Papers by Author: Leszek B. Magalas

Abstract: In this paper, we compare the values of the resonant frequency computed according to the OMI algorithm, DFT, and interpolated DFT methods for a set of 100 free decaying oscillations. It is unequivocally demonstrated that the performance of the different methods can be listed in the following order: (1) OMI, (2) YM, (3) YM_C, (4) Agrež, and finally (5) the well known Yoshida method, Y. For very short signals the order of the best methods is different: (1) OMI, (2) YM_C. It is pointed out that the DFT methods, including the Yoshida method, are discouraged for analysis of signals that are too short. This effect is explained in terms of spectral leakage. By contrast, short free decaying signals can be successfully analyzed with the OMI and the YM_C method. We conclude that the use of the OMI and the YM, i.e. the interpolated DFT method, can substantially increase the resolution of low-frequency resonant mechanical spectrometers (the decrease in dispersion of experimental points and the minimization of relative errors can be readily obtained.) For this reason a much more precise estimation of the logarithmic decrement is also simultaneously feasible.

Abstract: In this work, we present the comparison between different methods used to compute the logarithmic decrement, δ . The parametric OMI method and interpolated DFT (IpDFT) methods are used to compute the δ from free decaying oscillations embedded in an experimental noise typical for low-frequency mechanical spectrometers. The results are reported for δ = 5×10^-4, = 1.12345 Hz and different sampling frequencies, = 1 kHz and 4 kHz. A new YM algorithm yields the smallest dispersion in experimental points of the logarithmic decrement and the smallest relative errors among all investigated IpDFT methods. In general, however, the IpDFT methods suffer from spectral leakage and frequency resolution. Therefore it is demonstrated that the performance of different methods to compute the δ can be listed in the following order: (1) OMI, (2) YM, (3) YM_C, and (4) the Yoshida method, Y. For short free decays the order of the best performers is different: (1) OMI and (2) YM_C. It is important to emphasize that IpDFT methods (including the Yoshida method, Y) are discouraged for signals that are too short. In conclusion, the best methods to compute the logarithmic decrement are the OMI and the YM. These methods will pave the way toward high-resolution mechanical spectroscopy HRMS.

Abstract: Characteristic low-temperature mechanical loss peaks are reported in cold-rolled steel sheets. Similar mechanical loss peaks are observed in both metallic and paper substrates covered with thin oil films. The surface induced origin of these peaks is elucidated through direct comparison of mechanical loss peaks observed in the as-received, cold-rolled samples with loss peaks observed in metallic and paper substrates covered with thin films of the arachis oils. In all of these instances, similar low-temperature mechanical loss peaks are observed in the temperature range from 180 K up to 300 K in both low-frequency resonant and low-frequency sub-resonant mechanical spectrometers. It is concluded that low-temperature mechanical loss peaks are generated by surface induced effects that arise from the oil film itself.

Abstract: The advantages of the OMI algorithm to compute the logarithmic decrement and the resonant frequency from free decaying oscillations is reported. The OMI algorithm is proved to be the best solution in the computation of the logarithmic decrement and the resonant frequency for high damping levels.

Abstract: The on-line control unit is used to ensure high-accuracy computations of the logarithmic decrement. It is shown that the excitation process should be on-line low level real-time controlled during mechanical loss measurements to obtain high precision of the computations and to ensure a short-time excitation process.

Abstract: Extrinsic resonance effects observed in a low-frequency subresonant mechanical spectrometer are reported. High resolution of the mechanical spectrometer enables precise measurement of small values of the mechanical loss tangent ( tanϕ = 1- 2 × 10-4) and the apparent high values of the loss tangent detected for extrinsic resonance effects in the vicinity of the eigen frequencies of the spectrometer. Forced oscillations can be used as a ‘signal quality test’ to validate the quality of exponentially damped harmonic oscillations in the resonant mode.

Abstract: The concept of the ‘zero-point drift’, ZPD, is introduced and analyzed on the basis of mechanical loss measurements carried out in a low-frequency mechanical spectrometer – inverted torsion pendulum. It is demonstrated that the ZPD, which modifies damped harmonic oscillations leads to false values of the logarithmic decrement computed from several widely accepted algorithms.

Abstract: The mechanical loss spectra obtained in metallic samples covered with various thin films of arachis oils containing different sulphur concentrations, different consistency, and different oil fractions are observed in the low-temperature range from 180 K to 300 K. The observed mechanical loss spectra are identified as the surface induced mechanical loss phenomena. It is demonstrated that mechanical loss spectra are induced by the presence of oil films on metallic substrates. It is shown that the shape, the peak location, and the generation of the constituent low-temperature peaks can be controlled by the state of the arachis oil films.

Abstract: The Snoek-Köster (SK) relaxation and the dislocation-enhanced Snoek Effect (DESE) in deformed ultra-high purity Fe-C alloys and industrial low-carbon cold-rolled steel sheets as well as a quantitative comparison between experimental data and computer simulations have been presented.

Abstract: The H(D) atom’s interaction with one another, ‘heavy’ interstitial atoms (O, N, C), and substitutional atoms is analyzed on the basis of strain-induced (elastic) interaction. The interaction energies are calculated for bcc, fcc, and hcp metal solid solutions with regard to the discrete atomic structure of the host lattice. The elastic constants, Born-von Karman constants of the host lattice, and concentration expansion coefficients of the solid solution lattice due to solute atoms, are used as the parameters for numerical input. It is shown that the interaction is long-range, oscillating, and anisotropic. In all cases, the coordination shells of both types - with attraction and with repulsion - exist. The interaction energy dependence on the distance is due mainly to the crystal lattice type. The strain-induced interaction should be supplemented by repulsion in the nearest coordination shells for the case of interstitial-interstitial interaction and by chemical interaction in the case of H-substitutional interaction. Two examples are given for the use of the strain-induced interaction energies in calculations relaxation processes.