Continuum modeling of the reverse Hall–Petch effect in nanocrystalline metals under uniaxial tension: how many grains in a finite element model?

Owing to their very high strength, nanocrystalline metals have been extensively studied over the recent years. The direct Hall–Petch law, empirically proportioning the material strength to the inverse square root of its grain size has been shown to break down below a grain size of the order of tenths of nanometers. This phenomenon has been widely rationalized as a gradual switch from intragrain mediated deformation mechanisms to grain boundary mediated deformation mechanisms. This transition has been observed in many finite element simulations, despite the intrinsic restriction of necessarily limiting the nanocrystalline representative assembly to only a few grains. Such a limitation is generally overlooked, and its influence on an uniaxial tension test – when compared to a complete sample of millions of grains – ignored. We propose here to quantify the approximation done by considering a finite number of grains by means of a simple analytical model based on the early work of Stevens [R.N. Stevens, Philos. Mag. 23 (1971) p. 265]. The finite element approximation is demonstrated to be relatively good, even down to only three grains in width, and a method to “correct” the stress-strain curves of small representative volumes is proposed.     

Banking Seed: Use and Value in the Conservation of Agricultural Diversity

Since the 1930s it has been widely acknowledged that agricultural crop diversity is being lost at an alarming rate. The international response to this genetic erosion has principally taken the form of ex situ genebanks. In these facilities and in the international regulatory frameworks that now surround them, it is genetic diversity that is the focus of conservation efforts. This focus, however, passes over the many other important diversities—both biological and social (or ‘biosocial’)—that exist in, and depend on, agricultural environments. These diversities cannot be conserved in genebanks. In addition to failing to actually conserve agricultural diversity (in any full sense of the term), ex situ banking projects also produce important potential inequalities—in terms of which material is banked and who has access to it. If, however, we refuse to accept an exclusive focus on the genetic components of plants, and instead insist on a brand of conservation that includes whole biosocial, more-than-human communities, then the role of banked resources must be radically rethought. Genebanks might instead take the place of central nodes in networks of diversity sharing, helping to keep plant varieties growing and circulating. This focus, in turn, requires that we also pay more critical attention to the various economic, legal and other mechanisms that prevent or stifle the flow and development of plant genetic resources in/to agricultural communities—especially those of peasant and indigenous farmers that play such a crucial role in conserving the world's (agro)biodiversity.     

Understanding scale-dependent yield stress of metals at micrometre scales

The mechanical softening behaviour of micrometre-scale metals (free-standing metal foils and wires) with decreasing either the geometrical or the microstructural scale (i.e. the smaller, the weaker) has recently been reported. Here, we present a mechanism-based model to understand the softening behaviour, which is based on competition between the effects of surface grain relaxation and the interior grain strengthening effect. The model describes well the yield stress of Cu foils with different ratios of the thickness to grain size.     

Confidence in accuracy moderates the benefits of retrieval practice

ABSTRACT

The effect of Retrieval Practice refers to the phenomenon that taking a practice test is more effective for learning than re-study, probably due to the benefit from processes underlying successful retrievals during practice. However, it is rarely studied whether other processes (e.g., metacognitive monitoring) during retrieval practice may also play an important role. In two experiments, we examined whether the effectiveness of retrieval was affected by subjects’ confidence in their retrieval success. Subjects studied word-pairs and rated their confidence after each practice test trial (cued-recall in Exp.1, multiple choice in Exp.2), or re-study trial. In both experiments, we observed the classic retrieval practice effect. Moreover, in Exp.2, the benefits differed as a function of subjects’ confidence in their retrieval performance: the effect of retrieval practice only emerged for high-confidence trials with higher than 56% confidence. In summary, the retrieval practice only facilitates the retention of correct answers with high confidence.     

Retrieval practice benefits memory precision

ABSTRACT

Although previous research on retrieval practice (RP) has predominantly featured stimuli with discrete right-or-wrong answers, continuous measures offer potentially greater sensitivity in assessing the effects of RP on memory precision. The present study used a colour gradient (125 points ranging from magenta to yellow) as a continuous response variable. The colours of different images were learned through either RP or restudy and either one or three cycles of practice after initial study. On a delayed final test, participants’ memory was assessed for each item’s colour. Participants also created per-item intervals representing the region where they believed the correct colour most likely to have been. We found that repeated rounds of RP enhanced the correspondence between responses and the correct colour. In addition, RP led to participants creating more accurate (correct answers were more likely to be within the participant-specified intervals) and more precise (narrower) intervals relative to restudy, suggesting that RP enhances the precision of memories.     

Comparing two perceived characteristics of autobiographical memory: Memory detail and accessibility

Four samples of participants recalled autobiographical memories. While some evidence emerged from regression analyses suggesting that judgements of the amount of detail contained in each memory and judgements of the ease with which events could be recalled were partially independent, the analyses generally showed that these judgements were similarly predicted by various event characteristics (age, typicality, self-importance, emotional intensity at event occurrence, rehearsal types). Co-occurrence frequency data yielded similar conclusions, showing that while ease ratings and detail ratings occasionally diverged, they were more often consistent with each other. Finally, the data also suggested that events that prompted emotional ambivalence were not judged to be more easily recalled, or to contain more detail, than non-ambivalent events.     

Remembered study mode: Support for the distinctiveness account of the production effect

The production effect is the finding that words spoken aloud at study are subsequently remembered better than are words read silently at study. According to the distinctiveness account, aloud words are remembered better because the act of speaking those words aloud is encoded and later recovery of this information can be used to infer that those words were studied. An alternative account (the strength-based account) is that memory strength is simply greater for words read aloud. To discriminate these two accounts, we investigated study mode judgements (i.e., “aloud”/”silent”/”new” ratings): The strength-based account predicts that “aloud” responses should positively correlate with memory strength, whereas the distinctiveness account predicts that accuracy of study mode judgements will be independent of memory strength. Across three experiments, where the strength of some silent words was increased by repetition, study mode was discriminable regardless of strength—even when the strength of aloud and repeated silent items was equivalent. Consistent with the distinctiveness account, we conclude that memory for “aloudness” is independent of memory strength and a likely candidate to explain the production effect.     

Living in History in Lebanon: The influence of chronic social upheaval on the organisation of autobiographical memories

The Living in History (LiH) effect is a litmus test for the degree to which historical events reorganise autobiographical memory. The LiH effect was studied in two Lebanese samples: a Beiruti sample that lived in the epicentre of the 15-year Lebanese Civil War (1975–1990) and another group from the Bi'qa region who lived in an area that was indirectly exposed for most of the civil war but experienced one short-term period of war during the Israeli invasion. Using the two-phase word-cueing task to elicit dated autobiographical memories, we observed a significantly stronger LiH effect in the Beirut sample but also a significant yet weaker LiH effect in the Bi'qa sample. In addition to the main finding we offer evidence that the LiH effect waxes and wanes with the level of conflict in an area and that reported personal experiences of war exposure predict the strength of the LiH effect. Our findings suggest that collective transitional events which produce a marked change in the fabric of daily living engender historically defined autobiographical periods which give structure and organisation to how individuals remember their past.     

The effects of local and global processing orientation on eyewitness identification performance

Recent work has demonstrated that performance on a simultaneous target-present photographic line-up can be enhanced by prior global processing orientation, and hindered by prior local processing orientation induced by processing Navon letter stimuli. A series of studies explore the generality of this processing bias effect using either videotaped scenarios or live interactions. Five experiments demonstrate that these effects are seen across a range of test stimuli, test formats, and test instructions. These data inform the processes engaged in by witnesses when making line-up identifications and indicate that it may be possible to improve the accuracy of witnesses making such judgements.