Investigation of Linear and Nonlinear Torsional Guided Waves in Hollow Circular Cylinders for Damage Detection

Abstract

Non-destructive testing plays an important role in structural health monitoring. One of the promising options is the use of guided wave for damage detection in engineering applications, such as pipeline and truss system. Common types of damage in the structures include cracks and corrosion. Guided wave is sensitive to cracks up to micro scale. Long range inspection is the other benefit of using guided wave. The overall aim of this thesis is to present a systematic investigation of guided wave in pipe-like structures to gain physical insights into linear and nonlinear features associated with torsional guided wave interaction with damage. This thesis includes a number of published and prepared journal papers under the same topic. The overview of linear and nonlinear guided wave, and guided wave mixing is introduced in Chapter 1. A computational model using one-dimensional time-domain spectral finite element with cracked element is presented in Chapter 2. This chapter mainly focuses on linear features of guided wave, such as scattering and mode conversion phenomena. The results show that the proposed cracked model has good agreement between the experimental results and three-dimensional (3D) finite element (FE) simulations. Nonlinear guided wave is highly sensitive to early stage of micro cracks. Material nonlinearity is one of the nonlinear phenomena in the presence of the micro cracks. It can induce higher-order harmonics of guided wave. Guided wave mixing is the advanced version of nonlinear guided wave since the generation of combinational harmonics at sum and difference frequencies can minimise the effect due to the nonlinearity generated by equipment. Chapter 3 analyses the nonlinear characteristics of two interacting fundamental torsional guided wave modes numerically and experimentally. Chapter 4 and Chapter 5 are the extension parts based on the work in Chapter 3 since pipe-like structures are commonly used in many circumstances, for example embedded pipes and pre-stressed hollow structures. Comprehensive studies in this thesis can gain more understanding for the real applications. In Chapter 4, a 3D FE embedded pipe model with the implementation of nonlinear strain energy function is established to simulate the energy leakage of guided wave propagation due to the existence of soil media. The use of 3D laser scanning system receives guided wave signals from the surface of the pipe for verification. Both numerical and experimental results indicate a significant decline in the interested harmonics at mixed frequency and single frequency. On the other hand, acoustoelastic effect is studied in Chapter 5. A series of case studies are carried out to observe the group velocity change with respect to different levels of loading. The nonlinear features, such as combinational harmonics and second harmonics, are also investigated numerically and experimentally.