L.G. Ullate

Porosity estimation of concrete by ultrasonic NDE

The increasing number of concrete structures with symptoms of premature deterioration due to environmental action demands procedures to estimate the durability of this type of component. Concrete durability is related to porosity, which determines the intensity of interactions of the material with aggressive agents. The pores and capillaries inside the structure facilitate the destructive processes that generally begin in the surface. In this work, an ultrasonic NDE technique to estimate the porosity of concrete is developed. The method is based on the analysis of the mechanical behaviour of mortar probes built with calibrated sand, in which the concentration of water-cement mixture has been varied. In this sense, data of sound velocity are correlated with data of porosity, which have been previously measured by destructive measurements.

NDE ultrasonic methods to characterise the porosity of mortar

Premature damage of mortar and concrete structures, due to environmental action, demands procedures to estimate durability of this type of components. Mortar or concrete composition (e.g. grain size, type and percentage of sand) may have some influence in the durability, but it is mainly related to porosity, which determines the interaction between aggressive agents and material. In this work, several IDE ultrasonic methods to estimate porosity of mortar are presented and evaluated. In these methods, porosity is related to (1) the material structural noise, (2) sound velocity and (3) ultrasonic attenuation. In all these methods, mortar is consider to be formed by only two phases: solid and pores.

A new algorithm to calculate the transient near-field of ultrasonic phased arrays

An algorithm valid for an accurate calculation of the near-field in the scanning plane of ultrasonic phased arrays is presented. Using the classical time-domain impulse response approach, a simple analytical expression for the impulse response at points lying in the central plane of a narrow rectangular aperture is decided. An expression for the array impulse response is then obtained by superposition. The proposed solution is useful for an efficient computation of transient and continuous wave (CW) pressure fields without requiring any far-field or paraxial approximations. Moreover, the convolution-impulse response approach applied to phased arrays constitutes an important tool for the analysis of array fields. Some numerical examples are presented, in which the advantages of using the array impulse response in the field analysis are shown. Several aspects of array fields not currently described in literature are included in the examples.<<ETX>>

2D array design based on Fermat spiral for ultrasound imaging

The main challenge faced by 3D ultrasonic imaging with 2D array transducers is the large number of elements required to achieve an acceptable level of quality in the images. Therefore, the optimisation of the array layout, in order to reduce the number of active elements in the aperture, has been a research topic in the last years. Nowadays, array technology has made viable the production of 2D arrays with larger flexibility on elements size, shape and position, allowing to study other configurations different to the classical matrix organisation, such as circular, archimedes spiral or polygonal layout between others. In this work, the problem of designing an imaging system array with large apertures and a very limited number of active elements (N(e)=128 and N(e)=256) using the Fermat spiral layout has been studied. As summary, a general discussion about the most interesting cases is presented.

64 Elements two-dimensional piezoelectric array for 3D imaging

Ultrasound has a large potential on non-invasive inspection with main applications in medical imaging and non-destructive testing (NDT). The increasing interest in 3D imaging applications leads to investigate new solutions for two-dimensional (2D) ultrasonic arrays with an affordable number of electronic channels without resolution degradation. 2D segmented annular arrays (SAAs) are a good compromise between resolution--image quality--and number of electronically active channels. A 1-3 piezoelectric composites are used as basis material to manufacture the array transducers due to their low planar coupling and high electromechanical coupling coefficients. A 1.5 MHz SAA of 64 elements and 20 mm of diameter was designed, manufactured and tested. The design key point is the use of a flexible circuit with electrodes and tracks that define the array geometry. The piezocomposite was used as a monolithic support. Soft backing and one matching layer were used. The array elements have been tested electrically and acoustically showing good agreement with a KLM-based simulation model. Acoustical field measurements in water at different steering angles were made and compared with simulations performed with a model that uses an exact solution of the impulse response approach. Side lobes are important because the array geometry used was designed to work in metals for NDT purposes. Smaller array elements should be made for medical applications.

Application of micromechanics to the characterization of mortar by ultrasound.

Mechanical properties of concrete and mortar structures can be estimated by ultrasonic non-destructive testing. When the ultrasonic velocity is known, there are standardized methods based on considering the concrete a homogeneous material. Cement composites, however, are heterogeneous and porous, and have a negative effect on the mechanical properties of structures. This work studies the impact of porosity on mechanical properties by considering concrete a multiphase material. A micromechanical model is applied in which the material is considered to consist of two phases: a solid matrix and pores. From this method, a set of expressions is obtained that relates the acoustic velocity and Youngs modulus of mortar. Experimental work is based on non-destructive and destructive procedures over mortar samples whose porosity is varied. A comparison is drawn between micromechanical and standard methods, showing positive results for the method here proposed.

A small 2D ultrasonic array for NDT applications

A prototype of two-dimensional transducer array with reduced number of elements, based on segmented annular distribution is presented. The capability of this array to produce volumetric imaging is compared to the equivalent conventional 2D squared matrix array. The comparison between both apertures is made for the cases of the full-array emission/reception mode and SAFT mode. From the analysis it is deduced that the segmented annular arrays produce lower grating lobes than squared arrays, improving the image contrast. The fabrication process of a segmented annular array of 64 elements and the experimental work made with this array transducer is also presented in this paper.

Beam Steering with Segmented Annular Arrays

Two-dimensional (2-D) arrays of squared matrix have maximum periodicity in their main directions; consequently, they require half wavelength (lambda/2), interelement spacing to avoid grating lobes. This condition gives rise to well-known problems derived from the huge number of array elements arid from their small size. In contrast, 2-D arrays with curvilinear configuration produce lower grating lobes and, therefore, allow the element size to be increased beyond lambda/2. Using larger elements, these arrays have the advantage of reducing the number of elements arid of increasing the signal-to-noise ratio (SNR). In this paper, the beamforming properties of segmented annular phased arrays are theoretically analyzed and compared with the equivalent squared matrix array. In the first part, point-like elements are considered in order to facilitate the field analysis with respect to the array structure. Afterward, the effect of the element size on the steered beam properties also is presented. In the examples, it is shown that the segmented annular array has notably lower grating lobes than the equivalent squared matrix array and that it is possible to design segmented annular arrays with interelement distance higher than lambda whose beam characteristics are perfectly valid for volumetric imaging applications

Application of digital signal processing techniques to synthetic aperture focusing technique images

Abstract Synthetic aperture focusing technique (SAFT) has become a popular alternative to transmission–reception focused arrays (TRFA) in order to reduce hardware complexity and cost associated to ultrasonic (UT) imaging systems. A shortcoming of SAFT processing is that it introduces artifacts that distort the images. However, as SAFT is sequential, efficient digital processing algorithms can be used to improve image quality. In this paper, several digital processing techniques are proposed including apodization, deconvolution, dynamic focusing, deflection and envelope extraction. A pipeline architecture which allows execution of these algorithms in parallel for real-time imaging is also proposed.

Reduction of grating lobes in SAFT images

In this work we present a new synthetic aperture technique, derived from the conventional SAFT, where only one element in emission and two in reception are used (2R-SAFT). This new technique suppresses the grating lobes of the conventional SAFT and allows obtaining images with a better contrast and signal-noise ratio, at the same time that minimizes the hardware requirements of the imaging systems and maintains a high frame rate.