J.J. Anaya

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.

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.

Sand/cement ratio evaluation on mortar using neural networks and ultrasonic transmission inspection

The quality and degradation state of building materials can be determined by nondestructive testing (NDT). These materials are composed of a cementitious matrix and particles or fragments of aggregates. Sand/cement ratio (s/c) provides the final material quality; however, the sand content can mask the matrix properties in a nondestructive measurement. Therefore, s/c ratio estimation is needed in nondestructive characterization of cementitious materials. In this study, a methodology to classify the sand content in mortar is presented. The methodology is based on ultrasonic transmission inspection, data reduction, and features extraction by principal components analysis (PCA), and neural network classification. This evaluation is carried out with several mortar samples, which were made while taking into account different cement types and s/c ratios. The estimated s/c ratio is determined by ultrasonic spectral attenuation with three different broadband transducers (0.5, 1, and 2 MHz). Statistical PCA to reduce the dimension of the captured traces has been applied. Feed-forward neural networks (NNs) are trained using principal components (PCs) and their outputs are used to display the estimated s/c ratios in false color images, showing the s/c ratio distribution of the mortar samples.

Multi-pattern adaptive inverse filter for real-time deconvolution of ultrasonic signals in scattering media

This work provides a method for real-time deconvolution, which considers the signal distortion of the ultrasonic (UT) wave-front travelling in dispersive materials. The deconvolution method is based on an inverse filter, whose coefficients are computed by an adaptive algorithm from a set of pattern signals scanned at different depths of the tested material. Several experiments carried out on aeronautic composite laminates show that a valuable improvement of axial resolution is obtained at the different depths of the test piece.

Ultrasonic wave propagation in cementitious materials: a multiphase approach of a self-consistent multiple scattering model.

This paper examines ultrasonic wave propagation through strongly heterogeneous materials such as cementitious materials, and deals meanly with the formulation of a multiphase approach of a self-consistent multiple scattering model, the so-called dynamic generalized self-consistent model (DGSCM) proposed by Yang [J. Appl. Mech. 70(2003) 575-582]. This extended model can describe the influence of the size and volume fraction of aggregates on cementitious materials, as well as the interaction, contribution, and influence of entrapped air voids together with the aggregates on frequency-dependent parameters such as the phase velocity and the attenuation coefficient. To show the performance of this approach, theoretical predictions were compared with experimental ultrasonic measurements over a wide frequency range from several mortar specimens with different features in their microstructure properties and concentrations of aggregates up to 60%. The multiphase approaches of both the DGSCM and the Waterman-Truell model (WT) were also compared. The obtained results of the multiphase DGSCM were found to be significantly better than those obtained from the N-phase WT model for ultrasonic measurements from cementitious materials at high aggregate concentrations. The feasibility of material characterization using the multiphase approach of DGSCM was also discussed.

A high-resolution object recognition ultrasonic system

Abstract An ultrasonic system for object identification based on a single sensor operating in the pulse-echo mode is presented. Instead of direct processing of the echoes, the impulse response to the diffraction of the objects is first extracted by means of a deconvolution process. Since the impulse response has high axial resolution it offers more possibilities to find discriminant parameters to achieve the object recognition task. For this purpose, the impulse response is segmented and a search is done on values involving the whole impulse response, each one of the intervals and relationships between values in these intervals. To test the proposed method, an experimental framework has been arranged. Results show a quite good performance when the identification procedure is based on discriminant parameters obtained from the impulse response.

A three-transducer ultrasonic system for object location in air

Abstract A B-mode ultrasonic imaging system using three transducers mounted in a static triangular configuration is presented. The system determines the position of single or multiple reflecting points placed in the inner part of a triangle. Signals propagate through air, as is currently found in robotic applications. The low resolution expected in low-density media has to be compensated by a clever processing of the signals. Thus, the range resolution is drastically improved if impulse responses are obtained by deconvolution. The location of objects is then accomplished by means of a focusing procedure. In order to obtain real-time images, computation is carried out by a parallel digital architecture based on transputers, which also supports a digital signal processor for fast numerical treatment of ultrasonic signals.

Signal-to-noise ratio enhancement based on the whitening transformation of colored structural noise

In the ultrasonic testing and evaluation of highly scattering materials (i.e. non-homogeneous media such as composites, layered and clad materials) structural noise is an important limitation to the visibility of flaw echoes. This noise cannot be reduced by conventional linear filtering or by time-averaging techniques. In order to enhance the defect-to-background noise ratio (SNR), many different algorithms have been developed over the years. This work analyzes three new strategies for SNR enhancement based on the whitening transformation of the colored structural noise. By using this transformation, the small spectral differences between noise and flaw echoes are exploited, thereby allowing an improvement in the visibility of the flaw.

Performances of remote digital processing in automated non-destructive evaluation

Abstract Recent advances in transducers, electronic techniques, computer architectures and digital signal processing provide substantial improvements in automated non-destructive evaluation (ANDE) techniques. ANDE must fulfil very strict specifications, especially related to reliability (i.e., the quality of images, repetitiveness of acquisitions, etc.) and to the inspection speed. Multi-channel operation and digital signal processing provide a good solution to overcome strict NDE demands. In this paper, an approach based on a multi-channel remote system (RSENDAS 1 ) is presented, in which both signal conditioning and digital processing are done locally. The remote system performs complex algorithms for noise suppression, resolution improvement and smart data reduction at a sustained global processing rate of 10M samples per second. The central computer operates in parallel to attend to other jobs such as movement control, image display, etc. Several features of this solution related to noise immunity, system flexibility, system parallelism and other advantages are analysed in the paper.

Numerical simulation of ultrasonic tomography inspections of highly heterogeneous materials

This paper deals with the simulation of ultrasonic transmission tomography systems in water-immersed to nondestructively inspect highly heterogeneous materials with fractures. The time-domain Elastodynamic Finite Integration Technique (EFIT) was employed for all numerical simulations because is able to reliably simulate this type of ultrasonic problems. The EFIT code was implemented using OpenCL and PyOpenCL. Several ultrasonic tomography inspection setups were numerically simulated under different conditions varying the number of ultrasonic sources and their size and number and different operation schemes. Sinograms of concrete scenarios were computed and compared for each configuration, using homogeneous materials with similar fracture types and experimentally validated.