Structural and Mechanical Strength of Proton Radiation Processed Polyethylene Terephthalate

The change in structural and mechanical behavior of polyethylene terephthalate (PET) due to 2.4 MeV proton has been studied. Radiation processing of PET polymer is carried out using different low doses such as 0.2, 2.0, and 20 kGy. The Physics of microstrain and radiation-induced mesophase formation are analysed. X-ray investigation indicates that proton-induced structural modification takes place in the material. Apart from usual diffraction peaks, a low intensity broad peak is observed at small angle of about 2q =10º, when the fibre axis is mounted parallel to the X-ray direction. Such peak is absent in the diffraction spectrum when the fibre axis is mounted perpendicular to the beam direction. The appearance of the extra peak in a particular orientation confirms that, the phase is 2-dimensionally oriented (mesophase). The Young’s modulus (Y) of this irradiated PET sample is found to be more than that of the virgin sample with the highest value recorded for a dose of 2.0 kGy. The decrease in Y for higher dose (20 kGy) may be due to enhanced ion-induced microstrain in the sample, causing degradation in mechanical strength.

Helically twisted photonic crystal fibres

Recent theoretical and experimental work on helically twisted photonic crystal fibres (PCFs) is reviewed. Helical Bloch theory is introduced, including a new formalism based on the tight-binding approximation. It is used to explore and explain a variety of unusual effects that appear in a range of different twisted PCFs, including fibres with a single core and fibres with N cores arranged in a ring around the fibre axis. We discuss a new kind of birefringence that causes the propagation constants of left- and right-spinning optical vortices to be non-degenerate for the same order of orbital angular momentum (OAM). Topological effects, arising from the twisted periodic ‘space’, cause light to spiral around the fibre axis, with fascinating consequences, including the appearance of dips in the transmission spectrum and low loss guidance in coreless PCF. Discussing twisted fibres with a single off-axis core, we report that optical activity in a PCF is opposite in sign to that seen in a step-index fibre. Fabrication techniques are briefly described and emerging applications reviewed. The analytical results of helical Bloch theory are verified by an extensive series of ‘numerical experiments’ based on finite-element solutions of Maxwell's equations in a helicoidal frame. This article is part of the themed issue ‘Optical orbital angular momentum’.

Strength and toughness of structural fibres for composite material reinforcement

The characterization of the strength and fracture toughness of three common structural fibres, E-glass, AS4 carbon and Kevlar KM2, is presented in this work. The notched specimens were prepared by means of selective carving of individual fibres by means of the focused ion beam. A straight-fronted edge notch was introduced in a plane perpendicular to the fibre axis, with the relative notch depth being a 0 / D ≈0.1 and the notch radius at the tip approximately 50 nm. The selection of the appropriate beam current during milling operations was performed to avoid to as much as possible any microstructural changes owing to ion impingement. Both notched and un-notched fibres were submitted to uniaxial tensile tests up to failure. The strength of the un-notched fibres was characterized in terms of the Weibull statistics, whereas the residual strength of the notched fibres was used to determine their apparent toughness. To this end, the stress intensity factor of a fronted edge crack was computed by means of the finite-element method for different crack lengths. The experimental results agreed with those reported in the literature for polyacrylonitrile-based carbon fibres obtained by using similar techniques. After mechanical testing, the fracture surface of the fibres was analysed to ascertain the failure mechanisms. It was found that AS4 carbon and E-glass fibres presented the lower toughness with fracture surfaces perpendicular to the fibre axis, emanating from the notch tip. The fractured region of Kevlar KM2 fibres extended along the fibre and showed large permanent deformation, which explains their higher degree of toughness when compared with carbon and glass fibres. This article is part of the themed issue ‘Multiscale modelling of the structural integrity of composite materials’.

Biomechanics of fibrous proteins of the extracellular matrix studied by Brillouin scattering

Brillouin light scattering (BLS) spectroscopy is a technique that is able to detect thermally excited phonons within a material. The speed of propagation of these phonons can be determined from the magnitude of the Brillouin frequency shift between incident and scattered light, thereby providing a measure of the mechanical properties of the material in the gigahertz range. The mechanical properties of the extracellular matrices of biological tissues and their constituent biopolymers are important for normal tissue function and disturbances in these properties are widely implicated in disease. BLS offers the prospect of measuring mechanical properties on a microscopic scale in living tissues, thereby providing insights into structure–function relationships under normal and pathological conditions. In this study, we investigated BLS in collagen and elastin—the fibrous proteins of the extracellular matrix (ECM). Measurements were made on type I collagen in rat tail tendon, type II collagen in articular cartilage and nuchal ligament elastin. The dependence of the BLS spectrum on fibre orientation was investigated in a backscattering geometry using a reflective substrate. Two peaks, a bulk mode arising from phonon propagation along a quasi-radial direction to the fibre axis and a mode parallel to the surface, depending on sample orientation relative to the fibre axis, could be distinguished. The latter peak was fitted to a model of wave propagation through a hexagonally symmetric elastic solid, and the five components of the elasticity tensor were combined to give axial and transverse Young's, shear and bulk moduli of the fibres. These were 10.2, 8.3, 3.2 and 10.9 GPa, and 6.1, 5.3, 1.9 and 8 GPa for dehydrated type I collagen and elastin, respectively. The former values are close to those previously reported. A microfocused BLS approach was also applied providing selection of single fibres. The moduli of collagen and elastin are much higher than those measured at lower frequency using macroscopic strains, and the difference between them is much less. We therefore believe, like previous investigators, that molecular-scale viscoelastic effects are responsible for the frequency dependence of the fibre biomechanics. Combining BLS with larger-scale mechanical testing methods therefore should, in the future, provide a means of following the evolution of mechanical properties in the formation of the complex structures found in the ECM.

Wupatkiite from the Cameron Uranium District, Arizona, a new member of the halotrichite group

Wupatkiite (Co,Mg,Ni)Al2(SO4)4.22H2O is a new member of the halotrichite group. It occurs 8 miles ESE of Gray Mountain, Arizona, near the prehistoric pueblo dwelling Wupatki. It occurs as cross-fibre veinlets with fibres up to 8mm long. Colour: empire rose (RHS48C), streak white, H. = 1 ½. G = 1.92, D(calc) = 1.87g/cm3. Nonpleochroic with α = 1.477, γ = 1.484, Z^fibre axis = 12°. Wet chemistry gave MgO 1.63, CaO 0.10, MnO 0.17, FeO 0.15, NiO 0.52, CuO 0.12, CoO 3.41, Al2O3 11.30, SO3 35.97, H2O 42.26 (total 95.63%); rem. = montmorillonite and quartz. Parameters refinedo from the powder pattern are close to those of halotrichite. Wupatkiite is monoclinic P21/c with a = 6.189 Å, b = 24.23 Å, c = 21.20 Å, β = 100.33°. Strongest lines are 4.790, 100 (024); 4.295, 27 (140); 3.945, 26 (025); 3.768, 33 (062); 3.494, 92 (124, 063).

Metamunirite, a new anhydrous sodium metavanadate from San Miguel County, Colorado

Metamunirite, β-NaVO3, is found in cavities in sandstone in San Miguel County, Colorado, occurring as fine, fibrous, colourless needles. X-ray powder and precession photographs show the crystals to be orthorhombic, space group Pnma, with a = 14.134(7), b = 3.648(2), c = 5.357(2) Å. They are optically biaxial positive with nα = 1.780(2) (‖c), nβ = 1.800(2) (‖a), nγ ≫ 185 (‖b, fibre axis; positive elongation), 2Vz moderate. A crystal structure analysis, confirming the previously determined structure of β-NaVO3, shows the presence of (VO3)n chains made up of zig-zag VO5 square pyramids. Metamunirite is probably formed by dehydration of munirite, NaVO3.2H2O.

The hydrodynamic stability of boundary-layer flow over a class of anisotropic compliant walls

This paper examines the linear stability of zero-pressure-gradient boundary-layer flow over a class of anisotropically responding compliant walls. The anisotropic wall behaviour is derived from a material anisotropy which is characterized by relatively high tensile and compressive strength along a certain direction, termed the fibre axis. When the material stiffness along the fibre axis is sufficiently high, the resulting correlation between the horizontal and the vertical components of wall displacement induces at the flow–wall interface a Reynolds shear stress of a sign that is predetermined by the angle with which the fibre axis makes with the direction of the flow. The notion that anisotropic surface response could be employed to produce turbulent Reynolds shear stresses of predetermined sign at a surface was first explored by Grosskreutz (1971) in an experimental study on turbulent drag reduction. The present paper examines the implications of this interesting idea in the context of two-dimensional flow stability over anisotropic compliant walls. The study covers single- and two-layer compliant walls using the methodology described in Yeo (1988). The effects of wall anisotropy, as determined by the orientation of the fibre axis and the material stiffness along the fibre axis, on flow stability are examined for a variety of walls. The potential of some anisotropic compliant walls for delaying laminar–turbulent transition is investigated, and the contribution of the anisotropy to transition delay is appraised.