Effects of uneven moisture distribution on the strength of and wave velocity in concrete

Earlier findings showed that the effects of moisture (liquid or free water) in hardened concrete on its behavior, especially the lesser known effects ofuneven moisture distribution, can (a) be significant, and (b) vary from property to property. This distribution, for instance whether or not the surface layer is drier than the overall average moisture content, can be characterized by the difference between the velocity of the longitudinal wave (pulse velocity) measured in the standard through-thickness manner, and the velocity of the longitudinal wave propagating on the concrete surface. The summary of earlier findings on the effects of moisture distribution is followed by a recent investigation on pulse velocity in the special case, occurring frequently in practice, when the distribution is uneven because the liquid is concentrated in cracks in the concrete.     

Strengthening of Polypropylene Strips by Orientational Stretching in Liquids

Data on the orientational stretching of polypropylene strips in process liquids serving as plasticizers of polypropylene are presented. Such a stretching allows one to avoid many complications characteristic of stretching in air and to increase the maximum stretch ratio significantly (up to 15). The stretching of strips is accompanied by changes in the crystalline structure of polypropylene and by transformation of the amorphous phase into a crystalline one. It is established that the specific strength of the orientationally stretched strips as a function of stretch ratio has a maximum. This is caused by the competing processes of macromolecule orientation in the stretching direction and the destruction of molecular bonds, which leads to the origination of microdamages in the strip surface layer and to a growing content of amorphous phase. It is shown that the extraction of process liquids from the stretched strips increases their specific strength by 20–30%.__________Translated from Mekhanika Kompozitnykh Materialov, Vol. 41, No. 2, pp. 251–264, March–April, 2005.     

Estimation of the Tensile Strength Reduction of a Composite Laminate with a Hole

The strength problem for a body with an artificial defect in the form of a hole is considered. To solve this problem, an approach is suggested according to which the local strength of a material in the area of stress concentration is assumed to depend on the size of this area. The scale of the problem is introduced via the ratio between the characteristic sizes of the deformed area and the characteristic length of the material. This approach is used to estimate the strength of composite laminates weakened by holes and notches. Expressions for the failure stress are obtained, which can be applied to isotropic and orthotropic laminates both in quasi-brittle failure and in failure associated with significant inelastic deformations. A comparison between the calculated _c values and the known experimental data for glass/epoxy and graphite/epoxy laminates is presented.     

Emission spectra and stimulated emission characteristics of [N2]2--N2 molecular dimer

It has been proved by ab initio calculation and theoretical analysis that there exist [N2]2--N2 molecular dimers with D2h symmetry group, and there also exists an electric dipole excimer-like transition a1B2g→a1B3u. The theoretical spectra accord with the experimental results for transition a1B2g→a1B3u. The stimulated emission characteristic of N2 molecular dimer was researched through the microwave excited highly pure nitrogen and the method of amplified spontaneous emission. The experimental results show that N2 molecular dimer has stimulated emission characteristics when the microwave power is more than 100 W and the N2 pressure is in the range from 260 Pa to 2200 Pa.     

Characteristics of strength control of adaptive structure with electromechanical coupling

Based on the programming method, an electromechanical coupling adaptive statically indeterminate truss structure is controlled for increasing its load capacity. Several main parameters during the process of design of the adaptive structure are selected for a study of its characteristic during the control stage. The curves of each parameter for the effect of control results are plotted and corresponding conclusions are drawn. Thus, the theoretical basis is presented for optimal design, manufacture and control of the adaptive structure. Project supported by the National Natural Science Foundation of China (10072005), Beijing Educational Committee (99LG-11) and Beijing Natural Science (3002002) Foundation.     

High‐modulus,high‐strength,thick polypropylene monofilament production: Response surface analysis approach

Response surface analysis was used in optimizing spinning and drawing conditions for the production of a high‐modulus, high‐strength, thick polypropylene (PP) monofilament. Rotatable octagonal central composite design was adopted with two independent variables (take‐up velocity and draw ratio), and variations in the diameter, tensile modulus, and tenacity of the PP monofilament were observed. Quadratic response surface functions described the variations in those properties with quite high correlation coefficients. Canonical analysis combined with contour plots revealed the variation sensitivity: the diameter varied more markedly with the take‐up velocity, and the tensile modulus varied somewhat equally with the two variables, whereas the tenacity varied more markedly with the draw ratio. From such contour plots, the optimum processing conditions for desired properties were predicted without intensive structural analysis. Case studies showed that under the suggested processing conditions, the PP monofilament fulfilled the property requirements, such as a tensile modulus of 10.4 GPa and a tenacity of 672 MPa with a diameter still above 400 μm. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1175–1182, 2003     

Thermodynamic parameters for the protonation of carboxylic acids in aqueous tetraethylammonium iodide solutions

The protonation constants of 21 carboxylic acids (formic, acetic, propionic, benzoic, phenoxyacetic, salicylic, oxalic, malonic, succinic, itaconic, malic, tartaric, oxydiacetic, thiodiacetic, thiodipropionic, phthalic, maleic, citric, 1,2,3-tricarboxylic, 1,2,4-tricarboxylic and 1,2,4,5-tetracarboxylic), have been determined potentiometrically, by pH-metric measurements, at several temperatures and ionic strengths, 5T55°C, 0<11 mol-dm^–3, using tetraethylammonium iodide as background salt. General equations for the dependence on ionic strength of thermodynamic parameters have been found. The statistical significance of results and the possibility of using a simple model for the thermodynamics of carboxylic acids protonation, is discussed.     

Statistical analysis of HDPE fatigue lifetime

Fatigue lifetime of HDPE structures such as pipes is recognized to show a large scatter. This study aims to compare different statistical methods and distributions, in order to give convenient modeling of tensile and fatigue test results of commercially available polyethylene compression molded sheets. The median rank, the maximum likelihood and the Kolmogorov-Smirnov fitting are compared for the estimation of Weibull parameters. The choice of the best distribution to fit fatigue lifetime is discussed on the basis of the goodness-of-fit results. It is found that whether the three-parameter distributions of Weibull and lognormal types are suitable for lifetime prediction, the two-parameter Weibull is more conservative for probabilistic fatigue design.     

Coupling Strength and System Size Induce Firing Activity of Globally Coupled Neural Network

We investigate how firing activity of globally coupled neural network depends on the coupling strength C and system size N. Network elements are described by space-clamped FitzHugh-Nagumo (SCFHN) neurons with the values of parameters at which no firing activity occurs. It is found that for a given appropriate coupling strength, there is an intermediate range of system size where the firing activity of globally coupled SCFHN neural network is induced and enhanced. On the other hand, for a given intermediate system size level, there exists an optimal value of coupling strength such that the intensity of firing activity reaches its maximum. These phenomena imply that the coupling strength and system size play a vital role in firing activity of neural network.