Silvio Elton Kruger

Surface acoustic wave depth profiling of elastically inhomogeneous materials

The potential of Rayleigh wave spectroscopy for the in-depth reconstruction of elastic properties of multilayers for materials with a continuous profile of elastic properties is explored. Two models to calculate the surface acoustic wave (SAW) dispersion spectrum from the profile of the elastic parameters are elaborated and compared. It is found that the relevant elastic parameters for Rayleigh wave dispersion in multilayers are the “effective” Rayleigh velocities, i.e., the Rayleigh velocities calculated for virtually semi-infinite layers. For the solution of the inverse problem, a neural network and a singular value decomposition model are proposed and tested on simulated SAW spectra. The reconstruction techniques are applied to reconstruct the elastic depth profile of shot-peened steel samples from laser-generated and laser-detected SAW data.

Rescaled range analysis and detrended fluctuation analysis study of cast irons ultrasonic backscattered signals

RS Analysis and DF Analysis have been applied to backscattered ultrasonic signals obtained from three different cast iron samples in order to investigate the fractal nature of the microstructure of these materials. The results show a scenario with two distinct regions whose calculated parameters can be used to estimate their fractal dimensions.

Hydrogen damage detection by ultrasonic spectral analysis

The non-destructive testing technique known as ultrasonic spectral analysis was applied to steel samples for the detection of minute cracks produced by hydrogen in a H2S environment. The cracks were characterized and quantified by metallographic analysis and compared with the ultrasonic results. The backwall echoes and the backscattered signals were analyzed in the frequency domain and the characteristics of these spectra were evaluated by second order moments, which emphasizes the higher frequency components. The second order moments showed a greater variability for hydrogen attacked materials than for non-attacked ones, denoting good sensitivity to this type of material degradation.

Investigation of the microstructure of cast iron by laser ultrasonic surface wave spectroscopy

We have used an all-laser surface wave ultrasonic method to investigate cast iron samples with a variety of morphologies of included graphite precipitation. The relation between ultrasonic attenuation and the microstructure of cast iron is discussed. The frequency dependence of ultrasonic attenuation was obtained in a broad frequency band including the three characteristic regions of scattering. The scatterer size of the samples could be evaluated from the measurement, and good agreement was found between the average scatterer size estimated from our ultrasonic data and micrographic data. The demonstrated sensitivity of the ultrasonic propagation parameters to the microstructural properties of cast iron illustrates the strong potential of this technique for non-destructive microstructural characterization of polycrystalline materials.

CHARACTERIZATION OF CAST IRON MICROSTRUCTURE THROUGH THE STATISTICAL FLUCTUATION AND FRACTAL ANALYSES OF ULTRASONIC BACKSCATTERED SIGNALS

This work aims to identify the different microstructures presented by cast iron, namely, lamellar, vermicular and spheroidal microstructures, through the statistical fluctuation and fractal analyses of backscattered ultrasonic signals. The signals were obtained with a broadband direct contact ultrasonic probe with a central frequency of 5 MHz. The statistical fluctuations of the ultrasonic signals were analyzed using Hurst and detrended‐fluctuation analyses (DFA), and the fractal analyses were carried out by applying the minimal cover and box‐counting techniques to the signals. The curves obtained for the statistical fluctuations and fractal analyses, as function of time window, were processed by using two pattern classification techniques, namely, principal‐component analysis (PCA) and Karhunen‐Loeve expansion. For the Karhunen‐Loeve expansion, an approximately 100% success rate has been reached for the classification of the different microstructures, for the training and the testing sets of events. The ...

Shot peening treatment characterization by surface acoustic waves

Shot peening is a classical mechanical surface treatment used to improve the properties of components, principally the fatigue and stress corrosion resistance. The possibility of characterization of shot peening treatment efficacy was assessed by leaky Rayleigh SAW generated and detected by a large aperture, broadband and lens-less focused transducer. The broadband pulses propagated in the material surface were processed in order to obtain the dispersion curves, and were compared for different samples that were submitted to different treatment conditions. The results showed that the dispersion curve is sensitive to these treatment parameters. The possibility of depth profiling of the elastic properties changes due the shot peening are discussed on base of the fact that the penetration of SAW depends on the frequency and so the dispersion curve should have information of the elastic properties in different depths. The source of these elastic changes is also analyzed in terms of plastic deformation, residual stresses, roughness and microstructural changes. A comparison of the dispersion curves with a laser based SAW system was also performed showing equivalence of the methods.

Measuring cast iron graphite size by ultrasonic attenuation

Precipitated graphite geometry is one of the main microstructural parameters that contribute to cast iron mechanical properties. In this paper we address the measurement of the graphite size by ultrasonic attenuation. Samples of different cast iron microstructures were characterized by quantitative metallographic techniques and further correlated with ultrasonic parameters. Longitudinal ultrasonic waves were generated and detected by commercial broadband piezoelectric transducers on direct contact and the signals were processed in order remove system effects. For surface acoustic waves, a large aperture lens-less focalized piezoelectric transducer was used. The experimental attenuation spectra were fitted with simple scattering models and both longitudinal and surface wave techniques showed very good quantitative correlations with graphite size, independent of its geometry.

BROADBAND ULTRASONICS APPLIED TO PHASE TRANSITION CHARACTERISATION IN CHROMIUM ANTIFERROMAGNETIC ALLOY

LABOEND/PEMM/EE/COPPE, Federal University of Rio de Janeiro, P.O.Box 68505, CEP 21945-970 Rio de Janeiro-RJ, Brazil(Received October 4, 2000)(Accepted in revised form January 17, 2001)Keywords: Acoustic methods; Phase transformations; Nondestructive testing; Magnetic structureIntroductionUltrasonic pulse-echo techniques are being widely used in the investigation of magnetic phasetransitions in solid materials [1,2]. In general, changes in the ultrasonic pulse amplitude and velocity aremeasured, and then the ultrasonic attenuation and elastic constants are determined for fixed frequenciesgiven by the odd harmonics of the transducer coupled to the sample. It is difficult to reproduce the sameconditions of coupling between transducer and sample, therefore most analyses are limited to a fewfixed frequencies. An investigation of the ultrasonic pulse propagation close to a magnetic phasetransformation, for a continuous frequency range, is presented using wide band transducers and digitalspectral analysis. The material chosen is a single crystal of Cr-0.18at%Re, which has a magnetic phasetransformation from the paramagnetic (PM) to an antiferromagnetic (AFM) state at 318 K [3,4].Similarly to pure chromium, the ultrasonic attenuation and internal friction for this alloy have acontribution due to the coupling between the spin density wave in the AFM phase and the oscillatoryelastic strain introduced by the ultrasonic pulse. Since the sixties the ultrasonic broadband analysis isbeing widely used for many applications [5–8]. In this paper, we have shown that the ultrasonic spectralanalysis approach is highly advantageous in many aspects for the investigation of phase transitions.Experimental MethodThe Cr-0.18at%Re single crystal used was obtained from a larger ingot previously investigated byneutron scattering [9] and cut with faces parallel to the crystallographic directions [001], [110] and[110] with dimensions of 10 3 12 3 6 mm. The full half width at half maximum (FWHM) of the Braggpeak was found to be less than 0.5° after standard procedure of electropolishing with oxalic acid.The measurement was performed using the immersion in water technique with a broadbandKrautkra¨mer transducer model IAP 50.2.1, which has a good response in the frequency interval from5 MHz to 40 MHz. The generator/receptor excites the transducer with a wide band pulse, and thegenerated ultrasonic pulse propagates through the water/sample system. Part of the pulse reflects at

Methods of beam divergence corrections on broadband direct contact ultrasonic technique

The separation of intrinsic attenuation and velocity parameters from extrinsic factors is a complex task in nondestructive ultrasonic characterization of materials. The effect of the ultrasonic beam divergence due to finite source is discussed both theoretically and experimentally in this work. The objective is to evaluate different techniques of attenuation and velocity measurements with beam divergence corrections on direct contact configuration. Both amplitude and phase spectra for signals generated and detected by commercial NDE broadband transducers are to be corrected. The effectiveness of each technique is evaluated with different transducers and sample geometry. The results show that a smaller effective transducer diameter than the nominal must be assumed in theoretical corrections [P. H. Rogers and A. L. Van Buren, J. Acoust. Soc. Am. 55, 724–728 (1974)] and that the experimental determination of calibration correction for a particular transducer by a reference sample can be a good alternative.