Research On Reconstruction Methods And Analysis Of Defects In Pipelines Based On Ultrasonic Guided Waves

The pipeline structures are indispensable in the industrial transport and daily life.The inner and external surface of pipelines is eroded,which can lead to thinning defects and cavities,so there are potential security problems caused by broken pipelines.The traditional detection of pipelines is not effective or efficient,so a novel detecting scheme must be proposed to enhance detection efficiency and reconstruction precision,which can be applied to practical engineering conveniently.Currently,ultrasonic guided waves are usually used to complete the inspection of large structures,depending on the advantages of propagating in long distances and slow attenuation.At the same time,researchers utilize guided waves to detect pipelines.However,it is difficult to analyze the interaction mechanism of defects and guided waves in pipes,due to complicated modes and coupling properties.Therefore,flaws in pipes are still analyzed qualitatively.In order to solve these problems,a novel project that is used to reconstruct defects accurately and efficiently is proposed,which meets engineering requirements.This project consists of four parts:(1)By means of the theory elastic wave,solving the dispersion equation and fundamental solution of a semi-analytical form;(2)based on semi-analytical finite element method and traditional finite element method,calculating the scattered fields in pipelines;(3)according to reciprocity in elastodynamics,constructing boundary integral equations of defects and reconstructing axisymmetric defects and non-axisymmetric defects of pipeline structures;(4)denoising in the reconstruction by the wavelet analysis.In this thesis,five innovations are presented for accomplishing these projects as following:(1)the dispersion of guided waves in arbitrary curved surface is studied by the semi-analytical FEM;(2)the hybrid finite element method is established depending on semi-analytical finite element method and finite element method;(3)the function of defects is derived based on the boundary integral equation;(4)a new method,called as quantitative detection of Fourier transform(QDFT),is proposed,which is suitable to inspect surface defects of most of two-dimensional structures;(5)according to the wavelet denoising,a scheme is designed to process the noise of signals.To verify the rationality and practicality of these innovation points,many calculation simulations and theoretical proofs are shown in this thesis.