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Acoustic emission sources from fast dislocation motion
Hora, Petr ; Machová, Anna ; Červ, Jan ; Uhnáková, Alena
Acoustic emission from the fast dislocations emitted from an edge crack in 3D bcc iron crystal is studied via atomistic simulations by molecular dynamics technique. Acoustic emission patterns arising from the fast dislocation motion in molecular dynamics are visualized via the local kinetic energies of individual atoms and further modeled as a moving source of the stress waves in the anisotropic continuum.
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Analysis of acoustic emission source location precision for general sensor configurations
Chlada, Milan ; Gális, P. ; Převorovský, Zdeněk
Proper sensor placement is the crucial step and a premise for precise acoustic emission (AE) source location estimate. Using the algorithm for finding the shortest ways in discretely defined bodies it is possible to design three parallel tools how to evaluate problematic areas, namely the location sensitivity, similarity and ambiguity maps, available even for discontinuous or anisotropic bodies. To check the numerical forecast of localization capabilities for given sensor configuration, theoretical results were compared with the data measured on the real steam pressure vessel.
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Nonlinear ultrasonic porosimetry of 3D printed metallic parts
Převorovský, Zdeněk ; Krofta, Josef ; Kober, Jan ; Kirschner, A.
Additive manufacturing process optimization requires a feedback from nondestructive testing (NDT) and evaluation. In this contribution are discussed NDT results obtained by nonlinear elastic wave spectroscopy methods to classify presence of defects in Ti-6Al-4V prismatic samples fabricated by electron beam printer. Quantitative calibration of porosity was realized by metallography. The best porosity classification results were obtained by nonlinear wave modulation spectroscopy with chirp excitation and by ESAM (Excitation Symmetry Analysis).
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Recent progress in numerical methods for explicit finite element analysis
Kolman, Radek ; Kopačka, Ján ; Gonzalez, J. ; Gabriel, Dušan ; Cho, S.S. ; Plešek, Jiří ; Park, K.C.
In this paper, a recent progress in explicit finite element analysis is discussed. Properties and behaviour of classical explicit time integration in finite element analysis of elastic wave propagation and contact-impact problems based on penalty method in contact-impact problems are summarized. Further, stability properties of explicit time scheme and the penalty method as well as existence of spurious oscillations in transient dynamics are mentioned. The novel and recent improving and progress in explicit analysis based on a local time integration with pullback interpolation for different local stable time step sizes, bipenalty stabilization for enforcing of contact constrains with preserving of stability limit for contact-free problems and using a direct inversion of mass matrix are presented. Properties of the employed methods are shown for one-dimensional cases of wave propagation and contact-impact problems.
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Computational time reversal method based on finite element method: influence of temperature
Mračko, Michal ; Kolman, Radek ; Kober, Jan ; Převorovský, Zdeněk ; Plešek, Jiří
Time reversal method is used to focus elastic waves to the location of the original source and reconstruct its source time function. The procedure consists of two steps: Frontal task and Reversal task. In the Frontal task, the medium is excited by an arbitrary source, elastic waves propagate through a body of interest and the dynamic response at few points on boundary is recorded. In the second step (say the Reversal task) the response signal is reversed in time and transmitted back into the medium resulting in focusing in the original source location. It is of practical importance to investigate a case when the medium changes its properties between the frontal and reversal wave propagation steps. An example is a problem of transferring experimentally recorded data to a computational model, where discrepancies in geometry, elastic properties and boundary conditions are expected. Our motivation is to develop a methodology for computation of time reversal problems in commercial finite element software. The results prove that this method is extremely sensitive to the change of temperature and one have to pay special attention to tuning of elastic parameters relevant to the\nexperiment.
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Finite element modeling of the signal propagation in a thin tube and comparison with experimental data
Kruisová, Alena ; Kolman, Radek ; Trnka, Jan ; Mračko, Michal
In finite element modeling of wave propagation problems, both the spatial and temporal discretization lead to dispersion errors. It means that the phase velocity of propagated wave is related to its frequency. In framework of temporal-spatial dispersion analysis, the time step size for implicit time integration method based on the Newmark method is proposed for linear and quadratic serendipity plane finite elements. In this paper, we verify the theoretical dispersion analysis by elastic wave propagation in thin tube, where experimental results are known. Such time step size was used in finite element modeling of the stress wave propagating in this thin steel tube, the results of simulations were compared with experimental results.
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Localization of continuous acoustic emission sources using time reversal procedure
Převorovský, Zdeněk ; Krofta, Josef ; Kober, Jan ; Chlada, Milan
A new solution with the help of Time Reversal (TR) signal processing approach is suggested in this paper. It allows planar location of burst AE sources under high background noise using only one transducer, and continuous AE sources with two transducers, both under high background noise from other sources. TR procedure was in this approach applied to long random noise signals for the first time. Suggested method was verified by means of experimentally simulated AE sources on a steel plate and tube. Source location results certified high robustness of suggested approach. Certain disadvantage of the new method is requirement of detailed scanning of region around a presumed source. Scanning may be eliminated if we use numerical simulations with TR signals put into a computer model of the structure. Localization accuracy of suggested procedure (approx. 1 mm) is better than the wavelength and transducers aperture.
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Optimization of sensor selection for acoustic emission source location
Chlada, Milan ; Převorovský, Zdeněk
Similarly to Global Positioning System (GPS), location of material defects by acoustic emission method meets the geometrical dilution of precision (GDOP) phenomena. Besides the several attitudes how to mathematically express its measure and compare particular sensor configurations in accordance with desired location precision, recently proposed method brings extended possibilities for detection of critical regions characterized by strong sensitivity of location results to signal arrival time changes or errors. Analogically to GDOP parameter it yields the sensitivity map available even for non-continuous or anisotropic materials. During the computation it uses the algorithm for finding the shortest ways in discretely defined bodies, which furthermore enables to view the problem through the so-called similarity and ambiguity maps.
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Ultrasonic biopolymer characterisation by hysteresis quantification
Zatloukalová, Jana ; Tokar, Daniel ; Kopecká, Karolína ; Převorovský, Zdeněk
Nonlinear characteristics are advanced indicators of the structure of viscoelastic materials. In this contribution, we focus on biopolymers (such as articular cartilage and skin tissue), and their typical amount of energy lost by mechanical loading/unloading represented by a hysteresis curve. For hysteresis modeling we use the PreisachMayergoyz space model, and restricted power distribution (Guyer extended distribution). Besides the power distribution parameters, the stress protocol is required as an input to obtain the modeled hysteresis curve, and to compare with experimental results. Knowing the experimental curve, an iterative numerical procedure for identification of true density function of opening and closing pressures was applied, and an optimization algorithm sought for the best (characteristic) distribution parameters for the specific soft tissue. Material properties can be evaluated based on the described quantification, and potentially used for medical purposes and healing or cosmetics treatment evaluation.
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