Influence of step errors (truncation errors) on results of molecular dynamics simulations

Step errors (local errors, called also truncation errors) of the algorithms used in molecular dynamics simulations may result in non-physical correlations between particle velocities, as well as in errors of thermodynamic properties of simulated systems (energy, pressure). The simulations of the Lennard-Jones liquid showed, that the influence is especially high for the Verlet velocity algorithm. Beeman's technique decreases the correlations between the velocities, but at high densities the values of the errors of general averages are close to that of the Verlet method. The influence of step errors can be decreased by about two orders of magnitude by applying the Cowell-Numerov 4-th order implicit method (equivalent to the Gear 4-th order method treated as an implicit one). The method is very stable (more stable than the Verlet one), and can be highly optimized by restricting iteration to the closest neighbors of a given particle. As a result, the method becomes more efficient than the higher order explicit symplectic methods

Ultrasound imaging of stiffness with two frequency pulses

Nowadays there are new modalities in ultrasound imaging allowing better characterization of tissue regions with different stiffness. We are proposing a novel approach based on compression and rarefaction of tissue simultaneously with imaging. The propagating wave is a combination of two pulses. A low frequency pulse is expected to change the local scattering properties of the tissue due to compression/rarefaction while a high frequency pulse is used for imaging. Two transmissions are performed for each scanning line. First, with the imaging pulse that propagates on maximum compression caused by a low frequency wave. Next, the low frequency wave is inverted and the imaging pulse propagates over the maximum rarefaction. After the processing of the subtracted echoes from subsequent transmissions including wavelet transform and band-pass filtering, differential images were reconstructed. The low frequency wave has a visible impact on the scattering properties of the tissue which can be observed on a differential image

Evaluation of trabecular bone properties using ultrasonic scanne

Signals scattered in trabecular bone contain information about properties of the bone structure. Evaluation of this properties may be essential for osteoporosis diagnosis and treatment monitoring because the standard densitometry does not provide complete information about the bone strength. It was previously demonstrated that using numerical model of backscattering in trabecular bone it is possible to estimate some microstructural characteristics of bone. Model predicts departures from the Rayleigh statistics of the scattered signal envelope depended on the scatterer physical parameters and its shape uniformity. This study concerns examination of trabecular bone (calcaneus) in vivo. Ultrasonic bone scanner operating at frequency of 1,5 MHz was used to collect backscattered signals. Data were processed in order to obtain the statistical properties of the signal envelope and to compare them with histograms resulting from modeling. This study is an approach towards developing a tool for the investigation of scattering in trabecular bone that can potentially provide clinically useful information about bone strength and condition

Statistics of envelope of high frequency ultrasound signal backscattered in human dermis

The scattering of ultrasonic waves depends on the size, shape, acoustical properties and concentration of scatterers in tissue. In these study K distribution of the ultrasound backscatter envelope was used to assess the structural properties of the skin tissue. The custom-designed high frequency ultrasonic scanner was applied to obtain RF B-scans of the skin in vivo at the frequency of 20 ÷ 30MHz. The results are encouraging. The K distribution models the envelope statistics very well. The parameters of the K-distribution, namely, the effective number of scatterers may be useful for the skin characterization

The influence of the transducer bandwidth and double pulse transmission on the encoded imaging ultrasound

An influence effect of fractional bandwidth of ultrasound imaging transducer on the gain of compressed echo signal being the complementary Golay sequences (CGS) with different spectral widths is studied in this paper. Also, a new composing transmission method of CGS is discussed together with compression technique applied in order to increase the signal-to-noise ratio (SNR) and penetration. The CGS with two different bit lengths, one-cycle and two-cycles are investigated. Two transducers with fractional bandwidth of 25% and 80% at centre frequency 6 MHz are used. The experimental results are presented, clearly proofing that increasing of the code length leads to compressed echo amplitude enhancement. The smaller the bandwidth is the larger is this effect; the pulse-echo sensitivity of the echo amplitude increases by 1.88 for 25% fractional bandwidth and 1.47 for 80% while preserving time resolution. The presented results of double transmission of short codes show the penetration and SNR improvement while maintaining dead zone

Cellular Automata Based Approach to Corrosion and Passivity Related Phenomena

International audienc

Comparative assessment of Young's modulus measurements of metal-ceramic composites using mechanical and non-destructive tests and micro-CT based computational modeling

It is commonly known that the available non-destructive and mechanical methods of the Young modulus measurement yield different results. This paper presents comparison of the results of experimental determination and numerical modeling of the Young modulus of Cr-Al2O3-Re composites (MMC) processed by a powder metallurgical method (SPS). In the computational model a finite element analysis is combined with images of the real material microstructure obtained from micro-computed tomography (micro-CT). Experimental measurements were carried out by four testing methods: three-point bending, resonance frequency damping analysis (RFDA), ultrasonic pulse-echo technique, and scanning acoustic microscopy. The paper also addresses the issue which of the four experimental methods at hand gives results closest to the theoretical predictions of the micro-CT based FEM model

Experimentelle Ermittlung des Elastizitätsmoduls von Cr(Re)/Al2O3-Verbundwerkstoffen

Im Maschinenbau und der Werkstoffwissenschaft kommt dem Elastizitätsmodul eine grundlegende Bedeutung zu. Die Bestimmung des E-Moduls kann mittels Zugversuch, Drei-Punkt-Biegung und Ultraschall-Impuls-Echo-Verfahren erfolgen. Entwickelt wurden weitere experimentelle und numerische Verfahren, die alle Vor- und Nachteile haben und teilweise unterschiedliche Werte für den E-Modul eines Werkstoffs liefern. Es werden die Ergebnisse experimenteller Untersuchungen sowie numerischer Modellierungen am Verbundwerkstoff Cr(Re)/Al2O3 (MMC) vorgestellt und verglichen. Die erhaltenen Ergebnisse belegen die Schwierigkeit einer genauen Ermittlung des Elastizitätsmoduls, insbesondere von Verbundwerkstoffen. Mögliche Gefügeinhomogenitäten dieser Werkstoffe wirken sich offensichtlich verschieden intensiv auf die angewandten unterschiedlichen Messprinzipien des E-Modules aus. Alle vier Verfahren nutzen verschiedene physikalische Vorgänge, um Rückschlüsse auf die elastischen Konstanten des Werkstoffs zu ziehen. Dabei spielen nicht nur lokale Phänomene wie Spannungsverteilung, Verfomungsamplitude und -geschwindigkeit eine entscheidende Rolle, auch eine akkurate Probenvorbereitung ist für die Minimierung des Ausmaßes an Fehlmessungen sehr bedeutend. Da die Drei-Punkt-Biegung sehr empfindlich auf Gefüge- und Oberflächenfehler reagiert, sollte sie nur dann eingesetzt werden, wenn keines der zerstörungsfreien Prüfverfahren zur Verfügung steht oder ein Vergleich mit schon vorhandenen Messungen dieses Verfahren erfordert. Die beiden vorgestellten FEM-Modelle können hilfreiche Hinweise bei der Werkstoffentwicklung liefern. Sie erlauben eine gute Vorhersage der elastischen Konstanten eines noch nicht entwickelten Verbundwerkstoffs