Dynamic characterisation of interlaminar fracture toughness in carbon fibre epoxy composite laminates

In this work, the rate dependence of mode I interlaminar fracture toughness for two different materials systems, IM7/8552 and IM7/M91, both unidirectional UD carbon-fibre epoxy composite laminates have been examined over a wide range of loading rates from 0.5 mm/min up to 2000 mm/s at room temperature. Quasi-static fracture tests were performed using a DCB (double-cantilever beam) method with a screw-driven testing machine, while the dynamic tests were carried out using a WIF (wedge-insert fracture) specimen loaded dynamically in a hydraulic system. For performing the tests at high displacement rates, a special setup was designed and manufactured which allowed the insertion of the wedge within the DCB specimens at different cross-head displacement rates. The experimental technique used a pair of strain gauges attached to the bending surface of one of the arms of the cantilever beams and far from the initial crack tip. The peak values of the recorded strain were used to determine the fracture toughness under dynamic conditions through use of the compliance calibration method. A finite element model was developed to check the consistency of the measurements and validate the data reduction method used. The results exhibited rate insensitive behaviour in the case of the IM7/8552 laminates while IM7/M91 showed the contrary behaviour with maximum peak at 500 mm/s of displacement rate, with a toughness increase of 95% with respect to the quasi-static conditions.The research leading to these results has received funding from the European Union as Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 722096, DYNACOMP project

Tetramolecular G-quadruplex formation pathways studied by electrospray mass spectrometry

Electrospray mass spectrometry was used to investigate the mechanism of tetramolecular G-quadruplex formation by the DNA oligonucleotide dTG5T, in ammonium acetate. The intermediates and products were separated according to their mass (number of strands and inner cations) and quantified. The study of the temporal evolution of each species allows us to propose the following formation mechanism. (i) Monomers, dimers and trimers are present at equilibrium already in the absence of ammonium acetate. (ii) The addition of cations promotes the formation of tetramers and pentamers that incorporate ammonium ions and therefore presumably have stacked guanine quartets in their structure. (iii) The pentamers eventually disappear and tetramers become predominant. However, these tetramers do not have their four strands perfectly aligned to give five G-quartets: the structures contain one ammonium ion too few, and ion mobility spectrometry shows that their conformation is more extended. (iv) At 4°C, the rearrangement of the kinetically trapped tetramers with presumably slipped strand(s) into the perfect G-quadruplex structure is extremely slow (not complete after 4 months). We also show that the addition of methanol to the monomer solution significantly accelerates the cation-induced G-quadruplex assembly

Conserved presence of G-quadruplex forming sequences in the Long Terminal Repeat Promoter of Lentiviruses

G-quadruplexes (G4s) are secondary structures of nucleic acids that epigenetically regulate cellular processes. In the human immunodeficiency lentivirus 1 (HIV-1), dynamic G4s are located in the unique viral LTR promoter. Folding of HIV-1 LTR G4s inhibits viral transcription; stabilization by G4 ligands intensifies this effect. Cellular proteins modulate viral transcription by inducing/unfolding LTR G4s. We here expanded our investigation on the presence of LTR G4s to all lentiviruses. G4s in the 5'-LTR U3 region were completely conserved in primate lentiviruses. A G4 was also present in a cattle-infecting lentivirus. All other non-primate lentiviruses displayed hints of less stable G4s. In primate lentiviruses, the possibility to fold into G4s was highly conserved among strains. LTR G4 sequences were very similar among phylogenetically related primate viruses, while they increasingly differed in viruses that diverged early from a common ancestor. A strong correlation between primate lentivirus LTR G4s and Sp1/NF\u3baB binding sites was found. All LTR G4s folded: their complexity was assessed by polymerase stop assay. Our data support a role of the lentiviruses 5'-LTR G4 region as control centre of viral transcription, where folding/unfolding of G4s and multiple recruitment of factors based on both sequence and structure may take place

Damage signature of fatigued fabric reinforced plastics in the pulsed ultrasonic polar scan

This study investigates the use of both the amplitude and time-of-flight based pulsed ultrasonic polar scan (P-UPS) for the nondestructive detection and evaluation of fatigue damage in fiber reinforced composites. Several thermoplastic carbon fabric reinforced PPS specimens (CETEX), loaded under various fatigue conditions, have been scanned at multiple material spots according to the P-UPS technique in order to extract material degradation in a quantitative way. The P-UPS results indicate that shear dominated fatigued carbon/PPS goes with a reduction of shear properties combined with large fiber distortions. The P-UPS results of the tension-tension fatigued carbon/PPS samples on the other hand reveal a directional degradation of the stiffness properties, reaching a maximum reduction of -12.8% along the loading direction. The P-UPS extracted damage characteristics are fully supported by simulations, conventional destructive tests as well as visual inspection. The results demonstrate the excellent capability of the P-UPS method for nondestructively assessing and quantifying both shear-dominated and tension-tension fatigue damage in fabric reinforced plastics

Phreatische fauna in Ljubljansko polje (Ljubljana-Ebene, Jugoslavien) - ihre ökologische Verteilung und zoogeographische Beziehungen

The phreatic basin of Ljubljansko polje (polje = plain, field) recharges its water supply mainly from the Sava river-bed and at a few other locations where connections with karstic subterranean waters might exist and only up to 15% from precipitation. An important zone of infiltration in the river-bed is the bottom and not the bank which is to a large extent watertight due to organic debris (rests of Sphaerotilus e.g.). The main water-body moves about 10 m/day, there are however some local jets with far higher speeds. Yearly amplitudes of water temperatures are high near the river but in the centre of the plain only a couple of centigrades. Oxygen saturation is in the open river-water 100%, dropping to 40-60% just 1 m into the phreatic. True stygopsammal animals are represented here only by a few species and specimen in spite of the fact, that the interstices in the gravel are mostly filled with finer sediments. Remarkable is also the scarcity of Nematodes and the near absence of Acarina (compare with Danielopol 1976). Only a few specimen of the river benthos (Chironomidae, Tipulidae, Leuctra supp., Baetìs spp.) penetrate the interstitial water (compare with Ruffo 1961, Danielopol 1976) and only Naididae are more frequent there. However, many epigean animals occur in interstitial waters in the periodically flooded gravel-banks; one can explain this with oscillations of the water level. Some epigean animals (creno- and troglophilic) are quite regularly represented in the phreatic near the river, but have not been found in the river-bed. The distribution of phreatic species within the studied water-body seems to be controlled mainly by the presence of food supplies and the consequent competition among species. The same is true for the speed of the water current and some other factors which are less easily defined. The characteristics of the substratum as well as O2-saturation and other characteristics of the water seem to have little influence on the fauna. The energetically (food-) rich neighbourhood of the river is inhabited by a number of species in quite dense populations while the central parts of the phreatic water body exhibit a great poverty of species and of specimen. However, some species live here, which don’t occur in the presence of larger food supplies — and of greater competition (Niphargus serbicus). The higher current speed seems to prevent settlement of some species (Cyclopoida, Proasellus deminutus) while some are bound to such habitats (Proasellus vulgaris). Some species exhibit a high degree of euryvalency inside the stygopsephale habitats (Niphargus longidactylus e.g.), while some are highly specialized. Some of them form dense populations (comparatively dense even in energetically poor places) while others exhibit even in most favourable conditions very low densities (Niphargus jovanovici multipennatus). The present fauna is zoogeographically very diverse. Some species are distributed throughout Europe; some reach from Central Europe to the borders of Dinaride Karst (Bogidiella albertimagni) and some even penetrate it (Trichodrilus pragensis, Acanthocyclops kiefer). Bogidiella semidenticulata, Niphargus pectinicauda, Hadziella deminuta seem to be limited to the higher reaches of the Sava River. All of the above mentioned animals live regularly in interstitial waters and only sporadically in karstic hypogean waters. Niphargus stygius is here the only animal of a certainly karstic provenience; inside the plain it is limited to a completely special habitat. It is very likely that the entire Proasellus-deminutus group has developed in interstitial waters of larger plains which are in contact with karstic areas; some species penetrated from the plains into the karst rather than the reverse. To the contrary (judging from the distribution of the genera) karstic waters seem to be the cradle of Hauffenia and Hadziella. Such a sharp delimitation between cave- and interstitial fauna resp. in this area is very noteworthy. Both faunas live here in abundance and in close contact. It is very probable that particularly high competition and specialization of both faunas, caused by their richness and diversity, prevent mixing of species

The Discovery of Proteus-eggs (Proteus anguinus Laurenti, Amphibia) in Seminatural Conditions

Proteus-eggs were found for the first time in the nature, drifted out of a karstic spring. They were obtained from the Vir Spring al Sticna, 30 km ESE of Ljubljana. The hydrological and faunistical data indicate that Proteus lays its eggs also in “unsheltered”, energetically rich groundwater habitats

A microlevel capillary study in composite materials by X-ray tomography

One of the main problems associated with the manufacture of composite materials by liquid moulding (injection, infusion, etc.) is the generation of porosity. The optimization of the properties of the composites passes through a better understanding of the mechanisms of pore formation and transport. This pore formation is associated with a non-homogeneity of resin flow during the manufacturing process as a consequence of the double-scale porosity present in textile preforms. In the channels between bundles of fibers, the resin flows controlled by the pressure gradient and its viscosity. However, within the fiber bundles, the resin flows as a result of the capillary pressure. As a consequence, if the viscous and capillary forces are not balanced, non-homogeneous local flow occurs and the formation of pores, either between the fiber bundles, or internally to the bundles is produced.In this work a systematic study of the spontaneous wetting process of an E glass fiber bundle through wicking tests, using several fluids with different properties, viscosity, surface tension and contact angle is presented. The visualization of the front position of the resin flow at microscopic scale, as well as the formation of the menisci between fibers has been performed using X-ray tomography techniques using synchrotron radiation. The reconstructed volumes have made it possible to obtain detailed quantitative information on the geometry of the menisci which occur as a consequence of the surface tension operating at such scale. These characterization techniques constitute a fundamental tool for the optimization of the processing of composite materials buy liquid moulding