Anisotropic Elastic-Viscoplastic Properties at Finite Strains of Injection-Moulded Low-Density Polyethylene

Injection-moulding is one of the most common manufacturing processes used for polymers. In many applications, the mechanical properties of the product is of great importance. Injection-moulding of thin-walled polymer products tends to leave the polymer structure in a state where the mechanical properties are anisotropic, due to alignment of polymer chains along the melt flow direction. The anisotropic elastic-viscoplastic properties of low-density polyethylene, that has undergone an injection-moulding process, are therefore examined in the present work. Test specimens were punched out from injection-moulded plates and tested in uniaxial tension. Three in-plane material directions were investigated. Because of the small thickness of the plates, only the in-plane properties could be determined. Tensile tests with both monotonic and cyclic loading were performed, and the local strains on the surface of the test specimens were measured using image analysis. True stress vs. true strain diagrams were constructed, and the material response was evaluated using an elastic-viscoplasticity law. The components of the anisotropic compliance matrix were determined together with the direction-specific plastic hardening parameters.     

Biological weighting functions as a tool for evaluating two ways to measure UVB radiation inhibition on photosynthesis

To estimate the inhibitory effect of the changing UVB radiation (UVBR, 280-315nm) on earth's ecosystems, an understanding of its wavelength dependency is needed. The tool used for these estimations is the biological weighting function (BWF), whereby the inhibition of different wavelengths is calculated. BWFs were determined for three algae species from different classes, Phaeodactylum tricornutum (Bacillariophyceae), Dunaliella tertiolecta (Chlorophyceae) and Rhodomonas sp. (Cryptophyceae), using polychromatic irradiation, where the UVBR spectra were varied with cut-off filters. For each alga, BWFs were determined for two photosynthetic parameters; the quantum yield measured as fluorescence from Photo System II in a pulse-amplitude-modulation (PAM) fluorometer, and the fixation of (14)C-labelled carbon dioxide. The BWFs were calculated with the Rundel method, using the radiation data between 270 and 360nm with 1nm resolution. The results show that the UVBR damages were generally higher when using the carbon fixation measurements than when measuring with the PAM technique. When using PAM, P. tricornutum in particular had a sensitivity intermediate between the sensitive Rhodomonas sp. and the more tolerant D. tertiolecta, but was as sensitive as, or even more sensitive, than Rhodomonas sp. when using carbon fixation. D. tertiolecta was shown to be less sensitive when using both techniques and the inhibition of its photosynthesis was almost as high when using PAM as when using carbon fixation. We concluded that, although the PAM technique has advantages such as being cleaner and easier to use, it is unable to substitute the carbon fixation measurements. Not only are the algae less sensitive when measured with PAM than they are when measured as carbon fixation, the relationship between the effects on the algae measured with the two techniques also differs. As fixation of carbon dioxide integrates a larger part of the photosynthetic machinery, it should be favoured as a measure of photosynthesis.     

TOF-OFF: A method for determining focal positions in tightly focused free-electron laser experiments by measurement of ejected ions

Pulse intensities greater than 10¹⁷ Watt/cm² were reached at the FLASH soft X-ray laser in Hamburg, Germany, using an off-axis parabolic mirror to focus 15 fs pulses of 5–70 μJ energy at 13.5 nm wavelength to a micron-sized spot. We describe the interaction of such pulses with niobium and vanadium targets and their deuterides. The beam produced craters in the solid targets, and we measured the kinetic energy of ions ejected from these craters. Ions with several keV kinetic energy were observed from craters approaching 5 μm in depth when the sample was at best focus. We also observed the onset of saturation in both ion acceleration and ablation with pulse intensities exceeding 10¹⁶ W/cm², when the highest detected ion energies and the crater depths tend to saturate with increasing intensity.A general difficulty in working with micron and sub-micron focusing optics is finding the exact focus of the beam inside a vacuum chamber. Here we propose a direct method to measure the focal position to a resolution better than the Rayleigh length. The method is based on the correlation between the energies of ejected ions and the physical dimensions of the craters. We find that the focus position can be quickly determined from the ion time-of-flight (TOF) data as the target is scanned through the expected focal region. The method does not require external access to the sample or venting the vacuum chamber. Profile fitting employed to analyze the TOF data can extend resolution beyond the actual scanning step size.     

The ArNe-N2O van der Waals trimer: a high resolution spectroscopic study of its rotational spectrum,structure and dynamics

A new ternary van der Waals complex of the type rare gas-rare gas'-linear molecule, ArNe-N₂O, was investigated using a pulsed molecular beam cavity Fourier transform microwave spectrometer. The rotational spectra of six isotopomers of the trimer were studied in detail. These include Ar²⁰Ne-¹⁴N¹⁴NO, Ar²²Ne-¹⁴N¹⁴NO, Ar²⁰Ne-15N¹⁴NO, Ar²²Ne-¹⁵N¹⁴N0, Ar²⁰Ne-¹⁴N¹⁵NO and Ar²²Ne-¹⁴N¹⁵NO. Nuclear quadrupole hyperfine structures of the rotational transitions that are due to the one or two ¹⁴N nuclei were resolved and analysed. The resulting spectroscopic constants were used to provide structural and dynamical information about the trimer. Based on the quartic centrifugal distortion constants, a harmonic force field analysis was performed to estimate the frequencies of the van der Waals vibrational modes. A perturbation of the electronic charge distribution at the site of the central ¹⁴N nucleus of N²0 upon complex formation was detected and discussed. Differences of structural parameters of the trimer as compared to those of the respective dimer units are indicative of the presence of significant three-body non-additive contributions to the interaction energy.     

Explosions of xenon clusters in ultraintense femtosecond x-ray pulses from the LCLS free electron laser

Explosions of large Xe clusters ( ~ 11,000) irradiated by femtosecond pulses of 850 eV x-ray photons focused to an intensity of up to 10(17) W/cm(2) from the Linac Coherent Light Source were investigated experimentally. Measurements of ion charge-state distributions and energy spectra exhibit strong evidence for the formation of a Xe nanoplasma in the intense x-ray pulse. This x-ray produced Xe nanoplasma is accompanied by a three-body recombination and hydrodynamic expansion. These experimental results appear to be consistent with a model in which a spherically exploding nanoplasma is formed inside the Xe cluster and where the plasma temperature is determined by photoionization heating.     

Precision steering of an optical trap by electro-optic deflection

We designed, constructed, and tested a single-beam optical trapping instrument employing twin electro-optic deflectors (EODs) to steer the trap in the specimen plane. Compared with traditional instruments based on acousto-optic deflectors (AODs), EOD-based traps offer a significant improvement in light throughput and a reduction in deflection-angle (pointing) errors. These attributes impart improved force and position resolution, making EOD-based traps a promising alternative for high-precision nanomechanical measurements of biomaterials.     

The porous structure of pulp fibres with different yields and its influence on paper strength

The porous structure of the interior of papermaking fibres is a well-known important property of the fibres. Changes of this structure will influence tensile and burst strength of paper formed from the fibres and a change in pore size of the pores within the fibre wall is also important for the ability of molecules to diffuse in and out of the fibre wall. Relevant examples of this latter effect are the removal of lignin during cooking and the addition of performance chemicals during papermaking. In this paper, pore sizes and the pore size distribution of unbleached softwood fibres have been studied. A well-characterised fibre material consisting of laboratory cooked spruce and pine pulp of various lignin contents was used. Pore size and pore size distribution were measured by studies of the relaxation behaviour of ²H in fibres saturated with ²H₂O. Beside this the total and surface charge of the fibres were also measured together with strength properties of papers from unbeaten fibres. For both pulps, there is a maximum in pore radius at a yield around 46%. Calculations of fibre wall volume from water retention values and yield levels show that there is a discontinuity in pore radius as a function of the fibre wall volume around a yield of 51%. It is suggested that this discontinuity is caused by the breakdown of the hemicellulose/lignin matrix within the fibre wall at this yield level. The strength of the papers formed from the fibres shows a correlation with the surface charge of the fibres. Based on the change in surface charge with yield and the change in total charge with yield, this correlation is suggested to be due to an opening up of the external part of the fibre wall. This stresses the importance of the chemical composition and physical structure of the outer layer of the fibre wall.     

Robust chaos synchronization based on adaptive fuzzy delayed feedback ${\bf{\mathcal{H}}}_{\bf{\infty}}$ control

In this paper, we propose a new adaptive H_￥\mathcal H_\infty synchronization strategy, called an adaptive fuzzy delayed feedback H_￥\mathcal H_\infty synchronization (AFDFHS) strategy, for chaotic systems with uncertain parameters and external disturbances. Based on Lyapunov–Krasovskii theory, Takagi–Sugeno (T–S) fuzzy model and adaptive delayed feedback H_￥\mathcal H_\infty control scheme, the AFDFHS controller is presented such that the synchronization error system is asymptotically stable with a guaranteed H_￥\mathcal{H}_{\infty } performance. It is shown that the design of the AFDFHS controller with adaptive law can be achieved by solving a linear matrix inequality (LMI), which can be easily facilitated by using some standard numerical packages. An illustrative example is given to demonstrate the effectiveness of the proposed AFDFHS approach.