Lamb wave diffraction tomography for imaging material inhomogeneities in isotropic plates

Rohde, Andrew (2007). Lamb wave diffraction tomography for imaging material inhomogeneities in isotropic plates PhD Thesis, School of Engineering, University of Queensland.

       
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Author Rohde, Andrew
Thesis Title Lamb wave diffraction tomography for imaging material inhomogeneities in isotropic plates
School, Centre or Institute School of Engineering
Institution University of Queensland
Publication date 2007
Thesis type PhD Thesis
Supervisor Associate Professor Martin Veidt
Subjects 0913 Mechanical Engineering
Abstract/Summary This thesis examines the implementation and feasibility of Lamb wave diffraction tomography for digitally reconstructing flexural defects, i.e. those that do not produce any mode coupling, in structural plate components. The technique relies upon the damage been modelled as a pure flexural inhomogeneity and is well suited to low-levels of corrosion damage in metallic materials and barely visible impact damage in layered composites. Unlike previous Lamb wave imaging strategies, this approach explicitly includes the two-dimensional effects of wave diffraction, making it also more sensitive to damages that are comparable in size with the wavelengths of the interrogating field. This has typically been a problematic area for conventional nondestructive evaluation techniques as they often rely upon large damage severities for positive identification. Thus, diffraction tomography should be useful as a complimentary technique for the early detection and characterisation of low severity damage. The imaging is investigated both numerically and experimentally in three fundamental areas of quantitative nondestructive evaluation, viz. locating, sizing and quantifying the severity of a single damage on an otherwise undamaged plate. In the numerical studies, computer simulations of the wave-defect interaction are used to generate synthetic scatter data which are directly input into the imaging system. For this purpose, several modelling strategies that employ the analytical approximations of Mindlin plate theory are implemented and compared with experimental measurements for cylindrical damage geometries. The results demonstrate the usefulness of Mindlin's equations, as opposed to exact equations from three-dimensional elasticity, as an accurate and tractable framework for Lamb wave scattering problems. The simulations are also used to investigate the parametric sensitivities of wave-defect interactions, which are important for quantifying key experimental parameters in the first prototype imaging system, viz. minimum number of receivers, optimal actuator-damage-receiver configurations and interrogation pulse design for maximum signal-to-noise ratio. For the experimental investigations, various severities of manufactured damage on an aluminium plate are used to test the prototype imaging systems. Initially, damages are simulated by bonding small weights identically to both the upper and lower plate surfaces. This way the damages are symmetric about the plate mid-plane, which means that there shouldn't be any mode conversion in the scattering, i.e. a flexural incident wave gives rise to only flexural scattered waves. This is an important consideration for the feasibility studies as the analytical model used for the imaging does not account for mode coupling. Next, uniform reductions of plate thickness over a circular area are considered. These are also introduced identically to both sides of the plate. This type of damage is used as a first approximation to more realistic corrosion damage which, it is envisaged, will be one of the damage modes to be targeted by the technique. The results show that plate-wave diffraction tomography enables the quantitative reconstruction of location, size and severity of plate damage with excellent sensitivity. They also demonstrate the robustness of the technique for a variety of damage configurations, which sometimes do not exactly satisfy the underlying assumptions of the flexural inhomogeneity model. Thus, a fundamentally improved understanding of Lamb wave imaging for plate damage has been achieved. Apart from demonstrating the potential of diffraction tomography as an advanced quantitative nondestructive evaluation technique, the thesis also lays the foundation for future research on this topic, which will address the transition of the technique to realistic structures, e.g. curved panels, stiffened plates, sandwich structures, damages with mode conversion etc.

 
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Created: Fri, 21 Nov 2008, 16:22:25 EST