A dynamical approach to the spatio-temporal features of the Portevin-Le Chatelier effect

We show that the extended Ananthakrishna's model exhibits all the features of the Portevin - Le Chatelier effect including the three types of bands. The model reproduces the recently observed crossover from a low dimensional chaotic state at low and medium strain rates to a high dimensional power law state of stress drops at high strain rates. The dynamics of crossover is elucidated through a study of the Lyapunov spectrum.

Dislocation dynamics in alpha titanium Dynamique des dislocations dans le titane alphaVersetzungsdynamik in alpha-Titan

The activation area and activation enthalpy are determined as a function of stress and temperature for alpha titanium. The results indicated that plastic flow below about 700°K occurs by a single thermally activated mechanism. Activation area determined by differential-stress creep tests falls in the range 80−8b2 and does not systematically depend on the impurity content. The total activation enthalpy derived from the temperature and strain-rate dependence of flow stress is 1.15 eV. The experimental data support a lattice hardening mechanism as controlling the low-temperature deformation in alpha titanium.

Chaotic and power law states in the Portevin-Le Chatelier effect

Recent studies on the Portevin-Le Chatelier effect report an intriguing crossover phenomenon from low-dimensional chaotic to an infinite-dimensional scale-invariant power law regime in experiments on CuAl single crystals and AlMg polycrystals, as function of strain rate. We devise fully dynamical model which reproduces these results. At low and medium strain rates, the model is chaotic with the structure of the attractor resembling the reconstructed experimental attractor. At high strain rates, power law statistics for the magnitudes and durations of the stress drops emerge as in experiments and concomitantly, the largest Lyapunov exponent is zero.

A dynamical model for the Portevin-le Chatelier bands

We show that an extension of Ananthakrishna's model to include spatial degrees of freedom produces spatially uncorrelated bands, hopping type and the continuously propagating type with increasing applied strain rate. The velocity of the continuously propagating bands is found to vary linearly with applied strain rate. (C) 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Correlation between band propagation property and the nature of serrations in the Portevin-Le Chatelier effect

We investigate the correlation between the band propagation property and the nature and amplitude of serrations in the Portevin-Le Chatelier effect within the framework of the Ananthakrishna model. Several significant results emerge. First, we find that spatial and temporal correlations continuously increase with strain rate from type C to type A bands. Consequently, the nature of the bands also changes continuously from type C to A bands, and so do the changes in the associated serrations. Second, even the smallest extent of propagation induces small amplitude serrations. The spatial extent of band propagation is directly correlated with the duration of small amplitude serrations, a result that is consistent with recent experiments. This correspondence allows one to estimate the spatial extent of band propagation by just measuring the temporal stretch of small amplitude serrations. Therefore, this should be of practical value when only stress versus strain is recorded. Third, the average stress drop magnitude of the small amplitude serrations induced by the propagating bands remains small and nearly constant with strain rate. As a consequence, the fully propagating type A bands are in a state of criticality. We rationalize the increasing levels of spatial and temporal correlations found with increasing strain rates. Lastly, the model also predicts several band morphologies seen in experiments including the Luders-like propagating band. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

CheepSync: A Time Synchronization Service for Resource Constrained Bluetooth LE Advertisers

Clock synchronization is highly desirable in distributed systems, including many applications in the Internet of Things and Humans. It improves the efficiency, modularity, and scalability of the system, and optimizes use of event triggers. For IoTH, BLE - a subset of the recent Bluetooth v4.0 stack - provides a low-power and loosely coupled mechanism for sensor data collection with ubiquitous units (e.g., smartphones and tablets) carried by humans. This fundamental design paradigm of BLE is enabled by a range of broadcast advertising modes. While its operational benefits are numerous, the lack of a common time reference in the broadcast mode of BLE has been a fundamental limitation. This article presents and describes CheepSync, a time synchronization service for BLE advertisers, especially tailored for applications requiring high time precision on resource constrained BLE platforms. Designed on top of the existing Bluetooth v4.0 standard, the CheepSync framework utilizes low-level time-stamping and comprehensive error compensation mechanisms for overcoming uncertainties in message transmission, clock drift, and other system-specific constraints. CheepSync was implemented on custom designed nRF24Cheep beacon platforms (as broadcasters) and commercial off-the-shelf Android ported smartphones (as passive listeners). We demonstrate the efficacy of CheepSync by numerous empirical evaluations in a variety of experimental setups, and show that its average (single-hop) time synchronization accuracy is in the 10 mu s range.