Jack N. Potter

Diffuse field full matrix capture for near surface ultrasonic imaging

ABSTRACT This article reports a technique for near‐surface ultrasonic array imaging. Information equivalent to an undelayed full matrix of inter‐element responses is produced through cross‐correlation of a later time diffuse full matrix. This reconstructed full matrix lacks the nonlinear effects of early time saturation present in a directly acquired response. Consequently the near‐surface material information usually obscured by this effect is retrieved. Furthermore it is shown that a hybrid full matrix formed through a temporally weighted sum of coherent and reconstructed matrices allows for effective near‐surface and bulk material imaging from a single direct‐contact experimental realisation.

Nonlinear phased array imaging

A technique is presented for imaging acoustic nonlinearity within a specimen using ultrasonic phased arrays. Acoustic nonlinearity is measured by evaluating the difference in energy of the transmission bandwidth within the diffuse field produced through different focusing modes. The two different modes being classical beam forming, where delays are applied to different element of a phased array to physically focus the energy at a single location (parallel firing) and focusing in post processing, whereby one element at a time is fired and a focused image produced in post processing (sequential firing). Although these two approaches are linearly equivalent the difference in physical displacement within the specimen leads to differences in nonlinear effects. These differences are localized to the areas where the amplitude is different, essentially confining the differences to the focal point. Direct measurement at the focal point are however difficult to make. In order to measure this the diffuse field is used. It is a statistical property of the diffuse field that it represents the total energy in the system. If the energy in the diffuse field for both the sequential and parallel firing case is measured then the difference between these, within the input signal bandwidth, is largely due to differences at the focal spot. This difference therefore gives a localized measurement of where energy is moving out of the transmission bandwidth due to nonlinear effects. This technique is used to image fatigue cracks and other damage types undetectable with conventional linear ultrasonic measurements.

Quasi-Active Damping

This article introduces a new method of system design using semi-active dampers named quasi-active damping. This technique is the design of dynamical systems and controllers using multiple semi-active dampers such that, at any instant, the desired control force can always be achieved using a combination of the dampers. The motivation is to produce levels of performance approaching those of active systems but with the energy consumption and stability of semi-active systems. The concept is demonstrated in simulation through the design of a stiffness-cancelling suspension system. It is also shown how the robustness of the system can be improved through the addition of a secondary, closed-loop, controller.Copyright

Development of a nonlinear ultrasonic NDE technique for detection of kissing bonds in composites

The development of low-cost bonded assembly of composite aerospace structures ideally requires an NDE method to detect the presence of poor quality, weak bonds or kissing bonds. Such interfaces can introduce nonlinearity as a result of contact nonlinearity where an ultrasonic wave is distorted when it interacts with the interface. In general, the nonlinear elastic behaviour of these interfaces will generate harmonics but they can be lost among the harmonics generated by other nonlinearities present in the experimental system. The technique developed in this research is a non-collinear method; this involves the interaction of two ultrasonic beams, and it allows the removal of virtually all system nonlinearity except for that produced in the region where the two beams overlap. The frequencies of the two beams and the angle between are varied during the experiment. By measuring the nonlinear mixing response as these two parameters are swept through a ‘fingerprint’ of the nonlinear properties in the interaction region can be obtained. This fingerprint has been shown to contain information about the bulk material and the interface status. Work is ongoing to understand which features in the fingerprints reliably correlate with particular material or interface properties. To build this understanding a greatly simplified kissing bond, a compression loaded aluminium-aluminium interface, has been tested. Modelling of the nonlinear behaviour of the aluminium interface has also been conducted.

Near surface ultrasonic imaging

Ultrasonic phased arrays offer excellent performance for detecting and classifying defects; however when inspecting near the array, there is typically a deadzone where electrical cross talk saturates the response making it impossible to measure. In many situations this can be mitigated through the use of a physical standoff, however for permanently installed systems or in situ inspection of components in access restricted areas such as gas turbines such a solution is impossible. In addition, such a standoff typically reduces the amplitude of the received signals degrading the signal to noise ratio (SNR). This paper reports on an approach that allows ultrasonic measurements to be made of the near surface region. Specifically by measuring the diffuse response of the system it is possible to reconstruct the greens function between any pair of transducers. As this is created from data that is not saturated there is no deadzone in the resulting image. When combined with advanced sampling techniques using hadam...

Ultrasonic array imaging of contact-acoustic nonlinearity

Classically, ultrasonic arrays achieved physical beam-forming through the application of phase delays to the parallel transmission of elements. Alternatively, beam-forming may be emulated through post-processing of sequentially transmitted array data. Though exactly equivalent for linear systems, these parallel and sequential fields differ in their nonlinear propagation. Information pertaining to elastic nonlinearity is encoded onto the relative parallel/sequential field by interference between the field components of individual element transmissions. A family of nonlinear imaging techniques has been developed which exploit differences between parallel and sequential fields in order to image elastic nonlinearity. Diffuse energy and coherent scattering variants of the technique are demonstrated for the imaging and characterisation of contact-acoustic nonlinearity in application to the detection and sizing of closed fatigue cracks.

Ultrasonic phased array imaging of contact-acoustic nonlinearity

Classically, ultrasonic arrays achieved physical beam-forming through the application of phase delays to the parallel transmission of elements. Alternatively, beam-forming may be emulated through post-processing of sequentially transmitted array data. Though exactly equivalent for linear systems, these parallel and sequential fields differ in their nonlinear propagation. Information pertaining to elastic nonlinearity is encoded onto the relative parallel/sequential field by interference between the field components of individual element transmissions. A family of nonlinear imaging techniques has been developed which exploit differences between parallel and sequential fields in order to image elastic nonlinearity. Diffuse energy and coherent scattering variants of the technique are demonstrated for the imaging and characterization of contact-acoustic nonlinearity in application to the detection and sizing of closed fatigue cracks.