Investigation of Lamb Wave Propagation in Pre-stressed Plates with Applications to Structural Health Monitoring

Abstract

The evaluation of applied, residual and thermally-induced stresses, along with the non-destructive detection of damage, plays a critical role in the assessment of the integrity and residual life of engineering structures. There is currently a growing demand from many industries to develop and incorporate on-line stress monitoring capabilities into existing as well as future Structural Health Monitoring (SHM) systems. These capabilities would allow for a significant improvement to failure risk management and life forecasting, which currently largely rely on stress values obtained at the design or testing stage. It is also very attractive from cost considerations to share the same network of sensors for both damage detection and stress evaluation. Guided wave based techniques are among the most promising options to meet these demands and expectations. Although SHM systems utilising guided waves are already in operation in a number of applications, the design, integration and implementation of in situ stress monitoring techniques are still under development. The overall aim of this thesis is to develop a better understanding of the phenomena associated with the propagation of Lamb waves in pre-stressed plates. Such an understanding is of paramount importance in the development of in situ stress monitoring procedures based on Lamb waves and the operation of existing and future guided wave based SHM systems. The specific objectives of this thesis include the extension of the classical theory of acoustoelasticity to guided waves propagating in pre-stressed plates, the development of a new experimental procedure for the evaluation of third-order elastic constants and the analysis of the residual noise due to the effect of applied or thermally-induced stress after baseline signal subtraction. In particular, it was demonstrated that this residual noise is not negligible and has a similar impact on the operation of SHM systems as ambient temperature variations. The main body of this thesis is comprised of a combination of ‘published’ and ‘accepted for publication’ journal articles. These articles are united by the same framework, the theory of acoustoelasticity, which is applied to a range of problems, each representing a separate chapter of this thesis (Chapters 3 - 6). In addition, Chapter 1 provides brief introduction to the research area, along with a broad literature review which is not intended to duplicate the specific literature reviews incorporated as part of the journal articles. Chapter 2 presents the mathematical background, including the basic equations of nonlinear elasticity, relevant to the theoretical developments in Chapters 3 – 6. The thesis is concluded with Chapter 7 which summarises the main outcomes of the research undertaken and outlines the future work. The main outcomes of the thesis include the development of: (1) a new analytical model to study the propagation of Lamb waves in pre-stressed compressible plates, (2) a new approach to analyse the effect of a large pre-deformation on the propagation of Lamb waves in incompressible plates, (3) a new analytical model to estimate the effect of stress induced variations on damage detection with Lamb waves, and (4) a new method to evaluate third-order elastic constants utilising Rayleigh waves.