An investigation of pore cracking in titanium welds

Two welded Ti-6A1-4V pressure vessels leaked prematurely in service. The leaks were caused by cracks emanating from weld porosity. The cracks originated during fabrication, with subsequent growth in service leading to the formation of the leak paths. Pore cracking is thought to be caused by a mechanism that involves both sustained-load and cyclic contributions, with the former being the more prominent. It is shown that the tendency for cracking is influenced by pore position and that pore size is not a deciding factor in that regard. The factors that govern pore cracking are discussed, and the possible role of interstitial embrittlement is assessed.     

Crystal orientation of β-phase isotactic polypropylene induced by magnetic orientation of N,N′-dicyclohexyl-2,6-naphthalenedicarboxamide

Melt crystallization of isotactic polypropylene (iPP), containing crystallites of N,N′-dicyclohexyl-2,6-naphthalenedicarboxamide (DCNDCA) as a nucleating agent of the the β-phase iPP crystal, is carried out under a magnetic field (6 T) to obtain the alignment of the iPP crystal induced by magnetic alignment of DCNDCA. In a previous paper, DCNDCA was reported to undergo magnetic alignment in a liquid suspension. The obtained iPP sample exhibits alignment of the β-phase crystal with the c-axis aligned perpendicular to the magnetic field. The comparison of this alignment of iPP with the reported magnetic alignment of DCNDCA indicates that the β-phase crystal grows epitaxially on the DCNDCA crystal. The (330)_β plane of the iPP crystal lies on the bc-plane of the DCNDCA crystal in which the direction of the c-axis of the iPP coincides with the direction of the b-axis of the DCNDCA crystal.     

强化管理机制推进机械制造企业快速发展

随着国有企业改革的不断深化,市场竞争愈加激烈,针对机械行业的现状,从3个方面探讨了强化管理机制,推进机械制造企业可持续快速发展的途径.     

Mechanism for the Peritectic Reaction and Growth of Aligned Grains in YBa2Cu3O6+x

It is experimentally observed that the peritectic reaction, 211 + liquid → 123, can be driven essentially to completion in 1 h at an undercooling of only ∽30°C. The kinetic data, together with the observed microstructures, are inconsistent with the normal mechanism of the peritectic reaction. It is proposed that the mechanism of the reaction involves dissolution of 211 particles into the liquid and precipitation of solid 123. The aligned grain structure is explained through sympathetic nucleation of new 123 grains on existing grains.     

Microstructure of Silicon Carbide Whiskers Synthesized by Carbothermal Reduction of Silicon Nitride

The microstructure of silicon carbide whiskers synthesized by carbothermal reduction of silicon nitride has been studied using transmission electron microscopy. All of the whiskers examined are single crystals, and grow in the (111) crystallographic direction. Two different forms of stacking faults and microtwins were observed; in one the planar defects are normal to the whisker growth direction, and the other has the defect planes at an angle of about 70° to the growth axis, while both forms of the defects are on the [111] closed-packed planes. Without the addition of catalyst, droplets containing metallic impurities were not found at the tips of the whiskers synthesized by the present process. A core and outer regions were observed in the single-crystal whiskers, which may be evidence that the whiskers were formed by a two-stage mechanism.     

影响长廊阻抗式调压室配筋的几个因素分析

采用三维非线性有限元方法,对四川天全县干溪坡水电站调压室结构特性进行研究,并对几个影响配筋的因素进行敏感性分析,得出各个因素对配筋结果的影响,为调压室结构设计提供科学依据。     

INHIBITION OF BEER-SPOILAGE LACTIC ACID BACTERIA BY NISIN

149 strains of bacteria, mostly brewery contaminants able to spoil wort or beer, and 12 brewing strains of yeast (8 ale and 4 lager strains) have been screened using a well-test assay for sensitivity to the food preservative, Nisin (E234), Nisin inhibited growth of 92% of the gram-positive strains, predominantly lactic acid bacteria of the genera Lactobacillus and Pediococcus. In contrast, all 32 gram-negative strains tested, except 3 Flavobacter strains, were Nisin-resistant; in addition none of the brewing yeasts showed Nisin-sensitivity. Therefore. Nisin has potential applications in preventing spoilage of worts or beers by lactic acid bacteria.     

Polyethylene mixed crystal infra-red spectroscopy: The influence of matrix isotopic species on guest conformation

The chain conformation of solution-crystallized polyethylene has previously been confirmed by infra-red spectroscopy, using the CD₂ bending bandshape for samples with a minority of the deuterated species^1–3. The method of calculating the i.r. band profile from molecular conformational models is now developed to give a more accurate representation of doublet splittings and to include the singlet component. The CH₂ bending profile is calculated for samples with a deuterated matrix, and the good agreement with experimental data indicates that the same molecular model is applicable for samples with the deuterated polymer as either guest or matrix, for identical crystallization conditions. This result implies that isotopic fractionation is unimportant under these conditions.     

Cyclic variations in the permeability of the cell wall of Saccharomyces cerevisiae.

To study cell-cycle-related variations in wall permeability of Saccharomyces cerevisiae, two approaches were used. First, an asynchronous culture was fractionated by centrifugal elutriation into subpopulations containing cells of increasing size. The subpopulations represented different stages of the cell cycle as judged by light microscopy. Cell wall porosity increased when these subpopulations became enriched with budded cells. Secondly, synchronous cultures were obtained by releasing MATa cells from alpha-factor induced G1-arrest. These cultures grew synchronously for at least two generations. The cell wall porosity increased sharply in these cultures, shortly before buds became visible and was maximal during the initial stages of bud growth. It decreased in cells which had completed nuclear migration and before abscission of the bud had occurred. The porosity reached its lowest value during abscission and in unbudded cells. We examined the incorporation of mannoproteins into the wall during the cell cycle. SDS-extractable mannoproteins were incorporated continuously. However, the incorporation of glucanase-extractable mannoproteins, which are known to affect cell wall porosity, showed cyclic oscillations and reached its maximum after nuclear migration. This coincided with a rapid decrease in cell wall porosity, indicating that glucanase-extractable mannoproteins might contribute to this decrease.     

On the improvement of detection characteristics of trans-stilbene crystals by improving crystal perfection

Organic molecular scintillating crystals are noted for their good timing and particle discrimination process. Trans-stilbene is one such candidate noted for its good particle detection characteristics for the past five decades. Progressive strengthening of detection characteristics of trans-stilbene has been attempted by improving crystal perfection. A series of timing resolution studies have been carried out for the Bridgman grown trans-stilbene crystals under different experimental conditions. The results were compared with the previously reported values. Pulse shape discrimination process has been carried out for ²⁴¹Am and ²⁵²Cf sources and good discrimination has been obtained for gamma-alpha and gamma-neutron sources from the grown organic phosphor crystal. Electronic Publication