A mechanism for transgranular stress-corrosion cracking

A model is proposed to explain transgranular-stress corrosion cracking (T-SCC) in face-centered cubic (fcc) materials. Crack propagation is shown to be anisotropic, in that growth near {110} < 001> is discontinuous due to crack arrest by dislocation blunting whereas growth away from this growth orientation is continuous. For the former case, renucleation of arrested cracks involves active dissolution of shear bands at the crack tip, which changes the stress state at Lomer-Cottrell locks, causing them to fail by cleavage. Once the crack is nucleated, its instantaneous macroscopic crack-growth velocity is considered to be comprised of multiple nucleation of microcracks with intervening arrests. This microcracking results from the interaction of the stress fields from neighboring cracks which are forming simultaneously, the crack-opening constraint due to ligaments which act as “bridges” behind the crack front, and the localized dissolution at the microcrack tip which affectsK _IC and leads to the “cobblestone” appearance. Experimental evidence and theoretical considerations are presented to support the model. The system studied was Cu-25 at. pct Au in 0.6 M NaCl solution at potentials between 300 and 400 mV (sce), which precludes hydrogen embrittlement. This article is based on a presentation made in the symposium “Quasi-Brittle Fracture” presented during the TMS fall meeting, Cincinnati, OH, October 21–24, 1991, under the auspices of the TMS Mechanical Metallurgy Committee and the ASM/MSD Flow and Fracture Committee.     

Architectural,construction and environmental matters of Bahrain's International Formula 1 Circuit

The Bahrain International Circuit (BIC) is considered as one of the best international racing car track in terms of technical aspects and architectural quality. Two Formula 1 races have been hosted in the Kingdom of Bahrain, in 2004 and 2005, at BIC. The BIC had recently won the award of the best international racing car circuit.     

Mutations can cause large changes in the conformation of a denatured protein

Deletion of 13 amino acids from the carboxyl terminus of staphylococcal nuclease (WTSNase delta) results in a denatured, partially unfolded molecule that lacks significant persistent secondary structure but is relatively compact and monomeric under physiological conditions [Shortle & Meeker (1989) Biochemistry 28, 936-944; Flanagan et al. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 748-752]. Because of these and other properties of the SNase delta polypeptide, it is a useful model system for investigating the conformation of the denatured state of a protein without using extreme temperature or solvent conditions. Moreover, since the modification is a carboxyl-terminal deletion, SNase delta may also resemble a transient state of the polypeptide chain as it emerges from a ribosome prior to its folding. In the present study, we have examined the sizes and conformations of mutated forms of SNase delta, using small-angle X-ray scattering and circular dichroism spectroscopy. Seven mutated forms were studied: four with single substitutions, two with double substitutions, and one triple substitution. When present in the full-length SNase, each of these mutated forms exhibited unusual behavior upon solvent or thermal denaturation. In the case of the truncated form (SNase delta), the small-angle scattering curves of the mutated forms fall into two classes: one resembling the scattering curve of compact native nuclease and the other having features consistent with those expected for an expanded coil-like polymer. In contrast, the scattering curve of WT SNase delta exhibits features intermediate between those observed for globular proteins and random polymers. The amino acid substitutions that gave rise to compact, native-like versions of SNase delta were all of the m--type (m-substitutions are predicted to decrease the size of the denatured state). Those which gave rise to versions of SNase delta that were more extended and coil-like than WT SNase delta were of the m+ type (m+ substitutions are predicted to increase the size of the denatured state). Estimates of the residual secondary structure present in WT SNase delta, as well as both the m+ and m-substituted versions of SNase delta, as determined by CD, suggest that the formation of secondary structure and compaction of the polypeptide chain occur concurrently. Our results show that single amino acid substitutions can radically alter the conformational distribution of a partially condensed polypeptide chain.(ABSTRACT TRUNCATED AT 400 WORDS)     

Tissue variation in hydrocarbon composition in the rabbit

This study, which deals with the distribution of hydrocarbons in seven types of rabbit tissues, was done for the purpose of providing information that might help shed light on the biological relevance of the hydrocarbons in mammalian metabolism. Liver, kidneys, brain, spleen, skeletal muscle, perinephric adipose, and a sample of blood serum were collected from a single animal for analysis of their hydrocarbon composition. The analytical methodology consisted of solvent extraction, saponification (adipose), elution chromatography on hydrated alumina, and combined gas chromatography-mass spectrometry. Hydrocarbons were detected in all of the tissues examined at concentrations estimated to range from 0.1 to 0.01% of the total lipid extracted. Three quite distinct distribution modes were recognized. The bulk of the identified components consisted of normal, saturated, nonterpenoid hydrocarbons in the C₁₆ to C₃₃ range. Squalene, phytene, phytadiene, and pristane were the only terpenoids detected. Nonterepenoid branched (iso andanteiso) hydrocarbons were identified unequivocally and in significant amounts in the muscle only. The adipose was the only tissue which was relatively rich in monoalkenes, and its overall hydrocarbon composition closely resembled that of the feed. The results of the study are not consistent with metabolic inertness. The observed qualitative and quantitative differences might reflect function and metabolic activities of the individual organs in a way yet to be elucidated. Presented in part at the AOCS Spring Meeting, New Orleans, April, 1976.     

Hydrogen absorption characteristics of Pd1−xLix (x = 0.055 and 0.072) alloys and ordered Pd7Li

Thermodynamic quantities were determined for the absorption of hydrogen in Pd-5.5at.%Li and Pd-7.2at.%Li alloys, the latter alloy being in the two-phase field of saturated -Pd(Li,s) solid solution and the ordered Pd₇Li phase. Data were also determined for the Pd-10.6at.%Li alloy which consists only of the ordered Pd₇Li phase. The data were determined from measurements of pressure-composition isotherms at temperatures between 273 K and 463 K and hydrogen pressures up to 1000 Torr. The Pd---Li alloys dissolve considerable amounts of hydrogen and form a more stable hydride phase than Pd despite the lattice contraction which occurs on alloying Pd with Li. The hydrogen solubility in the ordered Pd₇Li phase alone is a little smaller than that in the two-phase mixture. The large hydrogen absorption of Pd---Li alloys, which does not depend on the presence of ordered Pd₇Li, may be attributed to (i) an attractive H---Li pair interaction, (ii) a decrease in the strain energies necessary for hydrogen occupation of the interstices, because of the similar lattice parameters of the -Pd(Li,s) and ordered Pd₇Li phases, and the large compressibility of Pd---Li alloys, and (iii) the valence of 1 of Li in Pd compared with 3 for, for example, Y in Pd.     

Solvent interactions with n ring systems in proteins

The interaction of water molecules with apolar amino acids isan important aspect of the hydrophobic effect and hence of proteinfolding. Our distributed multipole electrostatic model for waterinteracting with phenylalanine dipeptides shows that minimumenergy sites exist above the aromatic ring such that a solventmolecule can interact with the electrons, but only when thissite is not blocked by mainchain atoms or disturbed by main-chainpolar atoms. This is consistent with the experimental evidenceof others that water can hydrogen bond to aromatic n electrons.In contrast, our analysis of solvent interactions with phenylalanineresidues based on 48 high-resolution, well-refined protein structuresshows that the dominant interaction of solvent molecules iswith the edge of the ring and not with the 7i electrons. Asthe faces of phenylalanine rings tend to be buried, and solventinteractions with neighbouring polar atoms are more favourable,the interaction of water molecules with the faces of aromatic rings appears not to occur frequently in proteins     

Reversal bending fatigue test system for investigating vibration integrity of spent nuclear fuel during transportation

Transportation packages for spent nuclear fuel (SNF) must meet safety requirements under normal and accident conditions as specified by federal regulations. During transportation, SNF experiences unique conditions and challenges to cladding integrity due to the vibrational and impact loading during road or rail shipment. Oak Ridge National Laboratory (ORNL) has been developing testing capabilities that can be used to improve the understanding of the impacts on SNF integrity due to vibration loading, especially for high burn-up SNF in normal transportation operation conditions. This information can be used to meet the nuclear industry and U.S. Nuclear Regulatory Commission needs in the area of safety and security of SNF storage and transportation operations. The ORNL developed test system can perform reversal bending fatigue testing to evaluate both the static and dynamic mechanical response of SNF rods under simulated loads. The testing apparatus is also designed to meet the challenges of hot cell operation, including remote installation and detachment of the SNF test specimen, in situ test specimen deformation measurement, and implementation of a driving system suitable for use in a hot cell. The system contains a U frame set-up equipped with uniquely designed grip rigs to protect the SNF rod sample and to ensure valid test results, and uses three specially designed linear variable differential transformers to obtain the in situ curvature measurement. A variety of surrogate test rods have been used to develop and calibrate the test system as well as in performing a series of systematic cyclic fatigue tests. The surrogate rods include stainless steel (SS) cladding, SS cladding with cast epoxy and SS cladding with alumina pellet inserts simulating fuel pellets. Testing to date has shown that the interface bonding between the SS cladding and the alumina pellets has a significant impact on the bending response of the test rods as well as their fatigue strength. The failure behaviours observed from tested surrogate rods provide a fundamental understanding of the underlying failure mechanisms of the SNF surrogate rod under vibration, which has not been achieved previously. The newly developed device is scheduled to be installed in the hot cell in summer 2013 to test high burn-up SNF.