Viscosity of Silica and Doped Silica Melts: Evidence for a Crossover Temperature

Silica is known as the archetypal strong liquid, exhibiting an Arrhenius viscosity curve with a high glass transition temperature and constant activation energy. However, given the ideally isostatic nature of the silica network, the presence of even a small concentration of defects can lead to a significant decrease in both the glass transition temperature and activation energy for viscous flow. To understand the impact of trace level dopants on the viscosity of silica, we measure the viscosity-temperature curves for seven silica glass samples having different impurities, including four natural and three synthetic samples. Depending on the type of dopant, the glass transition temperature can vary by nearly 300 K. A common crossover is found for all viscosity curves around ~2200-2500 K, which we attribute to a change of the transport mechanism in the melt from being dominated by intrinsic defects at high temperature to dopant-induced defects at low temperatures

Material-independent crack arrest statistics: Application to indentation experiments

An extensive experimental study of indentation and crack arrest statistics is presented for four different brittle materials (alumina, silicon carbide, silicon nitride, glass). Evidence is given that the crack length statistics can be described by a universal (i.e. material independent) distribution. The latter directly derives from results obtained when modeling crack propagation as a depinning phenomenon. Crack arrest (or effective toughness) statistics appears to be fully characterized by two parameters, namely, an asymptotic crack length (or macroscopic toughness) value and a power law size dependent width. The experimental knowledge of the crack arrest statistics at one given scale thus gives access to its knowledge at all scales

Local Suppression of T Cell Responses by Arginase-Induced L-Arginine Depletion in Nonhealing Leishmaniasis

The balance between T helper (Th) 1 and Th2 cell responses is a major determinant of the outcome of experimental leishmaniasis, but polarized Th1 or Th2 responses are not sufficient to account for healing or nonhealing. Here we show that high arginase activity, a hallmark of nonhealing disease, is primarily expressed locally at the site of pathology. The high arginase activity causes local depletion of L-arginine, which impairs the capacity of T cells in the lesion to proliferate and to produce interferon-γ, while T cells in the local draining lymph nodes respond normally. Healing, induced by chemotherapy, resulted in control of arginase activity and reversal of local immunosuppression. Moreover, competitive inhibition of arginase as well as supplementation with L-arginine restored T cell effector functions and reduced pathology and parasite growth at the site of lesions. These results demonstrate that in nonhealing leishmaniasis, arginase-induced L-arginine depletion results in impaired T cell responses. Our results identify a novel mechanism in leishmaniasis that contributes to the failure to heal persistent lesions and suggest new approaches to therapy

TPB Test Provides New Insight to Fiber Strength, Quality

Discusses methods and processes involved in using the two-point bend technique to identify the properties and quality of glasses. Characterization of the effects of melt history on the nominal homogeneity of the resulting glasses; Effects of melt processing near the liquidus temperature; Measurement of the intrinsic strength and quality of glasses

The Effects of Melt History on the Failure Characteristics of Pristine Glass Fibres

The failure strains (εf) of silicate glass fibres drawn from melts with different thermal histories have been characterised using the two-point bend (TPB) technique. Narrow distributions, with Weibull parameters m\u3e100, result from well conditioned melts, and distributions broaden (m decreases) when melt temperatures approach the liquidus temperature. These results indicate that the TPB technique is sensitive to the presence of strength limiting heterogeneities in glass

The Intrinsic Strength and Fatigue of Oxide Glasses

Recent studies on the strength of glass fibers suggest that the time is ripe for new, fundamental studies in this area which may significantly advance our understanding of the intrinsic strength of glasses. In order to set the stage, in this paper we define various terms (intrinsic and extrinsic strength and inert and environmental fatigue) and analyze techniques for their measurement. We illustrate and evaluate these parameters by means of literature data on silica and E-glass. In addition we present some preliminary new data on E-glass fibers using 2-point bending. These new data report higher strength than previously reported and some possible reasons for this are given. While these comments deal primarily with the science of strength, a better understanding of these issues may lead to improvements in glass technology and glass products

Using the Two-Point Bend Technique to Determine Failure Stress of Pristine Glass Fibers

Two-point bending (2pb) is the simplest technique for the measurement of failure strains of glass fibers under a variety of experimental conditions. There is little chance of damage to fibers even when testing in liquid nitrogen, leading to reproducible and precise measurements of failure strain with Weibull moduli of greater than 100 measured routinely. However, a limitation of 2pb is that it measures failure strain not failure stress, and thus the Young\u27s modulus of the sample must be known at the failure strain in order to evaluate the failure stress. In this paper the failure strains, under both inert and ambient conditions, for a number of conventional glasses (commercial silica, soda-lime silicate, and E-glass), as well as a number of simple glasses, including a nepheline glass and a range of binary sodium and potassium silicate glasses are presented. These strain values are converted to failure stresses using known or estimated non-linear modulus parameters and compared with strength values found in the literature. For silica optical fibers, the failure stresses calculated from 2pb failure strains vary from 12.1 ± 0.2 to 14.4 ± 0.3 GPa in inert (liquid nitrogen, 77 K) conditions and 7.0 to 7.3 ± 0.1 GPa in ambient conditions (room temperature, 50% RH), compared to reports of 11-14 GPa for liquid nitrogen and 4-5 GPa ambient tensile strength measurements. For a commercial E-glass, the calculated failure stress from 2pb, is 5.1 to 5.2 ± 0.1 GPa in inert conditions and 3.7 to 3.8 ± 0.1 GPa in ambient conditions, compared to reported tensile strengths of 5.3 GPa and 3.0-3.8 GPa, respectively. The failure stresses for binary alkali silicate glasses calculated from 2pb failure strains are 2-3 times greater than those reported in the literature