Study On Surface Characteristics And Mechanical Properties Of Corroded H Steel Beam Under Coupling Action Of Load And Underground Environment

The underground corrosive environment is mainly caused by sulfate.The serious corrosion of steel components such as steel beam and steel column will threaten the engineering safety severely in underground environment.In this paper,the corrosion ratio,surface feature,bearing capacity and reliability of the H type steel beam under the coupling of load and sulfate attack is systematic studied through method of combination of physical test,numerical simulation and theoretical analysis.Pitting corrosion is a common corrosion mode.Corrosion pits of different sizes are appeared on steel surface after corrosion.The highly irregularity of corrosive steel surface will bring difficult to the measurement of corrosion pit size,minimum cross section area and so on.Therefore,this paper first produces elliptic blind holes of different sizes by drilling and milling methods to simulate corrosion pits,then effects of corrosion pits size,depth,width,distribution and other factors to the Q235 steel mechanical properties(nominal strength,nominal ultimate strength and elongation)were analyzed.The influence of those factors to the load-displacement curve of Q235 steel under tension was also studied.The results show that,the depth and distribution of corrosion pits affect the nominal yield strength and elongation obviously.The ultimate strength of steel with corrosion pits is relation to its minimum cross section area.Corrosion pits have great impact on load-displacement curve of Q235 steel.The deeper of the corrosion pits,the denser of the corrosion pits distribution leads to the more serious degradation of the yield plateau.In addition to this,the fracture factor is defined and the fracture position of Q235 steel with corrosion pits is analyzed.It turned out that the fracture is occurred at the section with larger fracture factor.Steel plate modes with one corrosion pit and multi corrosion pits of different sizes are established through ABAQUS.The influence law of corrosion pits depth,width,distance between corrosion pits,angle between corrosion pits and other factors to the maximum equivalent stress,the maximum plastic strain and stress triaxiality of corrosion pits is studied.And the first break point of the steel plate with corrosion pits was analyzed.Results indicate that the equivalent stress of the corrosion pits bottom and the middle lateral surface of corrosion pits which perpendicular to load direction is larger than the rest of the part through the whole loading process.The elements of corrosion pits bottom are failure at first,then it will extend transversely which leads to the fracture of steel plate.The equivalent stress of corrosion pits bottom increase linearly as time grows before yield.And it will turn to nonlinear increase after yield.After necking down,the equivalent stress of corrosion pits keeps increasing due to its triaxial stress state.The equivalent plastic strain of corrosion pits bottom increase linearly as time grows before necking down.And it will turn to nonlinear increase after necking down.The larger the stress triaxiality of corrosion pits bottom,the smaller the equivalent plastic strain is when steel plate fracture.The ductile fracture model of steel with corrosion pits is obtained through analyzing the effect of the curvature of corrosion pits bottom,distance and other factors to stress triaxiality and improving the existing ductile fracture model of metal.The development process of corrosive steel surface morphology is analyzed through comparing the corrosion of Q235 steel under sulfate environment with different load ratio.The quantitative characterization is conducted by using differential box counting methods in fractal theory.The results show that the development of surface morphology contains three stages no matter it is stressed part or non-stressed part: initial corrosion stage,middle corrosion stage and final corrosion stage.Local pitting corrosion appeared in initial stage,uniform corrosion exists with local pitting corrosion in middle corrosion stage and the final corrosion is mainly uniform corrosion.The load accelerates the development of the first two stages.The surface morphology of both stressed part and non-stressed part has fractal feature.Differential box counting can describe the roughness level of the corrosive steel surface.Through statistical analysis,the remaining thickness of steel plate after corrosion obeys.Hence,the normal distribution model for the remaining thickness of corrosive steel is established.The corrosion ratio of Q235 steel is represented by using weight loss method.The effect of different load ratio to Q235 corrosion ratio is studied.The result shows that the corrosion ratio increase nonlinearly with stress grows.The corrosion ratio calculation model for Q235 steel is deduced under loading condition.The fracture feature and mechanical properties such as ultimate load and elongation under the coupling effect of load and sulfate attack are obtained through uniaxial tension test.The effects of corrosion ratio and load ratio to corrosive steel fracture feature and other mechanical properties are analyzed.It turns out that the corrosive test pieces are of four fracture features: flat fracture feature,oblique fracture feature,stepped fracture feature and multiple fracture feature.The occurrence rate of stepped fracture feature and multiple fracture feature increase with the augment of corrosion degree.The load displacement curve of corrosive steel changed obviously as compare to non-corrosive steel.The yield plateau of corrosive steel become shortage or even disappear.And the necking down stage shows the same phenomenon as well.The nominal ultimate strength is decrease linearly with the augment of corrosive ratio.The exist of load ratio would accelerate the reducing of nominal ultimate strength for corrosive steel.The nominal yield strength of corrosive steel increase linearly with the growth of its corrosion ratio.The elongation decreases nonlinearly as corrosion ratio increase.Meanwhile,the relationship between corrosion ratio,load ratio and mechanical properties are studied.The empirical mechanical properties calculation formula for corrosive Q235 steel is established through quadratic regression analysis.The corrosive H type steel beams with different load ratio are obtained under sulfate attack and sustained load.The failure mode and bearing capacity of H type steel beam with different load ratio and corrosion ratio are analyzed through four-point bending test.The result shows that the local buckling of flanges occurs around loading points and the whole steel beam does not lose its overall stability.The ultimate bearing capacity and mid-span deflection decrease with the growth of corrosion ratio and load ratio.The load-displacement curve of corrosive steel beam can be divided into three stages which are linearly rising stage,approximately parallel stage and transitional stage.The slop of linearly rising stage decreases with growing corrosion ratio and load ratio.The slop of approximately parallel stage shows the same trend as that of linearly rising stage.The load-carrying properties of local corrosive steel beam is analyzed by using ABAQUS.The results indicate that the failure mode of steel beam is changed with the corrosion of flanges and web at pure bending section or shear span section.The corrosion of flanges and webs will lead to the bearing capacity decline no matter its position.The reducing magnitude of bearing capacity caused by corrosion at shear span section is larger than that at pure bending section.The empirical formula and the theoretical formula of the corrosive steel H type beam are obtained by neural network method and analytical method.The average section method and the method of average section considering stress are respectively used to deduce computational formula of bearing capability of corrosive H steel beam.The results show that the computational results getting from those two methods are consistent with test results.Finally,the relationship between the reliability of corroded H type steel beams and the corrosion ratio is analyzed.It is found that the reliability of corroded H type steel beam decreases nonlinearly with the increase of corrosion ratio,and the decreasing ratio of reliability becomes slowly.