Advanced non-contacting ultrasonic techniques for non-destructive testing

This thesis describes research towards the development of ultrasonic methods to test samples that are difficult to test using conventional techniques, with particular emphasis being given to non-contacting methods. The samples investigated in detail were adhesively bonded structures and zircalloy (a zirconium-niobium alloy). The adhesively bonded structures were investigated ultrasonically using an ultrasonic resonance technique (referred to as ultrasonic spectroscopy) to analyse suitable ultrasonic waveforms. This thesis starts by explaining a new approach to ultrasonic spectroscopy, and then describes a number of transduction techniques (both contacting and non-contacting) that were devised to obtain waveforms suitable for spectroscopic analysis. These including conventional piezoelectric transducers, laser generation of ultrasound, EMAT reception of ultrasound, and a novel couplant-free transducer. Tests were undertaken on a variety of samples under a number of different conditions, with the experimental results comparing well with those predicted by theory. Zircalloy was investigated next in an effort to evaluate non-destructively the concentration of hydride in the alloy. This was performed using velocity-temperature measurements (at temperatures up to 500°C) for both shear and longitudinal waves, and by dilatometry (thermal expansion) measurements. Both sets of tests successfully determined the hydride concentrations of test samples. A separate chapter is devoted to the description of some of the novel transducers developed during the course of this research, including a couplant-free transducer, and several transducers for airborne ultrasound. These transducers were found to operate well, the couplant-free transducer being particularly successful (subsequently finding a number of industrial applications). The final experimental chapter describes the building of both a photoelastic, and a schlieren rig that were used to visualise ultrasound, with the intention of giving an insight into some of the ultrasonic phenomena that were associated with the rest of the work. The results obtained were invaluable in analysing the results from previous chapters

Optimisation of an acoustic resonator for particle manipulation in air

An acoustic resonator system has been investigated for the manipulation and entrapment of micron-sized particles in air. Careful consideration of the effect of the thickness and properties of the materials used in the design of the resonator was needed to ensure an optimised resonator. This was achieved using both analytical and finite-element modelling, as well as predictions of acoustic attenuation in air as a function of frequency over the 0.8 to 2.0 MHz frequency range. This resulted in a prediction of the likely operational frequency range to obtain particle manipulation. Experimental results are presented to demonstrate good capture of particles as small as 15 µm in diameter

Using a magnetite/thermoplastic composite in 3D printing of direct replacements for commercially available flow sensors

Flow sensing is an essential technique required for a wide range of application environments ranging from liquid dispensing to utility monitoring. A number of different methodologies and deployment strategies have been devised to cover the diverse range of potential application areas. The ability to easily create new bespoke sensors for new applications is therefore of natural interest. Fused deposition modelling is a 3D printing technology based upon the fabrication of 3D structures in a layer-by-layer fashion using extruded strands of molten thermoplastic. The technology was developed in the late 1980s but has only recently come to more wide-scale attention outside of specialist applications and rapid prototyping due to the advent of low-cost 3D printing platforms such as the RepRap. Due to the relatively low-cost of the printers and feedstock materials, these printers are ideal candidates for wide-scale installation as localized manufacturing platforms to quickly produce replacement parts when components fail. One of the current limitations with the technology is the availability of functional printing materials to facilitate production of complex functional 3D objects and devices beyond mere concept prototypes. This paper presents the formulation of a simple magnetite nanoparticle-loaded thermoplastic composite and its incorporation into a 3D printed flow-sensor in order to mimic the function of a commercially available flow-sensing device. Using the multi-material printing capability of the 3D printer allows a much smaller amount of functional material to be used in comparison to the commercial flow sensor by only placing the material where it is specifically required. Analysis of the printed sensor also revealed a much more linear response to increasing flow rate of water showing that 3D printed devices have the potential to at least perform as well as a conventionally produced sensor

Additively-manufactured piezoelectric devices

A low-cost micro-stereolithography technique with the ability to additively manufacture dense piezoelectric ceramic components is reported. This technique enables the layer-wise production of functional devices with a theoretical in-plane resolution of ∼20 μm and an out-of-plane resolution of <1 μm without suffering a significant reduction in the piezoelectric properties when compared to conventionally produced ceramics of the same composition. The ability to fabricate devices in complex geometries and with different material properties means that conventional limits of manufacturing are not present. A hollow, spherical shell of the piezoelectric material 0.65Pb(Mg⅓Nb⅔)O3–0.35PbTiO3, built without tooling or recourse to additional equipment or processes, is shown generating ultrasound in the MHz range

Narrow-Band Hybrid Pulsed Laser/EMAT System for Non-Contact Ultrasonic Inspection Using Angled Shear Waves

Conventional ultrasonic testing (UT) using angled shear waves to locate and size potentially critical cracks and flaws in power generation and refinery equipment has become a widely utilized industrial tool. Because this technique uses piezoelectric transducers it requires intimate surface contact and fluid couplants. Therefore, conventional UT has the important drawback that it is difficult to use on surfaces at elevated temperature and, as a result, may require costly plant shut downs to implement. The development of non-contact techniques for angled shear wave UT would represent a significant improvement in the ability to test hot vessels and pipes

Ultrasonic propagation in highly attenuating insulation materials

Experiments have been performed to demonstrate that ultrasound in the 100−400 kHz frequency range can be used to propagate signals through various types of industrial insulation. This is despite the fact that they are highly attenuating to ultrasonic signals due to scattering and viscoelastic effects. The experiments used a combination of piezocomposite transducers and pulse compression processing. This combination allowed signal-to-noise levels to be enhanced so that signals reflected from the surface of an insulated and cladded steel pipe could be obtained

Thermosonic inspection of carbon fibre reinforced polymer composites using an airborne haptic ultrasonic phased array

This paper reports the development of a contactless non-destructive evaluation technique using an air-coupled haptic ultrasonic phased array to induce thermosonic frictional heating in damaged carbon fibre reinforced polymer composites. Haptic ultrasonic systems consist of controllable, narrowband, and high-power piezoelectric transducer arrays that are capable of electronically steering and shaping the ultrasonic beam on the surface of test samples. Localised thermal images of the damaged area were observed using an infrared camera. It was found that the intensity of the thermosonic heating reduced with increased distances between the ultrasonic excitation location and the damage. This approach allowed the ultrasonic focal point to be moved across the sample to identify the areas of damage, without moving either the array or the infrared camera, thus significantly decreasing the time needed for inspection

A simple, low-cost conductive composite material for 3D printing of electronic sensors

3D printing technology can produce complex objects directly from computer aided digital designs. The technology has traditionally been used by large companies to produce fit and form concept prototypes (‘rapid prototyping’) before production. In recent years however there has been a move to adopt the technology as full-scale manufacturing solution. The advent of low-cost, desktop 3D printers such as the RepRap and Fab@Home has meant a wider user base are now able to have access to desktop manufacturing platforms enabling them to produce highly customised products for personal use and sale. This uptake in usage has been coupled with a demand for printing technology and materials able to print functional elements such as electronic sensors. Here we present formulation of a simple conductive thermoplastic composite we term ‘carbomorph’ and demonstrate how it can be used in an unmodified low-cost 3D printer to print electronic sensors able to sense mechanical flexing and capacitance changes. We show how this capability can be used to produce custom sensing devices and user interface devices along with printed objects with embedded sensing capability. This advance in low-cost 3D printing with offer a new paradigm in the 3D printing field with printed sensors and electronics embedded inside 3D printed objects in a single build process without requiring complex or expensive materials incorporating additives such as carbon nanotubes

Acoustic field visualisation using local absorption of ultrasound and thermochromic liquid crystals

Acoustic field and vibration visualisation is important in a wide range of applications. Laser vibrometry is often used for such visualisation, however, the equipment has a high cost and requires significant user expertise, and the method can be slow, as it requires scanning point by point. Here we suggest a different approach to visualisation of acoustic fields in the kHz – MHz range, using paint-on or removable film sensors, which produce a direct visual map of ultrasound displacement. The sensors are based on a film containing thermochromic liquid crystals (TLC), along with a backing/underlay layer which improves absorption of ultrasound. The absorption generates heat, which can be seen by a change in colour of the TLC film. A removable sensor is used to visualise the resonant modes of an air-coupled flexural transducer operated from 410 kHz to 7.23 MHz, and to visualise 40 kHz standing waves in a Perspex plate. The thermal basis of the visualisation is confirmed using thermal imaging. The speed and cost of visualisation makes the new sensor attractive for use in condition monitoring, for fast assessment of transducer quality, or for analysis of acoustic field distribution in power ultrasonic systems

High Temperature Non Destructive Evaluation of Hydrided Metal Tubing

The testing of CANDU Zr-Nb pressure tubes for the presence of hydrides is a problem which is of current interest, as the premature replacement of pressure tubes could occur if the estimation of hydride levels is not made correctly. It has been shown that changes in the ultrasonic and thermal expansion properties of the material occur at a particular temperature, known as the terminal solid solubility (TSS) temperature [1]. Here, stable zirconium hydride platelets present at low temperatures dissociate to form free hydrogen on heating, and also reform from free hydrogen on cooling. Note that the TSS temperature is likely to be different for heating and cooling, due to hysteresis and thermodynamic effects, and will be dependent on the heating or cooling rate. The TSS temperature is known to be a function of the concentration of hydrogen present, and in fact follows a well known phase diagram [2], where the two-phase system becomes a single phase plus free hydrogen on heating (see Figure 1). The region of low atomic % is represented at the extreme left of this phase diagram. At a given value of atomic % of hydrogen (or the equivalent in parts per million (ppm)), a phase change will occur on heating from the two phase system containing the hydride to the single phase plus hydrogen. A transition in the reverse direction will occur on cooling. The temperature at which this happens is the TSS temperature, the value of which depends on the % of hydrogen present. Hence, determination of the TSS temperature leads directly to an estimation of hydride content