Elastic slow dynamics, consisting in a reversible softening of materials when an external strain is applied, was experimentally observed in polycrystalline metals and presents analogies with the same ...phenomenon more widely observed in consolidated granular media. Since the effect is extremely small in metals, precise experimental techniques are needed. Reliable measurement of relative velocity variations of the order of 10−7 is crucial to perform the analysis. In addition, the grain structure and the nature of grain boundaries in metals is very different from that in rocks or concrete. Therefore, linking relaxation elastic effects to the microstructure is needed to understand the physical origin of slow dynamics in metals. Here, interpreting the relaxation phenomenon as a multirelaxation process, we show that it is sensitive to the spatial scale at the microstructural level, up to the point of allowing the identification of the existence of features at different spatial scales, particularly distinguishing damage from microstructural inhomogeneities.
This paper studies theoretical results of a nonlinear ultrasonic method based on interaction of two elastic waves of different frequencies. A virtual Nonlinear Wave Modulation Spectroscopy experiment ...is performed in the vicinity of a crack described by a model combining classical and hysteretic nonlinearity. Quasistatic response to two frequency excitation was computed and harmonic and intermodulation components were studied. The influence of driving signal parameters and nonlinear parameters on the response is thoroughly discussed. A general way of hysteretic response description based on scaling properties is explained. In case of the combined nonlinear model, an analysis of nonlinear spectral components is performed in complex plane. Based on the complex interaction of classical and hysteretic parts, a method of their separation is proposed.
•Crack described by a model combining classical and hysteretic nonlinearity.•Spectral analysis on quasistatic response to two-frequency excitation was performed.•General description of hysteretic response is based on amplitude and frequency ratios.•Method of hysteretic parameter extraction from combined model response is proposed.
•Reference-free in situ determination of AE transducers frequency responses.•New calibration method based on reciprocal calibration and time reversed acoustics.•Universally applicable for reversible ...transducers.•Conditionally applicable for irreversible or one-way transducers.
Conventional methods of calibration of acoustic emission transducers require an already calibrated reference transducer and/or special test specimen. Moreover the calibration results are not directly transferrable to practice, since they are dependent on bonding conditions and mechanical properties of the medium. In this paper a reference-free in situ calibration method is proposed. It is based on a combination of time reversed acoustics and reciprocal calibration and allows accurate determination of transducers’ frequency responses. The method was developed and successfully experimentally verified assuming that frequency response of used transducers is the same for transmission and reception. Subsequently the analysis was extended considering irreversible transducers and applied to experimental data. Of five tested transducers, three were found reversible and two demonstrated resonance frequency shift in lower frequency region between reception and transmission.
The Hopkinson split pressure bar (HSPB) was used for the testing of three polymers at strain rates between 102 to 103 s-1. Higher strain rates were achieved using the direct Hopkinson test. ...Experimental data were evaluated in time as well as in the frequency domain. A more detailed analysis in the frequency domain showed that the description of tested polymers can be described in the framework of the linear viscoelasticity. The use of the direct Hopkinson test showed the occurrence of a permanent strain.
Článek Historické právní informace a jejich databázové zpřístupnění zkoumá zpřístupňování výrazně specifického druhu právních informací – historických právních informací. V své první části se článek ...zabývá všeobecnými otázkami zpřístupňování historických právních informací a nakládání s nimi. Druhá část popisuje současnou českou situaci a zmiňuje současné praktické obtíže. Třetí část článku přináší kritický přehled aktivit, které byly dosud na poli digitalizace ve zkoumané oblasti uskutečněny. Čtvrtá část článku tyto projekty sumárně hodnotí na podkladu tohoto hodnocení formuluje optimální požadavky na podobu dalších aktivit. Z těchto požadavků vychází podoba vytvářeného projektu Národní právní dědictví, stručně popsaná částí pátou. Možnosti systému podrobně popisuje rozsáhlá část šestá o způsobech jeho využívání. Část část sedmá pak doplňuje část šestou přiblížením roviny vybraných technických parametrů řešení systému.
Selective electron beam melting (SEBM) is an additive manufacturing process for the production of complex metallic parts. To optimize their mechanical properties, pores and fusion defects, since not ...completely avoidable, have to be detected and evaluated. The generated microstructure may contain not only voids, but also weak bonds, sharp interfaces and loose particles causing nonlinear elasticity. This paper reports on Nonlinear Impact Modulation Spectroscopy (NIMS) experiments applied to a set of Ti–6Al–4V samples, each of which contains an internal zone with well-defined porosity ranging from 0.2 to 20%, respectively. Resonance spectra, attenuation and indicators of material nonlinearity were evaluated, which allowed the localization, dimensioning and assessment of porous zones. Properties of used linear and nonlinear methods are discussed.
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