The Study Of Failure Mode And Fracture Prediction For Beam-to-column Connections Under Ultra-low Cycle Fatigue Loads

Abroad research for fracture failure of steel frame structures was caused in engineering filed by the site investigations after 1994 Northridge earthquake in American and 1995 Kobe earthquake in Japan.The failure mode of steel connections is attributed to the ultra-low cycle fatigue(ULCF)failure in the stress concentration region.The rotation capacity and ductility design of steel connections can be evaluated efficiently by accurate prediction of ULCF life for steel connections.Meanwhile,it is an important reference index for the improvement measure of the connections' seismic design.It follows that there is a great research value for the ULCF failure prediction of steel connections.This paper was based on a relatively accurate ULCF prediction model determined by comparison among three models.And a study of failure mode and fracture prediction for beam-to-column connections was conducted under ultra-low cycle fatigue loads.In this paper,Micromechanical models used to predict ULCF lifetime of steel connections were sorted,which are CVGM,improved CVGM(ICVGM),and ductile crack length calculation(DCLC)formula.The 3 fracture criteria were employed for the prediction of the ULCF lifetime for 2 T-shaped sectional specimens from the bottom flange and a portion of the connection region of beam-to-column connections.The predicted results were compared with the experimental results to verify the applicability of above three criteria for the ULCF lifetime prediction of steel connections and a relatively accurate prediction model was determined.In addition,the finite element models were built by solid element and shell element respectively,in order to prove that shell element is available.The most effective model ICVGM is employed for the ULCF failure prediction of artificial cracks in local region specimens of beam-to-column connections,where the vertical unfused interface between the backing bar and column flange or beam flange acts as an open crack.The artificial crack region with the highest fracture possibility was estimated to study the failure mode of beam-to-column connection under severe earthquake.Set the ICVGM as a criterion,existed kiln tail beam-to-column connection as a research object.And the effect of access hole configurations to the kiln tail connection's ULCF performance was analyzed.Finally,the failure mode of kiln tail connection was studied.And its anti-seismic behavior and ULCF performance were analyzed to determine whether the connection satisfied the requirement of ductility design.The analysis results showed that,the ULCF prediction from ICVGM was relatively accurate.As for the failure modes of beam-to-column connections,the main reason is the stress concentration caused by the vertical unfused interface between the backing bar and column flange acting as an artificial crack and the connecting region between the access hole and the beam flange.And it can be avoided for this kind of ULCF failure modes to place a continuous fillet weld along the backing bar and to improve the configurations of the access hole.In terms of the application of ULCF criterion in engineering filed,the ductility coefficient of the studied kiln tail beam-to-column connection is greater than 3,the total angle of rotation is greater than 5% rad and the plastic angle of rotation is greater than 3%,which agrees well with the requirement of ductility design.The anti-seismic performance of kiln tail connection behaves well under severe earthquake.