Characterizations of pore and constituent particle populations in 7050-T7451 aluminum plate alloys

Although qualitative relationships between fatigue lives and the sizes of the microstructural features, such as pores and particles, are well known, the quantitative models are lacking because of the unavailability of the required detailed microstructural data. The purpose of this work was to obtain such data for the high porosity (HP) and reduced porosity (RP) variants of the aluminum 7050-T7451 thick-plate alloys. Both alloys had similar tensile and fracture properties, but the reduced porosity variant showed superior fatigue performance attributed to the smaller sizes of the fatigue crack initiating particles and pores. Those size differences, as well as the differences in the through-thickness size gradients, have been characterized in this work. The sizes, shapes, and orientations of particles and pores were analyzed first on the plane sections and then converted to the true three-dimensional (3-D) characteristics using the moment method. In the conversions, the particle and pore shapes have been assumed as triaxial ellipsoids and their size distributions as lognormal. The spatial distributions were quantified using the nearest neighbor spacing method. Results confirmed that the reduced porosity alloy had smaller particles and pores than the high porosity variant. The size distributions in the former were also more confined. In both alloys, the largest particles and pores were at the plate centers and the smallest at the surface. Their spatial distributions could be categorized as random with clusters.     

Physical chemistry of some microstructural phenomena

Gibbs equation for stressed solids is extended to include constrained chemical equilibria among components whose chemical potentials may not be individually definable. Strain volume is introduced as a new tensorial extensive variable. Solubility enhancements in the presence of misfit inclusions of spherical as well as parallelepipedal shapes are cal-culated an compared with experiments involving excess nitrogen around TiN₂ platelet inclusions in ferrite. Diffusion-induced stresses or the chemical stress effect is treated for some simple shapes such as thin slabs, cylinders, and spheres. A programmed dif-fusion scheme is devised to minimize both the diffusion time and the stress developed dur-ing diffusion. The effective diffusivity of an isotropic solute due to diffusion-induced stress gradients is given. Stress induced diffusion is exemplified by a new creep test in which diffusion is localized around an indenter. Finally, the chemical kinetics of the annealing of defects are discussed with new experiments reported on the recovery kinetics of shearbands produced in an amorphous polymer.     

Ovarian follicles of normal NMRI mice and homozygous "nude" mice. 1. Quantitative methodological study in the pubescent "nude" mouse

It is possible to determine statistically the repartitions of shapes or sizes in a population of particles from the recurrence of shapes or sizes of their outlines in microscopical sections. This method was applied to a population of 1326 follicular profiles randomly sampled from 30 ovaries of pubescent homozygous "Nude" mice. Under experimental conditions, the follicles were not spherical but could be assimilated to prolate ellipsoids with a mean eccentricity of 0.81. The average radius of the follicles taken as a whole was 20.2+/-0.7 (S.E.) mum. From these, the average radius of only the non-primary follicles was 58+/-1.3 (S.E.) mum. The volumetric repartition of the ovarian follicles indicated that 62% of the total follicular mass was made of follicles with a radius greater than 115 mum, although these represent only 2% of the total number of follicles.     

Behavior of Ellipsoids of Two Sizes

The influence of particle shape on granular material response is examined by using the discrete element method. Triaxial drained and undrained tests were performed on specimens of ellipsoids of two sizes. The triaxial test boundary conditions were simulated with a recently developed boundary mechanism. Different loading paths including axial compression, axial extension, lateral compression, and true extension were employed. The specimens were composed of 1,170 ellipsoids having two types of particles. The specimen is made up of 50% by weight of Type I particles that have an aspect ratio of 1.2. The aspect ratio of the Type II particles varies between 1.5 and 2. The specimens were consolidated isotropically before shearing. Comparing with the behavior of specimens of mono-size particles, a higher friction angle and a more complex particle shape effect were observed. The friction angles from the drained tests (axial extension, true extension, and lateral compression tests) were similar and the values are higher than that of the axial compression test. All simulated results are in good agreement with laboratory observation of sands.     

Effect of 17beta-estradiol on somatostatin receptor expression and inhibitory effects on growth hormone and prolactin release in rat pituitary cell cultures

Predictions of the minimal size an organism must have to swim along stimulus gradients were used to compare the relative advantages of sensory systems employing spatial (simultaneous) and temporal (sequential) gradient detection mechanisms for small free-swimming bacteria, leading to the following conclusions: 1) there are environmental conditions where spatial detection mechanisms can function for smaller organisms than can temporal mechanisms, 2) temporal mechanisms are superior (have a smaller size limit) for the difficult conditions of low concentration and shallow gradients, but 3) observed bacterial chemotaxis occurs mostly under conditions where spatial mechanisms have a smaller size limit, and 4) relevant conditions in the natural environment favor temporal mechanisms in some cases and spatial mechanisms in others. Thus, sensory ecology considerations do not preclude free-swimming bacteria from employing spatial detection mechanisms, as has been thought, and microbiologists should be on the lookout for them. If spatial mechanisms do not occur, the explanation should be sought elsewhere.     

Dynamic prototypicality effects in visual search.

In recent studies, researchers have discovered a larger neural activation for stimuli that are more extreme exemplars of their stimulus class, compared with stimuli that are more prototypical. This has been shown for faces as well as for familiar and novel shape classes. We used a visual search task to look for a behavioral correlate of these findings regarding both simple geometrical shapes and more complex, novel shape classes. The latter stimulus set enabled us to control for the physical properties of the shapes, establishing that the effects are solely due to the positions of the particular stimuli in a particular shape space (i.e., more extreme versus more central in shape space) and not to specific shape features. The results indicate that finding an atypical instance of a shape class among more prototypical ones is easier and faster than the other way around. The prototypical status of a shape in our experiment could change very quickly, that is, within minutes, depending on the subset of shapes that was shown to the participants. Manipulating the degree of familiarity toward the shapes by selectively increasing familiarity for the extreme shapes did not influence our results. In general, we show that the prototypical status of a stimulus in visual search is a highly dynamic property, depending on the distribution of stimuli within a shape space but not on familiarity with the prototype. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

The relaxation of residual stresses with postweld heat treatment in a high-performance weld measured with neutron diffraction

The residual stresses in a cylindrical weldment of HP-9-4-30 steel were measured with neutron diffraction in the as-welded (AW) state and after postweld heat treatment (PWHT). Large residual stresses are present in the interior of the material in the as-welded condition. The maximum principal stresses measured were found around the edges of the cap-pass heat-affected zone and reached up to 1045 MPa (76 pct of the base metal yield strength) in the as-welded condition. The principal stress directions for the residual stress tensors do not in general follow the hoop, axial, and radial axes of the weld and change from position to position within the weld, although the highest values are generally in the hoop direction. The postweld heat treatment relaxed the largest residual stresses, with the maximum value being 30 pct of the base metal yield stress. The need for position-dependent stress-free standards and the implications of stress gradients over the measurement volumes are discussed.     

The shapes of the motor domains of two oppositely directed microtubule motors, ncd and kinesin: a neutron scattering study

The shapes of the motor domains of kinesin and ncd, which move in opposite directions along microtubules, have been investigated. Using proteins expressed in Escherichia coli, it was found that at high salt (> 200 mM) Drosophila ncd motor domain (R335-K700) and human kinesin motor domain (M1-E349) were both sufficiently monomeric to allow an accurate determination of their radii of gyration (Rg) and their molecular weights. The measured Rg values of the ncd and kinesin motor domains in D2O were 2.06 +/- 0.06 and 2.05 +/- 0.04 nm, respectively, and the molecular weights were consistent with those computed from the amino acid compositions. Fitting of the scattering curves to approximately 3.5 nm resolution showed that the ncd and kinesin motor domains can be described adequately by triaxial ellipsoids having half-axes of 1.42 +/- 0.38, 2.24 +/- 0.44, and 3.65 +/- 0.22 nm, and half-axes of 1.52 +/- 0.23, 2.00 +/- 0.25, and 3.73 +/- 0.10 nm, respectively. Both motor domains are described adequately as somewhat flattened prolate ellipsoids with a maximum dimension of approximately 7.5 nm. Thus, it appears that the overall shapes of these motor domains are not the major determinants of the directionality of their movement along microtubules.     

Dynamic aspects concerned with the mechanism of separating motile sperm from nonmotile sperm, leukocytes, and debris with the use of high-density Percoll gradients

OBJECTIVE: To investigate the dynamics and the efficacy of separating motile from nonmotile sperm with the use of Percoll gradients as a function of centrifugation time. DESIGN: In vitro laboratory experiments on human sperm. SETTING: Male Infertility Institute. PATIENT(S): Fresh sperm specimens collected from donors and patients. INTERVENTION(S): Aliquots of sperm specimens, spread on top of 40% and 80% Percoll layers in test tubes, were centrifuged for 5-60 minutes. MAIN OUTCOME MEASURE(S): After centrifugation, samples were aspirated from the bottom, interface, and top layers for microscopic analysis. RESULT(S): Within 5 minutes, only fast-moving sperm reached the bottom as a result of both centrifugation force and their own tail movements. From 10 minutes onward, nonmotile sperm, slowed down by Percoll, joined them and gradually reversed the separation process. Leukocytes, debris, and abnormal sperm remained in the upper levels and never reached the bottom. CONCLUSION(S): Separation of motile sperm with the use of Percoll gradients is strongly dependent on centrifugation time. The separation is the gap formed between down-swimming sperm, accelerated by centrifugation, and nonmotile sperm, decelerated by Percoll. This gap gradually disappears with extended spinning.     

Residence time calculation for chemotactic bacteria within porous media

Local chemical gradients can have a significant impact on bacterial population distributions within subsurface environments by evoking chemotactic responses. These local gradients may be created by consumption of a slowly diffusing nutrient, generation of a local food source from cell lysis, or dissolution of nonaqueous phase liquids trapped within the interstices of a soil matrix. We used a random walk simulation algorithm to study the effect of a local microscopic gradient on the swimming behavior of bacteria in a porous medium. The model porous medium was constructed using molecular dynamics simulations applied to a fluid of equal-sized spheres. The chemoattractant gradient was approximated with spherical symmetry, and the parameters for the swimming behavior of soil bacterium Pseudomonas putida were based on literature values. Two different mechanisms for bacterial chemotaxis, one in which the bacteria responded to both positive and negative gradients, and the other in which they responded only to positive gradients, were compared. The results of the computer simulations showed that chemotaxis can increase migration through a porous medium in response to microscopic-scale gradients. The simulation results also suggested that a more significant role of chemotaxis may be to increase the residence time of the bacteria in the vicinity of an attractant source.     

Modulated dispersion explains changes in arrhythmia vulnerability during premature stimulation of the heart

BACKGROUND: Previously, we have shown that a premature stimulus can significantly modulate spatial gradients of ventricular repolarization (ie, modulated dispersion), which result from heterogeneous electrophysiological properties between cells. The role modulated dispersion may play in determining electrical instability in the heart is unknown. METHODS AND RESULTS: To determine if premature stimulus-induced changes in repolarization are a mechanism that governs susceptibility to cardiac arrhythmias, optical action potentials were recorded simultaneously from 128 ventricular sites (1 cm2) in 8 Langendorff-perfused guinea pig hearts. After baseline pacing (S1), a single premature stimulus (S2) was introduced over a range of S1S2 coupling intervals. Arrhythmia vulnerability after each premature stimulus was determined by measurement of a modified ventricular fibrillation threshold (VFT) during the T wave of each S2 beat (ie, S2-VFT). As the S1S2 interval was shortened to an intermediate value, spatial gradients of repolarization and vulnerability to fibrillation decreased by 51+/-9% (mean+/-SEM) and 73+/-45%, respectively, compared with baseline levels. As the S1S2 interval was further shortened, repolarization gradients increased above baseline levels by 54+/-30%, which was paralleled by a corresponding increase (37+/-8%) in vulnerability. CONCLUSIONS: These data demonstrate that modulation of repolarization gradients by a single premature stimulus significantly influences vulnerability to ventricular fibrillation. This may represent a novel mechanism for the formation of arrhythmogenic substrates during premature stimulation of the heart.     

Influence of pH on Morphology and Formation Mechanism of CeO2 Nanocrystalline

Nanoparticles of cerium oxide were prepared by common precipitation method using cerium nitrate solution and ammonia reagent. Cerium oxide particles with different morphologies were synthesized through adjusting pH values of the solution. TEM and BET results showed that spherical crystal was gained in acid solution, with the specific surface of 148. 1944 m2·g^-1. The cerium oxide appeared in the form of spherical and rod-like grains under neutral condition, and the specific surface changed to 114.7975 m^2·g^-1. Moreover, in alkaline solution, cerium oxide powders were exhibited in rod-like form with the specific surface of 106.2465 m^2·g^-1. Precipitation formation mechanism of different morphologies was also discussed, which followed decomposition precipitation mechanism and topology reaction mechanism in acid and alkaline solution, respectively.     

A within and between modality analysis of blocking.

Ran 4 groups of 6 homing pigeons to examine the role of modality relationships in blocking. In the within-modality condition Ss were pretrained on a line-tilt discrimination prior to line-tilt plus wavelength compound training. In the between-modality condition, auditory pretraining was followed by compounding with the same color cues. All Ss in pretrained conditions were run until they reached a criterion based on the proportion of responses emitted to the positive stimulus. Comparison of wavelength generalization gradients produced by Ss in pretrained conditions to those produced by Ss which received only compound training revealed that both pretraining conditions attenuated the development of control by wavelength cues (i.e., blocking occurred). Since the between- vs within-modality comparison was not significant, it appears that modality relationships are not critical for blocking. (French summary) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Deducing polymeric structure from aqueous molecular dynamics simulations of oligosaccharides: predictions from simulations of hyaluronan tetrasaccharides compared with hydrodynamic and X-ray fibre diffraction data

Molecular dynamics simulations of the two hyaluronan tetrasaccharides in water predict that over a period of 500 ps, their central linkages populate a single primary minima. Over the same period the peripheral linkages explore this minima, but also a secondary minima. Structures constructed using the primary minima were found to be extended left-handed helices of axial rise per disaccharide (h) 0.8 to 1.0 nm and 2.8 to 4.5 disaccharides per turn (n), in good agreement with n=3 and n=4 helices found by X-ray fibre diffraction studies. We have used the predicted average conformation from molecular dynamics to calculate the translational diffusion coefficients of the oligosaccharide series up to decasaccharide, and compared these with experimental measurements obtained using the method of capillary dispersion. Our calculated values are found to be in good agreement with experiment beyond the size of a tetrasaccharide. A partial digest of hyaluronan in the molecular mass range 10 to 100 kDa was fractionated by gel chromatography. Molecular weights were determined by in-line laser light-scattering measurements, and the translational diffusion coefficients of selected fractions were determined by dynamic laser light-scattering. A similar experiment was performed on hyaluronan with a molecular mass greater than 1MDa. The data suggest a change from rod-like to stiff coil behaviour beyond a molecular weight of 10 kDa. We have also examined the conformations available using the secondary minima, found at the peripheral linkages. In contrast to the extended structures previously described we have found left and right-handed helices with high values of n (5-10) and low values of h. Although there is no experimental evidence for these structures, they are of interest as, over short stretches, they would introduce folds, loops, and turns into the hyaluronan molecule. Such shapes may play an important role in the hydrodynamics of hyaluronan and its interaction with lipids and proteins.     

Long-term relationship between weight and aerobic-fitness change in children.

The relationship between changes in relative weight and fitness was assessed 5 years after children began treatment for obesity. Multivariate regression analysis showed that two factors were independently related to fitness change: (a) maintenance of weight loss from the end of 6 months of treatment to the 5-year follow-up and (b) initial fitness level. Children who showed the largest long-term changes in relative weight and the lowest initial fitness showed the largest improvements in fitness. Short-term weight loss was not related to long-term fitness change. These results show that weight loss and fitness are related over 5 years. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Demonstration of safety of probiotics -- a review

Probiotics are commonly defined as viable microorganisms (bacteria or yeasts) that exhibit a beneficial effect on the health of the host when they are ingested. They are used in foods, especially in fermented dairy products, but also in pharmaceutical preparations. The development of new probiotic strains aims at more active beneficial organisms. In the case of novel microorganisms and modified organisms the question of their safety and the risk to benefit ratio have to be assessed. Lactic acid bacteria (LAB) in foods have a long history of safe use. Members of the genera Lactococcus and Lactobacillus are most commonly given generally-recognised-as-safe (GRAS) status whilst members of the genera Streptococcus and Enterococcus and some other genera of LAB contain some opportunistic pathogens. Lactic acid bacteria are intrinsically resistant to many antibiotics. In many cases resistances are not, however, transmissible, and the species are also sensitive to many clinically used antibiotics even in the case of a lactic acid bacteria- associated opportunistic infection. Therefore no particular safety concern is associated with intrinsic type of resistance. Plasmid-associated antibiotic resistance, which occasionally occurs, is another matter because of the possibility of the resistance spreading to other, more harmful species and genera. The transmissible enterococcal resistance against glycopeptide antibiotics (vancomycin and teicoplanin) is particularly noteworthy, as vancomycin is one of the last effective antibiotics left in the treatment of certain multidrug-resistant pathogens. New species and more specific strains of probiotic bacteria are constantly identified. Prior to incorporating new strains into products their efficacy should be carefully assessed, and a case by case evaluation as to whether they share the safety status of traditional food-grade organisms should be made. The current documentation of adverse effects in the literature is reviewed. Future recommendations for the safety of already existing and new probiotics will be given.     

The moderating influence of physical fitness on age gradients in vigilance and serial choice responding tasks.

The moderating influence of physical fitness on age gradients in measures obtained from vigilance and serial choice responding tasks is examined in a sample of 90 postal workers. Physiological data relating to aerobic fitness determined fitness level within 2 age groups: younger participants ages 18 to 30 years (M ?=?25.19; 24 men, 24 women) and older participants ages 43 to 62 years (M ?=?49.19; 20 men, 22 women). A performance decrement across time was found in several measures, and some variation as a function of age was apparent. However, post hoc statistical analyses did not indicate this was due to older adults underperforming younger adults. According to predictions, significant Age?×?Fitness interactions showed older less fit workers to consistently underperform other participants. The findings suggest that older less fit individuals have lower signal sensitivity and processing speed than older fitter people and younger individuals. Results are discussed in relation to underlying physiological mechanisms. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Thrust Analysis of a Fish Robot Actuated by Piezoceramic Composite Actuators

In this work, a three-dimensional (3D) Computational Fluid Dynamics (CFD) model was built to simulate the tail fin motion of a fish robot actuated by a piezoceramic composite actuator, and to determine the maximum thrust tail-beat frequency. A simulation of the tail fin at a tail-beat frequency was performed to confirm measured thrust data from a previous study. The computed and measured thrusts were in good agreement. A series of thrust simulations were conducted for various tail-beat frequencies to confirm the maximum thrust frequency that was obtained from thrust measurements in the previous study. The largest thrust was calculated at a tail-beat frequency of 3.7 Hz and vortices around the tail were fully separated. The calculated maximum thrust tail-beat frequency was in good agreement with the measured frequency. Flow characteristics during tail fin motion were examined to explain why the largest thrust occurred at this particular tail-beat frequency.     

The equilibrium shape of a misfitting precipitate

We examine the equilibrium morphologies of precipitates with either a tetragonal or purely dilatational misfit in an elastically anisotropic medium with cubic symmetry under conditions of plane strain. We find that particles with a dilatational misfit are nearly spherical at small sizes, take on four-fold symmetric shapes at intermediate sizes and then undergo a supercritical symmetry-breaking bifurcation to two-fold symmetric shapes aligned along the elastically soft directions of the crystal. A tetragonal misfit breaks this four-fold to two-fold supercritical bifurcation when the direction of the tetragonality is coincident with one of the elastically soft directions of the crystal. Such a tetragonal misfit can lead to two-fold equilibrium particle shapes which are local energy minima, or metastable, and in some cases have large negative interfacial curvatures. When the tetragonality is not in an elastically soft direction, the supercritical bifurcation is not broken and the particles can take on unusual diamond-like or S-shaped morphologies.