Jennifer E. Michaels

Acquisition and analysis of angle-beam wavefield data

Angle-beam ultrasonic testing is a common practical technique used for nondestructive evaluation to detect, locate, and characterize a variety of material defects and damage. Greater understanding of the both the incident wavefield produced by an angle-beam transducer and the subsequent scattering from a variety of defects and geometrical features is anticipated to increase the reliability of data interpretation. The focus of this paper is on acquiring and analyzing propagating waves from angle-beam transducers in simple, defect-free plates as a first step in the development of methods for flaw characterization. Unlike guided waves, which excite the plate throughout its thickness, angle-beam bulk waves bounce back and forth between the plate surfaces, resulting in the well-known multiple “skips” or “V-paths.” The experimental setup consists of a laser vibrometer mounted on an XYZ scanning stage, which is programmed to move point-to-point on a rectilinear grid to acquire waveform data. Although laser vibro...

Ultrasonic Methods for Detection of Micro Porosity in Composite Materials

Porosity that is detrimental to the performance of a composite structure is often much smaller than can be detected using conventional ultrasonic testing methods. The direct ultrasonic reflection can be too small to detect, and the back wall and through transmission signals may not be significantly changed in amplitude. Thus, improved inspection methods to detect and quantitatively measure micro porosity are of considerable interest. The goals of the study reported here were to (1) evaluate conventional pulse echo and through transmission methods for detection and characterization of micro porosity in composite materials, (2) develop signal processing methods for measuring micro porosity content, and (3) compare results of developed algorithms to conventional inspection results.

Characterization of guided wave velocity and attenuation in anisotropic materials from wavefield measurements

The behavior of guided waves propagating in anisotropic composite panels can be substantially more complicated than for isotropic, metallic plates. The angular dependency of wave propagation characteristics need to be understood and quantified before applying methods for damage detection and characterization. This study experimentally investigates the anisotropy of wave speed and attenuation for the fundamental A0-like guided wave mode propagating in a solid laminate composite panel. A piezoelectric transducer is the wave source and a laser Doppler vibrometer is used to measure the outward propagating waves along radial lines originating at the source transducer. Group velocity, phase velocity and attenuation are characterized as a function of angle for a single center frequency. The methods shown in this paper serve as a framework for future adaptation to damage imaging methods using guided waves for structural health monitoring.

Challenges in the separation and analysis of scattered waves in angle-beam wavefield data

The measurement of ultrasonic signals on a 2-D rectilinear grid resulting from a fixed source, referred to as wavefield imaging, is a powerful tool for visualizing wave propagation and scattering. Wavefield imaging provides a more complete picture of wave propagation than conventional single-point measurements, but creates more challenges for analysis. This work considers the development of wavefield-based methods for analyzing angle-beam wave propagation and scattering in plates. Methods of analysis focus on the separation of scattered waves from the total wavefield with the eventual goal of quantitative scatterer characterization in a laboratory environment. Two methods for wave separation are considered: frequency-wavenumber filtering and wavefield baseline subtraction. Frequency-wavenumber filtering is applied to wavefield data that are finely sampled in both space and time, whereas baseline subtraction is a technique that has typically been applied to individual signals recorded from fixed transducer...

Estimation and application of 2-D scattering matrices for sparse array imaging of simulated damage in composite panels

Reliable detection of damage in composites is critically important for failure prevention in the aerospace industry since these materials are more frequently being used in high stress applications. Structural health monitoring (SHM) via guided wave sensors mounted on or embedded within a composite structure can help detect and localize damage in real-time while potentially reducing overall maintenance costs. One approach to guided wave SHM is sparse array imaging via the minimum variance algorithm, and it has been shown in prior work that incorporating expected scattering from defects of interest can improve the quality of damage localization and characterization. For this study, simulated damage in the form of attached magnets was used for estimating scattering from recorded wavefield data. Data were recorded on a circle centered at the damage location from multiple incident directions before and after the magnets were attached. Baseline subtraction is used to estimate scattering patterns for each incide...

Dispersive matched filtering of ultrasonic guided waves for improved sparse array damage localization

Although bulk waves have served as the industry standard in nondestructive evaluation for many years, guided waves (Lamb waves in plates) have become the focus of many current research efforts because they are able to interrogate larger areas of a structure in less time. Despite this advantage, guided waves also have characteristics that obfuscate data interpretation. The first property of guided waves that complicates analysis is their dispersive nature: their wave speed is a function of frequency. The second is that they are multimodal: they propagate as multiple symmetric and antisymmetric modes. Using pulse-compression techniques and a priori calculations of theoretical dispersion curves, the dispersive matched filter attempts to take advantage of these otherwise undesirable characteristics by maximizing the autocorrelation for only one mode, ideally increasing both the signal-to-noise ratio and time-resolution of ultrasonic guided wave measurements. In this research, the responses from broadband chir...

Incremental scattering of guided waves from a notch originating at a through-hole

Cracks, which frequently initiate from fastener holes as a result of stress concentration, are one of the most common defects in metallic plate-like structures. Among a variety of methods for crack detection, ultrasonic guided waves have been shown to be effective. To examine the performance of guided wave methods in the laboratory, notches are often used to simulate cracks. While extensive research has focused on the scattering of guided waves from a notch as well as a hole, limited work has been done on the incremental scattering resulting from the addition of a notch to an existing hole. This scenario is of particular interest for in situ monitoring of fastener holes, where the goal is to detect changes in scattering caused by crack initiation and growth. An experimental approach is taken here where a broadband chirp excitation is applied to surface-mounted PZT transducers to generate guided waves in an aluminum plate, and the out-of-plane particle motion is measured by a laser vibrometer. Notches are ...

Design of distributed sparse arrays for Lamb wave SHM based upon estimated scattering matrices

A common practice in guided wave structural health monitoring is collecting measurements from a transducer array using the pitch-catch method. Among different array configurations, the spatially distributed array provides a cost-effective solution for rapid interrogation of large, plate-like structures. Several guided wave imaging techniques have been proposed and successfully demonstrated for damage detection and localization. However, the performance of these imaging methods can be compromised by a mismatch between a particular transducer array geometry and the scattering characteristics of a defect of interest. This study proposes a method, which is based upon estimating scattering matrices, to quantify the ability of a specific array geometry to interrogate a scatterer. Several array geometries are evaluated using this method, and a Monte Carlo simulation is then performed to vary the transducer locations to find the array geometry that is best matched to a specific directional scatterer. The efficacy...

Estimation of guided wave scattering matrices from spatially distributed transducer arrays

Because of their ability to travel relatively long distances with low attenuation, guided waves are being considered as a tool for the detection of defects in plate-like structures for aerospace, civil, and petrochemical applications. When a guided wave encounters a defect, a scattered field related to the characteristics of the defect is generated. The far field scattering behavior can be described by a scattering matrix that quantifies the amplitude of the scattered signal as a function of incident and scattered angles. Because of the mode and frequency dependence of guided waves interacting with defects, the scattering matrix is typically defined for specific guided wave modes (incident and scattered) at a designated frequency. Prior work has utilized finite element modeling and full wavefield scanning to estimate scattering matrices, but these approaches may be impractical because of either computational requirements or experimental issues. Here, we propose a methodology for estimating a scattering matrix based upon limited experimental data recorded from a spatially distributed transducer array. After applying baseline subtraction to extract changes in received signals resulting from the introduction of a scatterer, we further process differenced signals to obtain a limited number of scattering matrix data points corresponding to the incident and scattered angles for each transducer pair. We perform radial basis function interpolation of these initial points to estimate the complete scattering matrix and evaluate the efficacy of the proposed method via experiments with a glued-on linear scatterer.

SIMULTANEOUS ULTRASONIC MONITORING OF CRACK GROWTH AND DYNAMIC LOADS DURING A FULL SCALE FATIGUE TEST OF AN AIRCRAFT WING

An in situ ultrasonic angle beam method has been developed to detect and size fatigue cracks emanating from fastener holes. The method employs transducers mounted in a pitch‐catch configuration on each side of the hole, and the modulation of received signals with load is used to detect and size small fatigue cracks. The focus of this paper is the monitoring of a series of critical fastener holes during a full scale fatigue test of an aircraft wing. Essential to the success of this method is the ability to use the same ultrasonic signal as is used for crack monitoring to estimate the instantaneous load. The applied load causes a time shift in ultrasonic echo arrivals resulting from both dimensional changes and the acoustoelastic effect. Waveforms are recorded randomly during fatiguing, and the dynamically applied load associated with each waveform is estimated from the measured time shift. Additionally, small cracks near the hole open and close during fatigue loading, causing a modulation of the amplitude ...