Kevin R. Leonard

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.

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.

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.

Simulation of guided waves in complex piping geometries using the elastodynamic finite integration technique

Although many technologies exist for inspecting piping systems, they are most successful on straight pipes and are often unable to accommodate the added complexities of pipe elbows, bends, twists, and branches, particularly if the region of interest is inaccessible. This paper presents a numerical technique based on the elastodynamic finite integration technique for simulating guided elastic wave propagation in piping systems. Comparisons show agreement between experimental and simulated data, and guided wave interaction with flaws, focusing, and propagation in pipe bends are presented. These examples demonstrate the ability of the simulation method to be used to study elastic wave propagation in piping systems which include three-dimensional pipe bends, and suggest its potential as a design tool for designing pipe inspection hardware and ultrasonic signal processing algorithms.

Blind test of Lamb wave diffraction tomography

Lamb waves are guided ultrasonic waves capable of propagating relatively long distances in plates and 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 and fatigue cracks represent changes in effective thickness and local 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 to give a quantitative map of a quantity of interest, e.g. thickness loss due to corrosion. As a test of the Lamb wave scanning apparatus and diffraction tomography reconstruction algorithms, we have undertaken an experiment which is double-blind in the medi...

Wavelet Thumbprint Analysis of Time Domain Reflectometry Signals for Wiring Flaw Detection

We describe a signal processing technique for time‐domain reflectometry (TDR) detection of flaws in wiring. For subtle flaws the backscattered TDR voltage pulses are too slight to be identified by amplitude‐based peak‐detection methods. Here, a wavelet transform is used here to convert the 1D time traces into 2D binary “thumbprint” images. Flaws are then identified according to their unique 2D time‐scale patterns in these wavelet thumbprints. The method is demonstrated for RG58 coaxial cables with varying amounts of damage to the shielding.

Lamb Wave Helical Ultrasonic Tomography

Ultrasonic guided waves have been used for a wide variety of ultrasonic inspection techniques. We describe here a new variation called helical ultrasound tomography (HUT). This new technique, among other things, has direct application to advanced pipe inspection. HUT uses guided ultrasonic waves along with an adaptation of the tomographic reconstruction algorithms developed by seismologists for what they call “cross borehole” tomography. In HUT, the Lamb‐like guided waves travel in various helical crisscross paths between two parallel circumferential transducer arrays instead of the planar crisscross seismic paths between two boreholes. Although the measurement itself is fairly complicated, the output of the tomographic reconstruction is a readily interpretable map of a quantity of interest such as pipe wall thickness. We demonstrate the feasibility of the HUT technique via laboratory scans on steel pipe segments into which controlled thinnings have been introduced.

Lamb Wave Tomography of Pipes and Tanks Using Frequency Compounding

Lamb waves allow large sections of thin‐wall structures to be rapidly inspected for structural flaws such as corrosion. Tomography allows Lamb wave data to be efficiently rendered for direct interpretation. Here we describe two crosshole Lamb wave tomography geometries appropriate for thin‐wall cylindrical structures such as pipes and storage tanks. In both the Lamb waves propagate over a large number of helical criss‐cross pitch‐catch paths generated and received by either meridonal or circumferential pseudoarrays of transducers. In order to improve the quality of reconstructions we vary the frequency slightly of the sources and thus utilize tomographic frequency compounding. After compounding multiple frequency scans of a thick pipe segment, noise is reduced in the reconstructions and the flaw image becomes more pronounced.

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 time‐scale 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 dynamic wavelet fingerprint 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 aluminum plates.