Size Effects in Micro-bending Deformation of MEMS Devices Based on the Discrete Dislocation Theory

In the present research, the discrete dislocation theory is used to analyze the size effect phenomena for the MEMS devices undergoing micro-bending load. A consistent result with the experimental one in literature is obtained. In order to check the effectiveness to use the discrete dislocation theory in predicting the size effect, both the basic version theory and the updated one are adopted simultaneously. The normalized stress-strain relations of the material are obtained for different plate thickness or for different obstacle density. The prediction results are compared with experimental results.

Particulate Size Effects in the Particle-Reinforced Metal-Matrix Composites

The influences of I,article size on the mechanical properties of the particulate metal matrix composite;are obviously displayed in the experimental observations. However, the phenomenon can not be predicted directly using the conventional elastic-plastic theory. It is because that no length scale parameters are involved in the conventional theory. In the present research, using the strain gradient plasticity theory, a systematic research of the particle size effect in the particulate metal matrix composite is carried out. The roles of many composite factors, such as: the particle size, the Young's modulus of the particle, the particle aspect ratio and volume fraction, as well as the plastic strain hardening exponent of the matrix material, are studied in detail. In order to obtain a general understanding for the composite behavior, two kinds of particle shapes, ellipsoid and cylinder, are considered to check the strength dependence of the smooth or non-smooth particle surface. Finally, the prediction results will be applied to the several experiments about the ceramic particle-reinforced metal-matrix composites. The material length scale parameter is predicted.

Fracture Mechanisms And Size Effects Of Brittle Metallic Foams: In Situ Compression Tests Inside Sem

In situ compressive tests on specially designed small samples made from brittle metallic foams were accomplished in a loading device equipped in the scanning electron microscopy (SEM). Each of the small samples comprises only several cells in the effective test zone (ETZ), with one major cell in the middle. In such a system one can not only obtain sequential collapse-process images of a single cell and its cell walls with high resolution, but also correlate the detailed failure behaviour of the cell walls with the stress-strain response, therefore reveal the mechanisms of energy absorption in the mesoscopic scale. Meanwhile, the stress-strain behaviour is quite different from that of bulk foams in dimensions of enough large, indicating a strong size effect. According to the in situ observations, four failure modes in the cell-wall level were summarized, and these modes account for the mesoscopic mechanisms of energy absorption. Paralleled compression tests on bulk samples were also carried out, and it is found that both fracturing of a single cell and developing of fracture bands are defect-directed or weakness-directed processes. The mechanical properties of the brittle aluminum foams obtained from the present tests agree well with the size effect model for ductile cellular solids proposed by Onck et al. (C) 2008 Elsevier Ltd. All rights reserved.

Size effects on the melting of nickel nanowires: a molecular dynamics study

The melting process of nickel nanowires are simulated by using molecular dynamics with the quantum Sutten-Chen many-body force field. The wires studied were approximately cylindrical in cross-section and periodic boundary conditions were applied along their length; the atoms were arranged initially in a face-centred cubic structure with the [0 0 1] direction parallel to the long axis of the wire. The size effects of the nanowires on the melting temperatures are investigated. We find that for the nanoscale regime, the melting temperatures of Ni nanowires are much lower than that of the bulk and are linear with the reciprocal of the diameter of the nanowire. When a nanowire is heated up above the melting temperature, the neck of the nanowire begins to arise and the diameter of neck decreases rapidly with the equilibrated running time. Finally, the breaking of nanowire arises, which leads to the formation of the spherical clusters. (C) 2004 Elsevier B.V. All rights reserved.

Edge and Size Effects in Micro Composite Structure

Because of the load transfer effect of interface layer, the stress distribution inside the composite structure of film/substrate can be very different from the Timoshenko's model. In this paper, we give the derivation and analysis of such load transfer effect of shear-lag (S-L) model. The micro-structure size (boundary conditions) effect together with interface load transfer effect becomes more and more important as the microstructure size including the three dimensions of thickness, width and length shrinks. The microstructure size is also responsible for the so-called edge-induced stress. The edge effect and difference of S-L model and Timoshenko model are also demonstrated.

A unified guide to two opposite size effects in nano elastic materials

The microstructural variation near surface of nano elastic materials is analyzed based on different potentials. The atomic/molecular mechanism underlying the variation and its effect on elastic modulus are such that the nature of long-range interactions (attractive or repulsive) in the atomic/molecular potentials essentially governs the variation near surface (looser or tighter) and results in two opposite size effects (decreasing or increasing modulus) with decreasing size.

Group size effects on foraging and vigilance in migratory Tibetan antelope

Large group sizes have been hypothesized to decrease predation risk and increase food competition. We investigated group size effects on vigilance and foraging behaviour during the migratory period in female Tibetan antelope Pantholops hodgsoni, in the Kekexili Nature Reserve of Qinghai Province, China. During June to August, adult female antelope and yearling females gather in large migratory groups and cross the Qinghai-Tibet highway to calving grounds within the Nature Reserve and return to Qumalai county after calving. Large groups of antelope aggregate in the migratory corridor where they compete for limited food resources and attract the attention of mammalian and avian predators and scavengers. We restricted our sampling to groups of less than 30 antelopes and thus limit our inference accordingly. Focal-animal sampling was used to record the behaviour of the free-ranging antelope except for those with lambs. Tibetan antelope spent more time foraging in larger groups but frequency of foraging bouts was not affected by group size. Conversely, the time spent vigilant and frequency of vigilance bouts decreased with increased group size. We suggest that these results are best explained by competition for food and risk of predation. (C) 2007 Elsevier B.V. All rights reserved.

The Size-Dependent Elastic Properties of Nanofilms with Surface Effects

Size-dependent elastic constants are investigated theoretically with reference to a nanoscale single-crystal thin film. A three-dimensional _3D_ model is presented with the relaxation on the surface of the nanofilm taken into consideration. The constitutive relation of the 3D model is derived by using the energy approach, and analytical expressions for the four nonzero elastic constants of the nanofilm are obtained. The size effects of the four elastic constants are then discussed, and the dependence of these elastic constants on the surface relaxation and the ambiguity in the definition of the thickness of the nanofilm are also analyzed. In addition, the elastic moduli of the nanofilm in two kinds of plane problem are obtained and discussed in the case of a special boundary condition.

Size-Dependent Elastic Modulus And Fracture Toughness Of The Nanofilm With Surface Effects

The effective elastic modulus and fracture toughness of the nanofilm were derived with the surface relaxation and the surface energy taken into consideration by means of the interatomic potential of an ideal crystal. The size effects of the effective elastic modulus and fracture toughness were discussed when the thickness of the nanofilm was reduced. And the dependence of the size effects on the surface relaxation and surface energy was also analyzed.

Effects of diet and body size on phosphorus utilization of Liza haematocheila T. & S.

A 21-d laboratory experiment was conducted to study, the phosphorus (P) utilization of two different diets by redlip mullet Liza haematocheila T. & S. Sand-filtered water in salinity 30 and temperature 25 degrees C was used. Twenty-nine fish individuals were divided into three groups: 11 to group 1 (G1) fed on diet 1, 11 to group 2 (G2) fed on diet 2, and 7 to contrast group. Diet 1 was a commercial feed, more valuable in nutrition than diet 2 that similar to natural detritus. The results show the intake phosphorus (IP) of G1 was significantly higher than that of G2, and both increased linearly with body size at a certain amount of diet. The retention phosphorus (RP) in fish of G1 was lower than G2. The relationship between retention phosphorus and body size was positive and stronger in G2. Significant difference in faecal phosphorus (FP) was found between G1 and G2. Body size significantly impacted the excretion phosphorus (EP) in G1 but G2. The loss of intake phosphor-us in G1 was 10.83-20.27 mg per g fish weight gain, higher than that in G2 for 6.63-9.56. Of the phosphor-us, about 10% was allocated into growth, 50% in faeces, and the rest lost in excretion. The main part of phosphorus was lost in faeces but excretion. The phosphorus budget of the fish could be described as 100IP = 7.40RP + 47.39FP + 36.63EP (Diet 1) or 100IP = 11.93RP + 56.64FP + 21.76EP (Diet 2).

Size Effect and Geometrical Effect of Solids in Micro-Indentation Test

Micro-indentation tests at scales of the order of sub-micron show that the measured hardness increases strongly with decreasing indent depth or indent size, which is frequently referred to as the size effect. At the same time, at micron or sub-micron scale, another effect, which is referred to as the geometrical size effects such as crystal grain size effect, thin film thickness effect, etc., also influences the measured material hardness. However, the trends are at odds with the size-independence implied by the conventional elastic-plastic theory. In the present research, the strain gradient plasticity theory (Fleck and Hutchinson) is used to model the composition effects (size effect and geometrical effect) for polycrystal material and metal thin film/ceramic substrate systems when materials undergo micro-indenting. The phenomena of the "pile-up" and "sink-in" appeared in the indentation test for the polycrystal materials are also discussed. Meanwhile, the micro-indentation experiments for the polycrystal Al and for the Ti/Si_3N_4 thin film/substrate system are carried out. By comparing the theoretical predictions with experimental measurements, the values and the variation trends of the micro-scale parameter included in the strain gradient plasticity theory are predicted.

Mechanisms underlying two kinds of surface effects on elastic constants

ABSTRACT Recently, people are confused with two opposite variations of elastic modulus with decreasing size of nano scale sample: elastic modulus either decreases or increases with decreas- ing sample size. In this paper, based on intermolecular potentials and a one dimensional model, we provide a unified understanding of the two opposite size effects. Firstly, we analyzed the mi- crostructural variation near the surface of an fcc nanofilm based on the Lennard-Jones potential. It is found that the atomic lattice near the surface becomes looser in comparison with the bulk, indicating that atoms in the bulk are located at the balance of repulsive forces, resulting in the decrease of the elastic moduli with the decreasing thickness of the film accordingly. In addition, the decrease in moduli should be attributed to both the looser surface layer and smaller coor- dination number of surface atoms. Furthermore, it is found that both looser and tighter lattice near the surface can appear for a general pair potential and the governing mechanism should be attributed to the surplus of the nearest force to all other long range interactions in the pair po- tential. Surprisingly, the surplus can be simply expressed by a sum of the long range interactions and the sum being positive or negative determines the looser or tighter lattice near surface re- spectively. To justify this concept, we examined ZnO in terms of Buckingham potential with long range Coulomb interactions. It is found that compared to its bulk lattice, the ZnO lattice near the surface becomes tighter, indicating the atoms in the bulk located at the balance of attractive forces, owing to the long range Coulomb interaction. Correspondingly, the elastic modulus of one- dimensional ZnO chain increases with decreasing size. Finally, a kind of many-body potential for Cu was examined. In this case, the surface layer becomes tighter than the bulk and the modulus increases with deceasing size, owing to the long range repulsive pair interaction, as well as the cohesive many-body interaction caused by the electron redistribution.

Effects of seed layer on the realization of larger self-assembled coherent InAs/GaAs quantum dots

The size and shape evolution of self-assembled InAs quantum dots (QDs) influenced by 2.0 ML InAs seed layer has been systematically investigated for 2.0, 2.5, and 2.9 ML deposition on GaAs(100) substrate. Based on comparisons with the formation of large incoherent InAs islands on single-layer samples at late growth stage, the larger coherent InAs quantum dots at 2.9 ML deposition has been observed on the second InAs layer. A simple model analysis accounting for the surface strain distribution influenced by buried islands gives a stronger increment of critical QD diameter for dislocation nucleation on the second layer in comparison with the single-layer samples. Additionally, the inhibition of dislocation nucleation in InGaAs/GaAs large islands can also be explained by our theoretical results. (C) 2000 American Institute of Physics. [S0021-8979(00)08922-2].

Size-Dependent Adhesion Strength of a Single Viscoelastic Fiber

Nano-fibrillar adhesives can adhere strongly to surfaces as a gecko does. The size of each fiber has significant effects on the adhesion enhancement, especially on rough surfaces. In the present study, we report the size effects on the normal and shear strength of adhesion for a single viscoelastic fiber. It is found that there exists a limited region of the critical sizes under which the interfacial normal or tangential tractions uniformly attain the theoretical adhesion strength. The region for a viscoelastic fiber under tension with similar material constants to a gecko's spatula is 135-255 nm and that under torque is 26.5-52 nm. This finding is significant for the development of artificial biomimetic attachment systems.

Scaling, Dimensional Analysis, and Indentation Measurements

We provide an overview of the basic concepts of scaling and dimensional analysis, followed by a review of some of the recent work on applying these concepts to modeling instrumented indentation measurements. Specifically, we examine conical and pyramidal indentation in elastic-plastic solids with power-law work-hardening, in power-law creep solids, and in linear viscoelastic materials. We show that the scaling approach to indentation modeling provides new insights into several basic questions in instrumented indentation, including, what information is contained in the indentation load-displacement curves? How does hardness depend on the mechanical properties and indenter geometry? What are the factors determining piling-up and sinking-in of surface profiles around indents? Can stress-strain relationships be obtained from indentation load-displacement curves? How to measure time dependent mechanical properties from indentation? How to detect or confirm indentation size effects? The scaling approach also helps organize knowledge and provides a framework for bridging micro- and macroscales. We hope that this review will accomplish two purposes: (1) introducing the basic concepts of scaling and dimensional analysis to materials scientists and engineers, and (2) providing a better understanding of instrumented indentation measurements.