Grain boundary structural disruption under thermal stress

The qualitative experimental results presented here support the hypothesis of Read and Shockly that low angle grain boundaries may not be structurally stable under the action of an applied stress. Our results show that, at least in the constrained geometry of a thin foil, low angle grain boundaries may act as sources for lattice dislocations, eventually causing complete disruption of the structural integrity of the grain boundary.     

Sustained attention under thermal stress.

Discusses the effects of the thermal environment on vigilance. A reinterpretation of early and contemporary studies contradicts the existing notion that vigilance is facilitated in ambient temperatures that exceed a comfortable level. Rather, performance is degraded as thermal homeostasis of the observer is disturbed. Significant breakdown in capability becomes manifest with measurable perturbation to deep body temperature. This assertion requires that conditions induce a dynamic change, as performance is unaffected with no variation in deep body temperature and is facilitated when the observer is established in a static hyperthermic state. An attentional account of this phenomenon is offered that contrasts with previous arousal explanations. (112 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The formation and annealing of structural defects in lipid bilayer vesicles

It is shown that sonication of phospholipid-water dispersions below the crystalline leads to liquid crystalline phase transition temperature (Tc) produces bilayer vesicles with structural defects within the bilayer membrane, which permit rapid permeation of ions and catalyze vesicle-vesicle fusion. These structural defects are annihilated simply by annealing the vesicle suspension above Tc. The rate of annealing was found to be slow, of the order of an hour for T = 3 degrees C above Tc, but annealing is complete within 10 min for T = 10 degrees C above Tc. It is proposed that these structural defects are fault-dislocations in the bilayer structure, which arise from a population defect in the distribution of the lipid molecules between the outer and inner monolayers, when small bilayer fragments reassemble to form the small bilayer vesicles during the sonication procedure. Such a population defect can only be remedied by lipid transport via the inside in equilibrium outside flip-flop mechanism, which would account for the slow kinetics of annealing observed even at 3 degrees C above the phase transition.     

High prevalence of bihemispheric structural and functional defects in Sturge-Weber syndrome

Abnormal cerebral venous drainage is associated with hypoxia and glucose deprivation, which can account for progressive neurologic deterioration in Sturge-Weber syndrome. Although developmental delay is common in Sturge-Weber syndrome, bihemispheric calcification is uncommon. Computed tomography (CT) and magnetic resonance imaging (MRI) were used to study the neuroanatomy, while single photon emission computed tomography (SPECT) was used concurrently to evaluate perfusion and glucose metabolism using 99mTc hexamethylpropyleneamine oxime (HMPAO) and [18F] fluorodeoxyglucose (FDG), respectively. Ten patients (10 to 22 years of age) with previously diagnosed Sturge-Weber syndrome, port-wine nevi, and clinical evidence of seizures or stroke-like episodes were studied. Five children with onset of seizures in the first year of life had overall clinical severity comparable to that of children with later-onset seizures. Calcification was present in both hemispheres in one patient; six additional patients had other radiologic evidence of bihemispheric disease; SPECT studies detected bihemispheric disease in four cases. Our study is the first to concurrently evaluate structure, perfusion, and glucose metabolism in Sturge-Weber syndrome and to show a mismatch between functional and structural brain imaging in both cerebral hemispheres. Widespread abnormalities of cerebral perfusion and glucose metabolism might explain the high prevalence of developmental delay associated with Sturge-Weber syndrome. Longitudinal studies are needed to define better the natural history of neurologic deterioration and radiologic progression that relates to central nervous system circulatory dysfunction in Sturge-Weber syndrome.     

Predicting the critical stress intensity in structural steels

A method is proposed for predicting the critical stress-intensity coefficient K _1c of structural steels (their crack resistance) on the basis of uniaxial tensile tests of standard samples. The derivation of this method relies on the concepts of mechanical stability and embrittlement when nonuniform force fields act on a metal. The strain-hardening index n of the metal at the critical transition temperatures from the plastic state to the quasi-brittle state (T _c or T ₀) and from the quasi-brittle state to the brittle state (T _k2) plays a key role here. The proposed method may prove effective in monitoring the crack resistance of steels certified in scientific and plant laboratories.     

Combined influence of geometric defects and thermal gradients on tensile ductility

Finite element modeling of sheet tensile specimens has been performed in order to analyze the combined effect of initial geometric defects and thermal gradients on tensile stress-strain behavior. Isothermal tensile ductilities were predicted to decrease dramatically from 0.555 for a specimen with no taper to 0.116 for a specimen with 50 pct taper. The corresponding reductions in tensile ductility for assumed adiabatic conditions were found to be from 0.46 to 0.099. These results demonstrate that thermal gradients are most detrimental for initially uniform tensile specimens and have a smaller effect on tensile ductility for specimens with large initial geometric defects. As a proportion of total ductility, however, thermal gradients decrease ductility by a nearly constant value of 15.5 pct, independent of the size of the initial taper. Comparison of 2-D FEM results with a complete multi-element 1-D analysis indicates that the developed biaxial stress state in the neck accounts for increased total elongations. The increase in total ductility caused by the biaxial stress state in the neck increases with increasing size of initial geometric defect. These results question the value of 1-D analysis of sheet tensile test behavior. Formerly Graduate Student at The Ohio State University, Columbus, OH     

热采井用筛管热应力试验技术研究

随着油气勘探技术的进步,热采越来越多地应用于稠油的开采中.热采井中的筛管、套管及套管接头的循环耐热能力、实体拉伸与压缩能力是整个管柱的重点考量因素.根据油井管实体性能试验方法,结合热采井中筛管工况,总结出一种模拟热采井管柱服役条件的热循环、实体拉伸与压缩试验方法,并采用宝钢φ139.7 BG110H BC耐热套管为基管,由中海油田服务股份有限公司开发的CMS、CDS两种筛管产品进行了试验验证.该试验技术对热采油气并的筛管的设计、制造有指导意义.     

Ceramide induces structural defects into phosphatidylcholine bilayers and activates phospholipase A2

We studied the effects of bovine brain ceramide on the structure of dipalmitoylphosphatidylcholine (DPPC) bilayers and cobra venom phospholipase A2, (PL-A2) activity using 2H NMR and specific enzymatic assays. Addition of ceramide to DPPC at 45 degrees C induces lateral phase separation of the bilayers into regions of gel and liquid crystalline phases. The order parameters of the DPPC acyl chains in the liquid crystalline phase are only slightly affected by the presence of ceramide, indicating that the latter is largely partitioned in the gel phase of DPPC, whereas at 60 degrees C the presence of ceramide induced a large increase of the order parameters of DPPC side chains. The observed structural effects of ceramide correlated with ceramide-induced activation of cobra venom phospholipase A2 (PL-A2). Ceramide activated PL-A2 in a concentration-dependent manner, with a significant effect observed at 5 mol% ceramide, which caused an approximately 3-fold increase in PL-A2 activity. The results showing activation of PL-A2 by ceramide illustrate an additional feature of the biological effects of this second messenger and suggest the possibility of cross-talk between the sphingomyelinase and PL-A2 signal transduction pathways.     

Profilin is required for the normal timing of actin polymerization in response to thermal stress

We have used a fluorometric assay to determine the relative amounts of polymerized actin (F-actin) in wild-type and profilin mutant yeast cells. Our results indicate that profilin plays a role in maintaining normal F-actin levels in response to shifts to high temperature. Cells lacking profilin display a greater drop in F-actin levels upon such temperature shifts, and are slower to recover to initial F-actin levels than are wild-type cells. Interestingly, shifts to cold temperatures result in rapid increases of F-actin levels in wild-type and profilin null cells. We have further determined that shifting to high-osmolarity growth conditions causes a relatively slow decrease in F-actin levels in wild-type cells, and a small but rapid increase in the F-actin levels in profilin null cells. Profilin null cells contain normal concentrations of F-actin while growing exponentially at room temperature, indicating that profilin is not essential for maintaining F-actin concentrations during steady-state growth. Our data suggest that actin is inherently unstable in vivo at high temperatures, and that profilin helps to maintain actin in its filamentous state at these temperatures, perhaps by stimulating actin polymerization in a proper temporal and spatial fashion.     

Characterization of the thermal stress response of Campylobacter jejuni

Campylobacter jejuni, a microaerophilic, gram-negative bacterium, is a common cause of gastrointestinal disease in humans. Heat shock proteins are a group of highly conserved, coregulated proteins that play important roles in enabling organisms to cope with physiological stresses. The primary aim of this study was to characterize the heat shock response of C. jejuni. Twenty-four proteins were preferentially synthesized by C. jejuni immediately following heat shock. Upon immunoscreening of Escherichia coli transformants harboring a Campylobacter genomic DNA library, one recombinant plasmid that encoded a heat shock protein was isolated. The recombinant plasmid, designated pMEK20, contained an open reading frame of 1,119 bp that was capable of encoding a protein of 372 amino acids with a calculated molecular mass of 41,436 Da. The deduced amino acid sequence of the open reading frame shared similarity with that of DnaJ, which belongs to the Hsp-40 family of molecular chaperones, from a number of bacteria. An E. coli dnaJ mutant was successfully complemented with the pMEK20 recombinant plasmid, as judged by the ability of bacteriophage lambda to form plaques, indicating that the C. jejuni gene encoding the 41-kDa protein is a functional homolog of the dnaJ gene from E. coli. The ability of each of two C. jejuni dnaJ mutants to form colonies at 46 degreesC was severely retarded, indicating that DnaJ plays an important role in C. jejuni thermotolerance. Experiments revealed that a C. jejuni DnaJ mutant was unable to colonize newly hatched Leghorn chickens, suggesting that heat shock proteins play a role in vivo.     

梯度功能材料热应力的研究进展

梯度功能材料是一种高弹性模量、耐腐蚀、耐高温的新型非均匀复合材料,其热应力研究是目前梯度功能材料研究的热点之一。该文详细介绍了梯度功能材料热应力的理论分析、数值计算和试验研究方法,对比分析了目前常用的梯度成分分布模型和物性参数模型,讨论了材料的梯度层厚度和成分分布等因数对材料残余热应力的影响,介绍了直接测定残余热应力的2种非接触性实验分析方法;并对梯度功能材料热应力研究的发展趋势做了展望。