial positions(weld domain);which balance costs and accaracy.Structural stress,equivalent structural stress and fatigue life immune to meshsize effects indeed present mesh–insensitive,Matser S–N Curve can be applied to fatigue life estimation results that keep stable;low–cycle fatigue life of welded BCC approaches seismic response from low-cycle to high-cycle fatigue under different loading modes.Finally Matser S–N Curve test data have comprised welding residual stress which needn't be taken into account when employing Matser S–N Curve for fatigue life.Matser S–N Curve fatigue model provides accurate reference for fatigue resisting design of BCCs or other welded structures.