The use of phased array methods are commonplace in ultrasonic applications, where controlling the variation of the phase between the narrowband emitters in an array facilitates beam steering and focusing. An approach is presented here, whereby all emitters in a 1 dimensional array are pulsed simultaneously, with a controlled bandwidth to emit a 2 dimensional wave. The key result is that one can generate a smooth, continuous wave-front emitted from the array, over a large solid angle, whose frequency varies as a function of angle to the array. Analytic and finite element models created to simulate this phenomena have been validated with experimental results using ultrasonic waves in metal samples. This pulsed approach provides a rapid means of flooding a region of space with a wave-front, whereby any wave that scatters or reflects off a body to a detector will have a distinct arrival time and frequency. This is a general wave phenomena with potential applications in radar, sonar and ultrasound.
Time of flight diffraction and imaging (TOFDI) is based on time of flight diffraction (TOFD); it adds cross-sectional
imaging to examine the bulk of a sample. Multiple wave modes are generated by a pulsed laser beam, ablative source
and are received by a sparse array of non-contact electromagnetic acoustic transducers (EMATs). A B-scan is formed
from multiple data captures (A-scans), with time and scan axes, and colour representing amplitude. B-scans may contain
horizontal lines from surface waves propagating directly from emitter to receiver, or via a back-wall reflection, and
angled lines after reflection off a surface edge. A Hough transform (HT), modified to deal with the constraints of a Bscan,
can remove such lines. A parabola matched filter has been developed to identify features in the B-scan caused by
scattering from point-like features, reducing them to peaks. The processed B-scan is processed further to form a crosssectional
image, enabling detection and positioning of multiple defects. Phase correlation of camera images is used to
track the relative position between transducer and sample to sub-pixel precision.
A combined ultrasound and thermography defect detection system using a raster scanned Q-switched laser as a source of
heat and ultrasound has been developed for identifying surface breaking defects. Heat is generated on a sample surface
by a laser source and the resultant thermal image is examined by a thermal imaging camera. This can be done using a cw
or a pulsed laser, but for ultrasonic generation a pulsed laser beam is required. When a defect is present, the flow of heat
in the sample is disturbed and a change in shape of the thermal spot on the sample's surface can be detected. The pulsed
laser beam generates simultaneously an ultrasonic wave that can be detected by a suitable transducer, which in this case
is an electromagnetic acoustic transducer (EMAT). The presence of a defect changes both the amplitude and frequency
content of the received wave. Three dimensional finite element modelling of the interaction between Lamb waves and
defects have been studied and compared with experimental data, in order to optimise source and detector positions
around a defect. The approach can detect surface crack defects via the ultrasonic and thermography method in one
Some early designs of Electromagnetic Acoustic Transducers (EMATs) used electromagnets to provide the strong
magnetic field required for the transducer to operate. The advent of a new generation of permanent magnets such as
NdFeB, with magnetic fields approaching 1T, meant that many EMAT designs switched over to using these small,
compact and relatively inexpensive magnets. Typically, most modern EMATs make use of permanent magnets since
they can exert high magnetic fields with compact structures. There are certain limitations when using permanent magnets,
and their low Curie points of between 80-150C limit their practicality for high temperature testing without using water
cooled transducers. In this work we have employed a pulsed electromagnet to provide the magnetic field. Pulsing the
magnet dramatically reduces the average power required, keeping the supply more compact and less complex. It has the
added advantage on ferritic steels, of resulting in much larger amplitude ultrasonic signals and improved signal to noise
when compared with EMATs which use the strongest permanent magnets available.
The quantitative measurement of crystallographic texture through determination of the Orientation Distribution
Coefficients (ODCs) can provide critical information on a sample's suitability for being utilised in a particular
manufacturing process or can be used to measure changes in the microstructure of components in service. Ultrasonic
techniques have been developed by previous workers that measure three of the ODCs that describe the orientation
probability distribution function for an aggregate of cubic crystallites. Electron Backscatter Diffraction (EBSD), a
microscopic technique that measures the crystallographic orientations of individual crystals, has been utilised to offer an
alternative method to measuring the complete range of ODCs. As a technique, EBSD provides a much more detailed
measurement of texture than ultrasonic measurements ever could. Ultrasonic methods are however non-destructive, can
be used on components in service and are quicker in use and are less expensive to implement that EBSD measurements.
EBSD is a valuable method in validating ultrasonic measurements, and can help to guide us in determining the
limitations of the ultrasonic measurements. Ultrasonic measurement of texture is and will continue to be a useful
approach to measuring texture but it does have its limitations for application to real samples. Equally, one has to use
EBSD properly if one is to obtain accurate and representative data for the entire sample.
When an electromagnetic acoustic transducer (EMAT) is used to generate ultrasound in an electrically conducting
sample, eddy currents are generated in the sample's skin depth as the first stage in transduction. The resultant acoustic
wave amplitude is proportional to the amplitude of this eddy current, and so anything that we can do to increase the eddy
current will lead to the generation of larger amplitude ultrasonic waves. In eddy current testing, wire coils are often
wound onto a ferrite core to increase the generated eddy current, with the effect that inductance of the coil increases
greatly. When we are dealing with an EMAT, any increase in the coil inductance is usually unacceptable as it leads to a
reduction in the amplitude of a given frequency of eddy current from a limited voltage source. This is particularly
relevant where current arises from capacitor discharge, as is typically used in EMAT driver current circuitry. We present
a method for electromagnetic acoustic transduction where ferrite is used to increase eddy current amplitude, without
significantly increasing coil inductance or changing the frequency content of the eddy current or the generated acoustic
A technique has been developed whereby the thickness and elastic modulus of a coating applied to a substrate can be
calculated from measurement of the resonant bulk wave ultrasonic modes of the combined substrate and coating. The
density of the coating and the material properties of the substrate are required for this method. We have investigated the
difference between models that take account of material attenuation and simpler models that do not and have found that
there is little difference in the predicted resonant frequencies of the system for modes that can be observed in the
experimental data. We have applied the technique to explain the experimental measurements for a 100μm thick epoxy
resin coating curing on a 1mm thick aluminium substrate using wideband radially polarised SH shear waves. In this
dynamic system the elastic properties of the coating change and particular resonant modes not only shift but can
disappear or appear in the experimental data. A model is used to explain this behaviour and show that the technique has
potential for coating thickness measurement in other areas. We have taken samples of 220μm thick aluminium sheet
with a fully cured epoxy coating of nominal thickness 11μm, and have used ultrasonic measurements to calculate the
thickness of the epoxy coating layer.
We have developed a 'pitch-catch' low frequency-wideband Rayleigh wave EMAT system with a centre frequency of approximately 200kHz, extending to around 500kHz and study here its applicability to crack detection in the head of rail tracks. On the head of a rail, the generated waves are strictly speaking a type of guided wave mode as the propagation surface is not a flat halfspace. They propagate along the surface of the rail penetrating down to a depth of several millimetres. We have used this approach to demonstrate detection of gauge corner and longitudinal cracking in the rail head. On samples containing machined slots we have shown that crack depth can be estimated by measuring the proportion of the ultrasonic wave at a particular frequency that passes underneath the crack. The approach that we have used is fundamentally different to and has several advantages over conventional ultrasonic contact methods and should ultimately facilitate testing the rail head more thoroughly at higher speeds.
Air gap such as disbond and crack can be successfully detected by ultrasonic testing. But imperfect interface evaluation is still a challenge. The challenge arises from the uncertainty of formation mechanism, boundary condition and acoustical response. In the paper, samples of structure silicon/adhesive/lead-frame, typical in IC packaging, are fabricated with two adhesives, degraded through thermo cycling, examined by acoustical waveform and C-imaging, and compared to the measured optical microscopy image and the measured failure shear strength.
We present a model for transient ultrasonic wave generation by Electromagnetic Acoustical Transducers (EMATs). Analytical solutions are currently available only for few kinds of sources and our model combines these analytical solutions and numerical computation to predict the ultrasonic field generated by arbitrary sources. This model can be used to calculate bulk waves within samples as well as surface waves on sample surfaces with the advantages of explicit physical meaning and quick processing speed over pure numerical calculations such as the Finite Element Method (FEM). We use the model to explain how static and dynamic magnetic fields generate ultrasonic waves in a sample. We wish to characterize the EMAT source in detail in order to tailor sources for optimal configuration for specific NDE applications. A Michelson laser interferometer is used to measure out of plane surface displacement of sample, and results agree well with the modelling simulation. The modelling can be used for arbitrary source.
Epoxy resins are essential to the fabrication of carbon fiber reinforced composites (CFRCs). This paper investigates laser generated ultrasound in epoxy resins using three pulsed lasers: A TEA CO2, a fundamental Nd:YAG and a XeCl excimer. In the low power thermoelastic regime, the laser beam causes the surface of the sample to expand rapidly, in times that are comparable to the rise time of the laser pulse. In non-metals the phenomenon is dominated by the optical absorption depth, which is a function both of the properties of the material and the laser wavelength, and for epoxy resins, varies from a few microns to several millimeters. Compared to the thermoelastic source in metals, a bigger volume of the material is affected, the temperature rise is less and the amplitude of the longitudinal wave is greater. This condition is referred to as "a buried thermoelastic source". In CFRCs, the superficial layer of epoxy resin (typically 50-100 microns thick) plays an important role to the generation mechanism. At the Nd:YAG wavelength the epoxy is transparent and acts as a constrained layer. At the TEA CO2 and the XeCl excimer wavelengths both the epoxy and the underlying fibers absorb strongly. Experiments were carried out on CFRC and pure epoxy resin samples, comparative results and efficiency graphs are presented.