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.     

In vitro chronic hepatic disease characterization with a multiparametric ultrasonic approach

Although, high resolution, real-time ultrasonic (US) imaging is routinely available, image interpretation is based on grey-level and texture and quantitative evaluation is limited. Other potentially useful diagnostic information from US echoes may include modifications in tissue acoustic parameters (speed, attenuation and backscattering) resulting from disease development. Changes in acoustical parameters can be detected using time-of-flight and spectral analysis techniques. The objective of this study is to explore the potential of three parameters together (attenuation coefficient, US speed and integrated backscatter coefficient-IBC) to discriminate healthy and fibrosis subgroups in liver tissue. Echoes from 21 fresh in vitro samples of human liver and from a plane reflector were obtained using a 20-MHz central frequency transducer (6-30 MHz bandpass). The scan plane was parallel to the reflector placed beneath the liver. A 30 x 20 matrix of A-scans was obtained, with a 200-microm step. The samples were classified according to the Metavir scale in five different degrees of fibrosis. US speed, attenuation and IBC were estimated from standard methods described in the literature. Statistical tests were applied to the results of each parameter individually and indicated that it was not possible to identify all the fibrosis groups. Then a discriminant analysis was performed for the three parameters together resulting in a reasonable separation of fibrotic groups. Although the number of tissue samples is limited, this study opens the possibility of enhancing the discriminant capability of ultrasonic parameters of liver tissue disease when they are combined together.     

Ultrasonic sizing of short surface cracks

This paper presents a method for ultrasonic sizing of surface cracks based on time domain and frequency domain Rayleigh wave near-field analysis. The procedure allows for the entire range of ratio of crack depth to Rayleigh wavelength a/λ to be covered with one single measurement. In the time domain the time-of-flight method was extended to cracks smaller than the wavelength by correlation of the time delay of the transmitted Rayleigh wave with the crack depth. In the frequency domain, the inverse scattering problem was solved by comparison of the measured scattering coefficients and central frequencies of the reflected and transmitted Rayleigh waves with theoretical curves. The sizing procedure was demonstrated experimentally with narrow slots and real fatigue cracks. The out-of-plane displacement component was measured pointwise in the scattered near field by means of laser interferometry. The determination of the scattering parameters in the near field was enabled by a procedure that allows for the Rayleigh wave to be separated from the other modes scattered at the defect. The experimental results showed good accuracy and repeatability down to the smallest available ratio of crack depth to Rayleigh wavelength a/λ = 0.15.     

Lamb wave detection of limpet mines on ship hulls

This paper describes the use of ultrasonic guided waves for identifying the mass loading due to underwater limpet mines on ship hulls. The Dynamic Wavelet Fingerprint Technique (DFWT) is used to render the guided wave mode information in two-dimensional binary images because the waveform features of interest are too subtle to identify in time domain. The use of wavelets allows both time and scale features from the original signals to be retained, and image processing can be used to automatically extract features that correspond to the arrival times of the guided wave modes. For further understanding of how the guided wave modes propagate through the real structures, a parallel processing, 3D elastic wave simulation is developed using the finite integration technique (EFIT). This full field, technique models situations that are too complex for analytical solutions, such as built up 3D structures. The simulations have produced informative visualizations of the guided wave modes in the structures as well as mimicking directly the output from sensors placed in the simulation space for direct comparison to experiments. Results from both drydock and in-water experiments with dummy mines are also shown.     

Study of low-frequency ultrasonic cavitation fields based on spectral analysis technique

A kind of new spectral analysis method for analyzing ultrasonic cavitation fields is brought forward in this paper. This method extracts the line spectrum and the continuous spectrum from the cavitation noise spectrum respectively, then obtains estimations of the energy intensities of different cavitation components such as the stable cavitation and the transient cavitation through corresponding calculations. The method is also applied to investigate low-frequency ultrasonic cavitation fields and is compared with spectral level analysis method. The results show that this method can provide adequate information on the course of the cavitation process, give the delicate analysis on the transient cavitation and the stable cavitation, and help to gain an insight into the nonlinear process of cavitation fields. Moreover, it also shows that the criterion for characterization of cavitation intensity presented by Frohly through studying the high-frequency ultrasonic fields can be used to evaluate the low-frequency ultrasonic fields.     

A method to identify acoustic reverberation in multilayered homogeneous media

The presence of reverberation is a source of artifacts that can hinder the analysis of ultrasound signals and images. Besides compromising image generation, these artifacts can introduce errors in the quantitative parameter estimation in fields such as material and biological tissue characterization. A method that allows the separation between the first reflection on an interface and all the other reflections from the same interface (reverberation) could improve the quality of these images as well as the precision and accuracy in quantitative results. This work presents an algorithm for the identification of reverberating echoes in multilayered media, based on the comparison of their power spectra (estimated via FFT), through a least mean square approach, and on the temporal relationship among them. It considers that the global effect from attenuation, reflection and transmission coefficients for each layer causes spectral differences that could differentiate echoes that pass through one layer or another. The results of 10 simulations and of 60 experiments, carried out with 6 different phantoms (10 experiments with each one) are presented and discussed. It was found that the algorithm provided a correct identification for 85% of the simulated and 86.6% of the experimental echoes collected from the 60 experiments.     

Air-coupled ultrasonic evaluation of food materials

This research was performed with the aim of detecting foreign bodies and additives within food products, and to measure selected acoustic properties, without contact to the sample. This would allow use in manufacturing plants on production lines, where contacting the product for ultrasonic inspection would not be feasible. Images of internal structure are reported. The air-coupled system uses capacitive devices which are able to provide sufficient bandwidth for many measurements, including the detection of foreign bodies in cheese, the detection of deliberate additives to chocolate, the detection of fill level and content of metallic food cans, and measurements of frozen dough products. The approach demonstrates that ultrasound has the potential for application to many industrial food packaging environments where non-metallic objects within food need to be detected.     

Improving the transient response of a bolt-clamped Langevin transducer using a parallel resistor

This paper suggests a parallel resistor to reduce DC time constant and DC response time of the transient response, induced immediately after an AC voltage connected to a bolt-clamped Langevin transducer (BLT) is switched off. An equivalent circuit is first expressed. Then, an open-circuit transient response at the terminals induced by initial states is derived and measured, and thus parameters for losses of the BLT device are estimated by DC and AC time constants of the transient response. Moreover, a driving and measuring system is designed to determine transient response and steady-state responses of the BLT device, and a parallel resistor is connected to the BLT device to reduce the DC time constant. Experimental results indicate that the DC time constant greatly exceeds the AC time constant without the parallel resistor, and greatly decreases from 42 to 1 ms by a 100-kOmega parallel resistor.     

Thickness Determination for a Two-Layered Composite of a Film and a Plate by Low-Frequency Ultrasound

We present an ultrasonic method for determining the thickness of a composite consisting of a soft thin film attached to a hard plate substrate, by resonance spectra in the low frequency region, The interrogating waves can be incident only to the two-layered composite from the substrate side. The reflection spectra are obtained by FFT analysis of the compressive pulsed echoes from the composite, and the thicknesses of the film and the substrate are simultaneously inversed by the simulated annealing method from the resonant frequencies knowing other acoustical parameters in prior. The sensitivity of the method to individual thickness, its convergence and stability against experimental noises are studied, Experiment with interrogating wavelength 4 times larger than the film thickness in a sample of a polymer film （0.054mm） on an aluminium plate （6.24mm） verifies the validity of the method. The average relative errors in the measurement of the thicknesses of the film and the substrate are found to be -4.1% and -0.62%, respectively.     

A computer simulation study of imaging flexural inhomogeneities using plate-wave diffraction tomography

This paper investigates the feasibility of plate-wave diffraction tomography for the reconstruction of flexural inhomogeneities in plates using the results of computer simulation studies. The numerical implementation of the fundamental reconstruction algorithm, which has recently been developed by Wang and Rose [C.H. Wang, L.R.F. Rose, Plate-wave diffraction tomography for structural health monitoring, Rev. Quant. Nondestr. Eval. 22 (2003) 1615-1622] is investigated addressing the essential effects of applying the discrete form of the Fourier diffraction theorem for solving the inverse problem as discussed by Kak and Slaney [A.C. Kak, M. Slaney, Principles of Computerized Tomographic Imaging, IEEE Press, New York, 1988] for the acoustic case, viz. diffraction limited sensitivity, influence of weak scatterer assumption, damage location and scatter field data processing in time and Fourier space as well as experimental limitations such as finite receiver length and limited views. The feasibility of the imaging technique is investigated for cylindrical inhomogeneities of various severities and relative position within the interrogation space and a normal incident interrogation configuration. The results show that plate-wave diffraction tomography enables the quantitative reconstruction of location, size and severity of plate damage with excellent sensitivity and offers the potential for detecting corrosion thinning, disbonds and delamination damage in structural integrity management applications.     

Ultrasonic density measurement cell design and simulation of non-ideal effects

This paper presents a theoretical analysis of a density measurement cell using an unidimensional model composed by acoustic and electroacoustic transmission lines in order to simulate non-ideal effects. The model is implemented using matrix operations, and is used to design the cell considering its geometry, materials used in sensor assembly, range of liquid sample properties and signal analysis techniques. The sensor performance in non-ideal conditions is studied, considering the thicknesses of adhesive and metallization layers, and the effect of residue of liquid sample which can impregnate on the sample chamber surfaces. These layers are taken into account in the model, and their effects are compensated to reduce the error on density measurement. The results show the contribution of residue layer thickness to density error and its behavior when two signal analysis methods are used.     

Contribution to classification of buried objects based on acoustic impedance matching

Determination of material the buried objects are made of could contribute significantly to their recognition, or classification. This is important in detecting buried antipersonnel landmines within the context of humanitarian demining, as well as in a variety of other applications. In this article the concept has been formulated of the approach to buried object's material determination starting with ultrasonic impulse propagation analysis in a particular testing set configuration. The impulse propagates through a characterized transfer material in such a way that a part of it, a reflected wave, carries the information about the buried object's surface material acoustic impedance. The limit of resolution capability is theoretically analyzed and experimentally evaluated and the influencing factors described. Among these, the contact between clean surfaces of the transfer material and buried object is emphasized.     

Effect of anisotropy on acoustoelastic birefringence in wood

The ultrasonic velocity of shear waves propagating through radial direction of a wood plate specimen, transversely to the loading direction, was measured. By rotating an ultrasonic sensor, the oscillation direction of the shear waves was varied with respect to the wood plate axis and loading direction. The relationship between shear wave velocity and oscillation direction was examined to discuss the effect of anisotropy on the acoustoelastic birefringence in wood. The results obtained were summarized as follows. When the oscillation direction of the shear wave corresponded to the tangential direction of the wood specimen regardless of the stress direction, shear wave velocity decreased markedly and the relationship between shear wave velocity and rotation angle tended to become discontinuous. That is, when the shear waves oscillated in the anisotropic axis of the wood, the shear wave velocity peaked unlike in the case of oscillation in the stress direction. In an isotropic material (acrylic, aluminum 5052), on the contrary, when the shear waves oscillated in the stress direction of the specimen, the shear wave velocity peaked regardless of the main-axis direction of the specimen. On the basis of the discussion of these results, the ultrasonic shear wave propagating in wood under stress is confirmed to be polarized in the anisotropic axis of the wood.     

Bonding Interface Imaging and Shear Strength Prediction by Ultrasound

The propagation of a longitudinal ultrasonic wave normally incident upon an adhesively bonded structure is studied. The structure consists of adherend and adhesive layers with finite thickness. Interfaces between adherend and adhesive are regarded as distributed springs. Theoretical and experimental results show that resonant frequencies of the bonded structure vary sensitively with the interface stiffness constants and adhesive thickness, and these interface characteristics are inversed by the simulation annealing (SA) method. Furthermore, the distribution image of interface stiffnesses is compared with the state of fracture interface, and quantitative prediction of shear strength is achieved based on the distribution of interface stiffnesses and adhesive thicknesses by using a back-propagation neutral network. The average relative error of the shear strength from prediction to real value is 10.7%.     

Estimation of ultrasound attenuation and dispersion using short time Fourier transform

Determination of the acoustic attenuation and dispersion has important applications in ultrasound tissue characterization and non-destructive material testing. Current signal processing methods Fourier transform of ultrasound signals to get the spectra of amplitude and phase to estimate respectively the attenuation and dispersion of a given medium. These methods are frequency domain method and obsessed with ambiguity issue in the phase unwrapping calculation. Conventional ultrasound velocity measuring method detects the time of arrival of a pulse (or echo) signal, which is a time domain method to compute group velocity (not phase velocity). This paper presents a novel approach based on the short time Fourier transform (STFT)--a time-frequency analysis, to estimate the ultrasonic dispersion and attenuation. Only the amplitude information of the pulse-signal spectra is used. Based on the time-frequency presentation, the attenuation coefficient of the signal is obtained by computing the amplitude decay of pulse spectrum in time domain, while phase velocities are obtained based on the "time-of-flight" (TOF) of the mono frequency component of the pulse signals. As a result, we eliminate the ambiguity issue in phase angle calculation. Furthermore, the proposed method makes the phase velocity pedagogically intuitive for novice users. The paper presents experiments to evaluate demonstrate the performance of the proposed method.     

Coupling of Acoustical Plane p-Wave to a Cased Borehole

Theoretical and numerical study on the coupling acoustic field of the plane p-wave to a cased borehole is carried out. The medium outside the cased borehole is modeled as the porous medium. The scattering field characteristics in the cased borehole are investigated when a plane fast p-wave is incident in tilt to the cased borehole from the porous medium. The scattering fields inside and outside the cased borehole are analyzed and deduced by Biot＇s theory under the boundary conditions on each interface, and they are numerically studied. It is found that the scattering field has strong resonant characteristics and there exists a series of resonant frequencies and peaks. The effects of the frequency, radii of each interface, incident angle, porosity, and other parameters on the resonant acoustic field have been investigated in detail in the fast and slow formations respectively. The resonant characteristics of the scattering field are also analyzed from the physical sense.     

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.     

A semi-analytical model for predicting multiple propagating axially symmetric modes in cylindrical waveguides

A semi-analytical model for multiple mode axially symmetric wave propagation in finite solid cylindrical waveguides is presented. The model is designed as a tool for predicting and interpreting experimental signals. The model is based on a common experimental configuration and considers the excitation, propagation and reception of the ultrasonic signal in the waveguide. The Pochhammer-Chree solution for an infinite cylinder is the basis for the model. Extensions are made to enable comparison to experimental results. Comparisons with experiment are performed in the time, frequency and joint-time frequency domain for both narrow band and broad band excitation of the piezo-electric transducer.