Post-fire Mechanical Properties And Constitutive Study Of Q690D High Strength Steel

Steel whose nominal yield strength beyond 460 MPa is defined as high strength steel.Various advantages,such as high strength,well ductility and low price,entitle it keeps a lion’s share in engineering application.During employment,however,structures are easily suffered from wind load,atmosphere corrosion and fire hazard,degenerating mechanical properties of structure members and causing potential collapse.On the other hand,after suffering from accidential disasters,like earthquake or fire hazard,for a course of time,structure members,with unconspicious distortion,are still reliable;and dismantling them will waste money and workforce.Thus,considering the facts that the figure for fire events is increasing by year over the past 10 years,in this study we focused on post-fire mechanical properties of Q690 D high strength steel,the problems we have resolved are listed as follows:(1)Mingled with test data,this work studied the effects of high temperature and cooling methods on residual mechanical properties of Q690 D high strength steels,and proposed the corresponding predictive models with different reliability.First,specimens were heated to target temperatures and then cooled down through air,water and foam extinguisher cooling methods,respectively.Static tensile tests were then conducted on the specimens afterwards to determine their residual mechanical properties.After this,residual mechanical properties of heated specimens were compared with the as-received specimens to reveal the degradation to the cooling methods and the temperature.Later,true stress-strain curves were established to compare the difference between true yield stress and ultimate stress with engineering yield stress and ultimate stress.Futher,the data was compared with the existed studies to observe the differences of post-fire mechanical properties among Q234,Q345,Q420 Q690 and Q690 D steels.Finally,we determined the probability distribution of the residual mechanical properties of Q690 D steels,and thereby the predictive models with different reliability levels were proposed.(2)This work studied the influence of macro-characteristics,micro-characteristics and metallographic characteristics on post-fire mechanical properties of Q690 D steels.First,we observed the vatirty of macro-characteristics on the fracture surfaces to target temperatures and cooling methods.Then,with the help of SEM(scanning electron micrographs)details,we revealed the patterns of micro-characteristics toward target temperatures and cooling methods.Finally,we studied the optical micrographs and discussed the change of metallographic characteristics of specimens under different temperatures and cooling methods,showing the effect of heat tresatment on macro mechanical properties of Q690 D steels.(3)This study imoroved the existed monotonous tensile constitutive model and proposed a full-range tensile constitutive model for high strength steels.Firstly,we summarized the existed studies over the past 20 years and contrasted the features among these studies.Ather that,based on these features,the existed constitutive model was modified to suit the current data better.Finally,based on the results from existed studies,this work deduced the constitutive model for necking stage;combined with the improved model,we proposed a fullstage constitutive model for high strength steel.