The basis of test predictions for text material.

Investigated the relationship between test predictions and test performance for text material. Ss predicted test performance or made judgments about ease of comprehension both before and after reading short expository texts. As Ss gained more information about texts, the correlations between predictions and performance increased. Generally, test predictions were better predictors of test performance than were ease of comprehension ratings. Exps 2 and 3 showed that Ss use domain familiarity in their test predictions, but this declines from before to after reading. Increased accuracy of test predictions from before to after reading was interpreted as indicating that Ss use specific information gained from reading texts to make accurate predictions about their future test performance. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Structural and material mechanical properties of human vertebral cancellous bone

The structural Young's modulus (i.e. that of the cancellous framework) was determined by non-destructive compressive mechanical testing in the three orthogonal axes of 48 vertebral bone cubes. In addition, the material Young's modulus (i.e. of the trabeculae themselves) was estimated using an ultrasonic technique. Apparent and true density were determined by direct physical measurements. Significant mechanical anisotropy was observed: mean structural Young's modulus varied from 165 MPa in the supero-inferior direction to 43 MPa in the lateral direction. Structural Young's modulus correlated with apparent density, with power-law regression models giving the best correlations (r2 = 0.52-0.88). Mechanical anisotropy increased as a function of decreasing apparent density (p < 0.001). Material Young's modulus was 10.0 +/- 1.3 GPa, and was negatively correlated with apparent density (p < 0.001). In multiple regression models, material Young's modulus was a significant independent predictor of structural Young's modulus only in the supero-inferior direction. The data suggest the presence of two effects in vertebral bone associated with decreasing apparent density and, by implication, bone loss in general: (a) increased mechanical anisotropy, such that there is relative conservation of stiffness in the axial direction compared with the transverse directions; and (b) increased stiffness of the trabeculae themselves.     

高钨含量先进屏蔽材料的力学性能研究

采用粉末冶金法制备了具有吸收中子和屏蔽γ射线的优异屏蔽性能的钨硼铝材料,研究了高钨含量对钨硼铝材料显微组织结构和力学性能的影响。结果表明,在选定的钨含量范围内,钨硼铝材料中大部分Al和W以单相存在,其中出现了WAl12相,这对钨硼铝材料起到了有效的增强作用。而随着钨含量增加,抗拉强度呈现降低的趋势。铝含量的减小不利于钨硼铝材料的结合强度,当含钨量为80%时,即80W1.46B₄C/Al,具有最优的综合力学性能,抗拉强度R_m为252 MPa,屈服强度R_p0.2为209 MPa,伸长率A_50mm为2.1%,冲击韧性为3.0 J/cm²,硬度HV₅为95.7。     

机械球磨与烧结W基材料的组织与性能

采用机械球磨与热压工艺制备了W-TiC、W-Ni、W-CNTs(碳纳米管)和W-Ni-CNTs 4种W基材料。研究结果表明,机械球磨能显著降低复合粉的晶粒尺寸和增加晶格畸变。经机械球磨后热压的样品中W-TiC的致密度最好,密度达到18.36g/cm3;W-Ni和W-CNTs的密度分别为17.97g/cm3和18.23g/cm3,具有较好的致密性;W-Ni-CNTs样品密度为15.84g/cm3,致密度略低。微观组织分析表明:添加TiC粒子可以显著改善材料的烧结行为,但晶粒较大;添加少量Ni制备的样品,不仅致密度高,而且晶粒较小;添加CNTs可以改善W的烧结行为,同时能够抑制晶粒长大和对W晶界起到强化作用;同时添加Ni和CNTs样品的致密度较低,需要对Ni和CNTs的添加量及烧结工艺条件进一步优化。结合微观组织分析与显微硬度测试结果,发现W烧结体的显微硬度不仅和材料密度有关,而且和W晶粒大小及掺杂相有关。     

Influence of forming conditions on the mechanical properties of aluminum-lead powder metallurgy material

Translated from Poroshkovaya Metallurgiya, No. 6(306), pp. 45–49, June, 1988.     

Estimating the working life of steel beams on the basis of their mechanical properties

Forged steel 50 rollers are used in the roughing group of wire and small-rod mills. In intense rolling, the roller pin often breaks, since the pin experiences the maximum stress. It is established that the fracture is of fatigue type, and three disintegration zones are identified. Crack propagation is due to the stress in the crack tip. The rate of crack propagation in cyclic loading depends on the structure of the metal, the grain size, and the mechanical properties of the rollers. The unbroken rollers are characterized by high values of the impact strength, initial and final temperatures of ingot forging, and σ_y/σ_B. An empirical dependence of the number of roller rotations to pin fracture on the mechanical properties of the material and the loads is established. Small-grain structure of the metal matrix and purity of the metal ensure a good combination of strength and plasticity.     

The mechanical properties of superplastic materials

The relationship between stress and strain rate is often sigmoidal in superplastic materials, with a low strain rate sensitivity at low and high strain rates (regions I and III, respectively) and a high strain rate sensitivity at intermediate strain rates (region II) where the material exhibits optimal superplasticity. This relationship is examined in detail, with reference both to the conflicting results reported for the Zn-22 pct Al eutectoid alloy and to the significance of the three regions of flow. This paper is based on a presentation made at the symposium “On the Mechanical, Microstructural and Fracture Processes in Superplasticity” held at the annual meeting of the AIME in Pittsburgh, PA on October 7, 1980 under the sponsorship of the Flow and Fracture Activity of the Materials Science Division of ASM.     

The mechanical properties of cellular solids

The mechanical properties (elastic, plastic, creep, and fracture) of cellular solids or foams are related to the properties of the cell wall material and to the cell geometry. The properties are well described by simple formulae. Such materials occur widely in nature and have many potential engineering applications.     

Genetic basis for diabetes resistance in NOD/Wehi mice

The basis for diabetes resistance in low diabetes incidence NOD/Wehi mice was examined in a breeding study. NOD/Wehi mice were crossed with high diabetes incidence NOD/Lt mice producing F1 hybrid mice which expressed a low incidence of diabetes. To distinguish between genetic and environmental causes for diabetes resistance, these F1 mice were backcrossed to NOD/Lt mice resulting in BC1 hybrid mice which expressed an intermediate incidence of diabetes. Similar results were obtained by examining the severity of insulitis in the hybrid mice. As both the incidence of diabetes and severity of insulitis in the hybrid mice were consistent with a single dominant gene mediating diabetes resistance, an attempt to localize this gene was made. Although over 140 loci which display polymorphism amongst inbred strains were typed in both parental lines, only a single locus, D8Mit9, was found to differ. As heterozygotes at D8Mit9 were not over represented amongst 45 diabetic BC1 hybrid mice examined, it was concluded that a resistance gene was not linked to this locus.     

A mechanical device for studying mechanical properties of human muscles in vivo

A mechanical device, primarily devoted to biomechanical studies of human calf muscles during microgravity experiments is presented. It allows investigation of both contractile and visco-elastic properties of musculo-articular systems using, respectively, isokinetic movements, quick-release tests and sinusoidal perturbations. This device is a specifically designed ergometer associated to an experimental protocol designed for pre- and post-flight tests. The protocol was evaluated on 22 healthy subjects and typical results are briefly presented. Preliminary results are discussed in terms of agreement with currently available data and a detailed evaluation of test-retest measurements is provided for quick-release experiments. Complementary investigations are suggested and potential fields of research are indicated.     

Structural state and mechanical properties of the material in the sialon-titanium nitride system after radiant heating

Conclusions Short-term radiant heating of the material of the Sialon-titanium nitride system in the temperature range 1230–2300°C is accompanied by oxidation of the titanium nitride with oxidation of rutile and titanium oxynitride (the lattice spacing of these compounds increases with increasing temperature) and also by the breakdown of '-Si₃N₄ to -Si₃N₄ and '-phase with a higher aluminum and oxygen content. The formation of rutile reduces the hardness of the material whereas formation of the oxynitride increases this parameter. A liquid phase consisting mainly of silicon and titanium disilicate appears on the surface of the specimens at temperatures above 1500°C. The material retains satisfactory strength in air at temperatures of up to 1200°C.Translated from Poroshkovaya Metallurgiya, No. 5(305), pp. 60–66, May, 1988.The authors are grateful to V. V. Kovylyaev and V. V. Traskovskii for help in the experiments.     

A miniature mechanical ventilator for newborn mice

Transgenic/knockout mice with pre-defined mutations have become increasingly popular in biomedical research as models of human diseases. In some instances, the resulting mutation may cause cardiorespiratory distress in the neonatal or adult animals and may necessitate resuscitation. Here we describe the design and testing of a miniature and versatile ventilator that can deliver varying ventilatory support modes, including conventional mechanical ventilation and high-frequency ventilation, to animals as small as the newborn mouse. With a double-piston body chamber design, the device circumvents the problem of air leakage and obviates the need for invasive procedures such as endotracheal intubation, which are particularly important in ventilating small animals. Preliminary tests on newborn mice as early as postnatal day O demonstrated satisfactory restoration of pulmonary ventilation and the prevention of respiratory failure in mutant mice that are prone to respiratory depression. This device may prove useful in the postnatal management of transgenic/knockout mice with genetically inflicted respiratory disorders.     

Structure and mechanical properties of a multilayer carbide-hardened niobium composite material fabricated by diffusion welding

The structure, the bending strength, and the fracture mechanism of an artificial niobium-based composite material, which is fabricated by high-pressure diffusion welding of multilayer stacks assembled from niobium foils with a two-sided carbon coating, are studied. The microstructure of the composite material is found to consist of alternating relatively plastic layers of the solid solution of carbon in niobium and hardening niobium carbide layers. The room-temperature proportional limit of the developed composite material is threefold that of the composite material fabricated from coating-free niobium foils using the proposed technology. The proportional limit of the developed composite material and the stress corresponding to the maximum load at 1100°C are 500 and 560 MPa, respectively. The developed material is considered as an alternative to Ni–Al superalloys.     

机械球磨技术在材料制备中的应用

综述了机械合金化、机械研磨以及机械涂覆三种机械球磨技术在材料制备领域中的应用和作用机理,讨论了三种机械球磨技术在材料制备中的优势与弊端,展望了该技术的今后发展方向。     

The influence of grain boundaries on mechanical properties

The effect of interfaces on mechanical properties is considered. Elastic and plastic compatibilities at boundaries are treated. Specific influences at both low and high temperatures are discussed, with emphasis on dislocation mechanisms and the atomic scale structure of boundaries.     

The structure and mechanical properties of metallic nanocrystals

Metallic nanocrystals are ultrafine-grained polycrystalline solids with grain sizes in the range of 1 to 10 nm in at least one dimension. Because of the extremely small dimensions, a large fraction of the atoms in these materials is located at the grain boundaries, and thus, they possess novel, and often improved, properties over those of conventional polycrystalline or glassy materials. In comparison to more conventional materials, nanocrystalline materials show a reduced density; increased thermal expansion, specific heat, and strength; a supermodulus effect; and extremely high diffusion rates. Traditionally brittle materials can be made ductile by nano-structure processing. At present, there is considerabe confusion on the nature of the micro-structure and mechanical properties of the nanocrystalline materials, especially of the equiaxed (three-dimensional, 3-D) type. The present article reviews the current understanding of nanocrystals and evaluates the data available on structure and mechanical properties of nanocrystalline metals. This invited overview is based on a presentation made in the symposium “Structure and Properties of Fine and Ultrafine Particles, Surfaces and Interfaces” presesnted as part of the 1989 Fall Meeting of TMS, October 1–5, 1989, in Indianapolis, IN, under the auspices of the Structures Committee of ASM/MSD.     

A high sucrose diet decreases the mechanical strength of bones in growing rats

High sucrose diets alter mineral metabolism in humans and animals. We examined the effect of a high sucrose diet on bone growth, composition and mechanical strength in growing rats. Weanling Wistar rats received a high sucrose (43 g/100 g) diet (9 males, 11 females). In the control diet (8 males, 8 females), sucrose was replaced with potato starch, providing an equal energy value. At the onset of the experiment, bones were marked by tetracycline. After 5 wk, the tibias and femurs were weighed, and maximum breaking strengths were determined. Tibias were cut at the tibia-fibular junction; the widths of the bone at the start of the experiment, the periosteal bone formation during the experiment, the widths of the medullary cavity and the final bone width were determined from tetracycline lines. Bone ash weight, Ca and P contents were determined. The breaking strengths of both bones were significantly lower in the sucrose-fed groups of both sexes. In females, the weight of both bones and the final width of the tibias were significantly lower in the sucrose-fed group. The Ca concentration in both bones and the P concentration in tibias were significantly lower in the sucrose-fed group. It was concluded that the metabolic interference induced by sucrose was the reason for the differences. The alterations were more pronounced in females, but independent of body weight.     

The mechanical properties of zero- carbon ferrite-plus-martensite structures

A study has been made of an Fe-3Ni-3Mo alloy whose structure can be either ferrite or martensite or any combination of ferrite plus martensite. By being able to measure the mechanical properties of the ferrite and martensite phases separately it is possible, using Milieko's theory for composites of two ductile phases, to calculate the properties expected for ferrite-plus-martensite mixtures. This theory assumes: 1) the stressstrain relationship for all structures is a power law; 2) a perfect interface between the phases; and 3) the fibers are aligned parallel to the stress axis. The experimentally determined tensile strength and ductility of the two-phase structures are in good agreement with the theory even though the martensite is not in the form of aligned fibers. The ductilities obtained at tensile strengths greater than 620 MPA (90 ksi) are no better than those for conventional HSLA steels because of the low ductility of the ferrite and low strength of the martensite.