Spacing of edge fractures in layered materials

Opening-mode fractures developed from a free surface in a layered material often terminate at the interface that divides the fractured layer and the underlying layer. They also display regular spacing that is of the same order of magnitude as the thickness of the fractured layer. We have investigated the stress distribution between two adjacent edge fractures as a function of the ratio of fracture spacing to thickness of the fractured layer using a two-layer elastic model with a fractured top layer. The results show that when the ratio of fracture spacing to the layer thickness changes from greater than to less than a critical value the normal stress acting perpendicular to the fractures near the free surface changes from tensile to compressive. This stress state transition precludes further infilling of fractures unless they are driven by mechanisms other than a pure extension, or there are flaws that significantly perturb the local stress field between the fractures. Hence, the critical fracture spacing to layer thickness ratio defines a lower limit for fractures driven by extension, which also defines the condition of fracture saturation. The critical value of the fracture spacing to layer thickness ratio is independent of the average strain of the fractured layer, and it increases with increasing ratio of Young's modulus of the fractured layer to that of the underlying layer. The critical value increases with increasing Poisson's ratio of the fractured layer, but it decreases with increasing Poisson's ratio of the underlying layer. For the case with the same elastic constants for the fractured layer and the underlying layer, the critical spacing to layer thickness ratio is about 3.1. Delamination between the fractured layer and the underlying layer makes the critical spacing to layer thickness ratio much greater. Infilling fractures grow more easily from flaws located near the bottom of the fractured layer than from those located near the free surface when the spacing to layer thickness ratio is less than the critical value. The propagation of an edge flaw between adjacent edge fractures is unstable, but for the flaw to propagate to the interface, its height has to be greater than a critical size, that decreases with increasing fracture spacing to layer thickness ratio. The propagation behavior of an internal flaw with its lower tip at the interface depends on the edge fracture spacing to layer thickness ratio. The propagation is unstable, when the fracture spacing to layer thickness ratio is greater than a critical value; stable, when the fracture spacing to layer thickness ratio is less than another critical value; and first unstable, then stable, and/or unstable again, when the fracture spacing to layer thickness ratio is between these two critical values.     

A glow discharge unit to render electron microscope grids and other surfaces hydrophilic

We describe the design, construction, and operation of a simple glow discharge unit that can be used to make surfaces such as carbon-coated electron microscopy grids and glass coverslips hydrophilic. The use of a vacuum leak detector (Tesla coil) in place of a conventional high-voltage power supply and a small plastic desiccator for the vacuum chamber make the unit very inexpensive. Owing to the small volume of the chamber and the simplicity of the unit, the whole glow discharge process can be carried out in only 2 to 3 min, a time considerably shorter than that required for conventional vacuum evaporators. The hydrophilic surface improves adsorption of particles by several orders of magnitude in preparation for negative staining, freeze-drying, and other procedures.     

Small‐Scale Safety Testing of Ammonium Nitrate and Mixtures

Ammonium nitrate (AN), gunpowder (GP), and an ammonium nitrate gunpowder mixture (AN/GP) were studied for impact sensitivity by four laboratories using the drop hammer apparatus. Bruceton and Neyer methods were used as experimental protocols and for data reduction. The results are presented as 50 % probability of reaction (DH₅₀). For AN, the DH₅₀ values are widely varied among the participants, from sensitive to completely insensitive (limit of the equipment), with no real correlation among results. GP and the AN/GP mixture exhibited much more sensitivity overall and were in some cases within statistical values extrapolated from previous studies of RDX. The variability in results for the AN data is attributed to the difficulty in determining a positive reaction event for AN, as detailed by Neyer experiments and photography during positive reactions. Variability in results for the GP and AN/GP mixtures is attributed to equipment environment and detection criteria. This work was performed by the Integrated Data Collection Analysis (IDCA) program, a multi‐laboratory effort to standardize safety testing of improvised or homemade explosives funded by the Department of Homeland Security.     

Fissure formation and subsurface subsidence in a coalbed fire

Coalbed fires are uncontrolled subsurface fires that occur around the world. These fires are believed to be significant contributors to annual CO₂ emissions. Although many of these fires have been burning for decades, researchers have only recently begun to investigate physical mechanisms that control fire behavior. One aspect of fire behavior that is poorly characterized is the relationship between subsurface combustion and surface fissures. At the surface above many fires, long, wide fissures are observed. At a coalbed fire near Durango, Colorado, these fissures form systematic orthogonal patterns that align with regional joints in the Upper Cretaceous Fruitland Formation. Understanding the mechanisms that form these fissures is important, as the fissures are believed to play vital roles in sustaining the combustion in the subsurface. In some of the coalbed fire simulation models available today, these fissures are treated as fixed boundary conditions. We argue, using data collected, field observations and simulation result, that there exists a relationship between the location and magnitude of subsidence caused by the fire and the opening of fissures. The results presented suggest that fissures are believed to open when subsurface subsidence gives rise to tensile stresses around pre-existing joints.     

Fourier Transform Infrared and Ion-Chromatographic Sulfate Analysis of Ambient Air Samples

Fourier transform infrared (FTIR) spectrometry has been evaluated as a method for determining the sulfate concentration of ambient aerosol particulate samples. Samples were collected on Teflon filters. The filters were analyzed for sulfate by both FTIR and ion chromatography (IC). There is good agreement between sulfate analysis by IC and analysis of the FTIR transmission spectra of the air filters during the first 5 1/2 days of the Carbonaceous Species Methods Intercomparison Study in which the ambient sulfate concentrations were above the 5.8 μg/cm² lower limit of detection of the FTIR technique. A method to improve the FTIR lower limit of detection is discussed. The difficulties incurred during background subtraction of the infrared spectra are described.     

Active Micromachined Integrated Terahertz Circuits

Schottky barrier diodes have been integrated into on-chip rectangular waveguides. Two novel techniques have been developed to fabricate diodes with posts suitable for integration into waveguides. One technique produces diodes with anode diameters of the order of microns with post heights from 90 to 125 microns and the second technique produces sub-micron anodes with post heights around 20 microns. A method has been developed to incorporate these structures into a rectangular waveguide and provide a top contact onto the anode which could be used as an I.F. output in a mixer circuit. Devices have been fabricated and D.C. characterized.     

Microvascular and cellular responses in the retina of rats with acute experimental allergic encephalomyelitis (EAE)

The main mechanism causing catabolite repression in Escherichia coli is the dephosphorylation of enzyme IIAGlc, one of the enzymes of the phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS). The PTS is involved in the uptake of a large number of carbohydrates that are phosphorylated during transport, phosphoenolpyruvate (PEP) being the phosphoryl donor. Dephosphorylation of enzyme IIAGlc causes inhibition of uptake of a number of non-PTS carbon sources, a process called inducer exclusion. In this paper, we show that dephosphorylation of enzyme IIAGlc is not only caused by the transport of PTS carbohydrates, as has always been thought, and that an additional mechanism causing dephosphorylation exists. Direct monitoring of the phosphorylation state of enzyme IIAGlc also showed that many carbohydrates that are not transported by the PTS caused dephosphorylation during growth. In the case of glucose 6-phosphate, it was shown that transport and the first metabolic step are not involved in the dephosphorylation of enzyme IIAGlc, but that later steps in the glycolysis are essential. Evidence is provided that the [PEP]-[pyruvate] ratio, the driving force for the phosphorylation of the PTS proteins, determines the phosphorylation state of enzyme IIAGlc. The implications of these new findings for our view on catabolite repression and inducer exclusion are discussed.     

Microvascular and cellular responses in the optic nerve of rats with acute experimental allergic encephalomyelitis (EAE)

The optic nerve of rats with EAE was examined at various times to determine the integrity of the blood-brain barrier (BBB) and to assess monocyte-macrophage, T cell, and microglial responses. In naive control animals, leakage of horseradish peroxidase (HRP) and the presence of cells expressing major histocompatibility complex (MHC) class II antigen were evident in the meninges of the retrobulbar optic nerve. In rats with EAE, microglia in the region of the lamina cribrosa and in the regions adjacent to the meninges became activated from day 7 to 8 postinduction (pi). HRP leakage was also evident in the region of the lamina cribrosa from day 7 to 8 pi. On day 8 pi, infiltration of inflammatory cells and Monastral blue leakage were apparent in the myelinated region of the optic nerve. The intensity of these cellular and vascular changes peaked at day 12 pi, when signs of clinical disease became manifest. Monocytes-macrophages expressing MHC class II and the ED1 antigen, together with lymphocytes expressing the alphabetaT cell receptor, constituted the major proportion of cells associated with inflammatory lesions. Thus: (i) the inherent weakness of the BBB as well as the presence of both antigen (myelin) and MHC class II+ cells in the retrobulbar optic nerve are likely susceptibility factors for the frequent involvement of this region in EAE and multiple sclerosis; and (ii) activation of microglia occurs early in the pathogenesis of experimental optic neuritis.     

Allelochemicals in soil from no-tillage versus conventional-tillage wheat (Triticum aestivum) fields

Putative allelochemicals found in the soil of no-tillage and conventional-tillage wheat plots near Stillwater, Oklahoma, were obtained by a mild alkaline aqueous extraction procedure, bioassayed to determine their biological activity, purified, and analyzed with a capillary gas chromatography-mass spectrometry-data analysis system. The most significant inhibition was found in bioassays of extracts from soil collected immediately after harvest in June, July, and August. No-tillage soils produced significant inhibition during the rest of the year also. Mass spectrometry showed fatty acids as the most abundant compounds. However, when bioassayed authentic samples of the five free fatty acids showed no significant biological activity toward wheat.Journal Article No. 5650 of the Oklahoma Agricultural Experiment Station, Oklahoma State University, Stillwater, Oklahoma 74078.