Bruce W. Drinkwater

AIP Conference Proceedings

The quality of an ultrasonic array image, especially for anisotropic material, depends on accurate information about acoustic properties. Inaccuracy of acoustic properties causes image degradation, e.g., blurring, errors in locating of reflectors and introduction of artifacts. In this paper, for an anisotropic austenitic steel weld, an autofocus imaging technique is presented. The array data from a series of beacons is captured and then used to statistically extract anisotropic weld properties by using a Monte-Carlo inversion approach. The beacon and imaging systems are realized using two separated arrays; one acts as a series of beacons and the other images these beacons. Key to the Monte-Carlo inversion scheme is a fast forward model of wave propagation in the anisotropic weld and this is based on the Dijkstra algorithm. Using this autofocus approach a measured weld map was extracted from an austenitic weld and used to reduce location errors, initially greater than 6mm, to less than 1mm.

The detectability of kissing bonds in adhesive joints using ultrasonic techniques.

This paper concerns a study of the detectability of dry contact kissing bonds in adhesive joints using three ultrasonic inspection techniques. Conventional normal incidence longitudinal and shear wave inspection were conducted on dry contact kissing bonds using a standard damped ultrasonic transducer and an electro-magnetic acoustic transducer (EMAT) respectively. The detectability of the dry contact kissing bonds was assessed by calculating the reflection coefficient of the imperfect interface at varying loads for a number of surface roughnesses. A high power ultrasonic method was also employed to determine the non-linear behavior of the adhesive interface. The non-linearity of the interface was determined by the ratio of the amplitudes of the first harmonic and fundamental frequencies of the transmitted waveform. It was found that the high power technique showed the greatest sensitivity to these kissing bonds at low contact pressures, however at high loads conventional longitudinal wave testing was more sensitive. It was also noted that a combination of two or more techniques could provide enhanced information about the kissing bond compared to a single technique alone.

Strategies for guided-wave structural health monitoring

Structural health monitoring (SHM) using guided waves is one of the only ways in which damage anywhere in a structure can be detected using a sparse array of permanently attached sensors. To distinguish damage from structural features, some form of comparison with damage-free reference data is essential, and here subtraction is considered. The detectability of damage is determined by the amplitude of residual signals from structural features remaining after the subtraction of reference data. These are non-zero due to changing environmental conditions such as temperature. In this paper, the amplitude of the residual signals is quantified for different guided-wave SHM strategies. Comparisons are made between two methods of reference signal subtraction and between two candidate sensor configurations. These studies allow estimates to be made of the number of sensors required per unit area to reliably detect a prescribed type of damage. It is shown that the number required is prohibitively high, even in the presence of modest temperature fluctuations, hence some form of temperature compensation is absolutely essential for guided-wave SHM systems to be viable. A potential solution is examined and shown to provide an improvement in signal suppression of approximately 30 dB, which corresponds to two orders of magnitude reduction in the number of sensors required.

The temperature stability of guided wave structural health monitoring systems

It is desirable for any structural health monitoring (SHM) system to have maximum sensitivity with minimum sensor density. The structural health monitoring system described here is based on the excitation and reception of guided waves using piezoelectric elements as sensors. One of the main challenges faced is that in all but the most simple structures the wave interactions become too complex for the time domain signals to be interpreted directly. One approach to overcoming this complexity is to subtract a baseline reference signal from the measured system when it is known to be defect free. This strategy enables changes in the structure to be identified. Two key issues must be addressed to allow this paradigm to become a reality. First, the system must be sufficiently sensitive to small reflections from defects such as cracking. Second, it must be able to distinguish between benign changes and those due to structural defects. In this paper the baseline subtraction approach is used to detect defects in a simple rectangular plate. The system is shown to work well in the short term, and good sensitivity to defects is demonstrated. The performance degrades over the medium to long term. The principal reason for this degradation is shown to be the effect of change in temperature of the system. These effects are quantified and strategies for overcoming them are discussed.

Holographic acoustic elements for manipulation of levitated objects

Sound can levitate objects of different sizes and materials through air, water and tissue. This allows us to manipulate cells, liquids, compounds or living things without touching or contaminating them. However, acoustic levitation has required the targets to be enclosed with acoustic elements or had limited manoeuvrability. Here we optimize the phases used to drive an ultrasonic phased array and show that acoustic levitation can be employed to translate, rotate and manipulate particles using even a single-sided emitter. Furthermore, we introduce the holographic acoustic elements framework that permits the rapid generation of traps and provides a bridge between optical and acoustical trapping. Acoustic structures shaped as tweezers, twisters or bottles emerge as the optimum mechanisms for tractor beams or containerless transportation. Single-beam levitation could manipulate particles inside our body for applications in targeted drug delivery or acoustically controlled micro-machines that do not interfere with magnetic resonance imaging.

The use of non-collinear mixing for nonlinear ultrasonic detection of plasticity and fatigue

This letter reports on the application of the non-collinear mixing technique to the ultrasonic measurement of material nonlinearity to assess plasticity and fatigue damage. Non-collinear mixing is potentially more attractive for assessing material state than other nonlinear ultrasonic techniques because system nonlinearities can be both independently measured and largely eliminated. Here, measurements made on a sample after plastic deformation and on a sample subjected to low-cycle fatigue show that the non-collinear technique is indeed capable of measuring changes in both, and is therefore a viable inspection technique for these types of material degradation.

The wavenumber algorithm for full-matrix imaging using an ultrasonic array

Ultrasonic imaging using full-matrix capture, e.g., via the total focusing method (TFM), has been shown to increase angular inspection coverage and improve sensitivity to small defects in nondestructive evaluation. In this paper, we develop a Fourier-domain approach to full-matrix imaging based on the wavenumber algorithm used in synthetic aperture radar and sonar. The extension to the wavenumber algorithm for full-matrix data is described and the performance of the new algorithm compared with the TFM, which we use as a representative benchmark for the time-domain algorithms. The wavenumber algorithm provides a mathematically rigorous solution to the inverse problem for the assumed forward wave propagation model, whereas the TFM employs heuristic delay-and-sum beamforming. Consequently, the wavenumber algorithm has an improved point-spread function and provides better imagery. However, the major advantage of the wavenumber algorithm is its superior computational performance. For large arrays and images, the wavenumber algorithm is several orders of magnitude faster than the TFM. On the other hand, the key advantage of the TFM is its flexibility. The wavenumber algorithm requires a regularly sampled linear array, while the TFM can handle arbitrary imaging geometries. The TFM and the wavenumber algorithm are compared using simulated and experimental data.

The Use of Ultrasound in the Investigation of Rough Surface Interfaces

The measurement of ultrasonic reflection has been used to study the contact between rough surfaces. An incomplete interface will reflect some proportion of an incident wave; this proportion is known as the reflection coefficient, If the wavelength is large compared with the width of the gaps in the plane of the interface then the reflection mechanism can be modeled by considering the interface as a spring. The proportion of the incident wave reflected (reflection coefficient) is then a function of the stiffness of the interface and the frequency of the ultrasonic wave. The sensitivity of the ultrasonic technique has been quantified using a simple model, from which the stiffness of individual gaps and contacts are calculated and their effect on the ultrasonically measured stiffness predicted. The reflection of ultrasound at a static interface between a rough, nominally flat aluminum plate and a rough, nominally flat hardened steel punch has been investigated. Plastic flow on first loading was evident, while repeated loading was largely elastic. However, subsequent cycles indicate a small amount of further plasticity and contact irreversibility. The effect of surface roughness on the resultant contact has also been investigated. A simple plastic contact model is described which allows prediction of the average size of the asperity contacts and their number. This model shows that the average size of the contacts remains constant over most of the loading whereas the number of contacts increases almost linearly. The contact stiffness has also been modeled with two well known elastic rough surface contact models. These models predicted a lower interface stiffness than was observed in the experiments. However they provide a useful way of interpreting the ultrasonically measured interface stiffness data.

Maximum-likelihood estimation of damage location in guided-wave structural health monitoring

This paper describes the formulation of a maximum-likelihood estimate of damage location for guided-wave structural health monitoring (GWSHM) using a minimally informed, Rayleigh-based statistical model of scattered wave measurements. Also introduced are two statistics-based methods for evaluating localization performance: the localization probability density function estimate and the localizer operating characteristic curve. Using an ensemble of measurements from an instrumented plate with stiffening stringers, the statistical performance of the so-called Rayleigh maximum-likelihood estimate (RMLE) is compared with that of seven previously reported localization methods. The RMLE proves superior in all test cases, and is particularly effective in localizing damage using very sparse arrays consisting of as few as three transducers. The probabilistic basis used for modelling the complicated wave scattering behaviour makes the algorithm especially suited for localizing damage in complicated structures, with the potential for improved performance with increasing structure complexity.

Nondestructive testing of adhesively-bonded joints

One approach to testing the suitability of an adhesive joint for a particular application is to build and test to destruction a representative sample of the joint. In this way the best adhesive and surface treatment for a given application can be found. To reduce the costs of this approach, the designer will wish to call on previous experience with adhesives, surface treatments, and joint designs so as to reach a high probability of success before he builds and tests a structural prototype. If structures are expensive, it will be difficult to justify more than a very limited series of prototype tests before production begins. During the production phase, and also in service with critical structures, it is essential to use nondestructive tests to assess the quality and fitness for purpose of the product. The nondestructive test will not measure strength directly but will measure a parameter which can be correlated to strength. It is therefore, essential that a suitable nondestructive test is chosen and that its results are correctly interpreted. In this paper, typical defects found in adhesive joints are described together with their significance. The limits and likely success of current physical nondestructive tests are described, and future trends outlined.