Failure mechanism in porous materials under compression: crackling noise in mesoporous SiO2

The failure mechanism of porous materials under uniaxial stress has been investigated experimentally. Microporous SiO₂, Vycor, has been subjected to slowly increasing compressive uniaxial stress with stress rates between 0.2 and 12.2?kPa/s. With increasing stress the strain changes stepwise with acoustic emission correlated with each volume collapse. The acoustic emission followed the characteristics of ‘crackling noise’ with a power law distribution over an exceptionally large interval of 6 decades at the slowest stress rate. The power law exponent is ?1.39. Possible applications in mining industry and others are discussed.     

Changes of lumbosacral joint compression force profile when walking caused by backpack loads

Walking with backpack loads induces additional mechanical stress on the spine and has been identified as a risk factor of lower-back pain. This study evaluated the effects of walking with backpack loads on the lumbosacral joint compression force profile in both the magnitude and time domains. Ten male adults geared with anatomical markers and trunk surface electromyographic sensors walked along a walkway embedded with three force plates with no load and various backpack loads (5%, 10%, 15%, and 20% body weight). Lower-body movements, ground reaction forces, and trunk muscle activations were measured using a synchronized motion analysis, force plate, and surface electromyography system. The force profiles of identified gait cycles were predicted using an integrated inverse dynamic and electromyography-assisted optimization model and evaluated statistically. The results showed that as backpack load increased, the 10th, 50th, and 90th percentiles of force profiles escalated disproportionately. However, no significant changes were observed in the timing of the two peak force incidences. Such changes in the compression force might be an indication of the combined effects of the increase in both gravitational and mass moment of inertia of the system (body plus pack loads) when walking with a backpack. Pearson correlation coefficients of the force profiles between the five loading conditions were greater than 0.94. Strong associations between the force profiles at different backpack loads were confirmed.     

Intrinsic factor controlling the deformation and ductile-to-brittle transition of metallic glasses

The energetic driving force and resistance for shearing and cracking in metallic glasses (MGs) are quantitatively evaluated. A universal thermodynamic criterion is proposed for better understanding the intrinsic correlations between fracture toughness and Poisson’s ratio, the competitions between various deformation modes and the ductile-to-brittle transition in MGs and other materials. A new cooperation parameter δ is also introduced to depict quantitatively the relative propensity of shearing versus cracking. This work could provide insights into the long-standing issues of deformation mechanisms of glassy materials, and be helpful in searching for ductile and tough MGs.     

Limb Segment Load Inhibits the Recovery of Soleus H-Reflex After Segmental Vibration in Humans

We investigated the effects of vertical vibration and compressive load on soleus H-reflex amplitude and postactivation depression. We hypothesized that, in the presence of a compressive load, limb vibration induces a longer suppression of soleus H-reflex. Eleven healthy adults received vibratory stimulation at a fixed frequency (30 Hz) over two loading conditions (0% and 50% of individual's body weight). H-reflex amplitude was depressed ～88% in both conditions during vibration. Cyclic application of compression after cessation of the vibration caused a persistent reduction in H-reflex excitability and postactivation depression for > 2.5 min. A combination of limb segment vibration and compression may offer a nonpharmacologic method to modulate spinal reflex excitability in people after CNS injury.     

Deformation and failure of a heterogeneous high explosive

The deformation and failure of the pristine PBX 9501 high explosive are studied experimentally by combining Brazilian disk compression with the digital image correlation (DIC) technique. The Brazilian disk specimen was loaded by a pair of compressive forces across the diameter. The full-field deformation on the surface of the disk, during the loading process, was obtained using DIC on a series of random speckle images acquired during the test. In this series of experiments, we focused on the effect of temperature while keeping the loading speed quasi-static. Tests were conducted under three different temperatures, ?15°C, 23°C, and 50°C. For all testing conditions, it was observed that as the applied load reaches maximum, localized deformation starts at the two regions near the loading points. The localized deformation then proceeds along an irregular path toward the center of the specimen and coalesces. Finally, deformation of the Brazilian disk subjected to cyclic compressive load at ambient temperature was also measured and studied.     

Precipitate strengthening in nanostructured metallic material composites

Nanostructured metallic material (NMM) composites are a new class of materials that exhibit high structural stability, mechanical strength, high ductility, toughness and resistance to fracture and fatigue; these properties suggest that these materials can play a leading role in the future micromechanical devices. However, before those materials are put into service in any significant applications, many important fundamental issues remain to be understood. Among them, is the question of the strengthening of NMM using second phase particles and if the addition of precipitates will strengthen the structures in the same manner as in bulk crystalline solids. This issue is addressed in this work by performing molecular dynamics simulations on NMM with precipitates of various sizes and comparing the results with the same structure without precipitates. In this view, Cu/Nb bilayer thin films with spherical Nb particles inside the Cu layer were examined using molecular dynamics simulations and show a significant improvement on their mechanical behavior, compared to similar structures without particles. Furthermore, an analytical model is developed that explains the strengthening behavior of an NMM that has precipitates inside one layer. The theoretical results show a qualitative agreement with the finding of the atomistic simulations.     

Critical Poisson’s ratio between toughness and brittleness

We have a new understanding on a unified fracture criterion based on two kinds of strain energy density. A definite expression which is a function of Poisson’s ratio to describe a crucial parameter in the unified fracture criterion is derived. The critical value of Poisson’s ratio between brittle and ductile materials can be derived as 2/7 which is verified by experimental data. Besides, the effect of Poisson’s ratio on the fracture loci in stress spaces is studied using unified fracture criterion. It is found that the decrease in Poisson’s ratio increases the asymmetry of fracture loci between tensile stress states and compressive stress states in principal stress spaces and between Lode parameters are 1 and ?1 in deviatoric-stress spaces. In addition, stress triaxiality has a great effect on the fracture behaviour with different Poisson’s ratios.     

Understanding how people process health information: a comparison of tailored and nontailored weight-loss materials.

Health information tailored to meet individuals' unique needs has been shown to be more effective than generic information in promoting risk-reducing behavior changes. To explore mechanisms underlying tailoring's effectiveness, this study randomly assigned 198 overweight adults to receive weight-loss materials that were (a) tailored to the individual, (b) in an American Heart Association (AHA) brochure, or (c) AHA-content formatted to look like tailored materials. Participants who received tailored materials had more positive thoughts about the materials, positive personal connections to the materials, positive self-assessment thoughts, and positive thoughts indicating behavioral intention than those who received either of the untailored materials. The tailoring of health information can significantly improve the chances the information will be thoughtfully considered and can stimulate prebehavioral changes such as self-assessment and intention.     

Microstructure and mechanical behavior of nanostructured composite Cu60Fe40 alloy

In this study, bulk nanostructured composite Cu₆₀Fe₄₀ alloy is prepared by a combustion synthesis technique. The prepared Cu₆₀Fe₄₀ alloy consists of Cu(Fe) solid solution and Fe(Cu) solid solution phases. The large-scaled compositional segregation in the Cu-rich and Fe-rich phases is not observed, respectively. A few micron-sized dendrite (Fe(Cu) solid solution) is embedded into the nanostructured matrix (Cu(Fe) solid solution). The grain size of the matrix is in the range 50–300?nm. The yield and fracture strength of the Cu₆₀Fe₄₀ alloy are 540 and 1050?MPa, respectively, and the fracture strain obtained from the compression test is about 20.9%. The Cu₆₀Fe₄₀ alloy displays notable work hardening in the compressive deformation.     

Toughening of a brittle material by means of dislocation subboundaries

A novel technique to toughen brittle materials has been developed. Surfaces of a brittle material were damaged at room temperature, followed by annealing at a high temperature. During annealing, cracks, which were introduced during surface damage, disappear and dislocation subboundaries are formed. This treatment improves the fracture toughness K_Ic by a factor of two.     

The default mode network and altered consciousness in epilepsy

The default mode network has been hypothesized based on the observation that specific regions of the brain are consistently activated during the resting state and deactivated during engagement with task. The primary nodes of this network, which typically include the precuneus/posterior cingulate, the medial frontal and lateral parietal cortices, are thought to be involved in introspective and social cognitive functions. Interestingly, this same network has been shown to be selectively impaired during epileptic seizures associated with loss of consciousness. Using a wide range of neuroimaging and electrophysiological modalities, decreased activity in the default mode network has been confirmed during complex partial, generalized tonic-clonic, and absence seizures. In this review we will discuss these three seizure types and will focus on possible mechanisms by which decreased default mode network activity occurs. Although the specific mechanisms of onset and propagation differ considerably across these seizure types, we propose that the resulting loss of consciousness in all three types of seizures is due to active inhibition of subcortical arousal systems that normally maintain default mode network activity in the awake state. Further, we suggest that these findings support a general "network inhibition hypothesis", by which active inhibition of arousal systems by seizures in certain cortical regions leads to cortical deactivation in other cortical areas. This may represent a push-pull mechanism similar to that seen operating between cortical networks under normal conditions.     

Effects of enactment by professional actors at encoding and retrieval

This experiment investigated memory benefits similar to those found with subject-performed tasks (SPTs) but under widely differing circumstances. Almost all SPT research has shown that as long as enactment takes place at encoding, mode of recall (enactment vs. verbal recall) is immaterial. Using professional actors, the experimenters had previously shown (Noice & Noice, 1999) that movement that was not semantically congruent with the accompanying material produced enhanced recall at retrieval compared to a non-moving condition, a result that did not appear to be due to the fact that retrieval conditions closely resembled encoding conditions. Experiment 1 of the present study replicated and extended this finding, demonstrating that the effect can be found after a delay of five months with actors of varying age and experience, and that the enhancement is not dependent on physical context. Experiment 2 demonstrated that, even with purely verbal retrieval of material that had been equated for memorability, dialogue originally performed when the participants had been engaged in movement from place to place was better recalled than dialogue originally performed when the participants had remained in one location. Taken together, these results indicate that movement at retrieval is not necessary for the nonliteral enactment effect to occur, but that such movement can aid recall compared to a purely verbal mode of report.     

Generalizing test-enhanced learning from the laboratory to the classroom

Test-enhanced learning refers to the fact that taking an initial test on studied material enhances its later retention relative to simply studying the material and then taking a final test. Most research on the testing effect has been done with materials such as word lists, and the general finding has been that the benefits of testing are greater when the initial test is a recall (production) test rather than a recognition test. We briefly summarize three experiments that extend these results to educationally relevant materials, namely brief articles, lectures, and materials in a college course. All three experiments demonstrated a robust testing effect and also revealed that an initial short-answer test produced greater gains on a final test than did an initial multiple-choice test. Furthermore, one experiment revealed a positive effect of immediate feedback given with the initial test. The educational implications are that production tests (short answer or essay) and feedback soon after learning increase learning and retention. In addition, frequent testing probably has the indirect positive effects of keeping students motivated and leading them to space out periods of study.     

Factor pattern of test items and tests as a function of the correlation coefficient: Content,difficulty, and constant error factors

A dilemma was created for factor analysts by Ferguson (Psychometrika, 1941,6, 323–329) when he demonstrated that test items or sub-tests of varying difficulty will yield a correlation matrix of rank greater than 1, even though the material from which the items or sub-tests are drawn is homogeneous, although homogeneity of such material had been defined operationally by factor analysts as having a correlation matrix of rank 1. This dilemma has been resolved as a case of ambiguity, which lay in (1) failure to specify whether homogeneity was to apply to content, difficulty, or both, and (2) failure to state explicitly the kind of correlation to be used in obtaining the matrix. It is demonstrated that (1) if the material but (2) if content is homogeneous but difficulty is not, the homogeneity of the content can be demonstrated only by using the tetrachoric correlation coefficient in deriving the matrix; and that the use of the phi-coefficient (Pearsonianr) will disclose only the nonhomogeneity of the difficulty and lead to a series ofconstant error factors as contrasted withcontent factors. Since varying difficulty of items (and possibly of sub-tests) is desirable as well as practically unavoidable, it is recommended that all factor analysis problems be carried out with tetrachoric correlations. While no one would want to obtain the constant error factors by factor analysis (difficulty being more easily obtained by counting passes), their importance for test construction is pointed out.     

Scale-dependent fracture mode in Cu–Ni laminate composites

Fracture behavior of Cu–Ni laminate composites has been investigated by tensile testing. It was found that as the individual layer thickness decreases from 100 to 20 nm, the resultant fracture angle of the Cu–Ni laminate changes from 72° to 50°. Cross-sectional observations reveal that the fracture of the Ni layers transforms from opening to shear mode as the layer thickness decreases while that of the Cu layers keeps shear mode. Competition mechanisms were proposed to understand the variation in fracture mode of the metallic laminate composites associated with length scale.     

Deformation twins induced by multi-mode deformation in nanocrystalline copper

A multi-mode deformation model is used in a molecular dynamics simulation of nanocrystalline copper. Abundant deformation twin lamellae are developed by shearing the following compression to the elastic limit. Deformation twins (DTs) nucleate through two different mechanisms facilitated by Shockley partial slips. Interactions between DTs and Shockley partials are observed in this simulation.     

重复知盲效应的理论之争及脑机制

重复知盲现象自被发现至今已走过四分之一世纪, 这期间重复知盲的产生机制一直是争论的焦点, 并有标记个体化理论、类型节点不应期理论、竞争理论、提取失败理论和建构/归因理论等多种理论解释。大量研究证明, 重复知盲是一种视知觉失败现象, 但也有研究认为它是一种记忆失败现象。很多研究还发现, 重复知盲不仅受到呈现时间和间隔刺激个数的限制, 还受到实验材料的性质、刺激的空间位置及人们的反应方式等多种因素的影响。近些年来, 随着ERP和fMRI等技术的利用, 从更深层次揭示了重复知盲产生的脑机制。同时, 重复知盲也作为一种实验范式应用于视觉加工领域的研究中。目前关于重复知盲仍然有很多问题值得探索。     

回合式教学法的有效学习：来自自闭症儿童职前教师的证据

使用异步培训技术自主学习循证教育干预方法,或可帮助自闭症儿童职前教师应对新时代自闭症儿童融合教育高质量发展带来的挑战。采用3个实验探究了自闭症儿童职前教师在3种学习材料呈现方式下对回合式教学法的自主学习效果。结果发现,学习材料呈现方式对自闭症儿童职前教师自主学习回合式教学法的效果有显著影响：“字幕+画面+声音解说”组成的视听双通道材料——真实教学视频下的学习效果最好,“文本+图像”组成的PPT材料比同属视觉单通道材料的Word文本更有优势。结果表明,无论是在知识层面的学习(实验1、2和3),还是在实操层面的忠诚度提升(实验3),学习内容以视听双通道方式呈现时,自闭症儿童职前教师的自主学习效果最好。     

Muscular contribution to low-back loading and stiffness during standard and suspended push-ups

Push-up exercises are normally performed to challenge muscles that span upper extremity joints. However, it is also recognized that push-ups provide an effective abdominal muscle challenge, especially when the hands are in contact with a labile support surface. The purpose of this study was to compare trunk muscle activation levels and resultant intervertebral joint (IVJ) loading when standard and suspended push-ups were performed, and to quantify and compare the contribution of trunk muscles to IVJ rotational stiffness in both exercises. Eleven recreationally trained male volunteers performed sets of standard and suspended push-ups. Upper body kinematic, kinetic, and EMG data were collected and input into a 3D biomechanical model of the lumbar torso to quantify lumbar IVJ loading and the contributions of trunk muscles to IVJ rotational stiffness. When performing suspended push-ups, muscles of the abdominal wall and the latissimus dorsi were activated to levels that were significantly greater than those elicited when performing standard push-ups (p<.05). As a direct result of these increased activation levels, model-predicted muscle forces increased and consequently led to significantly greater mean (p=.0008) and peak (p=.0012) lumbar IVJ compressive forces when performing suspended push-ups. Also directly resulting from the increased activation levels of the abdominal muscles and the latissimus dorsi during suspended push-ups was increased muscular contribution to lumbar IVJ rotational stiffness (p<.05). In comparison to the standard version of the exercise, suspended push-ups appear to provide a superior abdominal muscle challenge. However, for individuals unable to tolerate high lumbar IVJ compressive loads, potential benefits gained by incorporating suspended push-ups into their resistance training regimen may be outweighed by the risk of overloading low-back tissues.