1,720,994 research outputs found
Linking high- and low-temperature plasticity in bulk metallic glasses II: use of a log-normal barrier energy distribution and a mean-field description of high-temperature plasticity
No full textA thermal activation model to describe the plasticity of bulk metallic glasses (Derlet and Maa beta, Phil. Mag. 93 (2013) p.4232) which uses a distribution of barrier energies and some aspects of under-cooled liquid physics is developed further. In particular, a log-normal distribution is now employed to describe the statistics of barrier energies. A high-temperature mean-field description of homogeneous macroplasticity is then developed and is shown to be similar to a thermal activation picture employing a single characteristic activation energy and activation volume. In making this comparison, the activation volume is interpreted as being proportional to the average mean-square-value of the plastic shear strain magnitude within the material. Also, the kinetic fragility at the glass transition temperature is shown to represent the effective number of irreversible structural transformations available at that temperature
A probabilistic explanation for the size-effect in crystal plasticity
No full textIn this work, the well-known power-law relation between strength and sample size, d(-n), is derived from the knowledge that a dislocation network exhibits scale-free behaviour and the extreme value statistical properties of an arbitrary distribution of critical stresses. This approach yields n = ( + 1) / ( + 1), where reflects the leading order algebraic exponent of the low-stress regime of the critical stress distribution and is the scaling exponent for intermittent plastic strain activity. This quite general derivation supports the experimental observation that the size effect paradigm is applicable to a wide range of materials, differing in crystal structure, internal microstructure and external sample geometry
Linking high- and low-temperature plasticity in bulk metallic glasses: thermal activation, extreme value statistics and kinetic freezing
No full textAt temperatures well below their glass transition, the deformation properties of bulk metallic glasses are characterized by a sharp transition from elasticity to plasticity, a reproducible yield stress and an approximately linear decrease of this stress with increasing temperature. In the present work, it is shown that when the well-known properties of the undercooled liquid regime, in terms of the underlying potential energy landscape, are assumed to be also valid at low temperature, a thermal activation model is able to reproduce the observed onset of macroscopic yield. At these temperatures, the thermal accessibility of the complex potential energy landscape is drastically reduced, and the statistics of extreme value and the phenomenon of kinetic freezing become important, affecting the spatial heterogeneity of the irreversible structural transitions mediating the elastic-to-plastic transition. As the temperature increases and approaches the glass transition temperature, the theory is able to smoothly transit to the high-temperature deformation regime where plasticity is known to be well described by thermally activated viscoplastic models.Alexander von Humboldt foundatio
Crystal size effect in two dimensions - Influence of size and shape
No full textBased on the statistics of both the stress and strain of a plastic event, the well known size-effect in strength can be linked to a crystal's critical stress distribution and the universal scaling exponent of intermittent plasticity. We successfully test these hypotheses with small-scale deformation experiments as a function of diameter and aspect ratio, and find that the latter affects the material's strength in a way that gives direct insight into the underlying critical stress distribution of the deforming volume. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved
Small-scale plasticity: Insights into dislocation avalanche velocities
No full textWe report direct measurements of displacement jump velocities in deforming nano- and microcrystals. These velocities exhibit large scatter and no correlation with plastic strain, crystal size or applied stress. Statistical analysis for the displacement jump velocities shows a cubic power-law decay, with a shoulder at low velocities. Our results indicate that the collective motion of dislocations in the deforming crystals is dominated by statistical fluctuations within the internal stresses, rather than by a mobility law that depends on the externally applied stress
Rate-dependent shear-band initiation in a metallic glass
No full textA statistical analysis of the rate-dependent pop-in behavior during spherical nanoindentation of a metallic glass is presented. Since the first pop-in of a test corresponds to the initiation of plasticity, this method provides insights into the fundamentals of shear-banding behavior. In this study, test series with different applied loading rates reveal that the initiation of a shear band is thermally activated with a rate-and stress-dependent activation volume. We further find a spatial inhomogeneity of the elastic modulus at the micrometer scale. This long-range structural heterogeneity does not strongly correlate with the shear-band initiation stress. (C) 2015 AIP Publishing LLC
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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