Ultrasonic Lamb wave diffraction tomography.

Ultrasonic guided waves, Lamb waves, allow large sections of aircraft structures to be rapidly inspected. Unlike conventional ultrasonic C-scan imaging that requires access to the whole inspected area, tomographic algorithms work with data collected over the perimeter. Because the velocity of Lamb waves depends on thickness the travel times of the fundamental modes can be converted into a thickness map of inspected region. Lamb waves cannot penetrate through holes and other strongly scattering defects and the assumption of straight wave paths, essential for many tomographic algorithms, fails. Diffraction tomography is a way to incorporate scattering effects into tomographic algorithms in order to improve image quality and resolution. This work describes the iterative reconstruction procedure developed for Lamb wave tomography and allowing for ray bending correction for imaging of moderately scattering objects.

Fan beam and double crosshole Lamb wave tomography for mapping flaws in aging aircraft structures.

As the worldwide aviation fleet continues to age, methods for accurately predicting the presence of structural flaws-such as hidden corrosion and disbonds-that compromise airworthiness become increasingly necessary. Ultrasonic guided waves, Lamb waves, allow large sections of aircraft structures to be rapidly inspected. However, extracting quantitative information from Lamb wave data has always involved highly trained personnel with a detailed knowledge of mechanical waveguide physics. The work summarized here focuses on a variety of different tomographic reconstruction techniques to graphically represent the Lamb wave data in quantitative maps that can be easily interpreted by technicians. Because the velocity of Lamb waves depends on thickness, for example, the traveltimes of the fundamental Lamb modes can be converted into a thickness map of the inspection region. This article describes two potentially practical implementations of Lamb wave tomographic imaging techniques that can be optimized for in-the-field testing of large-area aircraft structures. Laboratory measurements discussed here demonstrate that Lamb wave tomography using either a ring of transducers with fan beam reconstructions, or a square array of transducers with algebraic reconstruction tomography, is appropriate for detecting flaws in multilayer aircraft materials. The speed and fidelity of the reconstruction algorithms as well as practical considerations for person-portable array-based systems are discussed in this article.

Model for quantifying photoelastic fringe degradation by imperfect retroreflective backings.

In any automated algorithm for interpreting photoelastic fringe patterns it is necessary to understand and quantify sources of error in the measurement system. We have been considering how the various components of the coating affect the photoelastic measurement, because this source of error has received fairly little attention in the literature. Because the reflective backing is not a perfect retroreflector, it does not preserve the polarization of light and thereby introduces noise into the measurement that depends on the angle of obliqueness and roughness of the reflective surface. This is of particular concern in resolving the stress tensor through the combination of thermoelasticity and photoelasticity where the components are sensitive to errors in the principal angle and difference of the principal stresses. We have developed a physical model that accounts for this and other sources of measurement error to be introduced in a systematic way so that the individual effects on the fringe patterns can be quantified. Simulations show altered photoelastic fringes when backing roughness and oblique incident angles are incorporated into the model.

An analytic solution for energy deposition in model spherical tumours undergoing ultrasound-hyperthermia treatments.

In this paper, we develop a method to predict the energy deposition in tumours undergoing ultrasound-hyperthermia treatments. Energy depositions are calculated using an exact, analytic solution to the problem of ultrasound scattering from a spherical tumour. The biological tissues are modelled as solid, lossy elastic media so that (i) transverse-wave modes, in addition to longitudinal-wave modes, are considered, and (ii) mode coupling is fully accounted for during the scattering. The model tumour is of arbitrary size and no restrictions are placed on its material parameters relative to the surrounding tissue. Simpler analytic results are given for tumours that differ in densities and rigidities only slightly from the surrounding tissue. We briefly discuss how the above analysis could be extended to more complex systems, i.e., irregularly shaped tumours.

Ultrasound scattering from spherical tumours.

The interaction of biomedical ultrasound with spherical tumours in the human body is investigated using analytic methods which predict the angular distribution of the ultrasound scattered by the tumour. Both the tumour and the surrounding tissue are considered to be lossy elastic media, which support shear-wave modes in addition to the familiar compressional-wave acoustic modes. Exact expressions for the angular distribution of the ultrasonic energy scattered by the tumour are used to illustrate graphically the behaviour of plane ultrasound wave interactions.