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.     

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.     

The proximate memory mechanism underlying the survival-processing effect: Richness of encoding or interactive imagery?

Nairne and collaborators showed that assessing the relevance of words in the context of an imagined survival scenario boosts memory for these words. Although this survival-processing advantage has attracted a considerable amount of research, little is known about the proximate memory mechanism mediating this effect. Recently, Kroneisen and Erdfelder (2011) argued that it is not survival processing itself that facilitates recall but rather the richness and distinctiveness of encoding that is triggered by the survival-processing task. Alternatively, however, it is also conceivable that survival processing fosters interactive imagery, a process known to improve associative learning. To test these explanations we compared relevance-rating and interactive imagery tasks for survival and control scenarios. Results show that the survival advantage replicates in the relevance-rating condition but vanishes in the interactive imagery condition. This refutes the interactive imagery explanation and corroborates the richness-of-encoding hypothesis of the survival-processing effect.     

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.     

Adapting to test structure: Letting testing teach what to learn

We propose that we encode and store information as a function of the particular ways we have used similar information in the past. More specifically, we contend that the experience of retrieval can serve as a powerful cue to the most effective ways to encode similar information in comparable future learning episodes. To explore these ideas, we did two studies in which all participants went through study–test cycles of single category lists while we manipulated the nature of the recognition tests. The recognition tests either included only same-category lures or only different-category lures. The experience of repeated testing leads participants to avoid conceptual-based strategies but only when conceptual knowledge was poorly diagnostic for recognition (i.e., in the same-category lures condition). In a second study with a similar manipulation, we showed that repeated testing with lures from the same category as study items improved performance in a final recall surprise test compared to conditions in which different-category lures were used. Such a difference is akin to the one obtained when encoding instructions focus on distinctive item features compared to cases in which the focus is on relational processing. We suggest that testing requirements lead to adaptive changes at encoding.