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Direct measurement of sap flow enabled determination of the seasonal pattern of water use, ETc, of three date (Phoenix dactylifera L.) varieties irrigated with groundwater at different salinities: S1 at 5 dS m−1 and S3 at 15 dS m−1. The ETc at the higher salinity was 43–46% lower across all varieties. The crop factor, Kc, was computed from ETc / ETo, where ETo is the reference evapotranspiration. By proximal sensing using a light stick, we measured the fraction of light intercepted, LI, by the trees’ canopies. For all varieties and salinities, we found the ratio Kc LI−1 to be about 0.95, which enables proximal sensing to be used to predict ETc for all varieties and across salinities. These predictions can then be used to schedule irrigation the recommended rate of 1.5 ETc, which accounts for a 25% factor-of-safety and a 25% salt leaching fraction. For S1, the salt-tolerant ‘Lulu’ used 50 kL tree−1 y−1, the moderately tolerant ‘Khalas’ consumed 43.1 kL tree−1 y−1, and the salt-intolerant ‘Shahlah’ transpired 57.3 kL tree−1 y−1. Whereas the drop in ETc across all varieties was similar between S1 and S3, there were large differences in the drop in date production. Date production between S1 and S3 dropped 29% for ‘Lulu’, 43% for ‘Khalas’, and 52% for ‘Shahlah’. Analysis of the consumed water productivity, CWP (kg-dates kL−1) provides insight into the impacts of salinity on date yield. For the tolerant ‘Lulu’ the CWP for S3 was higher (2.21 kg-dates kL−1) than that for S1 (1.78 kg-dates kL−1), although production was higher with S1 (89.1 kg tree−1) than S3 (62.9 kg tree−1). The CWP for ‘Khalas’ was the same for both treatments (≈ 1 kg-dates kL−1). For the salt intolerant ‘Shahlah’, CWP dropped between S1 (1.5 kg-dates kL−1) and S3 (1.34 kg-dates kL−1). Based on the price of dates, the CWP can also be used to assess the economic value of irrigation water by variety and salinity.

The arid forests of Abu Dhabi are valuable but they require irrigating. Currently groundwater (GW) is the source of this water, but these subterranean reserves are being over-exploited. Law No 5 of 2016 on the regulation of GW has been passed by the Government of Abu Dhabi to reduce GW abstraction. Abu Dhabi has a supply of tertiary-treated sewage effluent (TSE) that could be used as an alternative for irrigation. We set up experiments near Madinat Zayed in the Al Dhafra region of Abu Dhabi on two arid-forest species: Al Ghaf (Prosopis cineraria) and Al Sidr (Ziziphus spina-christi). The trees were planted at 8 m x 8 m spacing. The TSE is significantly ‘sweeter’ than GW, as its electrical conductivity is

In the search for ever more sensitive non-destructive testing (NDT) techniques to detect progressively smaller defects in increasingly complex industrial materials, environments and geometries, conventional eddy-current testing (ECT) methods, have reached their sensitivity limit. Fortunately, advanced resonance-based techniques, demonstrated in recent years, promise to help extend that limit, but, before such techniques can be implemented with confidence, a detailed understanding of their sensitivity and stability must be achieved. In this study, statistical probability-of-detection (PoD) analysis was performed to assess the sensitivity of a novel single-frequency, near electrical resonance signal enhancement (SF-NERSE) technique relative to an equivalent conventional probe operation. This study was performed on 36 real fatigue defects in Titanium 6V-4Al (Ti6-4) with defects ranging from 0.10 − 6.48 mm in surface extent. In addition, a critical evaluation of background noise stability in the SF-NERSE technique (a common concern with NERSE-based methods) was also performed to establish the viability of such a technique in relation to industrial inspection and assessment criteria. The results of this study demonstrate a sensitivity enhancement of up to 20% for the SF-NERSE method over conventional operation and, through controls, confirmed that the effect is a result of the resonance-shifting phenomenon and not just an increase in operational frequency. In addition, an examination of background noise implies that a SF-NERSE method exhibits more stable background noise than conventional excitation. This study validates the SF-NERSE technique and methodology as a viable industrial inspection technique, able to significantly improve detection capabilities.

A new high-power electromagnetic acoustic transducer (EMAT) solid state pulser system has been developed that is capable of driving up to 4 EMAT coils with programmable phase delays, allowing for focusing and steering of the acoustic field. Each channel is capable of supplying an excitation current of up to 1.75 kA for a pulse with a rise time of 1 μs. Finite element and experimental data are presented which demonstrate a signal enhancement by a factor of 3.5 (compared to a single EMAT coil) when using the system to transmit a longitudinal ultrasound pulse through a 22.5 cm thick as-cast steel slab sample. Further signal enhancement is demonstrated through the use of an array of detection EMATs, and a demonstration of artificial internal defect detection is presented on a thick steel sample. The design of this system is such that it has the potential to be employed at elevated temperatures for diagnostic measurements of steel during the continuous casting process.

This paper investigates through experiment and finite element modelling, the interaction and mode conversion phenomenon of SH0 and SH1 guided wave modes on a metal plate with machined wall thinning. Quantitative analysis was performed by calculating the reflection and transmission coefficients at the leading and trailing linearly tapered edges, for incident SH0 and SH1 modes. Several geometries were evaluated by varying the taper length and depth. Experiments were performed with periodic permanent magnet array EMATs as transmitters and receivers, generating a single SH mode, whilst both SH0 and SH1 are received. Experimental and numerical data show good agreement, revealing that the interaction of SH guided waves with such defects is complex when mode conversion arises. The values of the reflection and transmission coefficients are non-monotonic along the thinning depth and edge angle ranges. The quantitative results provide insight into the capabilities and limitations of guided SH wave measurements for simple corrosion type defects, indicating that with current capabilities, inspection of real defects will be limited to screening type measurements rather than detailed quantification of the defect region.

The United Arab Emirates has a hyper-arid climate. Irrigation is essential for dates (Phoenix dactylifera L.), an important crop economically and culturally. Groundwater is relied on, yet it is a non-renewable resource at the rate it is being used. Furthermore, as the water-table drops, it is becoming more saline. Law no. 5 has been passed in Abu Dhabi to regulate the use of groundwater and set allocation limits for agriculture. For assessing the allocation of irrigation water to date farms under Law 5, we carried out measurements of tree water-use by the compensation heat-pulse method, complemented by measurements of the changing soil-water dynamics using time domain reflectometry and bulk soil electrical conductivity. Over four years we measured the hourly pattern of Lulu date-palm water use, ETc, at two levels of irrigation-water salinity: Treatment S1 at 5 dS m−1, and S3 at 15 dS m−1. The mid-summer ETc for the S1 Lulu trees is up to 190 L d−1, on average, whereas for the S3 trees ETc is lower at 130 L d−1 (68% of S1) because of the salt. Measurements of canopy radiation interception using a ‘light stick’ showed the S1 trees intercepted 26% of the incident radiation, whereas the S3 trees only intercepted 20% (ratio S3/S1 = 76%). The date yield of the S1 trees was 68 kg tree−1, but was 46 kg tree−1 for the S3 trees (ratio 68%). Current practice is to irrigate trees with 275 L d−1, irrespective of salinity. Our recommendation for Law 5 is to tailor irrigation to the seasonal demand in the reference evapotranspiration of ETo, and allow for a 25% factor-of-safety and a 25% salt leaching fraction. For S1 date palms this would mean an annual average of 210 L d−1, and for S3 just 137 L d−1. This represents savings of 25–50% from current practice.

The Electromagnetic acoustic transducer (EMAT) is used in a number of non-destructive testing applications [1–5]. The EMAT's operation is principally based on one of two mechanisms; the Lorenz force and magnetostriction mechanism [6–9]. The magnetostriction mechanism of an EMAT at elevated temperatures is reported in this paper. An optimized model is developed to describe the magnetostriction of polycrystalline iron, which is based on Brown's magnetic domain wall movement model [10] and Lee's magnetic domain rotation model [11]. The magnetostriction curves of polycrystalline iron for the temperature range 300 K–900 K are predicted, which reveal that the saturated magnetostriction coefficient changes from − 4 × 10 − 6 to approximately 12 × 10 − 6 . A non-linear, isotropic magnetostriction, finite element model is developed to simulate the Lamb waves generated in 4 mm thick steel plate by an EMAT, and the results show that the amplitude of S0 Lamb wave is greatly enhanced with an increase of temperature. In the experiments, a magnetostriction-based EMAT is used to generate Lamb waves in 4 mm thick steel plate. Experimental measurements verify that the contribution of the magnetostriction mechanism on steel rises as temperature increases in the range 298 K–873 K, while the contribution to ultrasonic generation from the Lorenz force mechanism decreases, as expected.

The arid forests of Abu Dhabi provide a variety of valuable provisioning, regulating and cultural ecosystem services. They require irrigating. And groundwater is the source. About 95 % of groundwater consumption is for agriculture and forestry. Over-extraction threatens groundwater resources. The Government of Abu Dhabi recently passed Law 5 to restrict groundwater abstraction. We have determined the minimum allocation required for the irrigation of two tree species. We carried out experiments at Madinat Zayed in the western desert on two arid-forest species: Al Ghaf (Prosopis cineraria) and Al Sidr (Ziziphus spina-christi), both planted on a 7 m × 7 m grid. We measured the actual evapotranspiration (ETc) using heat-pulse equipment in the trees. Saline groundwater with an electrical conductivity of about 8–10 dS m−1 was used for irrigation. Current practice is to drip irrigate with 60 L d‐1. Both species displayed distinct, and different, summer ‘deciduous behaviours’ that determine their seasonal pattern of ETc. A single crop-factor approach, using ETc predicted from Kc.ETo, where Kc is the crop factor and ETo is the reference evaporation, would not provide appropriate irrigation allocations. From our hourly measurements of ETc, made over 3 years, we quantified the seasonal pattern in Kc. For Al Ghaf, Kc ranged from 0.1 during February–July, to 0.15 in November–December; for the Sidr, Kc was at a minimum of 0.06 in May, and rose to 0.16 in December. Daily irrigation requirements were provided for Law 5. With a 25% factor-of-safety, and a 25% salt-leaching requirement, irrigation requirements for Al Ghaf ranged from 24.4 L d−1 in January to 52.8 L d−1 in July. For Al Sidr the range was from 33.8 L d−1 in April to 53.5 L d−1 in September. These are a 40% saving on current practice.

The interaction of guided waves with wall thinning can be complex, depending on the thinning geometry and the frequency. At a high frequency–thickness, when a shear-horizontal (SH) guided wave mode impinges upon a tapered wall thinning region, there is mode conversion to other propagating SH modes, either in reflection or transmission, which heavily depends on the shape of the taper. In this paper, we have combined the reciprocity theorem of elastodynamics and the theory of multiple reflections, in order to analytically calculate the scattered SH wavefield in plates, due to the interaction with an arbitrary tapered thinning. The taper is discretized into several sections and the formulation is addressed in matrix notation, in order to tackle several modes which arise due to mode interconversion distributed within the taper. The method was validated with experimental and numerical data at linear tapered thinning, in the high-frequency–thickness regime. It was also applied to provide understanding of the reflection behaviour within smoother taper profiles, namely, raised-cosine and Blackman window tapers, and to visualize the propagating field of each mode. It is shown that for a linear taper profile, the reflection within the taper is virtually constant, which produces an interference pattern in the overall reflection from the whole taper. Such a mechanism is broken with smoother tapers, since they impose lower reflection close to the taper ends. The method proves itself useful for analytically investigating the scattering from arbitrary wall thinning when mode-conversion arises.

The Magnetic Tomography Method (MTM), a passive Non-Destructive Testing (NDT) technique based on the magneto-mechanical effect, has been claimed to be able to detect defects at large (>1 m) stand-off distances. In this study, the MTM signal was studied experimentally on a 4140-L80 pipe sample, in which a flat bottom defect was electrochemically generated. The Residual Magnetic Leakage Field (RMLF) signal was recorded using an annular array of AMR sensors. The experimental results show perturbations due to the defect (about 12 dB above noise level) only when the AMR sensors were positioned at a very low stand-off. The presence of ferromagnetic objects near the sensors could cause perturbations many times larger than that from a defect (about 18 dB above the noise level). A Finite Element (FE) model validated the experimental results. The model showed that there is a significant risk of false indications due to foreign ferromagnetic objects when measuring at a large distance from the pipe.

The magneto-mechanical memory (MMM) method, that is often referred to as the metal magnetic memory method, has been reported to be a non-destructive testing technique capable of quantifying stress concentrations and detecting defects in ferromagnetic materials. The underlying mechanism behind MMM has been explained in the literature, but the sensitivity to stress concentration has not been satisfactorily investigated. In this paper, both the normal and tangential components of the stress-induced MMM signal were measured by permanently installed magnetic sensor arrays on specimens made from three grades of L80 alloy steel and 20 other structural steels; tests were also carried out on a pipe made from the 4140-L80 steel. As expected, the stress history affects the MMM signal, but the experimental results show that significant irreversible change of magnetization always occurs only in the first cycle of loading regardless whether the deformation is purely elastic or partially plastic. If the peak stress level is increased at a given point during cycling, the immediately following next cycle acts as a new “first” cycle at that peak stress level and causes additional significant irreversible change of magnetization, but there is no evidence that plastic deformation might build up a cumulative magnetization. The MMM effect is very small in the steel samples tested, indicating that it will not be useful in field applications. In un-notched specimens the irreversible change in magnetization caused a proportional change in the measured external magnetic field on the order of only 5–10 A/m, while in the case of notched specimens the leakage field was on the order of 30–60 A/m.\ud \ud

Bulk thickness measurements were performed at elevated temperatures on magnetite coated low carbon steel pipe and aluminium samples, using a permanent magnet electromagnetic acoustic transducer (EMAT). The design presented here exploits the non-contact nature of EMATs to allow continuous operation at elevated temperatures without physical coupling, sample preparation (in the form of oxide scale removal), or active cooling of the EMAT. A non-linear change in signal amplitude was recorded as the magnetite coated sample was heated in a furnace, whereas a steady decrease in amplitude was observed in aluminium. For a magnetite coated pipe sample, after a dwell time of 3 h, a SNR of 33.4 dB was measured at 450 °C, whilst a SNR of 33.0 dB was found at 25 °C. No significant EMAT performance loss was observed after one month of continuous exposure to 450 °C. EMAT-sample lift-off performance was investigated at elevated temperature on magnetite coated steel; a single-shot SNR of 31 dB for 3.0 mm lift-off was recorded at 450 °C, highlighting the suitability of this design for scanning or continuous fixed point inspection at high temperature.

The main disadvantage of electromagnetic acoustic transducers (EMATs) is their inefficiency in generating ultrasound. A new design of Rayleigh-wave EMAT is proposed with improved ultrasonic generation efficiency on non-ferromagnetic materials. The new EMAT's magnet is narrower than the meander-line coil used in the transducer and the coil itself has an uneven distribution of conductors. The principle of the new design is informed by finite element simulations, which show that the EMAT generates Rayleigh waves more efficiently by taking advantage of both horizontal and vertical magnetic fields of the magnet simultaneously. Experimental measurements verify that the maximum amplitude of the ultrasonic signal generated by the new design of EMAT working under a pulse-echo configuration has been enhanced by 90%, when compared to conventional designs.

The use of guided waves is now widespread in industrial NDT for locating metal loss in pipelines, that manifests as pitting, corrosion and general wall thinning. In this paper, a screening technique is assessed in terms of defect detection and defect sizing capability. Shear Horizontal (SH) guided waves propagate circumferentially around the pipe whilst the scanner is moved axially along the length. This type of tool is preferable to other methods, being applied to the exterior of the pipe, without requiring full circumferential access, and is able to operate through thin coatings (up to 1 mm thick). It is designed to provide a pipe screening tool for petrochemical pipelines both topside and subsea, particularly for detecting defects at pipe support areas. The system's efficacy in terms of detection and sizing of defects is considered via experimental measurements on artificially induced defects and in service corrosion patches, with results compared to finite element modelling of the interaction of the guided waves with artificial defects. Finite element modelling has been used to better understand the behaviour of different wave modes when they interact with defects, focusing on the mode conversions and reflections that occur.

An investigation was performed into the effects of operating an absolute eddy-current testing (ECT) probe at frequencies close to its electrical resonance. A previously undocumented defect signal enhancement phenomenon, resulting from associated shifts in electrical resonant frequency, was observed and characterized. Experimental validation was performed on three notch defects on a typical aerospace superalloy, Titanium 6Al–4V. A conventional absolute ECT probe was operated by sweeping through a frequency range about the electrical resonance of the system (1 � 5 MHz). The phenomenon results in signal-to-noise ratio (SNR) peak enhancements by a factor of up to 3.7, at frequencies approaching resonance, compared to those measured at 1 MHz. The defect signal enhancement peaks are shown to be a result of resonant frequency shifts of the system due to the presence of defects within the material. A simple, operational approach for raising the sensitivity of conventional industrial eddycurrent testing is proposed, based on the principles of the observed near electrical resonance signal enhancement (NERSE) phenomenon. The simple procedural change of operating within the NERSE frequency band does not require complex probe design, data analysis or, necessarily, identical coils. Therefore, it is a valuable technique for improving sensitivity, which complements other ECT methods. & 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license

Electromagnetic acoustic transducers (EMATs) can emit and receive ultrasound on a conducting sample without contact, but are usually kept within 3 mm lift-off from the sample surface, to achieve a sufficient signal-to-noise ratio (SNR). There are scenarios under which EMATs must scan a sample at high speed, with the EMAT-sample separation varying by more than the standard lift-off range, such as for detection of gauge corner cracks in rail. A new EMAT has been designed that allows the low weight and flexible EMAT coil to skim over the sample surface, while the heavier and bulkier magnet behind the coil has a lift-off that can vary over 10 mm whilst still achieving a reasonable SNR. In experiments conducted with the EMATs mounted on a train, scanning a rail, they were demonstrated as being sufficiently robust, with an SNR sufficient for defect detection.

Shear horizontal (SH) ultrasound guided waves are being used in an increasing number of non-destructive testing (NDT) applications. One advantage SH waves have over some wave types, is their ability to propagate around curved surfaces with little energy loss; to understand the geometries around which they could propagate, the wave reflection must be quantified. A 0.83mm thick aluminium sheet was placed in a bending machine, and a shallow bend was introduced. Periodically-poled magnet (PPM) electromagnetic acoustic transducers (EMATs), for emission and reception of SH waves, were placed on the same side of the bend, so that reflected waves were received. Additional bending of the sheet demonstrated a clear relationship between bend angles and the reflected signal. Models suggest that the reflection is a linear superposition of the reflections from each bend segment, such that sharp turns lead to a larger peak-to-peak amplitude, in part due to increased phase coherence.

Beam steering has been achieved using shear horizontal waves generated using a periodic permanent magnet (PPM) electromagnetic transducer (EMAT). Unlike phased arrays, where steering is achieved by carefully controlling the firing of individual elements, the spatial periodicity of the PPM EMAT is ultilised to steer the beam whilst exciting all elements simultaneously. Due to the periodic nature of the array, the interference of individual waves from each of the elements creates a highly frequency dependent angle of propagation, allowing the directivity to be changed by simply varying the frequency of the input signal. Simultaneous excitation precludes the need for complicated and expensive phased array hardware. A frequency domain model is developed so that the beam characteristics, such as steering angle and beam width, can be calculated, allowing for investigation into the beam steering qualities of the PPM transducer. Broadband pulsed generation is also demonstrated, showing how a wave is generated over a large range of angles, meaning a large area can be covered with a single pulse. Interesting properties of this wave, such as a variation of frequency as a function of angle, and how this can be useful, are also discussed.

Understanding of optimal signal generation and frequency content for electromagnetic acoustic transducers (EMATs) is key to improving their design and signal to noise ratio. Linear and meander coil designs are fairly well understood, but other designs such as racetrack or focused coils have recently been proposed. Multiple transmission racetrack coil EMATs, with focused and unfocused designs, were constructed. The optimum driving frequency for maximum detected signal was found to range between 1.1 and 1.4 MHz on aluminium for a 1.5 mm width coil. A simple analytical model based on the instantaneous velocity of a wave predicts a maximum signal at 1.44 MHz. Modelling the detection coil as a spatial square wave agrees with this, and predicts a general relation of f =0.761v/L between the optimum frequency f , the wave velocity v, and the coil width L. A time domain model of the detection coil predicts a 1.4-1.5 MHz peak for continuous wave excitation, with a frequency that decreases as the length of the wavepacket is decreased, consistent with the experimental data. Linear coil modelling using the same technique is shown to be consistent with previous work, with improving detection at lower wave frequencies, and signal minima at every integer multiple of the wavelength. Finite Element Analysis (FEA) is used to model the effects of the spatial width of the racetrack generation coil and focused geometry, and no significant difference is found between the focused and the unfocused EMAT response. This highlights the importance of designing the EMAT coil for the correct lift-off and desired frequency of operation. [Abstract copyright: Copyright © 2019 Elsevier B.V. All rights reserved.]

The amplitude of an ultrasonic signal generated by electromagnetic acoustic transducers (EMATs) is typically low when compared to those generated by contacting transducers, which restricts the application of EMATs in the fields of nondestructive testing and nondestructive evaluation. The transmission process of a surface wave EMAT is studied, based on a previously established 3-D model, with the aim of enhancing the amplitude of ultrasonic waves generated by the EMAT. The effect of changing various EMAT parameters on the surface wave is investigated, by utilizing the orthogonal test method. Results indicate that after optimization, the signal amplitude of the EMAT has increased by 25.2%.

Time of flight diffraction (ToFD) outputs B-scans using an ultrasound emitter and receiver at a constant separation, scanned over a sample surface parallel to the line between the transducers. The B-scan, with time and scan position axes, contains parabolic features characteristic of a point-like scatterer. Human vision effectively detects these shapes, but this is time consuming and requires training. A parabola matched filter has been developed that is simple to implement and transforms parabolic shapes to peaks whilst reducing noise in the scan. The scan can then be displayed as depth versus lateral position.

Surface breaking defects are detected using a scanning laser source to deposit heat into a sample surface. Any lateral flow of heat is disturbed by such a defect, with a change in thermal spot shape being detected by an infrared camera. An apparent increase in temperature as the laser passes over a defect is due to a localised change in emissivity, allowing defects to be distinguished from surface markings. Defects have been detected in both stainless steel and aluminium.

Time of flight diffraction (ToFD) B-scans may contain horizontal lines from surface waves propagating directly from emitter to receiver and angled lines after a reflection off a surface edge. A Hough transform (HT) can detect lines but assumes lines at less than 45° from the vertical consist of a series of connected vertical segments (“thick” lines). In a B-scan image, narrow (temporally sharp) lines at less than 45° from the vertical become “sparse” such that there are gaps in the times of arrival between scan positions. This paper describes a modified HT, used to successfully identify and remove such lines from B-scans.

Preliminary results obtained at a range of temperatures using a pulsed electromagnet Electromagnetic Acoustic Transducer (EMAT) show that this alternative approach overcomes the limitation of permanent magnets due to their relatively low maximum operating temperature. Operation on low carbon mild steel sample at temperatures up to 250 °C has been demonstrated. Furthermore, this transducer has shown a significant enhancement in the generated shear wave and better lift-off performance when compared with permanent magnet EMATs at room temperature on mild steel.

A model has been developed for the detection of ultrasonic waves using electromagnetic acoustic transducers (EMATs). EMATs are particle velocity sensors, which can be designed to have sensitivity to in-plane and, or out-of-plane ultrasonic displacements, by suitably arranging the magnetic field in the receiving EMATs relative to the orientation of the coil. Good agreement between the results from modeling and experimental measurements has been demonstrated.

Electromagnetic acoustic transducers (EMATs) are non-contact ultrasonic transducers capable of generating wide band ultrasonic surface waves on metallic samples. A lab-based laser-EMAT system has been developed to observe the ultrasonic surface wave propagation and interaction with surface breaking defects on the sample rail head surface. A wide band EMAT generating surface waves with a frequency content between approximately 50 and 500 kHz is used to propagate ultrasonic waves on the surface of a rail head down the length of the sample. A stabilised Michelson interferometer is used to measure the out-of-plane displacement of the surface wave. A complete picture of the ultrasonic surface wave on the sample surface over time is reconstructed using this technique, with exceptionally high spatial and temporal resolution. Despite the curvature of the rail head, the ultrasonic surface wave propagating down the rail is found to have similar properties to Rayleigh waves by direct comparison to those observed on flat samples using the same technique.

Electromagnetic acoustic transducers (EMATs) often have an electrically conducting surface behind the coil which we refer to as a back-plate, which could be there to act as an electromagnetic screen or could be a permanent magnet which is part of standard EMAT design. Eddy current probes can share many design features with EMATs, and both are designed to generate an eddy current in the electromagnetic skin-depth of a sample. In this paper we present an analytical solution for a quick and accurate calculation of the eddy current generated by a spiral coil and for the case of EMATs we also calculate the resultant Lorentz force which leads to ultrasonic generation. The Lorentz force can arise from an interaction of the eddy current with either a dynamic magnetic field from the coil or with a static magnetic field from a permanent of electromagnet. Theoretical predictions have shown that in the presence of a ferrite back-plate, the eddy current and the Lorentz force are enhanced greatly, and this has been verified experimentally. These developments will lead to improved EMAT and eddy current probe design.

A model has been developed which can be used to calculate the particle velocity waveforms at arbitrary positions in a solid immersed in a fluid when irradiated by a transmitting transducer, launching a short pulse of a longitudinal wave into the fluid. Mode-converted longitudinal and shear waves are generated at the interface between the solid and the fluid. Calculations performed agree well with the results of experimental measurements using a specially constructed miniature piezoelectric receiving probe. The model constructed has been used to explain the radiation of a transmitting transducer into an immersed solid and it can be used for the optimal design of a transducer and ultrasonic NDT device in immersion mode, that has been widely used for ultrasonic imaging and automatic inspection.

We present work on the development of an ultrasonic texture measurement system for sheet metals using non-contact transducers, suitable for use both in the laboratory and on the production line. Variation of the velocity of the zero-order symmetric (S0) Lamb wave is used to determine the crystallographic texture of polycrystalline metal sheets ranging in thickness from 0.1 to 3 mm. This system features improvements on previous state-of-the-art ultrasonic technology in that it probes velocity over a continuous range of angles using only two electromagnetic acoustic transducers (EMATs). This is demonstrated to offer a significant improvement in accuracy and allows the detection and investigation of asymmetric anisotropies in the sheets. Another advantage of the system is its potential for combining several different measurements using a single pair of transducers. The capability is demonstrated for through-thickness shear wave measurements as well as the zero-order symmetric Lamb wave measurements which are the primary means of determining the texture. The change between generating Lamb and through-thickness bulk waves can be made entirely by changing the electrical circuit connected to the EMATs without modifying the transducer assembly in any way. Measurement of all of the above waves can provide information on the sheet thickness and other physical properties of the sheet in addition to texture. Certain texture parameters can be calculated from both Lamb and shear wave velocities, allowing self-calibration of the system.

Electro-magnetic acoustic transducers (EMATs) generate ultrasonic waves in metals through an electromagnetic coupling mechanism. A concept for EMAT generation, using a coil alone without a permanent magnet, but with a pulse generator and a sample, is introduced. A simplified equivalent coil circuit is given, and has been validated by experimental measurements. Such an equivalent circuit is useful for excitation current calculations, which have often been neglected in previous publications in this area but have proved to be of great importance in considering the efficiency and frequency characteristics of ultrasonic generation. Due to the coil sample coupling, the equivalent coil inductance is dependent on the electrical conductivity and magnetic permeability of the metal sample, the lift-off, the coil orientation relative to the metal sample and the coil configuration. The use of a coil alone to generate ultrasound has limited applications as a suitable ultrasonic detector must also be used, but where appropriate this type of generation source provides a robust, non-contact and inexpensive means of ultrasonic generation without worrying the permanent magnet attracting ferromagnetic particles that can prevent from practical on-line application of EMATs.

The ability to detect defects in rail, and in particular surface defects such as gauge corner cracking, is essential. Current ultrasonic testing of the rail uses contacting transducers in a send-receive mode which are at present limited in testing speed to around 20-30 mph, and signals from serious surface defects may be masked by nearby shallow surface defects. Using Rayleigh-like surface waves generated and detected in a pitch-catch manner we can overcome the effect of this masking, and by using electro-magnetic acoustic transducers (EMATs) working at a small standoff we may be able to lift many of the current rail testing speed limitations with a high accuracy for detecting and gauging surface defects. We present experiments on rail samples with real and machined defects, using EMATs to generate and detect a low-frequency wide-band surface wave. The depths of the defects can be characterised through analysis of both the time domain signal amplitude and frequency dependent behaviour. The design of EMAT used is discussed, with consideration of signal, frequency content, directionality of the signal and the effect of standoff above the sample.

A model has been developed for the prediction of the transient displacement of a Rayleigh wave generated by an electromagnetic acoustic transducer (EMAT), operating on the Lorentz principle. This model can be used to calculate Rayleigh waves of arbitrary forces. A linear coil and a spiral coil are considered as two examples. Out-of-plane displacement of the Rayleigh wave is measured experimentally using a Michelson laser interferometer. Good agreement is observed between the calculated and the measured results, demonstrating the value of this method. This paper explains how the Lorentz forces due to the static and dynamic magnetic fields work constructively and destructively, indicating that the orientation of the external magnetic field must be correct to achieve an efficient generation of the Rayleigh wave.

In non-destructive testing for cracks it is not sufficient to simply detect the presence of a defect, but it is essential to have an accurate measure of the depth. Accurate calibration of the techniques used to gauge defect size is therefore necessary. Recent progress in the field of non-contact ultrasonic testing has led to the development of a practically viable system for generating and detecting wideband Rayleigh waves on electrically conducting or magnetic samples using electro-magnetic acoustic transducers (EMATs). This system has been used to gauge the depth and position of surface breaking defects, and has many applications including metal billet testing and detecting and sizing gauge corner cracking in rails. In this paper we report experiments calibrating the response of EMATs when a defect is present between the generator and receiver, using a calibration sample with slots machined perpendicular to the surface to simulate surface breaking cracks. The depth of the defect can be gauged in the time domain and frequency domain, with an accurate ‘fingerprint’ of the position given by an enhancement of the signal when the receiver is close to the defect. The best choice of EMAT design for different applications is discussed, as is the best position for the receive EMAT to avoid areas of interference between the Rayleigh wave and bulk waves diffracted from the crack tip.

This paper describes a new type of non-contact electromagnetic transducer (EMAT) that can be used to generate both Lamb and Rayleigh waves on metal samples. The generated waves are wideband and low frequency with a dominant frequency content centred at approximately 200 kHz extending to around 500 kHz. The transducers have been used on both aluminium and steel, but operate more efficiently on aluminium due to its lower electrical resistance and density when compared to steel.

The paper reports the first measurements of the single crystal elastic constants of the heavy rare earth metal thulium as a function of temperature and magnetic field. The constants were obtained from ultrasonic velocity measurements over a temperature range of 4.2–296 K and in applied magnetic fields of up to 5 T. The elastic constants; C 11 , C 33 , C 44 and C 66 =(C 11 –C 12 )/2 were determined from the ultrasonic velocities. Anomalies in the elastic constants were observed at 58 K from the c -axis propagated shear wave measurements and at 55 K from the c -axis propagated longitudinal wave measurements. Significant softening of the elastic constants C 33 and C 44 was observed close to T N . Application of a magnetic field (>2 T) along the c -axis direction induced further softening of the material. Electromagnetic acoustic transducers (EMATs) were also employed in addition to conventional piezoelectric quartz transducers. A marked increase in the EMATs acoustic coupling efficiency (generation and detection efficiency) occurred close to T N .

Non-destructive testing of welded metal components is an area where significant cost savings can be made if preparation costs can be minimised. The largest cost associated with preparation is grinding off the weld cap in order to facilitate the detection of defects that may be present just under the cap. In addition, there are also potential savings to be made if the actual testing time can be reduced or if the tests can be performed at elevated temperatures beyond which conventional contacting ultrasonic techniques can be practically used. This paper describes a non-contacting NDT method using a pulsed laser and an electromagnetic acoustic transducer (EMAT) which has the potential to interrogate the entire cross-section of a weld.