Identification And Three Dimensional Profile Reconstruction Of Crack Colony In Pipeline

Influenced by transmission media,soil movement,pipeline coating failure,manufacturing defect and electromagnetic corrosion,cracks such as stress corrosion cracks frequently appear as a colony in the surface of pipeline.There are interaction and superposition between crack colony signals leading a shield to adjacent cracks.Thus,it is difficult for traditional nondestructive testing method to accurately identify and size crack colony,which will influence the accurate evaluation of pipeline residual strength and lead a sudden accident.Thus,knowing the length,depth,separation and distribution of crack colony is especially important to pipeline residual strength evaluation,inpection and maintenance plan.In this dissertation,the identification and the 3D(Three Dimension)profile reconstruction of crack colony are studied through the ACFM(Alternating Current Field Measurement)technique.Visual detection technique and system for crack colony in pipeline are developed,which provides important research foundation and tehchique for the national pipeline residual lifetime evaluation and pipeline reliable running.Main achievements can be summarised as:(1)Theoretical study on the detection of crack colony through ACFMBased on the model of the ACFM for the single crack detection,the model of the ACFM for crack colony is proposed and a numerical simulation model for crack colony detection through the ACFM is developed.The electromagnetic field distribution around crack colony and the magnetic field superposition are studied.The principles of the current perturbation and the magnetic field superposition are proposed.The relationships between the excitation frequency,specimen material,crack sizes and the influence area of crack are studied.Especially,the influence of crack separation,number and distribution on the magnetic signal of crack colony are studied.The detection principle of crack colony through the ACFM is proposed.Then,crack colony is classified as ‘alignment type',‘stagger type' and ‘leaving type' for crack sizing.(2)Development of ACFM array probe for crack colonyAiming at the problems of multiple parameters and dense distribution of crack colony,the structure and configuration of the ACFM probe is designed and optimized through numerical simulation method.Encircling Helmholtz coil array probe,bobbin Helmholtz coil array probe and high resolution array probe are proposed to measure the surface magnetic signal of crack colony providing a hardware foundation for crack colony identification and 3D profile reconstruction.(3)Study for crack colony surface identificationBased on machine vision technology,a method for crack colony surface identification is proposed to intelligently identify crack number,crack separation,crack length and distribution.Image enhancement technqie and filter technique of local regression method for weighted linear least squares algorithm,high pass filtering and median filtering etc.are used to improve the signal to nosie ratio and recognition capability of obtained image in industrial field.The local dynamical threshold algorithm is used to obtain the profile of crack tip region.The number of crack is obtained through calculating the number of connected domain.The position of crack is obtained through calcutaing the centre of crack tip region.Finally,the surface profile of crack colony is drawn.Based on the number and postion of crack colony,the type of crack colony is determined through the support vector machine algorithm.(4)Study for 3D profile reconstruction of crack colonyThere are interaction and superposition between crack signals in colony.To accurately size the depth of crack colony,the decomposing method for crack colony singal is proposed according to the inverse process of the magnetic field superposition.A neural network aggressive spacing mapping algorithm is proposed using the single crack generalized regression neural network model as a ‘coarse model' and the crack colony numerical model as a ‘fine model' to achieve accurate and fast depth sizing.Based on the surface profile and depth of crack colony,the 3D profile of crack colony is reconstructed achieving the visual detection of crack colony.(5)System development and experimental studyBased on the crack colony identification and 3D profile reconstruction algorithms,the software system for crack colony identification and 3D profile reconstruction is developed.Based on the multiplexing technique,multiplexing module for the high speed acquisition of sensor array is developed.Conbaining with the ACFM crack colony array probe and related hardware,the complete ACFM detection system is established.Through experiment test,the encircling and bobbin ACFM pipeline array probe can pick up pipeline surface magnetic signal.The minimum crack depth and crack colony identification separation for high resolution array probe are 0.8 mm and 3mm respectively.The accuracy of proposed algorithm for crack colony depth sizing is above 90%.