Bogdan Piwakowski

A new approach to calculate the field radiated from arbitrarily structured transducer arrays

A efficient time-domain algorithm, based on the spatial pulse response approach, is proposed for the determination of the acoustic fields radiated by means of acoustical sources. The computations are performed by the discrete representation array modelling (DREAM) procedure, specially adapted to study the planar and arbitrarily structured multielement transducer arrays. DREAM, based on the discrete representation computational concept, acts as the generator of the array velocity potential impulse response, and thus, does not require any analytical solutions prior to the computations. The computations are valid for all field regions and may be performed for any excitation form. Apart from the classic case of rigid baffle conditions, the free and soft planar baffle also can be considered. The use of the time-domain solution for causal Greens function for lossy media enables the wideband absorption effects to be modeled. The accuracy of computations depends on temporal and spatial discretization and can be obtained as required. The quantitative rules, which determine the required discretizations to be predicted, are proposed. The computational examples show that DREAM allows the different and various transducers to be modeled. Its possibilities are illustrated by computations for the multielement transducers, including the beam-steered, amplitude-weighted sonar array, the focusing annular transducer, and the diverging and converging cylindrical array.

Detection of chemical damage in concrete using ultrasound

This research deals with a non-destructive method for characterizing the degraded cover of concrete structures using high-frequency ultrasound (0.5-1 MHz). Although such a frequency range is unusual in civil engineering, it is well suited to the kind of defect to be detected, as it corresponds to a thin near-to-surface layer with increased porosity and density of microcracks. In order to assess the feasibility of detecting concrete cover degradation, velocity and attenuation measurements were made on both halves of a concrete slab. One half was immersed into an acid solution for 15-45 days, while the other half remained sound. These measurements were made for longitudinal, transverse and surface waves. The results obtained show a 23% decrease of ultrasonic pulse velocity and a 1000% increase of attenuation in the degraded material relative to the sound material. It is thus possible to detect and characterize concrete cover degradation using high-frequency ultrasound. Although attenuation measurements in heterogeneous media are difficult, their sensitivity to degradation is very high.

Non destructive evaluation of degraded concrete cover using high-frequency ultrasound

In this paper, the feasibility of detecting and characterizing concrete cover degradation by ultrasonic testing is investigated. The on-site nondestructive evaluation of concrete cover is vital to monitor the integrity of concrete structures and prevent irreversible damage. Given the thickness of the degraded layer - which can be as small as a few millimeters - the sensitivity of ultrasound propagation to cover degradation is expected to increase with frequency. Therefore, high-frequency ultrasound (0.5 to 1 MHz) is used. In order to assess the feasibility of detecting concrete cover degradation, velocity and attenuation measurements were made on both halves of a mortar slab. One half was chemically degraded, while the other half remained sound. The pulse velocity and the attenuation coefficient were measured on both halves for compression, shear and surface waves. After 45 days of degradation, up to 24% velocity decrease was observed, while attenuation increased up to 10 times. High-frequency ultrasound is thus able to detect changes in the microstructure of the concrete cover even at an early stage of degradation.

Automated non destructive testing by non-contact surface waves

The paper presents an automated device designed for the non destructive control and characterization of materials using ultrasonic surface waves (SW). The system of transmission and reception of waves is non-contact, i.e. uses air as the coupling medium between transducers and samples. The solution allows to avoid distortion of signals by a coupling agent and enables quick and automatic measurement. The recorded SW are processed to obtain the velocity, attenuation and dispersion characteristic of SW which can be directly used in order to evaluate the physical parameters of inspected medium cover. Particularly useful in Non Destructive Testing (NDT) is the inversion of velocity dispersion characteristics which enables to evaluate the variations of shear velocity as a function of the concrete depth. An example of application of the technique can be found in accompanying paper by Piwakowski et al. in the same proceedings‥

Evaluation of concrete cover by surface wave technique: Identification procedure

Concrete cover degradation is induced by aggressive agents in ambiance, such as moisture, chemicals or temperature variations. Due to degradation usually a thin (a few millimeters thick) surface layer has porosity slightly higher than the deeper sound material. The non destructive evaluation of concrete cover is vital to monitor the integrity of concrete structures and prevent their irreversible damage. In this paper the methodology applied by the classical technique used for ground structure recovery called Multichanel Analysis of Surface Waves is discussed as the NDT tool in civil engineering domain to characterize the concrete cover. In order to obtain the velocity as a function of sample depth the dispersion of surface waves is used as an input for solving inverse problem. The paper describes the inversion procedure and provides the practical example of use of developed system.

Auscultation non destructive du chevetre du pont de la Marque (59) : Combinaison de techniques

ABSTRACT This article was the result of a series of measurements on a bridge wall near Lille carried out by the various teams of a national project supported by the RGC&U, entitled “evaluation of the degradation of cover concrete and assistance to the diagnosis and the repair of the works” which the goal is to try to quantify what kind of improvement can be expected from the combination of non-destructive techniques in order to assess the structural state of existing reinforced concrete structures. Three techniques were employed: radar, resistivity and ultrasonic. The various teams implied in the project have tested the aptitude of each technique to detect the alkali-aggregate reaction but also to qualify the state of deterioration of the concrete by the combination of the results obtained by these various techniques.

Measurement of ultrasonic attenuation and Rayleigh wave dispersion for testing concrete with subsurface damage

This paper describes a non-destructive method for characterizing the cover of concrete structures using high-frequency ultrasound (0.5 to 1 MHz). Although this frequency range is unusual for such a material, it is well suited to the kind of defect to be detected, i.e. a thin damaged subsurface layer. This research has been carried out in three directions: (i) characterization of concrete samples through measurement of porosity and ultrasonic pulse velocity, (ii) adequate signal processing to access the extremely high attenuation to be measured and (iii) testing of concrete samples using Rayleigh waves. The tested samples have been submitted to chemical degradation. Measured velocities of compression and shear waves are used to derive estimates of elastic moduli. Porosity measurements have also been performed, showing that the observed velocity and stiffness decrease are related to an increase of the damaged layer thickness, not to an increase of porosity in this layer. Furthermore, a frequency domain system identification approach is used to derive an estimate of ultrasonic attenuation from multiple transmitted signals. Finally, high-frequency Rayleigh waves are generated into mortar samples by the wedge method. Several transducer/wedge combinations are tested and the optimal configuration is used to yield dispersion curves. Rayleigh wave dispersion evolves as expected with increasing damage and gives access to depth-dependent characteristics of the degraded layer.

Generation of broadband surface acoustic waves using a dual temporal-spatial chirp method

Wideband surface acoustic wave (SAW) generation with a spatial chirp-based interdigital transducer was optimized for non-destructive characterization and testing of coatings and thin layers. The use of impulse temporal excitation (Dirac-type negative pulse) leads to a wide band emitter excitation but with significantly limited SAW output amplitudes due to the piezoelectric crystal breakdown voltage. This limitation can be circumvented by applying a temporal chirp excitation corresponding in terms of frequency band and duration to the spatial chirp transducer configuration. This dual temporal-spatial chirp method was studied in the 20 to 125 MHz frequency range and allowed to obtain higher SAW displacement amplitudes with an excitation voltage lower than that of the impulse excitation.

Ultrasonic reflectometry in air: Errors of sample positioning

The reflection coefficient of ultrasonic waves propagating in air and interacting with a plane surface of a solid is considered. The simulation of dependence of the reflection coefficient on errors of sample positioning is performed using a finite beam model along with an angular spectrum method, and next the results are validated experimentally. The simulations show that for the considered range of geometrical parameters, the role of the wave divergence for the reflection coefficient in air is insignificant. The important consequences of errors of the sample positioning are that the shift of the sample influences mostly the phase, while the errors of inclination of the sample mainly affect the magnitude of the reflection coefficient. The experiments confirm simulation results pointing out the necessity of high precision of measurements for ultrasonic reflectometry in air. The results can be used for assessment of precision, calibration, and reduction of errors in applications of the reflectometry tests.

Automated non-contact NDT by ultrasonic surface waves

An automated ultrasonic scanner based on the non-contact air-coupled technique is designed and developed in order to perform nondestructive characterization of the surface of different structures and to detect the surface defects. The measurement is carried out without any contact with the examined part. The paper presents the scanner and several examples of its applications: the on-site characterization of concrete walls; the inverse analysis of concrete wall cover and the detection of surface defects on a steel sample and along old used railway rail.