Long- and short-term creep of polyoxymethylene/polyurethane/alumina ternary composites by comparison

Ternary composites consisting of polyoxymethylene, polyurethane (PU), and boehmite alumina were produced by melt blending with and with out latex precompounding. The latex precompounding served to predisperse the alumina particles. The related advanced masterbatch (MBa) was produced by mixing the PU latex with the water-dispersible boehmite alumina. The dispersion of alumina and rubber particles was studied by using the scanning electron micros copy. The creep properties of the composites were determined from results of long- and short-term creep tests (per formed at various temperatures). The composites produced by the MBa technique out performed those made by direct melt compounding with respect to most of creep characteristics. The Findley power law was found to be fairly applicable to the experimental results obtained. Master curves (strain vs. time) were also constructed by employing the time-temperature super position principle.     

Experimental comparison of manufacturing techniques of toughened and nanoreinforced polyamides

Composites consisting of polyamide-6 (PA-6), nitrile rubber (NBR), and sodium fluorohectorite (FH) or alumina silicate (Sungloss; SG) were produced by different techniques with latex precompounding. Their tensile and thermomechanical properties were determined by using tensile tests and a dynamic-mechanical analysis, performed at various temperatures. The PA-6/NBR composite systems produced by the direct melt compounding outperformed those obtained by using the masterbatch technique with respect to the strength and ductility, but the latter ones had a higher storage modulus.     

Effect of nanofiller aspect ratio on the stress relaxation and creep response of toughened pom composites

Ternary composites composed of polyoxymethylene (POM), polyurethane (PU), and sodium fluorohectorite (FH) or sodium bentonite (BN) were produced by the melt compounding masterbatch (MB) technique. The related MB was produced by mixing the PU latex with water-swellable FH or BN. The dispersion of the nanofillers in the composites was studied by X-ray diffraction techniques. The crystallization of the POM-based systems was inspected by polarized optical microscopy (PLM). The stress relaxation and creep properties of the composites were determined in short-time stress relaxation and creep tests (creep at various temperatures), respectively. The POM/PU/FH composites produced by the MB technique outperformed the POM/PU blend and the POM/PU/BN system in respect to most of the stress relaxation and creep characteristics. This fact was attributed to the higher aspect ratio of FH compared with that of BN. The master curves (creep compliance vs. time) constructed by employing the time-temperature superposition principle showed that the Findley power law was fully applicable to the experimental results obtained.     

Rheological and dynamic thermomechanical propert ies of epoxy composites reinforced with single- and multiwalled carbon nanotubes

An epoxy resin (Epon 828) was filled with single- and multiwalled carbon nanotubes (SWCNT and MWCNT) in two steps by using the high shear mixing and ultrasonication techniques. The melt flow of the composites was characterized in a plate/plate rheometer. The thermomechanical properties of the composites were determined in dynamic mechanical analysis tests performed at various frequencies and temperatures. It was found that the incorporation of SWCNT or MWCNT increased the viscosity and stiffness of epoxy above its glass-transition temperature. The time-temperature superposition principle was employed to estimate the storage modulus of the composites as a function of frequency (f = 10^–33–10³ Hz) in the form of master curves.     

Self-adhesion of amorphous polymers and their miscible blends

The amorphous film surfaces of polystyrene (PS), poly(2,6-dimethyl 1,4-phenylene oxide) (PPO), and their miscible blends are brought into overlap contact below the glass transition temperature T _g for 10 min and 24 h in order to obtain PS—PS, PPO—PPO, and blend—blend self-adhesive joints. It is shown that after the contact of the blend surfaces, i.e., when the molecules of both PS and PPO are present at the interface, it is possible to attain higher values of shear strength as compared with those at PS—PS and PPO—PPO interfaces. This points to the contribution of a specific interaction between the segments of PS and PPO to the strength development at the interface. Submitted to the 11th International Conference on Mechanics of Composite Materials (Riga, June 11–15, 2000). Translated from Mekhanika Kompozitnykh Materialov, Vol. 36, No. 1, pp. 127–135, 2000.     

Multiwall carbon nanotube/epoxy composites produced by a masterbatch process

A masterbatch process based on a minicalander (three-roller mill) and a vacuum dissolver was developed in order to produce multiwall carbon nanotube/epoxy composites with loading fractions of 0.5, 1.0, and 2.0 wt.%. TEM and SEM analyses were performed to investigate the dispersion results. A contrast imaging in the SEM backscattering mode revealed a homogeneous distribution of carbon nanotubes in the whole volume of the material. Furthermore, an interesting correlation was found to exist between the network structure formed by the nanotubes in the epoxy matrix and the appearance of fracture surface of the nanocomposites. Furthermore, the nanocomposites exhibited an electrical conductivity in the regime of some 10⁻² S/m. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 42, No. 5, pp. 567–582, September–October, 2006.     

Processing,structure, and mechanical properties of alumina-nanofilled polystyrene composites

Binary composites composed of polystyrene (PS) and a synthetic boehmite alumina were produced by using the water-mediated melt compounding (WMC) and direct melt compounding (DMC) techniques. The alumina particles were dispersed in water at ambient temperature. The aqueous alumina suspension was injected into molten PS in a twin-screw extruder to prepare reinforced polymer composites. The dispersion of the alumina was studied by transmission and scanning electron microcopy techniques (TEM and SEM, respectively). The mechanical and thermomechanical properties of the composites were determined by employing a dynamic-mechanical thermal analysis (DMTA) and short-time creep and uniaxial static tensile tests. It was found that the direct melt compounding of the alumina with PS resulted in microcomposites, whereas the water-mediated melt compounding technique gave rise to nanocomposites. The incorporation of alumina into the PS nanocomposites increased their stiffness, tensile strength, and creep resistance. However, the elongation of the PS nanocomposites at break was smaller than that of the PS microcomposites.     

Measurement of dynamic characteristics of latex foams by the antiresonance method

Measuring and calculation methods for the dynamic characteristics of tubular specimens of low-modulus latex foams and composites based on them, under longitudinal vibrations in a low-frequency region (up to 200 Hz), are developed. The method is based on the effect of antiresonance. Data on the influence of the basic formulation-processing factors on the dynamic characteristics (gelatination time and content of the plasticizer and chopped fibers) are presented. The results obtained can be used to choose the composition of latex foams for lining acoustic channels, injectors, ejectors, and phase inverters, as well as to control products quality. This method can be applied directly to full-size articles. Translated from Mekhanika Kompozitnykh Materialov, Vol. 36, No. 2, pp. 249–258, March–April 2000.     

Variation of the structure and permeability of tensioned fiber layers during impregnation with a thermoplastic polymer melt

The influence of displacements of tensioned fibers on the impregnation of fibrous layers with a polymer melt and on the final composite structure is studied. Using computer simulation, it is shown that, during impregnation, the structure of tensioned fibrous layers changes considerably depending on the initial arrangement and tensioning of fibers. The consolidated regions formed under the melt front move inside the impregnated layer with the advancing melt front. Displacement of the tensioned fibers as well as the formation of “washouts” favors the impregnation of internal layers, but cause significant inhomogeneity of the polymer structure. The surface (on the side of the melt flow) regions are more saturated with the polymer than the internal ones. A difference in the melt percolation mechanisms at various impregnation regimes is revealed. The effective permeability coefficients of a tensioned fiber layer are not constant but depend on the conditions and regimes of impregnation. Submitted to the 11th Conference on the Mechanics of Composite Materials (Riga, June 11–15, 2000). Translated from Mekhanika Kompozitnykh Materialov, Vol. 36, No. 2, pp. 259–270, March–April, 2000.     

Inequalities of Maximum of Partial Sums and Weak Convergence for a Class of Weak Dependent Random Variables

In this paper, we establish a Rosenthal-type inequality of the maximum of partial sums for ρ^- -mixing random fields. As its applications we get the Hájeck -Rènyi inequality and weak convergence of sums of ρ^- -mixing sequence. These results extend related results for NA sequence and p^＊ -mixing random fields,     

Massively parallel differential evolution—pattern search optimization with graphics hardware acceleration: an investigation on bound constrained optimization problems

This paper presents a novel parallel Differential Evolution (DE) algorithm with local search for solving function optimization problems, utilizing graphics hardware acceleration. As a population-based meta-heuristic, DE was originally designed for continuous function optimization. Graphics Processing Units (GPU) computing is an emerging desktop parallel computing technology that is becoming popular with its wide availability in many personal computers. In this paper, the classical DE was adapted in the data-parallel CPU-GPU heterogeneous computing platform featuring Single Instruction-Multiple Thread (SIMT) execution. The global optimal search of the DE was enhanced by the classical local Pattern Search (PS) method. The hybrid DE–PS method was implemented in the GPU environment and compared to a similar implementation in the common computing environment with a Central Processing Unit (CPU). Computational results indicate that the GPU-accelerated SIMT-DE-PS method is orders of magnitude faster than the corresponding CPU implementation. The main contribution of this paper is the parallelization analysis and performance analysis of the hybrid DE–PS with GPU acceleration. The research results demonstrate a promising direction for high speed optimization with desktop parallel computing on a personal computer.     

Strong relaxation limit of multi-dimensional isentropic Euler equations

This paper is devoted to study the strong relaxation limit of multi-dimensional isentropic Euler equations with relaxation. Motivated by the Maxwell iteration, we generalize the analysis of Yong (SIAM J Appl Math 64:1737–1748, 2004) and show that, as the relaxation time tends to zero, the density of a certain scaled isentropic Euler equations with relaxation strongly converges towards the smooth solution to the porous medium equation in the framework of Besov spaces with relatively lower regularity. The main analysis tool used is the Littlewood–Paley decomposition.     

Single item lot-sizing with non-decreasing capacities

We consider the single item lot-sizing problem with capacities that are non-decreasing over time. When the cost function is (i) non-speculative or Wagner–Whitin (for instance, constant unit production costs and non-negative unit holding costs) and (ii) the production set-up costs are non-increasing over time, it is known that the minimum cost lot-sizing problem is polynomially solvable using dynamic programming. When the capacities are non-decreasing, we derive a compact mixed integer programming reformulation whose linear programming relaxation solves the lot-sizing problem to optimality when the objective function satisfies (i) and (ii). The formulation is based on mixing set relaxations and reduces to the (known) convex hull of solutions when the capacities are constant over time. We illustrate the use and potential effectiveness of this improved LP formulation on a few test instances, including instances with and without Wagner–Whitin costs, and with both non-decreasing and arbitrary capacities over time. This work was partly carried out within the framework of ADONET, a European network in Algorithmic Discrete Optimization, contract no. MRTN-CT-2003-504438. This text presents research results of the Belgian Program on Interuniversity Poles of Attraction initiated by the Belgian State, Prime Minister’s Office, Science Policy Programming. The scientific responsibility is assumed by the authors.     

Fundamental solution of generalized thermo-viscoelasticity using the finite element method

The present paper is aimed at an investigation of the temperature, displacement, and stress in a viscoelastic half space of Kelven–Voigt type. The formulation is applied according to three theories of generalized thermoelasticity: Lord–Shulman with one relaxation time, Green–Lindsay with two relaxation times, as well as the coupled theory. The nondimensional governing equations are solved by the finite element method. Numerical results for the temperature distribution, displacement, and thermal stress are represented graphically. Comparisons are made with the results predicted by the CD, L-S, and G-L theories in the presence and absence of the viscoelastic relaxation time.     

Characterization of Transfer Functions of Pritchard–Salamon or Other Realizations with a Bounded Input or Output Operator

We show that the transfer functions that have a (continuoustime) well-posed realization with a bounded input operator are exactly those that are strong-H² (plus constant feedthrough) over some right half-plane. The dual condition holds iff the transfer function has a realization with a bounded output operator. Both conditions hold iff the transfer function has a Pritchard–Salamon (PS) realization. A state-space variant of the PS result was proved already in [3], under the additional assumption that the weighting pattern (or impulse response) is a function (whose values are bounded operators). We illustrate by an example that this does not cover all PS systems, not even if the input and output spaces are separable.     

Entropy Theory from the Orbital Point of View

 Inspired by [17], we develop an orbital approach to the entropy theory for actions of countable amenable groups. This is applied to extend – with new short proofs – the recent results about uniform mixing of actions with completely positive entropy [17], Pinsker factors and the relative disjointness problems [10], Abramov–Rokhlin entropy addition formula [19], etc. Unlike the cited papers our work is independent of the standard machinery developed by Ornstein–Weiss [14] or Kieffer [12]. We do not use non-orbital tools like the Rokhlin lemma, the Shannon–McMillan theorem, castle analysis, joining techniques for amenable actions, etc. which play an essential role in [17], [19] and [10]. (Received 23 October 2000)     

Interdiffusion and adhesion at the inteface of a polystyrene-poly(2,6-dimethyl-1,4-phenylene oxide) blend below the glass transition temperature

Experiments on adhesion between the film surfaces of a polystyrene (PS)-poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) blend were carried out below the glass transition temperature of the bulk (T _g ), at T=T _g −13 and T=T _g −23°C. The contact time varied from 2 to 1440 min. The strength at the interface was then measured at room temperature. The strength at the blend-blend interface was calculated using an additive diffusion model which takes into account the strength at homopolymer-homopolymer interfaces (PS-PS, PS-PPO, and PPO-PPO), surface concentration of components, and its variation with depth. A fair agreement between the calculated and measured strength was found. The results of the study support the concept of decreasing glass transition temperature near the surface of polymer glasses as compared with T _g of the bulk. Translated from Mekhanika Kompozitnykh Materialov, Vol. 35, No. 5, pp. 661–670, September–October, 1999.     

On the mechanical and thermomechanical properties of low-density polyethylene/ethylene-α-octene copolymer blends

Blends of low-density polyethylene (LDPE) and ethylene-octene copolymer (EOC) were obtained. The effect of EOC content and absorbed radiation dose on the mechanical and thermomechanical properties of LDPE/EOC blends are investigated. Particular attention is given to a tensile stress-strain analysis and the “form-memory” effect of the blends. With growing LDPE content, the elastic modulus, the yield stress, and the thermorelaxation and residual stresses of the blends increase, but the ultimate elongation at break decreases, which is caused by the higher crystallinity of polyethylene. As a result of radiation-induced cross-linking, the elastic modulus, the yield stress (at a 1% strain), the ultimate yield strength, and the thermorelaxation and residual stresses increase, while the ultimate elongation at break and the melt flow-behavior index decrease, which is confirmed by the growing gel fraction in the blend. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 44, No. 2, pp. 279–286, March–April, 2008.     

A modified inexact operator splitting method for monotone variational inequalities

The Douglas–Peaceman–Rachford–Varga operator splitting methods (DPRV methods) are attractive methods for monotone variational inequalities. He et al. [Numer. Math. 94, 715–737 (2003)] proposed an inexact self-adaptive operator splitting method based on DPRV. This paper relaxes the inexactness restriction further. And numerical experiments indicate the improvement of this relaxation.      

Effect of long-term exposure to water on the viscoelastic properties of an epoxy-based composition

For modeling the effect of volume change on the viscoelastic properties of a polymer, an equation underlying the concept of free volume is used. The parameters of the moisture–time reduction function, characterizing changes in the viscoelastic properties of an epoxy-based composition, are determined from tests results. To verify the approach suggested, results of long-term (up to one year) tests on stress relaxation under compression in an epoxy composition (COMFLOOR), both in the initial state and after a long-term (up to one year) exposure to water, are used. A satisfactory agreement between the reduction functions based on predictions and determined experimentally is found to exist. Translated from Mekhanika Kompozitnykh Materialov, Vol. 45, No. 2, pp. 201–210, March–April, 2009.