Mark K. Hinders

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.

Ultrasonic Lamb wave tomography

Nondestructive evaluation (NDE) of aerospace structures using traditional methods is a complex, time-consuming process critical to maintaining mission readiness and flight safety. Limited access to corrosion-prone structure and the restricted applicability of available NDE techniques for the detection of hidden corrosion or other damage often compound the challenge. In this paper we discuss our recent work using ultrasonic Lamb wave tomography to address this pressing NDE technology need. Lamb waves are ultrasonic guided waves, which allow large sections of aircraft structures to be rapidly inspected for structural flaws such as disbonds, corrosion and delaminations. Because the velocity of Lamb waves depends on thickness, for example, the travel times of the fundamental Lamb modes can be converted into a thickness map of the inspection region. However, extracting quantitative information from Lamb wave data has always involved highly trained personnel with a detailed knowledge of mechanical waveguide physics. Our work focuses on tomographic reconstruction to produce quantitative maps that can be easily interpreted by technicians or fed directly into structural integrity and lifetime prediction codes. Laboratory measurements discussed here demonstrate that Lamb wave tomography using a square perimeter array of transducers with algebraic reconstruction tomography is appropriate for detecting flaws in 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 paper.

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.

Parallel projection and crosshole Lamb wave contact scanning tomography

Lamb waves are guided ultrasonicwaves capable of propagating relatively long distances in thin plates and thin laminated structures, such as airframe skins, storage tanks, and pressure vessels. Their propagation properties in these media depend on the vibrational frequency as well as on the thickness and material properties of the structure. Structural flaws such as disbonds, corrosion, fatigue cracks, and voids represent changes in effective thickness and material properties, and therefore measurement of variations in Lamb wave propagation can be employed to assess the integrity of these structures. Lamb wave measurements can be made for a number of relative transducer positions (projections) and an image of the flawed region can be reconstructed tomographically. This paper presents a new technique in which two contact piezoelectric transducers are independently scanned along parallel lines in a fashion analogous to that commonly used in seismic crosshole tomography. These results are compared to those for parallel projection Lamb wavetomography data collected with an automated contact scanning apparatus. The advantages and drawbacks of these two methods in the development of automated tomographic Lamb wave scanners for quantitative mapping of thickness variation in platelike materials are discussed.

Automatic multi-mode Lamb wave arrival time extraction for improved tomographic reconstruction

An ultrasonic signal processing technique is applied to multi-mode arrival time estimation from Lamb waveforms. The basic tool is a simplified timescale projection called a dynamic wavelet fingerprint (DWFP) which enables direct observation of the variation of features of interest in non-stationary ultrasonic signals. The DWFP technique was used to automatically detect and evaluate each candidate through-transmitted Lamb mode. The area of the DWFP was then used as a feature to distinguish false modes caused by noise and other interference from the true modes of interest. The set of estimated arrival times were then used as inputs for tomographic reconstruction. The Lamb wave tomography images generated with these estimated arrival times were able to indicate different defects in aluminium plates.

Ultrasonography in dentistry

This paper reviews diagnostic applications of ultrasound to dentistry, or dental ultrasonography, beginning with pioneering work of the 1960s up through present lines of research. Clinical, in vivo applications that are of direct interest to dental practice are reviewed here, including measurements of enamel thickness and periodontal pocket depth. In vitro research that involves destructive tooth preparation or procedures, such as sound speed measurements or scanning acoustic microscopy, also are included. Although dental ultrasonography has been studied for over 40 years, most methods are not quite ready for routine clinical use, and there remains much opportunity for diagnostic ultrasonography to significantly impact the practice of dentistry.

Multi-mode Lamb wave tomography with arrival time sorting.

Lamb wave tomography has been shown to be an effective nondestructive evaluation technique for platelike structures. A series of pitch-catch measurements between ultrasonic transducers can be taken from different orientations across the sample to create a map of a particular feature of interest such as plate thickness. Most previous work has relied solely on the first arriving mode for the time-of-flight measurements and tomographic reconstructions. The work described here demonstrates the capability of the Lamb wave tomography system to generate accurate reconstructions from multiple modes. Because each mode has different through-thickness displacement values, each is sensitive to different types of flaws, and the information gained from a multi-mode analysis can improve understanding of the structural integrity of the inspected material. However, one of the problems with the extraction of multi-mode arrival times is that destructive interference between two modes may cause one of the modes to seemingly disappear in the signal. The goal of the sorting algorithm presented in this work is to try and counteract this problem by using multiple frequency scans--also known as frequency walking--to sort the arrival times into their correct mode series.

Wavelet Fingerprinting of Radio-Frequency Identification (RFID) Tags

Unintentional modulations of the electromagnetic signal of radio-frequency (RF) emitters are used to identify individual sources of signals as unique from emitters of the same type in a procedure known as RF fingerprinting. It allows for the identification and tracking of physical threats, prevention of unauthorized access, and detecting cloning of sensitive devices. Machine learning techniques assist RF fingerprinting by providing automatic recognition of these unique aspects of individual RF emitters. RF identification (RFID) tags are a common RF emitter used to track supplies and are also present in credit cards and passports to allow for automatic recognition or monetary transfers. Despite advances in RFID cryptography, RFID tags can still be easily cloned and tracked. Here, we implement RF fingerprinting to authenticate individual RFID tags at the physical layer. Features are extracted using the dynamic wavelet fingerprint, and supervised pattern classification techniques are used to identify unique RFID tags with up to 99% accuracy.

Multiple-mode Lamb wave scattering simulations using 3D elastodynamic finite integration technique.

We have implemented three-dimensional (3D) elastodynamic finite integration technique (EFIT) simulations to model Lamb wave scattering for two flaw-types in an aircraft-grade aluminum plate, a rounded rectangle flat-bottom hole and a disbond of the same shape. The plate thickness and flaws explored in this work include frequency-thickness regions where several Lamb wave modes exist and sometimes overlap in phase and/or group velocity. For the case of the flat-bottom hole the depth was incrementally increased to explore progressive changes in multiple-mode Lamb wave scattering due to the damage. The flat-bottom hole simulation results have been compared to experimental data and are shown to provide key insight for this well-defined experimental case by explaining unexpected results in experimental waveforms. For the rounded rectangle disbond flaw, which would be difficult to implement experimentally, we found that Lamb wave behavior differed significantly from the flat-bottom hole flaw. Most of the literature in this field is restricted to low frequency-thickness regions due to difficulties in interpreting data when multiple modes exist. We found that benchmarked 3D EFIT simulations can yield an understanding of scattering behavior for these higher frequency-thickness regions and in cases that would be difficult to set up experimentally. Additionally, our results show that 2D simulations would not have been sufficient for modeling the complicated scattering that occurred.

Lamb wave characterization of corrosion-thinning in aircraft stringers: Experiment and three-dimensional simulation

The development of automatic guided wave interpretation for detecting corrosion in aluminum aircraft structural stringers is described. The dynamic wavelet fingerprint technique (DWFT) is used to render the guided wave mode information in two-dimensional binary images. Automatic algorithms then extract DWFT features that correspond to the distorted arrival times of the guided wave modes of interest, which give insight into changes of the structure in the propagation path. To better understand how the guided wave modes propagate through real structures, parallel-processing elastic wave simulations using the finite integration technique (EFIT) has been performed. Three-dimensional (3D) simulations are used to examine models too complex for analytical solutions. They produce informative visualizations of the guided wave modes in the structures and mimic the output from sensors placed in the simulation space. Using the previously developed mode extraction algorithms, the 3D EFIT results are compared directly to their experimental counterparts.