Fracture mechanisms of sapphire under bending

The fracture mechanisms of a rapidly advancing crack in a single crystal are under investigation. Thin sapphire plates parallel to the basal plane were used as a model material for this purpose. Tests in three point bending (3PB) were carried out with smooth and with notched thin strip-shaped specimens having three different orientations with respect to the (1010) plane. The effect of the orientation on the fracture mechanisms is discussed and explained. Unique behavior was observed in the 3PB loading configuration, resulted from the typical state of stress in bending, i.e., tension in the lower, and compression in the upper region of the beam, which affected the fracture mechanisms. The continual changes of the crack direction and energy revealed a large spectrum of fracture phenomena. The major phenomena are explained. The effect of the mechanical energy on fracture mechanisms and topology is discussed.     

Radiative transfer model for aerosols in infrared wavelengths for passive remote sensing applications

A comprehensive analytical radiative transfer model for isothermal aerosols and vapors for passive infrared remote sensing applications (ground-based and airborne sensors) has been developed. The theoretical model illustrates the qualitative difference between an aerosol cloud and a chemical vapor cloud. The model is based on two and two/four stream approximations and includes thermal emission-absorption by the aerosols; scattering of diffused sky radiances incident from all sides on the aerosols (downwelling, upwelling, left, and right); and scattering of aerosol thermal emission. The model uses moderate resolution transmittance ambient atmospheric radiances as boundary conditions and provides analytical expressions for the information on the aerosol cloud that is contained in remote sensing measurements by using thermal contrasts between the aerosols and diffused sky radiances. Simulated measurements of a ground-based sensor viewing Bacillus subtilis var. niger bioaerosols and kaolin aerosols are given and discussed to illustrate the differences between a vapor-only model (i.e., only emission-absorption effects) and a complete model that adds aerosol scattering effects.     

Optical properties of single-crystal sapphire fibers

Single-crystal sapphire fibers have been grown with the laser-heated pedestal-growth method with losses as low as 0.3 dB /m at 2.94 ?m. With the incorporation of a computer-controlled feedback system, fibers have been grown with less than +/-0.5 % diameter variation, or +/-1.5 ?m for a 300- ?m fiber. The losses in these fibers have been reduced further through a postgrowth anneal at 1000 degrees C in air, from 5.4 to 1.5 dB /m at 543 nm and from 0.4 -0.3 dB /m at 2.94 ?m. These fibers delivered 4.7 W at 10 Hz of Er:YAG laser power.     

Plastic scintillating fibers for track detection

A particle track detector consisting of plastic scintillating fibers and a gatable image intensifier system has been developed and tested with pion, proton and deuteron beams of 1 GeV/c. We have observed clear tracks of beams and scattered particles at a beam rate of up to 10⁵ particles per second. Tracks have been recorded by using a CCD video camera at a trigger rate of around 30 Hz. The energy and spatial resolutions have been investigated.     

Sulfide glass fibers for infrared transmission

Germanium sulfide glasses were drawn into fibers with optical losses lower than 1 dB/m. The glass fibers were coated with Teflon FEP or silicone resin according to their thermal properties. Loss increase caused by coating was not observed. The main problems in obtaining low-loss fibers were investigated. Sulphur evolution in fiber drawing was avoided by choosing the appropriate glass compositions and drawing conditions, OH and SH impurities were reduced to $\text{1}\text{4}$ of initial values using the S₂Cl₂ chemical reaction with water at 400–600°C. The possibility of ultra-low loss fibers using sulfide glasses is discussed in terms of intrinsic losses and impurity-related absorption losses.     

Plastic bending of an edge-cracked specimen

The plastic zones and the rotation of the cracked planes of an edge-cracked bend specimen are investigated by a numerical three-dimensional analysis. For an alloy-steel, for bending moments up to the first yielding moment of the uncracked specimen, it is shown that the crack tip is in small scale yielding.     

Growth and spectroscopic properties of Ti-doped sapphire single-crystal fibers

Titanium doped sapphire (Ti:Al₂O₃) crystal fibers have been grown by the micro-pulling-down (μ-PD) method using different pulling rate in the range (0.1–0.5 mm/min). The present work has investigated the crystal growth, including diameter, bubbles defects and segregation properties. The fiber’s absorption and IR emission experiments are analyzed. The lifetime of Ti³⁺ in the IR range obtained at room temperature is 3.1 μs. Moreover, the two blue emissions at 420 nm and 470 nm bands have been discussed.     

Theory of infrared microspectroscopy for intact fibers

Infrared microspectroscopy is widely used for the chemical analysis of small samples. In particular, spectral properties of small cylindrical samples are important in forensic analysis, understanding relationships between microstructure and mechanical properties in fibers or fiber composites, and development of cosmetics and drugs for hair. The diameters of the constituent cylinders are typically of the order of the central wavelength of light used to probe the sample. Hence, structure and material spectral response are coupled and recorded spectra are usually distorted to the extent of becoming useless for molecular identification. In this paper, we apply rigorous optical theory to predict the spectral distortions observed in IR microspectroscopic data of fibers. The theory is used, first, to compute the changes that are observed for cylinders of various dimensions under different instrument configurations when compared to the bulk spectrum from the same material. We provide a method to recover intrinsic material spectral response from fibers by correcting for distortion introduced by the cylindrical structure. The theory reported here should enable the routine use of IR microspectroscopy and imaging for the molecular analysis of cylindrical domains in complex materials.     

Plastic properties of matrix composites in bending

Using the methods of integrated cross-sections and elastic solutions, we solve an elastico-plastic problem of bending of a Kirchhoff inhomogeneous square plate. The elastico-plastic properties and the effective yield stress of the inhomogeneous plate are calculated on an electronic computer. The computational results form the basis for a qualitative analysis and for the conclusions made. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 70, No. 6, pp. 1045–1048, 1997. Original article submitted July 9, 1996.     

Plastic bending of CdxHg1−xTe

Preliminary results are reported of three-point plastic bending tests on Cd _x Hg_1–x Te single crystal samples, for an x value of about 0.2, conducted in air at strain rates of the order of 10^–5 sec^–1, and at temperatures in the range 303 K (30° C) to 363 K (90° C) (in the region of 0.35T _m ⁰ , where T _m ⁰ is the absolute melting point). Single crystal samples were cut from polycrystalline ingots, and the orientation, although measured in each case, was not consistent from sample to sample, being determined by the available grain shape. The stress-strain curves resemble those found for Group IV and III–V semiconductors. They display a yield drop, followed by a region of zero work hardening. All tests were stopped in this region, and in no case did the overall glide strain exceed 3%. The upper and lower yield stresses (outer fibre glide stress values) varied from 16 MN m^–2 and 10 MN m^–2, respectively, at 363 K (90° C) to 24 MN m^–2 and 17 MN m^–2, respectively, at 303 K (30° C).     

The potential of active fiber composites made from piezoelectric fibers for actuating and sensing applications in structural health monitoring

Active Fiber Composites made from piezoelectric fibers were originally developed as actuators for adaptive materials systems. Their properties, however, also allow their use as sensor or emitter-receiver in variours applications. After presenting selected characteristics, the potential of Active Fiber Composites for structural health monitoring is discussed in comparison with that of conventional piezoelectric elements.     

Electrospinning of aligned biodegradable polymer fibers and composite fibers for tissue engineering applications

Fibrous membranes of aligned poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) fibers have been made through electrospinning. A high-speed rotating drum was used as the fiber collector while the electric field was manipulated by using five knife-edged auxiliary electrodes. It was found that a high drum rotating speed of 3000 rpm could lead to a nearly perfect alignment of PHBV fibers during electrospinning. Multilayered fibrous structures with each layer having a different direction of fiber alignment could also be constructed through electrospinning. The electrospun PHBV fibers were further modified by incorporating carbonated hydroxyapatite (HA) nanospheres (up to 20% of HA) in the fibers. The fibrous membranes made of aligned PHBV fibers and made of HA/PHBV composite fibers should be very useful for the tissue engineering of different human body tissues.     

Static bending and impact behaviour of areca fibers composites

Natural fibers are considered to have potential use as reinforcing agents in polymer composite materials because of their principle benefits: good strength and stiffness, low cost, and be an environmental friendly, degradable, and renewable material. A study has been carried out to evaluate physical, flexural and impact properties of composite made by areca fibers with randomly distributed fibers. The extracted areca fibers from the areca husk were alkali treated with potassium hydroxide to get better interfacial bonding between fiber and matrix. Then composite laminates were fabricated by using urea formaldehyde, melamine urea formaldehyde and epoxy resins by means of compression molding technique with varying process parameters, such as fiber condition (untreated and alkali treated), and fiber loading percentages (50% and 60% by weight). The developed areca fiber-reinforced composites were then characterized by physical, bending and impact test. The results show that flexural and impact strength increases with increase in the fiber loading percentage. Compared to untreated fiber, significant change in flexural and impact strength has been observed for treated areca fiber reinforcement.     

Supported planar germanium waveguides for infrared evanescent-wave sensing

We have fabricated miniature planar IR waveguides with thicknesses of 30-50 mum, consisting of 12-mm long, 2-mm wide strips of Ge supported on ZnS substrates. Evidence for efficient propagation of broadband IR light through these waveguides is provided by the presence of characteristic high- and low-frequency optical cutoffs of Ge; by the observation of an oscillatory interference pattern in the transmittance spectrum, which exhibits a dependence on waveguide thickness and propagation angle that closely matches waveguide theory; and by the detection of strong evanescent-wave absorption from small (2 mm(2)) droplets of liquid, e.g., water, on the waveguide surface. As also predicted by theory, the surface sensitivity (detected light absorbance per unit area of sample-waveguide contact) is shown to increase as a function of incidence or bevel angle.     

Vision-based sensing and control for space robotics applications

The following problems arise in the precise positioning of payloads by space manipulators: (1) the precise measurement of the relative position and motion of objects in the workspace of the robot; (2) the design of a control system, which is robust and performs well in spite of the effects of structural flexibility and oscillations typically associated with space robots. This paper discusses the solution to the measurement problem by a vision system using photogrammatic image processing to determine the motion of objects in real time. Performance characteristics are presented. The control problem is addressed by a new technique dealing effectively with the challenge posed by the noncollocated sensor/actuator configuration on the flexible robot structure. The laboratory implementation of the measurement and control concepts is discussed. Preliminary results validate the concepts     

Narrow-linewidth, tunable ultraviolet, Ti:sapphire laser for environmental sensing

We describe a compact, narrow-linewidth, etalon-tuned titanium:sapphire laser cavity that is designed for field environmental sensing and is pumped by the second harmonic of a kilohertz Nd:YAG laser. The fundamental tunable range is from 690 to 1100 nm, depending on mirror reflectivities and the optics kit used. The conversion efficiency is at least 25% for the fundamental and 2-3% for intracavity frequency doubling from 3.5 to 4 W 532 nm pump power. The linewidth is <0.1 cm(-1), and the pulse width is 18 ns. Applications of this cavity include the measurement of trace gas species by laser-induced fluorescence, cavity ringdown spectroscopy, and micropulse lidar in the UV-visible regions.     

Observations of and applications for self-imaging in optical fibers

Experimental results of self-imaging in optical fibers are reported along with an analytical model that explains the observations. Some implications for sensor design are discussed.     

Dual-wavelength diode-seeded Ti:sapphire laser for differential absorption lidar applications

A dual-wavelength, diode-seeded, flash-lamp-pumped Ti:sapphire laser system has been developed. Injection seeding allowed the attainment of high output energies (130 mJ), narrow linewidth (250 MHz), and high reliability. Furthermore, an original method is presented for dual-wavelength operation, using diode current sweeping. These specifications are ideal for differential absorption lidar applications, and the example of NO(2) detection is presented. Extension to other pollutants is discussed.     

Silver halide photonic crystal fibers for the middle infrared

Photonic crystal fibers are normally holey silica fibers, which are opaque in the mid- and far-infrared. We have fabricated novel fibers by multiple extrusions of silver halide crystalline materials, which are highly transparent in the mid-infrared. These fibers are composed of two solid materials; The core consists of pure AgBr, and the cladding includes AgCl fiberoptic elements arranged in two concentric hexagonal rings around the core. Flexible fibers of outer diameter 1 mm and length of approximately 1 m were fabricated, and their optical properties were measured. These fibers exhibited core-clad behavior and would be extremely useful for IR laser power transmission, IR radiometry, and IR spectroscopy.