Steven E. Olson

Automated detection of delamination and disbond from wavefield images obtained using a scanning laser vibrometer

The paper presents signal and image processing algorithms to automatically detect delamination and disbond in composite plates from wavefield images obtained using a scanning laser Doppler vibrometer (LDV). Lamb waves are excited by a lead zirconate titanate transducer (PZT) mounted on the surface of a composite plate, and the out-of-plane velocity field is measured using an LDV. From the scanned time signals, wavefield images are constructed and processed to study the interaction of Lamb waves with hidden delaminations and disbonds. In particular, the frequency–wavenumber (f–k) domain filter and the Laplacian image filter are used to enhance the visibility of defects in the scanned images. Thereafter, a statistical cluster detection algorithm is used to identify the defect location and distinguish damaged specimens from undamaged ones.

A fracture mechanics methodology assessment for fretting fatigue

Abstract A fracture mechanics methodology was evaluated for a fretting fatigue geometry in which one end of a specimen clamped between fretting pads was loaded in axial fatigue. In previous work, results from experiments on Ti–6Al–4V pads and specimens were evaluated using finite element analyses where stress intensity factors were calculated assuming a single-edge tension, Mode I crack to form. In the present work, mixed-mode behavior was considered and a more realistic crack geometry was incorporated. K I and K II were calculated from stress fields determined from the finite element analysis using a weight function method and assuming a single-edge Mode I/Mode II inclined crack. A correction was then applied based on empirical crack aspect ratio data. K I and K II were analyzed for several experimentally determined combinations of contact pad geometry, specimen thickness, and loading conditions used to obtain a range of normal and shear forces, each corresponding to a fatigue life of 10 7 cycles. The fracture mechanics methodology was used to determine the conditions for propagation or non-propagation of cracks that initiate in the edge of contact region based on a mixed-mode driving force and a short crack corrected threshold. The coefficient of friction was also varied in the analyses. The fracture mechanics approach appears to be a better method for determining the threshold for fretting fatigue than a stress analysis because thresholds for K are better known than criteria for crack initiation in a gradient stress field.

An analytical particle damping model

Particle damping is a passive vibration control technique where multiple auxiliary masses are placed in a cavity attached to a vibrating structure. The behavior of the particle damper is highly non-linear and energy dissipation, or damping, is derived from a combination of loss mechanisms. These loss mechanisms involve complex physical processes and cannot be analyzed reliably using current models. As a result, previous particle damper designs have been based on trial-and-error experimentation. This paper presents a mathematical model that allows particle damper designs to be evaluated analytically. The model utilizes the particle dynamics method and captures the complex physics involved in particle damping, including frictional contact interactions and energy dissipation due to viscoelasticity of the particle material. Model predictions are shown to compare well with test data.

Impact localization in complex structures using laser-based time reversal:

This study presents a new impact localization technique that can pinpoint the location of an impact event within a complex structure using a time-reversal concept, surface-mounted piezoelectric transducers, and a scanning laser Doppler vibrometer. First, an impulse response function between an impact location and a piezoelectric transducer is approximated by exciting the piezoelectric transducer instead and measuring the response at the impact location using scanning laser Doppler vibrometer. Then, training impulse response functions are assembled by repeating this process for various potential impact locations and piezoelectric transducers. Once an actual impact event occurs, the impact response is recorded by the piezoelectric transducers and compared with the training impulse response functions. The correlations between the impact response and the impulse response functions in the training data are computed using a unique concept of time reversal. Finally, the training impulse response function, which gives the maximum correlation, is chosen from the training data set and the impact location is identified. The proposed impact localization technique has the following advantages over the existing techniques: (a) it can be applied to isotropic/anisotropic plate structures with additional complex features such as stringers, stiffeners, spars, and rivet connections; (b) only simple correlation calculation based on time reversal is required, making it attractive for real-time automated monitoring; and (c) training is conducted using noncontact scanning laser Doppler vibrometer and the existing piezoelectric transducers that may already be installed for other structural health–monitoring applications. Impact events on an actual composite aircraft wing and an actual aluminum fuselage are successfully identified using the proposed technique.

Effectiveness and Predictability of Particle Damping

In this paper, recent results of ongoing studies into the effectiveness and predictability of particle damping are discussed. Efforts have concentrated on characterizing and predicting the behavior of a wide range of potential particle materials, shapes, and sizes in the laboratory environment, as well as at elevated temperature. Methodologies used to generate data and extract the characteristics of the nonlinear damping phenomena are illustrated with interesting test results. Experimental results are compared to predictions from analytical simulations performed with an explicit code, based on the particle dynamics method, that has been developed in support of this work.

Lamb Wave Propagation in Varying Isothermal Environments

Military and commercial aerospace organizations are exploring structural health monitoring (SHM) systems to reduce maintenance costs and to verify the integrity of structural components exposed to harsh conditions. This technical note considers the use of Lamb waves to monitor plate and shell components of aerospace structures. For fielded applications, SHM systems will need to operate across a variety of environmental conditions, including large temperature ranges. Therefore, it is critical to understand the effects of temperature on Lamb wave propagation. The focus of this study is the effect of temperature on Lamb wave propagation in a constant-thickness metallic plate under isothermal conditions. Experimental measurements and analytical predictions are made over temperatures ranging from -18°C to 107°C. Results indicate that only small and predictable changes in the wave propagation behavior occur over the temperature range investigated. This is significant because it may allow SHM systems to be designed for aircraft systems operating within this range without the need for complex compensation techniques.

Fastener Damage Estimation in a Square Aluminum Plate

To reduce costs and meet turn-around goals of future space vehicles, structural health monitoring systems are needed to assess the health of the entire vehicle structure within hours of the completed mission. In particular, the thermal protection system must be in good condition before launch due to its critical role in protecting the vehicle’s primary structure and subsystems. The capability to detect fastener failure is critical, as the overall thermal protection system for a vehicle will likely include a number of adjacent panels that are mechanically fastened to the substructure. This article discusses experimental and analytical efforts focused on estimating fastener damage in a square aluminum plate test article. Fastener condition is estimated using classifiers based on statistical pattern recognition methods. The classifiers utilize features from measured vibration data with the ability to discriminate between damage conditions. Finite element analyses are used to provide a physical understanding of the structural dynamics of the aluminum plate and an interpretation of the selected features used for the classifier. Techniques to detect, locate, and assess damage resulting due to fastener failure have been investigated. The classification systems presented perform reasonably well in detecting and localizing damage, particularly if the damage is fairly severe. However, the classification system performance is much worse at assessing the severity of damage, particularly for lower levels of damage.

Design methodology for particle damping

Focused research in the area of Multi-Particle Impact Damping (MPID) has resulted in new methods of characterization and prediction. An analytical method has been developed, based on the particle dynamics method, that uses characterized particle damping data to predict damping in structural systems. A methodology to design particle damping for dynamic structures will be discussed. The complete design methodology has been validated in proof-of-methodology testing on a structural component in the laboratory.

A Comparison of 1D and 3D Laser Vibrometry Measurements of Lamb Waves

This paper compares and contrasts one-dimensional (1D) and three-dimensional (3D) scanning laser Doppler vibrometer (LDV) measurements of Lamb waves generated by lead zirconate titanate (PZT) transducers. Due to the large cost and capability differences between the previously mentioned systems, this study is provided to highlight differences between these systems. 1D measurements are defined here as measurements of only out-of-plane velocities which are well-suited for studying anti-symmetric Lamb wave modes. 3D measurements provide both in- and out-of-plane velocities, which are especially important when studying both symmetric and anti-symmetric Lamb wave modes. The primary reason for using scanning LDVs is that these systems can make non-contact, accurate surface velocity measurements over a spatially-dense grid providing relatively high resolution image sequences of wave propagation. These scans can result in a clear understanding of Lamb waves propagating in plate-like structures and interacting with structural variations.

Computational Lamb wave model validation using 1D and 3D laser vibrometer measurements

Lamb waves are being explored for structural health monitoring (SHM) due to their capability of detecting relatively small damage within reasonably large inspection areas. However, Lamb wave behavior is fairly complex, and therefore, various computational techniques, including finite element analysis (FEA), have been utilized to design appropriate SHM systems. Validation of these computational models is often based on a limited number of measurements made at discrete locations on the structure. For example, models of pitch-catch of Lamb waves may be validated by comparing predicted waveform time histories at a sensor to experimentally measured results. The use of laser Doppler vibrometer (LDV) measurements offers the potential to improve model validation. One-dimensional (1D) LDV scans provide detailed out-of-plane measurements over the entire scanned region, and checks at discrete sensor locations can still be performed. The use of three-dimensional (3D) laser vibrometer scans further expands the data available for correlation by providing in- and out-of-plane velocity components over the entire scanned region. This paper compares the use of 1D and 3D laser vibrometer data for qualitatively and quantitatively validating models of healthy metallic and composite plates.