An intermediate model method for obtaining a discrete relaxation spectrum from creep data

A convenient method is described for obtaining a discrete stress relaxation spectrum from linear viscoelastic creep data by means of a three-stage process. In stage one, a discrete retardation spectrum is fitted to the creep data using a least squares procedure, subject to the constraint that the discrete spectrum must be a specified order of polynomial function of the retardation time. In stage two, the resulting generalised Voigt model is solved numerically for an imposed step in strain, to determine the stress relaxation modulus function of time. In stage three, a discrete relaxation spectrum is fitted to the calculated stress relaxation modulus function. Although three stages are involved instead of the usual two, the procedure has been found to have certain practical advantages. These advantages make it suitable for the generation of relaxation spectra needed in viscoelastic stress analyses of solids, for example by the finite element method. In order to illustrate the proposed procedure it is applied to both artificial data and experimental creep data for poly(methyl methacrylate) at 70°C and at the glass transition.     

The preparation and characterization of silica gels doped with copper complexes

Silica gels doped with [Cu(C₅H₅N)₄]²⁺ (C₅H₅N = Py), CuCl₂-HOCH₂CH₂OH and Cu(CH₃COO)₂-NH₂(CH₂)₃Si-(OC₂H₅)₃ (CH₃COO = OAc, C₂H₅ = Et) have been prepared and examined by X-ray diffraction, electron spin resonance, transmission electron microscopy (TEM), thermal gravimetric analysis and Fourier transform infrared spectroscopy. Different dopants have different interactions with the silica alcogels depending on the chemistry of the dopants: [Cu(Py)₄]²⁺ forms chemical bonds with the hydroxyl oxygens on the surface of the silica matrix; in Cu(OAc)₂-NH₂(CH₂)₃Si(OEt)₃-doped alcogels, the siloamine enters into the sol-gel reaction and Cu(OAc)₂ coordinates to the -NH₂(CH₂)₃ ligand of the precursor. However, there is no evidence for bond formation between copper and the hydroxyl groups on the silica surface in the CuCl₂-HOCH₂CH₂OH-doped alcogels. The TEM images show a homogeneous and uniform distribution of copper particles on the reduced CuCl₂-HOCH₂CH₂OH- and Cu(OAc)₂-NH₂(CH₂)₃Si(OEt)₃-doped xerogel surface. The type of nitrogen adsorption isotherms of doped silica xerogels are unaffected by the dopant concentration. However, their surface areas do depend on the level of dopant. The microstructure of the silica matrix is primarily determined by the pH of the initial reactants, at least in the [Cu(Py)₄]²⁺-doped case.     

ColliderBit: a GAMBIT module for the calculation of high-energy collider observables and likelihoods

We describe ColliderBit, a new code for the calculation of high energy collider observables in theories of physics beyond the Standard Model (BSM). ColliderBit features a generic interface to BSM models, a unique parallelised Monte Carlo event generation scheme suitable for large-scale supercomputer applications, and a number of LHC analyses, covering a reasonable range of the BSM signatures currently sought by ATLAS and CMS. ColliderBit also calculates likelihoods for Higgs sector observables, and LEP searches for BSM particles. These features are provided by a combination of new code unique to ColliderBit, and interfaces to existing state-of-the-art public codes. ColliderBit is both an important part of the GAMBIT framework for BSM inference, and a standalone tool for efficiently applying collider constraints to theories of new physics.     

Decomposition of in vivo spatially offset Raman spectroscopy data using multivariate analysis techniques

The decomposition of spatially offset Raman spectra for complex multilayer systems, such as biological tissues, requires advanced techniques such as multivariate analyses. Often, in such situations, the decomposition methods can reach their limits of accuracy well before the limits imposed by signal‐to‐noise ratios. Consequently, more effective reconstruction methods could yield more accurate results with the same data set. In this study we process spatially offset Raman spectroscopy (SORS) data with three different multivariate techniques (band‐target entropy minimization (BTEM), multivariate curve resolution and parallel factor analysis (PARAFAC)) and compare their performance when analysing a spectrally challenging plastic model system and an even more challenging problem, the analysis of human bone transcutaneously in vivo. For the in vivo measurements, PARAFAC's requirement of multidimensional orthogonal data is addressed by recording SORS spectra both at different spatial offsets and at different anatomical points, the latter providing added dimensionality through the variation of skin/soft tissue thickness. The BTEM and PARAFAC methods performed the best on the plastic system with the BTEM more faithfully reconstructing the major Raman bands and PARAFAC the smaller more heavily overlapped features. All three methods succeeded in reconstructing the bone spectrum from the transcutaneous data and gave good figures for the phosphate‐to‐carbonate ratio (within 2% of excised human tibia bone); the PARAFAC gave the most accurate figure for the mineral‐to‐collagen ratio (20% less than excised human tibia bone). Previous studies of excised bones have shown that certain bone diseases (such as osteoarthritis, osteoporosis and osteogenesis imperfecta) are accompanied by compositional abnormalities that can be detected with Raman spectroscopy, the utility of a technique which could reconstruct bone spectra accurately is manifest. The results have relevance on the use of SORS in general. © 2014 Crown copyright. Journal of Raman Spectroscopy published by John Wiley & Sons, Ltd.     

Novel methods for synthesizing halide-free alane without the formation of adducts

Many of the current synthesis methods for aluminum hydride (alane—AlH₃) involve reacting AlCl₃ and LiAlH₄ in solvents. The reaction requires the formation of an alane adduct such as AlH₃⋅[(C₂H₅)₂O] prior to obtaining crystallized stable α-AlH₃. This process requires several hours of pumping in a vacuum system to remove the ether and convert the alane etherate into stable α-alane. This crystallization process is both costly and hazardous because a large amount of highly flammable material (e.g. ether) is removed by vacuum pumps over several hours. Conversely, the work presented herein describes novel methods to synthesize adduct-free alane. It is demonstrated here that AlH₃ can form by mixing AlCl₃ and LiAlH₄ in the solid state and heating to 75^∘C; only α-AlH₃ was obtained. The α-AlH₃ product can be washed with minimal solvents leading to zero formation of alane adducts. In addition, the unwanted LiCl by-product is also removed during the solvent wash, resulting in halide-free α-alane. Although simply mixing and heating the reactants led to a 40% yield of alane, having the reactants compacted and mechanically pressed while heating increases the yield to 60% crystalline α-AlH₃.     

Neck propagation in polyethylene

Intrinsic true stress–true strain response was evaluated at room temperature for three linear polyethylene samples deformed in conventional tensile tests. It was observed that high crystallinity is associated with a low rate of strain hardening that results in a sharp neck and a large drop in nominal stress. The maximum and minimum deformation loads are accounted for by Considère's analysis of neck initiation and stabilization, respectively. Following stabilization, neck propagation occurs at a load or nominal stress that is lower than the yield stress. The jump analysis of Ericksen and Hutchinson/Neale predicts steady state neck propagation stresses that are in very good (ca. 10%) agreement with experiment. Although the jump analysis is done in terms of uniaxial stress, the actual value of the propagation stress is established by the triaxial stress state in the neck shoulders. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2081–2091, 2004     

Plastic deformation of glassy polystyrene: A unified model of yield and the role of chain length

A study was made of yield and plastic flow in glassy polystyrene. A range of 12 linear atactic polystyrenes was studied: monodisperse, bimodal blends, and a polydisperse commercial sample. M_n varied between 66,000 and 490,000 g/mol. These were given standardized thermal treatments and then subjected to uniaxial compression tests in the glassy state over the temperature range 40 to 95 °C and nominal strain-rates 10⁻⁴ to 10⁻³ s⁻¹. Their constitutive responses were interpreted in terms of the physically based three-dimensional constitutive model for small or large deformations in amorphous polymers proposed earlier (Polymer 1995, 36, 3301–3312), including plastic strain-induced structural rejuvenation. In multimode form, the model captured closely both linear viscoelastic response and yield and plastic flow. When the reduction of Vogel temperature caused by chain ends was incorporated in the model, it predicted a fall in yield stress with reducing molecular length. This was also observed in experimental data, with the rate of fall approximately in agreement. The results provide further support for the model as a unifying framework for describing the physical properties of polymer glasses. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2027–2040, 2004