Effect of annealing and silylation on the strength of melt-spun Ni-Mn-Ga fibres and their adhesion to epoxy

Single crystals of ferromagnetic Ni-Mn-Ga shape memory alloys show magnetic-field and stress induced twinning, leading to shape memory. Adaptive composites can thus be produced by embedding single crystalline particles or bamboo-structured Ni-Mn-Ga fibres into a polymer matrix. To guarantee a durable performance of these composites, adhesion between reinforcement phase and matrix should be quantified and optimised. The influence of annealing and surface treatment with an aminosilane of melt-spun Ni-Mn-Ga fibres on their strength and adhesion to an epoxy matrix was investigated using single fibre tension and fragmentation tests. Annealing of the melt-spun Ni-Mn-Ga fibres changed the surface from a "pimpled" to a smooth microstructure. This resulted in a reduced adhesion of the annealed fibres in comparison to the as-spun fibres embedded in an epoxy matrix. As-spun fibres exhibited an interfacial shear strength (IFSS) comparable to the shear strength of the epoxy matrix so that the silylation did not change the adhesion significantly. For the annealed fibres, the silane treatment improved the IFSS by 67%. Furthermore, the silylation increased the fracture strength of the Ni-Mn-Ga fibres due to surface flaw healing or forming of a protective surface coating. (C) 2014 Elsevier Ltd. All rights reserved

Induction of aromatic ring: cleavage dioxygenases in Stenotrophomonas maltophilia strain KB2 in cometabolic systems

Stenotrophomonas maltophilia KB2 is known to produce different enzymes of dioxygenase family. The aim of our studies was to determine activity of these enzymes after induction by benzoic acids in cometabolic systems with nitrophenols. We have shown that under cometabolic conditions KB2 strain degraded 0.25–0.4 mM of nitrophenols after 14 days of incubation. Simultaneously degradation of 3 mM of growth substrate during 1–3 days was observed depending on substrate as well as cometabolite used. From cometabolic systems with nitrophenols as cometabolites and 3,4-dihydroxybenzoate as a growth substrate, dioxygenases with the highest activity of protocatechuate 3,4-dioxygenase were isolated. Activity of catechol 1,2- dioxygenase and protocatechuate 4,5-dioxygenase was not observed. Catechol 2,3-dioxygenase was active only in cultures with 4-nitrophenol. Ability of KB2 strain to induce and synthesize various dioxygenases depending on substrate present in medium makes this strain useful in bioremediation of sites contaminated with different aromatic compounds

A Novel Phase Variation Mechanism in the Meningococcus Driven by a Ligand-Responsive Repressor and Differential Spacing of Distal Promoter Elements

Phase variable expression, mediated by high frequency reversible changes in the length of simple sequence repeats, facilitates adaptation of bacterial populations to changing environments and is frequently important in bacterial virulence. Here we elucidate a novel phase variable mechanism for NadA, an adhesin and invasin of Neisseria meningitidis. The NadR repressor protein binds to operators flanking the phase variable tract and contributes to the differential expression levels of phase variant promoters with different numbers of repeats likely due to different spacing between operators. We show that IHF binds between these operators, and may permit looping of the promoter, allowing interaction of NadR at operators located distally or overlapping the promoter. The 4-hydroxyphenylacetic acid, a metabolite of aromatic amino acid catabolism that is secreted in saliva, induces NadA expression by inhibiting the DNA binding activity of the repressor. When induced, only minor differences are evident between NadR-independent transcription levels of promoter phase variants and are likely due to differential RNA polymerase contacts leading to altered promoter activity. Our results suggest that NadA expression is under both stochastic and tight environmental-sensing regulatory control, both mediated by the NadR repressor, and may be induced during colonization of the oropharynx where it plays a major role in the successful adhesion and invasion of the mucosa. Hence, simple sequence repeats in promoter regions may be a strategy used by host-adapted bacterial pathogens to randomly switch between expression states that may nonetheless still be induced by appropriate niche-specific signals

Strength variability of single flax fibres

Due to the typical large variability in the measured mechanical properties of flax fibres, they are often employed only in low grade composite applications. The present study aims to investigate the reasons for the variability in tensile properties of flax fibres. It is found that an inaccuracy in the determination of the cross-sectional area of the fibres is one major reason for the variability in properties. By applying a typical circular fibre area assumption, a considerable error is introduced into the calculated mechanical properties. Experimental data, together with a simple analytical model, are presented to show that the error is increased when the aspect ratio of the fibre cross-sectional shape is increased. A variability in properties due to the flax fibres themselves is found to originate from the distribution of defects along the fibres. Two distinctive types of stress–strain behaviours (linear and nonlinear) of the fibres are found to be correlated with the amount of defects. The linear stress–strain curves tend to show a higher tensile strength, a higher Young’s modulus, and a lower strain to failure than the nonlinear curves. Finally, the fibres are found to fracture by a complex microscale failure mechanism. Large fracture zones are governed by both surface and internal defects; and these cause cracks to propagate in the transverse and longitudinal directions

Mechanical properties of flax fibers and their composites

Flax fibers, along with a number of other natural fibers, are being considered as an environmentally friendly alternative of synthetic fibers in fiber-reinforced polymer composites. A common feature of natural fibers is a much higher variability of mechanical properties. This necessitates study of the flax fiber strength distribution and efficient experimental methods for its determination. Elementary flax fibers of different gauge lengths are tested by single fiber tension in order to obtain the stress-strain response and strength and failure strain distributions. The applicability of single fiber fragmentation test for flax fiber failure strain and strength characterization is considered. It is shown that fiber fragmentation test can be used to determine the fiber length effect on mean fiber strength and limit strain. Stiffness and strength under uniaxial tension of flax fiber composites with thermoset and thermoplastic polymer matrices are considered. The applicability of rule of mixtures and orientational averaging based models, developed for short fiber composites, to flax reinforced polymers is evaluated.Godkänd; 2006; 20061206 (pafi)</p

Mechanical properties of flax fibers and their composites

Flax fibers, along with a number of other natural fibers, are being considered as an environmentally friendlier alternative of synthetic fibers in fiber-reinforced polymer composites. A common feature of natural fibers is a much higher variability of mechanical properties. This necessitates study of the flax fiber strength distribution and efficient experimental methods for its determination. Elementary flax fibers of different gauge lengths are tested by single fiber tension in order to obtain the stress-strain response and strength and failure strain distributions. The applicability of single fiber fragmentation test for flax fiber failure strain and strength characterization is considered. It is shown that fiber fragmentation test can be used to determine the fiber length effect on mean fiber strength and limit strain. The effect of mechanical damage in the form of kink bands and of diameter variability on the strength of elementary flax fibers is considered. Stiffness and strength under uniaxial tension of flax fiber composites with thermoset and thermoplastic polymer matrices are studied. The applicability of rule of mixtures and orientational averaging based models, developed for short fiber composites, to flax reinforced polymers are evaluated. Both the quasi-static and time dependent mechanical properties of flax fiber/thermoplastic starch based composites are analyzed. The effect of temperature and relative humidity is investigated. It is found that microdamage accumulation in this type of composites is not significant. Results show that the composite elastic modulus and failure stress are linearly related to the maximum stress reached by the matrix in tensile tests. Simple material models are suggested to account for the observed nonlinear viscoelasticity and viscoplasticity.Godkänd; 2009; 20091029 (edgspa); DISPUTATION Ämnesområde: Polymera konstruktionsmaterial/Polymeric Composite Materials Opponent: Docent Kristofer Gamstedt, Kungliga Tekniska Högskolan, Stockholm Ordförande: Docent Roberts Joffe, Luleå tekniska universitet Tid: Onsdag den 9 december 2009, kl 10.00 Plats: E 231, Luleå tekniska universitet</p