Numerical analysis of the stress distribution in single-lap shear tests under elastic assumption—Application to the optimisation of the mechanical behaviour

The single lap joint is the most used test in order to analyse the behaviour of an adhesive in an assembly as on one hand, the manufacturing of such specimens is quite easy, and on the other hand they require only a classic tensile testing machine. However, such specimens are associated with complex loading of the adhesive, i.e. non-uniform shear stress along the overlap length, quite large peel stress at the two ends of the overlap and significant edge effects associated with geometrical and material parameters. In addition, the stress concentrations can contribute to fracture initiation in the adhesive joints and thus can lead to an incorrect analysis of the adhesive behaviour. Therefore, understanding the stress distribution in an adhesive joint can lead to improvements in adhesively bonded assemblies. The first part of this paper presents the influence of edge effects on the stress concentrations in single lap joints under elastic assumption of the material and using a pressure-dependent elastic limit of the adhesive. In the second part, some usual geometries, proposed in the literature about stress limitation, are compared with respect to the maximum load transmitted by single lap joint. The last part presents some geometries, which significantly limit the influence of edge effects and are more appropriate for analysing the behaviour of the adhesive.     

The development of a silica nanoparticle-based label-free DNA biosensor

A silica nanoparticle-based DNA biosensor capable of detecting Bacillus anthracis bacteria through the use of unlabelled ss-oligonucleotides has been developed. The biosensor makes use of the optical changes that accompany a nanoparticle-immobilized cationic conjugated polymer (polythiophene) interacting with single-stranded vs. hybridized oligonucleotides, where a fluorescence signal appears only when hybridized DNA is present (i.e. only when the ss-oligonucleotide interacting with the polymer has hybridized with its complement). In order to enhance the sensitivity of the biosensor, two different nanoparticle architectures were developed and used to elucidate how the presence of neighboring fluorophores on the nanoparticle surface affects F?rster-resonant energy transfer (FRET) between the polythiophene/oligonucleotide complex (FRET donor) and the fluorophores (FRET acceptors). We demonstrate that the silica nanoparticle-based FRET platform lowers the limit of detection at least 10-fold in comparison to the polythiophene itself, and allows the detection of ～2 × 10(-12) moles of ss-oligonucleotide in a 100 μL sample with a standard fluorimeter (i.e. has a limit of detection of ～2 nM ssDNA). Such nanoparticle-based biosensor platforms are beneficial because of the robustness and stability inherent to their covalent assembly and they provide a valuable new tool that may allow for the sensitive, label-free detection (the target DNA that produces the fluorescence signal is unlabelled) without the use of polymerase chain reaction.     

Failure mechanism of thin Al2O3 coatings grown by atomic layer deposition for corrosion protection of carbon steel

Combined analysis by electrochemical impedance spectroscopy (EIS), time-of-flight secondary ion mass spectrometry (ToF-SIMS) and field emission scanning electron microscopy (FESEM) of the corrosion protection provided to carbon steel by thin (50 nm) Al₂O₃ coatings grown by atomic layer deposition (ALD) and its failure mechanism is reported. In spite of excellent sealing properties, the results show an average dissolution rate of the alumina coating of ∼7 nm h⁻¹ in neutral 0.2 M NaCl and increasing porosity of the remaining layers with increasing immersion time. Alumina dissolution is triggered by the penetration of the solution via cracks/pinholes through the coating to the substrate surface where oxygen reduction takes place, raising the pH. At defective substrate surface sites of high aspect ratio and concentrated residual mechanical stress (along scratches) presumably exposing a higher steel surface fraction, localized dissolution of the coating is promoted by a more facile access of the solution to the substrate surface enhancing oxygen reduction. De-adhesion of the coating is also promoted in these sites by the ingress of the anodic dissolution trenching the steel surface. Localized corrosion of the alloy (i.e. pitting) is triggered prior to complete dissolution of the alumina film on the elsewhere still coated surface matrix.     

Vertically oriented nickel nanorod/carbon nanofiber core/shell structures synthesized by plasma-enhanced chemical vapor deposition

Plasma-enhanced chemical vapor deposition, without a nickel-containing gaseous precursor, was used to synthesize continuous nickel (Ni) nanorods inside the hollow cavity of carbon nanofibers (CNFs), thus forming vertically aligned Ni/CNF core/shell structures. Scanning and transmission electron microscopic images indicate that the elongated Ni nanorods originate from the catalyst particles at the tips of the CNFs and that their formation is due to the effect of extrusion induced by the compressive force of the graphene layers during growth. Different from previous work, each vertically-aligned core/shell structure reported is totally isolated from its neighbors. Continuous Ni nanorods are found to separate into smaller ones with increasing growth time, which was ascribed to (i) the limited amount of Ni available in the tip of the CNF, (ii) the polycrystalline nature of the Ni nanorods and (iii) the combined effects of the compressive stresses on the side of the Ni nanorods and of the tensile stress along their axis.     

Oxygen Vacancy Relaxation and Domain Wall Hysteresis Motion in Cobalt-Doped Barium Titanate Ceramics

Mechanical and dielectric loss measurements were carried out in the BaTiO₃ ceramics doped with Co at frequencies between 0.01 Hz and 1 MHz as a function of temperature from −150° to 150°C. The relaxation peak observed in the ferroelectric phase with an activation energy of 0.27 eV is assumed to be related to the motion of oxygen vacancies. This peak could be because of the reorientation of an electrical dipole made of oxygen vacancies and Co³⁺ ions in the lattice. Furthermore, another loss peak located just below the Curie temperature T _c could be interpreted as hysteretic motion of the domain walls in a regime where the domain wall density is changing.     

Silk maysin content and resistance of commercial corn [maize] hybrids to kernel contamination by aflatoxin

Several phenols and related compounds have been shown to have detrimental effects on insects while others have antibiotic activity against fungi which attack higher plants. Insects have also been implicated as contributors to preharvest contamination of corn [maize], Zea mays L, by aflatoxin. The objectives, therefore, were to determine (a) if commercial corn hybrids vary in their silk maysin content and aflatoxin contamination of the grain, and (b) if grain aflatoxin contamination is correlated with maysin and related compounds. During 4 years of testing, 16 corn hybrids varied significantly for silk maysin content and grain aflatoxin contamination. Based on correlations, grain aflatoxin content of the hybrids tested was not significantly associated with maysin, chlorogenic acid, and 3′-methoxymaysin contents. It was concluded that other untested phenolic compounds in the category of compounds analyzed in the present study could be involved in resistance to aflatoxin formation, and that other classes of compounds should also now be assessed to locate major chemical resistance components.     

Innovative private micro-hydro power development in Rwanda

Under the “Private Sector Participation in Micro-Hydro Development Project in Rwanda”, four newly registered Rwandan companies are each constructing a micro-hydro electricity plant (100–500 kW) and building a low-voltage distribution grid. These companies financed their plants through their own equity and debt with support from the PSP Hydro project. This support comprised a subsidy of 30–50% of investment costs, technical and business development assistance, project monitoring and financial controlling. The experiences gained so far have important implications for similar future micro-hydro energy sector development projects and this paper puts forward three key messages: (i) institutional arrangements rather than technical quality determine the success of such projects; (ii) truly sustainable rural electrification through micro-hydro development demands a high level of local participation at all levels and throughout all project phases, not just after plant commissioning; and (iii) real impact and sustainability can be obtained through close collaboration of local private and financial sector firms requiring only limited external funds. In short, micro-hydro projects can and will be taken up by local investors as a business if the conditions are right. Applying these messages could result in an accelerated uptake of viable micro-hydro activities in Rwanda, and in the opinion of the authors elsewhere too.     

Synthesis of Nickel Sulfides by Mechanical Alloying

The feasibility and kinetics of synthesizing various nickel sulfides by milling of elemental mixtures of Ni and S in a high-energy shaker mill have been investigated. The phases Ni₃S₂ and the high-temperature polymorph of NiS are formed readily via such processing. In distinction, it requires prolonged milling to obtain Ni₇S₆; NiS₂ can only be obtained as a minor reaction product; and Ni₃S₄ cannot be formed by milling for the conditions of this study. Structural evolution during synthesis and the kinetics of Ni₃S₂ formation are investigated in depth. S coats the Ni particles and sulfide formation takes place at the interface of the elements after a certain degree of microstructural refinement due to the plastic deformation accompanying milling. Ni₃S₂ forms rapidly at this stage. However, a stasis in the reaction is then observed. This is associated with NiS formation and a slight decrease in the amount of Ni₃S₂. The stasis is of approximately 5-min duration and is followed by a recurrence in the formation of Ni₃S₂ and a disappearance of the NiS phase. The kinetics can be mimicked through a model of the mechanical alloying process. The model is able to predict the time dependence of the initial and later stages of Ni₃S₂ formation and the effect of other parameters, such as mill atmosphere and use of premilled powder, on the reaction kinetics. The microstructures found in the intermediate to later stages of milling are similar to those associated with self-sustaining reactions.     

Role of surface-inoculated Debaryomyces hansenii and Yarrowia lipolytica strains in dried fermented sausage manufacture. Part 2: Evaluation of their effects on sensory quality and biogenic amine content

The aim was to study the effects of Debaryomyces hansenii and Yarrowia lipolytica strains, used with lactic acid starter cultures (Lactobacillus plantarum), in the manufacture of dried fermented sausages in order to understand their effects on volatile profile, biogenic amine content and sensory properties. The experimental data showed that every yeast strain produced a specific profile of volatile metabolic products. The yeasts also gave sausages with distinctive sensory properties. The degree of mincing also influenced these properties, but none of these factors had significant influence upon the accumulation of biogenic amines.