On the measurement of frequency-dependent ultrasonic attenuation in strongly heterogeneous materials
Introduction
Non-destructive testing (NDT) by ultrasounds in strongly heterogeneous such as cementitious materials is a growing area of research. It enables the non-destructive assessment of microstructural properties, structural quality, and the degradation state of building materials. The most widely used technique is based on the measurement of ultrasonic pulse velocity (UPV). However, attenuation or energy parameters are considered even more sensitive than UPV. These parameters are used to examine the frequency-dependent nature due to dispersive media and the involved mechanisms of absorption and scattering energy associated with wave propagation through such media [1]. Ultrasonic attenuation has been employed for the characterization of cementitious materials, specially, for the size estimation, concentration and distribution of aggregates [2], [3], [4], [5], [6], [7], [8], and quantification and assessment of micro-cracks and damages [1], [6], [9]. Unlike UPV, accurate measurements of ultrasonic attenuation depend largely on several features such as the geometry of the specimen, the heterogeneities at the microstructure level, coupling type, and the forces applied upon transducers and the specimen. Moreover, the determination of the frequency-dependent ultrasonic attenuation over a wide frequency range is of paramount significance to establish more accurate estimations of microstructural parameters using inverse ultrasonic scattering problems [7], [10], [11].
The problem of energy loss due to the coupling type and variations regarding the contact pressure is solved by performing attenuation measurements in immersion. Several authors have developed different techniques to measure the precise attenuation on cementitious materials even using contact transducers, but they basically used several pairs of transducers to sweep over a wide frequency range [4], [7], [12]. In these studies, broadband pulses and narrowband bursts were applied as excitation signals for generating ultrasonic signals through the broadband transducers. However, the use of swept–frequency signals (chirp signals) to drive the emitter transducer has been shown to give good performance for measuring attenuation curves in cementitious materials [2], [3], [13]. This type of signals improves the effective bandwidth covering a wide frequency range using only a unique emitter transducer with much less electrical signal voltages required. This signal excitation gives optimal detection capability and measurement accuracy when it is combined with suitable processing [14].
The aim of this paper is to provide a simple method to measure the frequency-dependent attenuation in heterogenous media, especially cement mortar specimens. This is performed using a time–frequency (T–F) procedure applied to the received ultrasonic signal when it is by driven the emitter transducer with a linear swept–frequency signal. Firstly, an overview of the corrections of the transmission and diffraction effects in a transmission inspection in immersion is presented. Later, a brief exposition of three ultrasonic excitation techniques is introduced for the attenuation measurements from cementitious materials as well as a description of a T–F procedure to extract the chirp signal required to determine the precise attenuation. Finally, a comparison among the three described excitation techniques is addressed.
Section snippets
Attenuation measurement in transmission mode in immersion
In a through-transmission inspection in immersion, frequency-dependent ultrasonic attenuation can be determined by the frequency response of a pulse propagating through a specimen and the frequency response of a reference signal, the pulse propagating only in water. Assuming that the response of the measurement system is linear, the reference signal in water is found to be . is the transfer function that includes the amplitude spectrum of the electrical signal and the
Excitation techniques
To generate ultrasonic signals, broadband pulses or a set of narrowband pulses can be used as excitation signals for an emitter broadband transducer. With broadband pulses, the attenuation can be determined only by means of the spectra of the propagating pulses (traveling pulse through the specimen and the reference pulse) and using Eq. (3). However, the transmitted ultrasonic pulse exhibits its maximum energy on the resonance frequency that was designed for the transducer, and the bandwidth
Comparison among excitation techniques
To compare the performance of the three excitation techniques described above, a transmission mode inspection in immersion was carried out using two 2 MHz-broadband transducers (Krautkramer H2K, 10 mm diameter). The narrowband signals (sinusoidal signals with Gaussian envelope of 6–8 cycles) and the chirp signals were generated using the digital oscilloscope Tie-Pie HS3 (signals with peak-to-peak of 8 V). The broadband pulses were generated using the ultrasonic pulser/receiver AIMS Ultimo 2000
Conclusions
In this paper the measurement of frequency-dependent ultrasonic attenuation in strongly heterogeneous materials was addressed. To determine the total attenuation accurately over a wide frequency range, it is necessary to have suitable excitation techniques to improve the transducer bandwidth and the signal-to-noise ratio. In this study, a linear swept–frequency signal was employed to drive an emitter transducer for a through-transmission inspection in immersion. With this type of signal, there
Acknowledgements
The financial support of the Spanish Science and Innovation Ministry (Project BIA 2006-15188-C03-01 and BIA 2009-14395-C04-01) and the Spanish Ministry of Public Works (FOM 01/07) is greatly acknowledged. M. Molero was supported by the department of education of the Community of Madrid and the European Social Fund, and The Mexican National Council for Science and Technology CONACYT: (186384). S. Aparicio is supported by the postdoctoral JAE-Doc program of the Spanish National Research Council
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