Research On The Structural Non-destructive Testing Of One-dimensional Structures Based On The Guided Waves

The guided wave nondestructive testing (NDT) is a novel NDT method which adopts the ultrasonic guided waves propagating along the structure over a long distance. Benefiting from the characteristics of the guided waves of low attenuation in propagation and high sensitivity to cracks, the guided wave method is very useful in the applications of structural health monitoring, e.g. for rails, oil and gas pipelines and other long-range structures. The main problems that restrict the application of the guided wave NDT are as follows:(1) The popular crack location method, i.e. the pulse-echo method, should use the guided waves with low frequency dispersion, which restricts the use of long-distance, crack-sensitive guided waves. (2) The existing guided wave methods cannot identify effectively the cracks according to their dynamic characteristics. (3) The effective model, which can be employed to describe both the propagating of guided waves and their interactions with the impedance discontinuities, does not exist.In order to solve these problems, a finite element model, which use analytical boundaries to reduce the calculation consumption, was established to simulate the waves propagating in 1-d structures for dozens of meters. Based on the FE model, the fluctuation characteristics, e.g. dispersive characteristics, attenuation, group velocity, of a beam are extracted and applied to choosing the type of the guided waves in NDT.By using the long-range guided wave analysis model, a crack locating method, considered as an improved pulse-echo method, was developed. This model adopts the time-frequency transform to ensure a large detection range and high locating accuracy. This method is suitable to develop an automatic crack locating strategy to find and locate all the structure discontinuities with proper signal progressing algorithm. Moreover, a practical strategy of crack locating was developed. In this method, the wave parameters are measured and used to replace the calculated ones to obtain more accurate locating results. In addition, in order to drop the hardware cost of the guided wave NDT, a novel method which adopts single-frequency signal guided waves was proposed. Simulation shows that this method can achieve the same accuracy as that of the pulse-echo method.By studying the interactions between the propagating waves and impedance discontinuities, a discontinuity identifying method was proposed. Firstly, the reflection and transmission properties near an impedance discontinuity were investigated. Then the reflection and transmission coefficients of an attached mass was derived for an instance. As a result, an analytical method was provided to identify the quantity of the mass through the reflection coefficients. Furthermore, as the identifying results from the analytical method are sensitive to the wave modal and detection noise, this thesis proposed an identification method which can use the energy of the reflection and transmission coefficients to distinguish discontinuities. In addition, research showed that the flexural waves are more sensitive to cracks than the longitudinal waves.In order to validate the wave analysis model and the locating method, experiments were carried out on a uniform beam to measure the wave number and locate a crack with 50% depth ratio. Moreover, experiments on a rail show that the discontinuities on a rail can be accurately located using the improved pulse-echo method. And the locating error is less than 1%.This thesis provides an analysis model which can study the propagating properties and the interaction between the guided waves and the impedance discontinuities. A novel locating and identifying method based on the guided wave is present to evaluate the structural health effectively for one-dimensional structures. Experiments show that the NDT strategy is capable of discontinuity detection on beams and rails.