Experimental Study On Stress State Detection Of Steel Bars Based On The Self-Magnetic Flux Leakage Effect

In the bridge engineering,the traditional methods commonly used for stress detection of reinforced concrete structures can be divided into two categories:lossy detection method and non-destructive testing method,but they all have certain limitations.Based on the self-magnetic flux leakage effect of metal materials,the variation law of the self-magnetic flux leakage field on the bare steel bars and the outer concrete reinforcement bars during the tension process is studied.The basis for judging the stress state of the steel bars and the calculation of the tensile stress of the steel bars in the elastic phase are proposed.The method provides new ideas and methods for stress detection of steel bars in actual engineering.The main research contents and conclusions of this paper include:(1)The experimental study on the relationship between the self-magnetic flux leakage field strength of the bare steel bar and the outer concrete steel bar with the tensile stress was carried out.The results.show that at the initial stage of axial loading,the tangential component of the self-magnetic flux leakage signal on the bare steel bar surface increases with the increase of the tensile force.When the tensile stress increases near 65%to the yield strength of the steel bar,the self-magnetic flux leakage signal appears extreme large value,of which the first derivative curve shows significant peaks in both the elastic stage and the yielding stage.For the outer concrete steel bars,the tangential component of the self-magnetic flux leakage signal decreases with the increase of the tensile force at the initial stage of loading.When the steel stress is close to 90%yield strength,the maximum value point appears,and the first derivative curve of the self-magnetic flux leakage signal appears the peak value only during the elastic stage.(2)By studying the relationship between the self-magnetic flux leakage field value of the bare steel bar and the outer concrete steel bar under different tensions with the axial position of the steel bar,this paper proposes the "axial self-magnetic flux leakage field fluctuation logarithm value" AT to reflect the maximum change amount of self-magnetic flux leakage field value in the axial direction of the steel bar.The test results show that there is a linear relationship between AT and the axial tensile force T of the test piece in the linear elastic stage,and the slope varies with the diameter of the steel bar,the strength of the outer concrete,and the lift height of the magnetic sensor.The linear relationship can provide a basis for the detection and characterization of the stresses inside a specific reinforced concrete structure.For the outer concrete steel bar,this paper proposes the "area ratio deviation parameter" SD.During the strengthening stage of the specimen,there is a significant linear relationship between the SD and the axial tensile force T of the specimen.The slope of the linear segment of the SD-T diagram varies with the diameter of the outer concrete steel bar,the strength of the outer concrete,and the lift height of the magnetic sensor.(3)By studying the influence of the diameter on the axial tensile stress test results of the steel bar,the results show that as the diameter of the steel bar increases,the tensile force corresponds to the maximum value point on the tangential component of the self-magnetic flux leakage signal curve of the bare steel bar and the outer concrete steel bar in the elastic phase increases.At the same time,the ratio of the tensile force to the tensile force corresponding to the yield strength of the diameter steel bar also increases as the diameter of the steel bar increases.And the slopes of the linear segments of the AT-T diagram of the two types of test pieces all show an increasing trend with the increase of the diameter of the steel bars.The slope of the linear section of the SD-T diagram of the outer concrete steel bar decreases as the diameter of the internal steel bar increases.Based on the self-magnetic flux leakage effect of metal materials,the relationship between the self-magnetic flux leakage signal and the tensile force on the steel surface is studied.The method and criterion for judging the stress state of the steel bar and calculating the tension of the elastic and strengthing stage by using the self-magnetic flux leakage signal are obtained,which can provide new methods and ideas foe the non-destructive testing of tensile forces.The method has low cost,simple equipment and easy operation,and has good application prospects.