Influence of precipitates on the mechanical response and substructure evolution of shock-loaded and quasi-statically deformed Al−Li−Cu alloys

The mechanical response and substructure evolution of two Al−Li−Cu alloys (Al-2.90 wt pct Li-1.00 pct Cu-0.12 pct Zr and Al-2.30 pct Li-2.85 pct Cu-0.12 pct Zr) subjected to shock-loading (strain rate έ> 10⁶ s^-1), Split-Hopkinson-Pressure-Bar compression (έ ~ 5 × 10³ s^-1), and quasi-static compression (έ ~ 1.5 × 10^-3 s^-1) were examined. The strain levels achieved in these three deformation paths were desined to be comparable,i.e., all ∼15 pct. Both alloys were either naturally or artificially aged to yield an underaged or overaged condition. Various precipitates, such as theδ' andT ₁ phase, of different sizes and volume fractions were dispersed in the matrix and at the grain boundaries. The substructure in all of the shock-loaded, Split-Hopkinson-Pressure-Bar, and quasi-static compression samples was characterized by localized slip bands and microbands with the exception of the overaged alloys. The density of dislocations and dislocation loops was higher, independent of the aging condition, in the shock-loaded specimens. Well-defined cell structures were not observed in any of the samples, independent of strain rate. The influence of precipitates, shearable or not, on the substructure development in Al−Li−Cu alloys during shock-loading was seen to be pronounced, even though the size and volume fraction of precipitates was small and low, respectively. Flow stress measurements showed that the shock-loaded samples have flow strengths 3 to 8 pct higher than the quasi-statically deformed samples. This small, but reproducible, strength increment, for alloys deformed to equivalent strains at low and high rates, indicates that the Al−Li alloys studied have a small rate sensitivity. Based upon comparison of the results of the shock-loaded and quasi-static samples, it is concluded that the fundamental deformation mechanisms and substructure evolution in all three loading paths are not drastically different, corroborating previous investigations.     

Systems Analysis and Design

Books on systems analysis and design fall into two categories — books by systems professionals who feel they have discovered the unique methodology or way of solving all systems problems and textbooks by university professors whose knowledge often is secondhand. Such books also seem to be a growing industry: more and more of them are published each year. This column reviews some of the many books available on this topic.     

A unified approach for encoding clean and noisy sources by means ofwaveform and autoregressive model vector quantization

Data compression by vector quantization is considered for sources which have been degraded by noise. It is shown that, by appropriately modifying the given distortion measure, the problem becomes a standard quantization problem for the noisy source and the modified distortion measure. For the special case of sources corrupted by statistically independent additive noise, the authors provide sufficient conditions on the original distortion measure and probability distributions of the source and the noise for convergence of the generalized Lloyd algorithm in designing the quantizers. The results are specialized to waveform and autoregressive model vector quantization using the weighted quadratic and the Itakura-Saito distortion measures, respectively     

The Strategic Computing Program at four years: Implications and intimations

Examining the Strategic Computing Program after four years, in the context of the crucial recognition that it is only a small part of the whole range of military artificial intelligence applications, suggests a number of clear implications and intimations about such crucial questions as: 1) the current roles of industry and the universities in developing high technology war; 2) the effects on political and military policy of high-tech weapons systems; and 3) the importance of advanced military computing to subtle philosophical dilemmas including human self-definition and the future of war.     

The formal specification of a small bookshop information system

A specification, and its development, for a small bookshop information system are discussed. the specification is presented using mathematics and the scheme calculus of C. Morgan and B. Sufrin (see ibid., vol.SE-10, no.2, p.128-142, 1984). An insight is given into how the specification was developed and why a formal specification is appropriate     

BRCA1 and BRCA2 in breast cancer families from Wales: moderate mutation frequency and two recurrent mutations in BRCA1

Mutations in the BRCA1/BRCA2 genes account for varying proportions of breast cancer families studied, and demonstrate considerable variation in mutational spectra coincident with ethnic and geographical diversity. We have screened for mutations in 17 families from Wales with two or more cases of breast cancer under age 50 and/or ovarian cancer. Eight out of 17 (47%) families had demonstrable mutations. Six out of 17 (35%) carried BRCA1 mutations and 2 out of 17 (12%) carried BRCA2 mutations. Two recurrent mutations in BRCA1 were identified, which appear to represent founder mutations in this population. These data support the existence of additional breast and ovarian cancer susceptibility genes.     

Role of mast cells, neutrophils and nitric oxide in endotoxin-induced damage to the neonatal rat colon

PURPOSE: Twelve synthetic spider toxin analogs were prepared in an effort to better understand the structure-activity relationships of the polyamine portion of argiotoxin-636 (Arg-636), a noncompetitive NMDA receptor (NMDAR) antagonist. METHODS: The 1,13-diamino-4,8-diazatridecane portion of the side chain of Arg-636 was systematically modified in an effort to further our knowledge of the structural requirements for the alkyl linker spacing between the amine nitrogens. Systematic isosteric replacement of each of the amine nitrogens in the polyamine moiety with either oxygen or carbon provided a series of compounds which were evaluated in vitro for NMDAR antagonist activity. RESULTS: One-half of the heteroatoms found in Arg-636 were removed to provide analogs which maintained in vitro potency below 1 microM. However, these simplified analogs produced similar or more pronounced effects on the cardiovascular system than Arg-636 in vivo. CONCLUSIONS: In this set of analogs, a minimum of three basic nitrogens in the side chain was required for maximum potency as NMDAR antagonists. Isosteric nitrogen substitutions in the polyamine chain reduced the in vitro potency of these analogs. An analog binding-conformation model was proposed to rationalize the inactivity of these isosterically substituted analogs.