| Steel materials are extensively used in the oil-gas pipeline and pressure vessel.Within the long-term uses of these equipments,there will appear pit and crack defects on the steel surface due to the oxidation,corrosion and external force,easily inducing the safety accidents,environmental pollution,and even casualties.Therefore,there is a necessity to make regular tests for the steel used in the areas mentioned above.The purpose of this research is to develop a magnetic flux leakage(MFL)detecting-based non-destructive testing and evaluation system for the steel material,which can realize the detection and assessment of the pit defect.The following work had been carried out:(1)A new method was proposed to build up a magnetization system which only needs a single permanent magnet.Depending on the ANSOFT MAXWELL software,a finite element analysis platform,the influential rules regarding the main geometries of the single magnet versus the magnetization effect were explored,as well as the influence of the liftoff value on the detecting effect of the magnetic flux leakage,and then a series of suitable geometric parameters for the signal permanent magnet,and an optimal liftoff value for the sensor were determined.(2)A small trolley was designed and fabricated to carry the magnetization system self-developed,on which a special wheel to record the travel distance of the trolley was used.The wheel was also able to help locate the position of the defect.Another useful equipment was designed to drive the trolley with the different speeds,and it was based on a step motor and a set of ball screws,notably,the device was conducive to realizing the control of the tesing point density within the space upon the concave pit defect.(3)High-performance ARM single-chip STM32 and a new type of 3-D magnetic sensor were combined to develop an embedded system to measure the MFL signal and realize the data storage and Blue Tooth-based wireless delivery.In the meanwhile,special software was developed to receive,manage,and analyze the MFL data from the embedded system.Moreover,a series of ellipse concave pit defects with different sizes were fabricated on the Q235 steel plate(10mm in thickness)by NC machine.Thus,an experimental platform was so far set up.(4)Based on the established magnetic flux leakage testing experimental platform,a series of trials were carried out,then the signal characteristic analysis for the MFL signal was performed.In this investigation,the closed area formed by the MFL signal and axes was viewed as a geometric graphics,and several geometric characteristics in the graphics were extracted and used to characterize the concave pit defect,thus a new method was novelly proposed,laying the essential basis for the geometric parameter prediction and non-destructive evaluation of the concave pit defect.(5)Through combining the support vector machine(SVM)regression method with the geometric characteristics of 3-D MFL signals,the prediction models for the macro axis,minor axis and depth of the ellipse pit defect were developed.Testing results of the cross-validation show that the method,with respect to utilizing the single permanent magnet to construct the magnetization system,and combining the geometric features of the 3D MFL signals and the SVM method to realize the non-destructive evaluation of the concave defect,processes a good prediction performance and it is feasible and effective.Compared with the traditional double permanent magnet-based magnetization system,the newly proposed system possesses a more simple structure,a better design flexibility,and a lower cost,exhibiting a wide application prospects. |
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