Tensile Test Of Steel Member With Crack Damage And Its Probability Analysis Method For Its Brittle Performance

Steel is generally regarded as a pure ductile material due to its light weight,high strength,good plasticity and good seismic performance,etc,and is widely used in engineering structures.However,a series of low-stress brittle failure accidents occurred in actual engineerings.Through a large number of accident analysis,researchers have found that the occurrence of brittle failure accidents is related to the existence of crack damage in the member.These crack damages may be inherent to the material itself,or may be produced during the process of material manufacturing and using.This finding shows that if the steel member has a certain length of crack damage,then the steel member with crack damage may exhibit brittle performance.However,for steel members with crack damage,there is still a lack of a simple method to analyze its brittle or ductile performance.Therefore,it is of very important research significance to carry out experimental research and theoretical analysis on steel components with crack damage.The mechanical analysis of steel members with crack damage can reveal that when the crack exists,the ultimate stress that the steel member with crack damage can withstand is the residual ultimate strength.The ultimate stress of the member can be obtained from the load-displacement curve of the tensile test,it can reflect the ductile or brittle performance of the material.In addition,the tensile test can also study the index of plastic properties of the material,such as the yield ratio and elongation after fracture of steel members with crack damage.Therefore,in order to study whether the steel member with crack damage presents brittle or ductile performance when there is a certain length of crack damage,and to make up for the lack of analysis methods for such problems,the tensile test research and theoretical analysis of steel members with crack damage are carried out.The main tasks and conclusions are as follows:(1)According to the test specifications,this paper designs crack-free test members and cracked test members of Q235 and Q345.The cracked test members are specially made by laser slitting to simulate the crack damage of steel members.This paper assumes that the residual ultimate strength of steel members with crack damage can be treated as the yield strength of the material,and the test results verify the validity of this assumption.Taking the ratio of the crack length to the width of the member as the relative crack length of the member,combined with the yield strength,tensile strength,elongation after fracture and load-displacement curve measured by the tensile test,the following conclusions are obtained:(1)With the increase of the relative crack length,the residual ultimate strength of the steel member with crack damage shows an approximately linear decrease.(2)The yield ratio reflects the safety,reliability and utilization of the steel.When the yield ratio is smaller,the ductile performance of the steel is better.On the contrary,the ductile performance becomes worse and the safety is reduced.However,the crack damage of the steel member will increase its yield ratio,which will reduce the safety of the steel member with crack damage.(3)The elongation after fracture is an index to measure the ductile performance of materials.With the increase of the relative crack length,the crack damage of the steel member will cause the elongation of the material to gradually decrease,which will lead to the decrease of the ductile deformation ability of the steel member,and the steel member with crack damage will show brittle performance.(4)When the relative crack length is small,the steel members with crack damage exhibits ductile performance;as the relative crack length increases,the material properties of the steel member will quickly change from ductile performance to brittle performance,some steel members with crack damage will show brittle performance at this time;when the relative crack length increases to a certain extent,the steel members with crack damage exhibit brittle performance.(2)Based on the residual ultimate strength,this paper theoretically proposes a relatively simple analysis method to calculate the probability of brittle performance of the member with crack damage.In this article,the proposed method is named as the R_b-R_d method based on residual ultimate strength.Among them,R_b represents the load-bearing capacity when the member with crack damage exhibits brittle performance,represents the load-bearing capacity when the member with crack damage exhibits ductile performance.For this method,there are the following two characteristics:(1)Assuming that both R_d and R_b are random variables,the ductile or brittleness performance of the member is measured by the size between the R_d and R_b of the member in this paper.And the probability of the member showing ductile performance is called the ductile conditional probability,which is expressed by the probability P{R_d<R_b};the probability of the member showing brittle performance is called the brittle conditional probability,which is expressed by the probability P{R_b<R_d}.(2)There is a certain correlation between R_d and R_b of members,and the Pearson linear correlation coefficient is very large.(3)The experimental results and the theoretical calculation results of the proposed method are compared and analyzed,the feasibility and validity of the R_b-R_d method based on the residual ultimate strength are verified.For the later reinforcement and repair of steel members or steel structures,this method has certain guiding significance.