Thomas E. Michaels

Detection of structural damage from the local temporal coherence of diffuse ultrasonic signals

Permanently mounted ultrasonic transducers have the potential to interrogate large areas of a structure, and thus be effective global sensors for structural health monitoring. Recorded signals, although very sensitive to damage, are long, complex, and difficult to interpret compared to pulse echo and through transmission signals customary for nondestructive testing. These diffuse signals also are quite sensitive to environmental effects such as temperature and surface condition changes. Waveform comparison methods such as time domain differencing and spectral analysis, although effective for detecting changes, are generally unsuccessful in discriminating damage from environmental effects. This paper considers the local temporal coherence as another means of comparing two waveforms in order to provide a quantitative measure of the change in shape of a signal compared to a reference as a function of time from transmit. Experimental results show that the local temporal coherence is effective in discriminating structural damage from both temperature changes and modest changes in surface conditions; results are compared to those obtained from time domain and spectrogram differencing. The advantages of this methodology are the simplicity of the transducers, the applicability to a wide range of structures, and the straightforward signal processing.

Frequency-wavenumber domain analysis of guided wavefields.

Full wavefield measurements obtained with either an air-coupled transducer mounted on a scanning stage or a scanning laser vibrometer can be combined with effective signal and imaging processing algorithms to support characterization of guided waves as well as detection, localization and quantification of structural damage. These wavefield images contain a wealth of information that clearly shows details of guided waves as they propagate outward from the source, reflect from specimen boundaries, and scatter from discontinuities within the structure. The analysis of weaker scattered waves is facilitated by the removal of source waves and the separation of wave modes, which is effectively achieved via frequency-wavenumber domain filtering in conjunction with the subsequent analysis of the resulting residual signals. Incident wave removal highlights the presence and the location of weak scatterers, while the separation of individual guided wave modes allows the characterization of their separate contribution to the scattered field and the evaluation of mode conversion phenomena. The effectiveness of these methods is demonstrated through their application to detection of a delamination in a composite plate and detection of a crack emanating from a hole.

An ultrasonic method for dynamic monitoring of fatigue crack initiation and growth

Attached ultrasonic sensors can detect changes caused by crack initiation and growth if the wave path is directed through the area of critical crack formation. Dynamics of cracks opening and closing under load cause nonlinear modulation of received ultrasonic signals, enabling small cracks to be detected by stationary sensors. A methodology is presented based upon the behavior of ultrasonic signals versus applied load to detect and monitor formation and growth of cracks originating from fastener holes. Shear wave angle beam transducers operating in through transmission mode are mounted on either side of the hole such that the transmitted wave travels through the area of expected cracking. Time shift is linear with respect to load, and is well explained by path changes due to strain combined with wave speed changes due to acoustoelasticity. During subsequent in situ monitoring with unknown loads, the measured time of flight is used to estimate the load, and behavior of the received energy as a function of load is the basis for crack detection. Results are presented from low cycle fatigue tests of several aluminum specimens and illustrate the efficacy of the method in both determining the applied load and monitoring crack initiation and growth.

Simulation and Measurement of Ultrasonic Waves in Elastic Plates Using Laser Vibrometry

The propagation of Lamb waves in elastic plates is analyzed both numerically and experimentally. A Scanning Laser Doppler Vibrometer (SLDV) is here used to detect and visualize transient waveforms propagating in an elastic plate at low ultrasonic frequencies. The waves are excited by a piezoelectric crystal glued to the plate surface and actuated by sinusoidal pulses of varying frequency. The pulse sequence is triggered by the SLDV internal controller so that phase and delay information are preserved. Such information allows visualization of the waveform pattern as it propagates over the plate surface. The experiment produces animated displacement maps where the interaction with discontinuities in the plate such as defects becomes apparent. This capability suggests application of the SLDV technique as part of an overall damage detection methodology which combines the recognized sensitivity of ultrasonic waves with the localization of damage via wavefield visualization. The interpretation of the experiment...

Multi-mode and multi-frequency guided wave imaging via chirp excitations

Guided wave imaging has shown great potential for structural health monitoring applications by providing a way to visualize and characterize structural damage. For successful implementation of delay-and-sum and other elliptical imaging algorithms employing guided ultrasonic waves, some degree of mode purity is required because echoes from undesired modes cause imaging artifacts that obscure damage. But it is also desirable to utilize multiple modes because different modes may exhibit increased sensitivity to different types and orientations of defects. The well-known modetuning effect can be employed to use the same PZT transducers for generating and receiving multiple modes by exciting the transducers with narrowband tone bursts at different frequencies. However, this process is inconvenient and timeconsuming, particularly if extensive signal averaging is required to achieve a satisfactory signal-to-noise ratio. In addition, both acquisition time and data storage requirements may be prohibitive if signals from many narrowband tone burst excitations are measured. In this paper, we utilize a chirp excitation to excite PZT transducers over a broad frequency range to acquire multi-modal data with a single transmission, which can significantly reduce both the measurement time and the quantity of data. Each received signal from a chirp excitation is post-processed to obtain multiple signals corresponding to different narrowband frequency ranges. Narrowband signals with the best mode purity and echo shape are selected and then used to generate multiple images of damage in a target structure. The efficacy of the proposed technique is demonstrated experimentally using an aluminum plate instrumented with a spatially distributed array of piezoelectric sensors and with simulated damage.

APPLICATION OF ACOUSTIC WAVEFIELD IMAGING TO NON-CONTACT ULTRASONIC INSPECTION OF BONDED COMPONENTS

Through transmission ultrasonic (TTU) methods are widely used for inspection of critical aerospace structural components because of their simplicity and the high sensitivity of TTU methods to bonding and delamination type defects. TTU inspection requires access to both sides of a component, which is generally not a problem before components are installed in final assemblies. However, limited access to the back side of a component on final assemblies usually precludes using TTU methods. Pulse echo (PE) methods are often used when only single side access is available, but PE ultrasonic methods have a limited penetration range from the outer surface, and do not have the sensitivity of TTU, particularly when surfaces are non‐parallel. Thus, structural assemblies are often disassembled when a thorough TTU inspection is required. The work presented here addresses the need for a field deployable ultrasonic inspection method which has the sensitivity of TTU methods, is non‐contact, i.e., couplant is not required, and does not require access to the back side of the part. These goals are accomplished by attaching a sparse array of ultrasonic transducers to the back side of a component or embedding them within the component. These transducers are excited to generate ultrasonic waves which propagate through the structure, and the resultant acoustic wavefields are imaged using a non‐contact, air‐coupled transducer. This ultrasonic wavefield imaging method is referred to as Acoustic Wavefield Imaging (AWI). Results are presented for a bonded aluminum plate specimen demonstrating that recorded wavefield images clearly show bonding flaws at internal interfaces.

COMPARISON OF THE EFFECTS OF APPLIED LOADS AND TEMPERATURE VARIATIONS ON GUIDED WAVE PROPAGATION

Most guided wave‐based damage detection methods for structural health monitoring rely upon detecting small damage‐induced changes in ultrasonic guided wave signals. However, the structure of interest is generally exposed to variable loads and temperatures during normal usage, and received signals can be significantly affected. These signal changes are of concern because of their potential to cause false alarms during in situ monitoring. Load‐induced signal changes are similar in some respects to those caused by temperature variations because both cause changes in bulk wave speeds and specimen dimensions. However, load‐induced changes, unlike temperature changes, can produce a slight anisotropy of the structure, causing the changes in velocities to depend on the direction of propagation. Here we present experimental results that show the anisotropic effect of applied loads on the direct arrivals of various Lamb wave modes and compare that to both theory and to the isotropic effect of temperature.

Enhanced Differential Methods for Guided Wave Phased Array Imaging Using Spatially Distributed Piezoelectric Transducers

A number of tomographic and phased‐array methods have been proposed for generating two dimensional images of plate‐like structures using sparse arrays of spatially distributed ultrasonic transducers. The phased array differential approach is considered here whereby pulse echo and through transmission signals are recorded before and after localized damaged is introduced, and differenced signals are combined using a focusing rule to produce an image of the plate. The application is structural health monitoring where the transducers are permanently bonded to the structure. The quality of the image is affected by many factors such as the number and location of the transducers, the characteristics of the damage, the signal‐to‐noise ratio, presence of edge reflections, and anything unrelated to damage that may perturb the ultrasonic signals such as temperature changes and transducer bonding variations. Two methods for enhancing image quality are implemented and then evaluated as to their effectiveness. In the first method, the windowing function is changed in width prior to phased signal addition to yield the best image quality. In the second method, signals are envelope‐detected prior to phased signal addition to eliminate phasing artifacts. Results are reported for artificial defects introduced in aluminum plates.

Sparse Ultrasonic Transducer Array for Structural Health Monitoring

A spatially sparse array of conventional piezoelectric transducers is attached to a part surface to monitor its structural health. Artificial flaws are incrementally added to simulate damage progression. The structure is flooded with ultrasonic energy by transmitting on a single transducer, and waveforms are recorded from other transducers in the array. Simple waveform differencing techniques between pre‐flaw baseline waveforms and post‐flaw waveforms show promise for determining the state of damage progression in both concrete and aluminum samples.

A comparison of feature-based classifiers for ultrasonic structural health monitoring

Diffuse ultrasonic signals received from ultrasonic sensors which are permanently mounted near, on or in critical structures of complex geometry are very difficult to interpret because of multiple modes and reflections constructively and destructively interfering. Both changing environmental and structural conditions affect the ultrasonic wave field, and the resulting changes in the received signals are similar and of the same magnitude. This paper describes a differential feature-based classifier approach to address the problem of determining if a structural change has actually occurred. Classifiers utilizing time and frequency domain features are compared to classifiers based upon time-frequency representations. Experimental data are shown from a metallic specimen subjected to both environmental changes and the introduction of artificial damage. Results show that both types of classifiers are successful in discriminating between environmental and structural changes. Furthermore, classifiers developed for one particular structure were successfully applied to a second one that was created by modifying the first structure. Best results were obtained using a classifier based upon features calculated from time-frequency regions of the spectrogram.