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Clamp-on ultrasonic flowmeters geometry includes four different materials in the ultrasound propagation path: wedge, coupling material, pipe and liquid. In this case, oblique incidence ultrasonic beam generated by the emitter transducer is split into several paths caused by multiple reflections in pipe wall or because of propagation mode conversion at interfaces. These multiple paths are recombined at the reception transducer aperture, producing a single signal whose amplitude and phase depend on which proportion each path contributes. The objective of this paper is to quantify this phenomenon and analyses the implications it has on the accuracy of Clamp-on ultrasonic flowmeters as a function of the fluid that flows inside the pipe. According to simulations, based on 2D ray tracing model and experimental measurements, the variations in the recombination signal at the reception transducer produces a linearity error that in case of the installation under test, Z-mode transducers configuration on PVC pipe, could reach a value of 3.2%.

This article explains how a multi-frequency ultrasonic beam path is split due to passing through dispersive materials and the implications that it has in flow measurement using a Clamp-on ultrasonic flowmeter. The importance of knowing this phenomenon is because dispersive materials are commonly used in the manufacture of transducer wedges and plastic pipes. Splitting of the ultrasonic beam path occurs when the incident angle is not normal to the surface of the pipe and a coupling wedge is necessary. The diffraction angle is produced at the boundaries among materials with different propagation velocities (according to Snell’s law), and furthermore, in dispersive materials, the propagation speed depends on frequency. Therefore, when the excitation signal contains several frequential components, each frequency travels with a different velocity. Under this circumstance, different diffraction angles are produced for each frequency at the boundaries between materials. As a result of splitting the path, the ultrasonic beam widens and the reception transducer aperture becomes too small to receive all the frequential components contained in the ultrasonic beam. Thus, when the ultrasonic beam is carried by the liquid flowing inside the pipe, the reception transducer receives different frequential contributions, producing change in its spectral composition. As a consequence of the mixing of these frequencies, the reception signal is modulated in phase and amplitude. Moreover, modulation will change as a function of frequential composition that is combined at the reception transducer. This is the origin of the linearity error in Clamp-on ultrasonic flowmeters.

Air coupled piezoelectric ultrasonic array transducers are a novel tool that could lead to interesting advances in the area of non-contact laminar material testing using Lamb wave’s propagation techniques. A key issue on the development of such transducers is their efficient coupling to air media (impedance mismatch between the piezoelectric material and air is 90 dB or more). Adaptation layers are used in order to attain good matching and avoid possible serious signal degradation. However, the introduction of these matching layers modify the transducer surface behaviour and, consequently, radiation characteristics are altered, making the usual idealization criteria (of uniform surface movement) adopted for field simulation purposes inaccurate. In our system, we have a concave linear-array transducer of 64 elements (electrically coupled by pairs) working at 0.8 MHz made of PZ27 rectangular piezoceramics (15 mm × 0.3 mm) with two matching layers made of polyurethane and porous cellulose bonded on them. Experimental measurements of the acoustic aperture of single excited array elements have shown an increment on the geometrical dimensions of its active surface. A sub-millimeter vibrometer laser scan has revealed an extension of the aperture beyond the supposed physical single array element dimensions. Non-uniform symmetric apodized velocity surface vibration amplitude profile with a concave delay contour indicates the presumed existence of travelling wave phenomena over the surface of the outer array matching layer. Also, asymptotic propagation velocities around 2500 m/s and attenuation coefficient between 15 and 20 dB/mm has been determined for the travelling waves showing clear tendencies. Further comparisons between the experimental measurements of single array element field radiation diagram and simulated equivalent aperture counterpart reveal good agreement versus the ideal (uniform displaced) rectangular aperture. For this purpose an Impulse Response Method (IRM) has been used.

Many ultrasonic sensors intended for non-destructive testing include a plastic or metal element, known as buffer rod, between the ultrasonic transducer and the material under analysis. Buffer rods are often terminated in the form of a conical tip for ultrasonic inspection of liquid-like substances. The conical tip is carefully shaped in a 45?? angle to favour the in phase reception of all the components of the ultrasonic wave reflected at the buffer tip, obtaining thus a maximum amplitude measurement signal. The effect of the buffer rod cone angle on measurement signals is studied in this work. A straightforward approximate formula for the effect of the error angle on measurement signal amplitude is used. Simulations are performed using a two-dimensional finite differences tool. Measurements are conducted with the same operating conditions and buffer rod materials and dimensions as those defined for both approximate formula evaluation and simulations. Thus, a comparison of the approximate formula, simulation and measurement results are established. Furthermore, the significance of some parameters such as ultrasonic transducer operating frequency and diameter, and buffer rod material are also analysed. The obtained results show that significant loss of the measurement signal amplitude is found only for cone error angles beyond the range of usual machining errors.

Buffer rods are usually plastic or metallic solid cylinders of known acoustic properties, which are placed between the ultrasonic transducer and the sample under study. For some applications, the buffer rod end in contact with the sample under inspection is shaped in a form of a conical tip. If the cone angle is fixed to 45°, the components of the ultrasonic wave reflected at the buffer conical tip are received at the transducer in phase, forming thus a maximum amplitude signal. Otherwise, the components of the ultrasonic wave are received out of phase, composing thus a weaker signal. In this work, the effect of the buffer rod cone angle on the measurement signals has been studied by means of simulations and experiments. The simulations were performed using a two-dimensional finite differences tool and the measurements were conducted with the same operating conditions and buffer rod materials and dimensions than those defined for simulations. The influence of some aspects such as ultrasonic transducer operating frequency and diameter, and buffer rod material has also been analyzed.

In Lamb waves inspection, an air-coupled transmitter transducer is oriented at a specific angle such that it generates a pure Lamb mode which propagates along the structure and interacts with any existing defects. For this inspection system, amplitude losses appears when small inclinations of the tested structure occurs. An important factor which affects directly these losses has been observed, it consists of the Lamb waves beam (LWB) deviation due to this bad alignment. In this work, a simple expression of LWB deviation has been deduced. This expression includes the test structure angle, phase velocity of generated Lamb mode, and the phase velocity of waves propagating in the coupled medium. A(0) Lamb mode is generated and detected in 1 mm thick aluminium plate sample using 1 MHz PZ27 piezoelectric transducers of 20 mm of diameter. Experimental LWB deviation angles are measured for different inclination angles of the test sample. A comparative study is released with theoretical results. For 1 degree of misalignment in the aluminium plate inclination, and transducers separation distance of 35 mm, LWB deviation angle is around 7 degrees and the amplitude is reduced by around 11%. Then, for a large separation distance, we must move the receiver transducer to detect the deviated LWB. It is shown that, for both theoretical and experimental studies, the LWB deviation and its measured amplitude are very sensitive to the alignment of the tested structure with respect to the transmitter-receiver transducers plane. In metal plates it is most satisfactory to use A(0) mode compared with S(0) mode since it is easy to excite and has a large amplitude and small deviation beam angles.

This work presents a processing subsystem for an air- coupled concave array NDT system, which uses Lamb waves. This processing system is able to equalize, delay and add the signals provided by a circular concave array used to receive Lamb waves in laminate materials, such as paper. A Field Programmable Gate Array (FPGA) is used in order to achieve shorter processing times and more flexibility than with DSP systems. The used array has circular concave shape and hence the delays used to get a plain wavefront are not lineal, thus a table with the delay values must be stored inside the FPGA. The Non-Destructive Testing is a growing field of interest for those industries where the high value of the developed components implies the need of a better quality control. In most of these cases the materials cannot be destroyed or altered, therefore a Non Destructive Test is needed. Ultrasonic testing is widely used to test a great variety of materials, but ultrasonic waves need a couplant to get a good transmission coefficient between the inspected material and the ultrasonic transducer. When air is used as couplant, new problems appear, like the higher attenuation that ultrasound suffers in this element. However, the greatest problem is the huge acoustic impedance mismatch that exists between air and the transducers. This mismatch, which is 40(PZT) to 0.006(Air), impedes a good transmission of the energy from the transducer to the inspected material. To improve the bad transmission coefficient, matching layers are included in the transducer (1) and improvements in the system's dynamic range are achieved (2). Lamb waves (3) are also widely used to inspect laminate materials. The main benefit which Lamb waves contribute in is the ability to inspect great portions of material in a short time, making them ideal for inspections where a great area has to be inspected. Moreover, the usefulness of Lamb waves in industrial applications where paper is tested has been demonstrated (4), (5). The usual way to produce a Lamb wave in a plate is to impact a plain wavefront with a certain angle respect to this plate. Lamb wave ultrasonic techniques are mostly based on the application of single element ultrasonic transducer with specific configurations, where the single element is specifically oriented to excite specific Lamb wave modes, see (6). The angular range where Lamb waves can be excited is very narrow in air (7), therefore a system that provides high angular resolution is needed. An array system would be a good solution to obtain this high angular resolution. It also has two main advantages, a transducer array will allow accurate beam steering, and the signal - to - noise ratio will improve. Furthermore, the array has a circular concave shape, so that it raises the angular steering margin of the array. The array used in this paper and its characteristics are deeply described in (8).

Rapid and non contact ultrasonic NDE techniques are of great industrial interest. This paper describes an air-coupled ultrasonic inspection system based on concave linear arrays working on pitch-catch configuration, The system has been designed for real-time characterisation of sheet and plate manufactured materials such as paper and resin-fibre composites. The proposed system is based on air-coupled Lamb wave excitation and reception that performs a rapid measurement of the optimum input angle of the incident beam impinging the material surface. No mechanical parts are used for tuning the plate wave excitation with the angle, doing that electronically by steering the acoustic beam. This solution increases the exploration velocity and the measurement repeatability and system reliability. The main contributions are related to the utilization of a 0.8 MHz ultrasonic air-coupled concave array transducer. This transducer, using only 32 elements, is able to generate a 2.5 square cm size flat wavefront, steering up to /spl plusmn/26 degrees with 1.6 degree resolution, keeping the distance of wave flight and the impact point constant. The angular resolution can be improved up to 0.2 degrees using a novel micro-deflection technique, without any increment of the system complexity.