Micromechanics of Failure Waves in Glass: II, Modeling

In an attempt to elucidate the failure mechanism responsible for the so-called failure waves in glass, numerical simulations of plate and rod impact experiments, with a multiple-plane model, have been performed. These simulations show that the failure wave phenomenon can be modeled by the nucleation and growth of penny-shaped shear defects from the specimen surface to its interior. Lateral stress increase, reduction of spall strength, and progressive attenuation of axial stress behind the failure front are properly predicted by the multiple-plane model. Numerical simulations of high-strain-rate pressure-shear experiments indicate that the model predicts reasonably well the shear resistance of the material at strain rates as high as 1 × 10₆/s. The agreement is believed to be the result of the model capability in simulating damage-induced anisotropy. By examining the kinetics of the failure process in plate experiments, we show that the progressive glass spallation in the vicinity of the failure front and the rate of increase in lateral stress are more consistent with a representation of inelasticity based on shear-activated flow surfaces, inhomogeneous flow, and microcracking, rather than pure microcracking. In the former mechanism, microcracks are likely formed at a later time at the intersection of flow surfaces. in the case of rod-on-rod impact, stress and radial velocity histories predicted by the microcracking model are in agreement with the experimental measurements. Stress attenuation, pulse duration, and release structure are properly simulated. It is shown that failure wave speeds in excess to 3600 m/s are required for adequate prediction in rod radial expansion.     

Reliability of serum measurements of lignans and isoflavonoid phytoestrogens over a two-year period

We examined the distribution and long-term reliability of serum measurements of the two main human lignans, enterolactone and enterodiol, and the isoflavonoid phytoestrogens daidzein, genistein, equol, and O-Desmethylangolensin in the New York University Women's Health Study, a prospective cohort study of sex hormones and breast cancer. Serum samples collected at three yearly visits in 30 premenopausal and 30 postmenopausal women who had not been diagnosed with cancer or cardiovascular disease were included in the study. Assays were carried out by ion-exchange chromatography and capillary gas chromatography-mass spectrometry. Levels of isoflavonoid phytoestrogens were low, often at or below the sensitivity level of the assay. The reliability coefficients for these compounds were also low (< or =0.30). The median levels of enterodiol and enterolactone were 1.52 nmol/liter and 20.2 nmol/liter, respectively, and were comparable with the levels observed in omnivorous Finnish women living in the Helsinki area. A substantial number of women, though, had fairly high levels: for instance, 15% of the assays showed levels of enterolactone greater than the mean level observed in vegetarian Finnish women, i.e., 89.1 nmol/liter (H. Adlercreutz et al., Cancer Detec. Prev., 18: 259-271, 1994). The reliability coefficient of a single measurement of enterolactone was moderately high (0.55), suggesting that serum measurements of this compound could be a useful tool in prospective epidemiological studies with access to repeated blood or serum specimens. For instance, the reliability coefficient of the average of three measurements of enterolactone would be 0.79, a level considered acceptable in light of the other sources of error that are present in epidemiological studies (W. Willett, Stat. Med., 8: 1031-1040, 1989).     

Corrosion inhibition in an industrial crystallization plant: Optimizing the process with a statistical method

An experimental statistical plan is utilized to solve corrosion problems of a section of an industrial plant for the production of caprolactam, consisting of a multistage flash evaporator entirely constructed in AISI 316L (X2CrNiMo 17 12 2). Preliminary tests show that the nitrates or acidity due to the sulfuric acid do not influence the corrosive phenomena and confirmed the positive, though costly, effect of adding hydroxylamine sulfate (SI) in the evaporator feed. An experimental plan was prepared to diminish the cost and dosage of SI in the system: the experimental statistical approach has allowed us to minimize the number of tests required in order to define the system rapidly and adequately, and to optimize the required amount of SI. The data processed have been obtained in a two-step experimental plan that allowed us to establish a relationship between the logarithm of critical corrosion current (i_cr) and the concentrations of some evaporator feeding components. In addition, it has been possible to clarify certain corrosion effects and to reduce the SI addition up to 1.4 g/l, thereby optimizing the chemical system and considerably reducing maintenance costs. A plausible explanation of the related effects of some of the feeding components is also proposed.     

On invariant integrals in the analysis of cracked plates

Bending of cracked plates is considered within a formulation in which transverse shear deformation is taken into account. Path-domain-independent (or invariant) integrals are constructed by separating the strain energies of bending and shear. This separation permits the expression of the three stress intensity factors, which can occur simultaneously under certain loadings, in terms of the integrals mentioned. A numerical implementation is proposed and illustrated by means of a known example. Future applications are discussed in order to solve more complicated configurations.

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Résumé On considère la flexion de tôles fissurées selon une formulation où la déformation par cisaillement transversal est prise en considération. On construit des intégrales indépendantes du domaine ou du parcours (ou intégrales invariantes) en séparant les énergies de déformation en flexion et le cisaillement. Cette séparation permet d'identifier trois facteurs d'intensité de contraintes qui peuvent se présenter simultanément sous certaines charges et qui s'expriment selon les intégrales mentionnées. On propose une présentation numérique et on l'illustre au moyen d'un exemple connu. On discute les applications futures en vue de résoudre des configurations plus complexes.

     

Dietary cholesterol induces changes in molecular species of hepatic microsomal phosphatidylcholine

After 21 days on a diet containing 1g% cholesterol and 0.5g% cholic acid, rats had an increased content of cholesterol in liver microsomal lipids. In liver, both cholesterol content and δ9 desaturase activity increased, whereas δ6 and δ5 desaturase activities decreased. These changes correlated with increases in oleic, palmitoleic, and linoleic acids and decreases in arachidonic and docosahexenoic acids in total microsomal lipids. Similar fatty acid changes were found in phosphatidylcholine (PC), the principal lipid of the microsomal membrane. In PC the predominant molecular fatty acid species (67% of the total) in the control rats were 18:0/20:4, 16:0/20:4, and 16:0/18:2; and they mainly determined the contribution of PC to the biophysical and biochemical properties of the phospholipid bilayer. The cholesterol diet decreased specifically the 18:0/20:4 species, and to a lesser extent, 16:0/20:4 and 18:0/22:6. The 18:1-containing species, especially 18:1/18:2 and less so 16:0/18:1 and 18:1/20:4, were increased. A new 18:1/18:1 species appeared. The independent effects of the presence of cholesterol and change of the fatty acid composition of the phospholipid bilayer of liver microsomes on the packing were studied by fluorescence methods using 6-lauroyl-2,4-dimethylaminonaphthalene, 1,6-diphenyl-1,3,5-hexatriene and 1-(4-trimethylammonium phenyl)-6-phenyl-1,3,5-hexatriene, which test different parameters and depths of the bilayer. Data showed that the increase of cholesterol in the membrane, and not the change of the fatty acid composition of phospholipids, was the main determinant of the increased bulk packing of the bilayer. The increase of fluid oleic- and linoleic-containing species almost compensated for the drop in 20:4- and 22:6-containing molecules. But the most important effect was that the general drop in essential n-6 and n-3 polyunsaturated fatty acids meant that this endogenous source for the needs of the animal decreased.     

Comparative evaluation of the antioxidant capacity of smoke flavouring phenols by crocin bleaching inhibition, DPPH radical scavenging and oxidation potential

The antioxidant capacity of the main phenolic compounds present in wood smoke and smoke flavourings used in the food industry was investigated by three methods, based on a kinetic and thermodynamic approach: the bleaching of the carotenoid crocin, the scavenging of the DPPH radical, and the determination of the oxidation potential. The reaction with the DPPH radical was evaluated calculating the effective concentration (EC₅₀) and the antiradical efficiency (AE). The compounds tested were 2-methoxyphenols (guaiacol, 4-methylguaiacol, 4-ethylguaiacol, 4-vinylguaiacol, 4-propylguaiacol, eugenol, isoeugenol, vanillin, acetovanillone, 2-propiovanillone), 2,6-dimethoxyphenols (syringol, 4-methylsyringol, 4-allylsyringol, syringaldehyde, acetosyringone) and dihydroxybenzenes (catechol, 3-methylcatechol, 4-methylcatechol, 3-methoxycatechol and hydroquinone). The trend in antioxidant capacity was similar in all the three methods, with dihydroxybenzenes > 2,6-dimethoxyphenols > 2-methoxyphenols, although some discrepancies in the ranking within the groups were present. Considering the overall ranking, isoeugenol was amongst the most active compound, like dihydroxybenzenes, evidencing the role of a conjugated double bond at para position for the stabilization of the phenoxyl radical in the radical scavenging process.     

Recent developments on the Kardar-Parisi-Zhang surface-growth equation

The stochastic nonlinear partial differential equation known as the Kardar-Parisi-Zhang (KPZ) equation is a highly successful phenomenological mesoscopic model of surface and interface growth processes. Its suitability for analytical work, its explicit symmetries and its prediction of an exact dynamic scaling relation for a one-dimensional substratum led people to adopt it as a 'standard' model in the field during the last quarter of a century. At the same time, several conjectures deserving closer scrutiny were established as dogmas throughout the community. Among these, we find the beliefs that 'genuine' non-equilibrium processes are non-variational in essence, and that the exactness of the dynamic scaling relation owes its existence to a Galilean symmetry. Additionally, the equivalence among planar and radial interface profiles has been generally assumed in the literature throughout the years. Here--among other topics--we introduce a variational formulation of the KPZ equation, remark on the importance of consistency in discretization and challenge the mainstream view on the necessity for scaling of both Galilean symmetry and the one-dimensional fluctuation-dissipation theorem. We also derive the KPZ equation on a growing domain as a first approximation to radial growth, and outline the differences with respect to the classical case that arises in this new situation.     

In Situ Electron Microscopy Four‐Point Electromechanical Characterization of Freestanding Metallic and Semiconducting Nanowires

Electromechanical coupling is a topic of current interest in nanostructures, such as metallic and semiconducting nanowires, for a variety of electronic and energy applications. As a result, the determination of structure‐property relations that dictate the electromechanical coupling requires the development of experimental tools to perform accurate metrology. Here, a novel micro‐electro‐mechanical system (MEMS) that allows integrated four‐point, uniaxial, electromechanical measurements of freestanding nanostructures in‐situ electron microscopy, is reported. Coupled mechanical and electrical measurements are carried out for penta‐twinned silver nanowires, their resistance is identified as a function of strain, and it is shown that resistance variations are the result of nanowire dimensional changes. Furthermore, in situ SEM piezoresistive measurements on n‐type, [111]‐oriented silicon nanowires up to unprecedented levels of ～7% strain are demonstrated. The piezoresistance coefficients are found to be similar to bulk values. For both metallic and semiconducting nanowires, variations of the contact resistance as strain is applied are observed. These variations must be considered in the interpretation of future two‐point electromechanical measurements.     

Studies about the use of semicircular beams as hinges in large deflection planar compliant mechanisms

Conventional hinge designs in planar compliant mechanisms have a limited deformation range because of the high stresses induced during deflection. To improve the range of motion of these mechanisms, hinges that allow for large displacement are highly desirable. This paper explores the use of curved beams as large displacement hinges in planar compliant mechanisms. To facilitate design, analytic expressions that predict deflections under different types of loads are introduced. These expressions are used in pseudo rigid link models of compliant mechanism designs. Predictions made by the analytic expressions are compared with the results of FEA simulations. To validate the proposed models, two planar compliant mechanism designs were prepared and experimental measurements of deflections under loads were made. Overall, results showed that analytic models and FEA predictions lie within 10% of experimental data for the planar mechanism geometry in which pseudo rigid motion models apply. FEA models of the second case, a more complex mechanism, make predictions that lie within 15% of experimental measurements. Results and ways to improve accuracy of models and designs are discussed at the end of the article.