Structural Health Monitoring Of Corrosion Defect In Pipes Using Circumferential SH Wave

Occurrence of defects caused by corrosion in pipes is a serious problem in the oil and chemical industries.Therefore,efficient and reliable structural health monitoring(SHM)methods are called for monitoring corrosion defects in pipes.Over the last three decades,the ultrasonic guided-wave-based damage identification approach has demonstrated a potential for SHM of pipes.As conventional inspection is based on axisymmetric wave modes propagating in the axial direction of a pipe,it has a difficulty in detecting the circumferential location of a defect.The circumferential guided waves can be used to identify the circumferential location of a defect and quantitatively evaluate it.Among them,the circumferential fundamental shear horizontal wave(CSH₀)is of practical importance in SHM due to its virtually non-dispersive characteristics.However,so far,there have been limited CSH₀ wave transducers which can be used to constitute a SHM system for pipes.Moreover,the CSH₀ wave's capability of sizing the corrosion-like defect has not yet been confirmed by experiments.In this thesis,a double-mode piezoelectric transducer which can selectively excite pure CSH₀ wave and first order circumferential shear horizontal(CSH₁)wave was developed.Then,the reflection and transmission characteristics of the CSH₀ wave and CSH₁ wave from corrosion defects in pipes were investigated.The main results are listed as follows:(1)After analyzing the mechanism of exciting CSH₀ wave in pipes,the bidirectional SH wave piezoelectric transducer's performance on exciting and receiving CSH₀ wave in pipes were explored by both finite element simulations and experiments.Results show that the bidirectional transducer is capable of exciting pure CSH₀ mode traveling in both circumferential directions of a 1-mm thick steel pipe from 100 to 300 k Hz.Moreover,this transducer can also serve as a sensor to detect CSH₀ mode only by filtering circumferential Lamb waves over a wide frequency range from 100 to 450 k Hz.Unidirectional CSH₀ wave was then obtained by using two bidirectional transducers in a phased array form.(2)By analyzing the load conditions of exciting CSH₁ wave,a method based on the bidirectional SH wave piezoelectric transducers was then proposed to excite pure CSH₀mode and CSH₁ mode.Experiments show that pure CSH₀ wave and CSH₁ wave can be selectively excited over a wide frequency range in thick-walled pipes.(3)A method of sizing a rectangular notch defect by using CSH₀ wave was proposed.Experiments on an 11-mm thick steel pipe show that the depth and circumferential extent of a notch can be accurately determined by using the proposed method.Experiments were then performed to investigate the reflection and transmission characteristics of CSH₀ wave from notches with different depths and at different frequencies.It was found that the transmission coefficient of CSH₀ wave first decreases monotonically with frequency when the frequency range is below the first cutoff frequency and then keeps almost constant when the frequency range exceeds the cutoff frequency of CSH₁ mode.Moreover,the transmission coefficient of CSH₀ wave decreases with the increase of notch depth.(4)The reflection and transmission characteristics of CSH₀ wave and CSH₁ wave from elliptical defects were investigated by experiments.It is found that:a)the transmitted CSH₀wave is dominant at different defect depths while the reflected wave is very small;b)CSH₁wave is mainly reflected when the frequency-thickness product at defects is below the first cutoff frequency-thickness product,and the transmitted wave becomes dominant when the frequency-thickness product exceeds the the first cutoff frequency-thickness product;c)the transmission coefficients of CSH₀ wave and CSH₁ wave both decrease with the increase of the depth.