Work of Fracture and Crack Healing in Glass

The work of fracture, γwoF, of a soda-lime-silica glass was determined as 5.5 J/m² in an inert atmosphere and 4.4 J/m² in air by using a short bar specimen with a chevron notch. The critical stress intensity factor K_IC calculated from γ_wof obtained in an inert atmosphere agreed with the value, 0.88 MN/m^3/2, determined by the indentation method. In unloading-loading cycles during stable crack growth, crack healing was observed both on the diagram of load vs load-point displacement and visually. Crack healing is more prevalent in an inert atmosphere, occurring only minimally in air. The energy for healing was measured as 0.65 J/m² in an inert atmosphere and ∼0.21 J/m² in air.     

Fracture Energy and Strength Behavior of a Sodium Borosilicate Glass-Al2O3 Composite System

Fracture energy and strength were determined for three series within a sodium borosilicate glass-Al₂O₃ dispersed composite system. The average particle sizes of the Al₂O₃ dispersions were
, and
μm. Within each series, composites containing 0.10, 0.25, and 0.40 vol fractions of the Al₂O₃ dispersed phase were vacuum hot-pressed. The fracture energy was determined at 77°K with the double cantilever specimen configuration. Strength was measured by a 4-point flexural test. A significant increase in fracture energy was observed (up to 5 times the fracture energy of the glass without second-phase dispersion). The fracture energy depended on the interparticle spacing and average particle size of the Al₂O₃ dispersion. These results could best be explained by a previously proposed model for the interaction of a crack front with a second-phase dispersion. Surface roughness also contributed to the increased fracture energy. Some composites were strengthened significantly relative to the glass without a dispersion. Calculation of the crack size showed that the Al₂O₃ dispersion increased the crack size of the glass by ∼1 to 3 times the average particle size of the Al₂O₃ dispersion. Thus, the dispersion increased both the fracture energy and the crack size. These opposing parameters ultimately determined the strength behavior of these composites.     

Fracture Surface Energy of Glass

Fracture surface energies of six glasses were measured using the double-cantilever cleavage technique. Values ranged from 3.5 to 5.3 J/m² depending on the chemical composition of the glass and the temperature of the test. The fracture surface energy increased with decreasing temperature and increasing Young's modulus; however, exceptions to this behavior were noted. The magnitude of the values obtained is discussed with respect to the theoretical strength of glass and possible irreversible effects at the crack tip such as stress corrosion and plastic deformation are considered.     

Use of Fracture Mirrors to Interpret Impact Fractures in Brittle Materials

The impact resistances and fracture mirror radii ( r_m ) of rods of several ceramic materials were measured. The fracture stresses (σ _f ) were determined from σ _f vs r_m^−1/2 curves obtained from fiexural strength tests. An analysis, based on the assumption that the principal factor contributing to the impact energy absorbed is the energy ( U_e ) required to deflect the specimen to the fracture stress, indicated that the impact energy absorbed ( U ) per unit of specimen cross-sectional area ( A ) increased in proportion to the square of the maximum stress. The analysis also indicated that the slopes of the curves of U/A vs σ _f ² are proportional to the reciprocal of Young's modulus. Experimental data for several materials are consistent with this analysis.     

Fracture Energies of Tape-Cast Silicon Nitride with β-Si3N4 Seed Addition

The fracture energies of the tape-cast silicon nitride with and without 3 wt% rod-like β-Si₃N₄ seed addition were investigated by a chevron-notched-beam technique. The material was doped with Lu₂O₃–SiO₂ as sintering additives for giving rigid grain boundaries and good heat resistance. The seeded and tape-cast silicon nitride has anisotropic microstructure, where the fibrous grains grown from seeds were preferentially aligned parallel to the casting direction. When a stress was applied parallel to the fibrous grain alignment direction, the strength measured at 1500°C was 738 MPa, which was almost the same as room temperature strength 739 MPa. The fracture energy of the tape-cast Si₃N₄ without seed addition was 109 and 454 J/m² at room temperature and 1500°C, respectively. On the contrary, the fracture energy of the seeded and tape-cast Si₃N₄ was 301 and 781 J/m² at room temperature and 1500°C, respectively, when a stress was applied parallel to the fibrous gain alignment. The large fracture energies were attributable primarily to the unidirectional alignment fibrous Si₃N₄ grains.     

Elevated-Temperature R-Curve Behavior of a Polycrystalline Alumina

The fracture behavior of a polycrystalline alumina was examined at temperatures ranging from ambient through 1400°C, using three-point bend bar test specimens. R -curves were determined at all temperatures studied, and when accompanied by renotching procedures, a wake removal technique, conclusive evidence was provided to support the existence of a following wake region in this monolithic ceramic material. The crack closure stresses identified in this region are responsible for all toughening with crack extension observed in this study. Room-temperature " K _IC" fracture toughness values of 4.5 MPa · m^1/2 for the chevron-notch specimen and 3.9 MPa · m^1/2 for the straight-notch configuration were obtained. The critical stress intensity factor of the renotched chevron-notch specimen compared very closely with that of the straight-notch specimen. These findings further confirm the toughening role of the microstructural features found in the following wake region. This paper considers, in detail, these observations in terms of the microstructure and its role in the toughening mechanism.     

Compositional Dependence of Infrared-to-Visible Upconversion in Yb3+- and Er3+-Codoped Germanate, Gallate, and Tellurite Glasses

Upconversion fluorescences of the green ⁴S_3/2→⁴I_15/2 and red ⁴F_9/2→⁴I_15/2 transitions of the Er³⁺ ion are studied for Yb³⁺- and Er³⁺-codoped sodium germanate, potassium tantalum gallate, and barium tellurite glasses by InGaAs laser-diode pumping. The phonon energies of the host glasses are determined by infrared-reflection measurements. Compositional effects on the Judd—;Ofelt parameters for the Er³⁺ ion, the spontaneous emission probability (SPE) of the ²F_5/2→²F_7/2 transition for the Yb³⁺ ion, and the phonon energy of the glass network are discussed in terms of glass structure. The factors that affect the upconversion fluorescence intensities of the Er³⁺ ion are discussed, using the phonon energy of the host glass and the SPE for the Yb³⁺ ion in the germanate, gallate, and tellurite glasses.     

Relation Between Strength, Fracture Energy, and Microstructure of Hot-Pressed Si3N4

A fracture mechanics approach was used to investigate the high strength of hot-pressed Si₃N₄. Room-temperature flexural strengths, fracture energies, and elastic moduli were determined for material fabricated from α- and β-phase Si₃N₄ powders. When the proper powder preparation technique was used, α-phase powder resulted in a high fracture energy (69,000 ergs/cm²), a high flexural strength (95,000 psi), and an elongated (fiberlike) grain morphology, whereas β-phase powder produced a low fracture energy (16,000 ergs/cm²), a relatively low strength (55,000 psi), and an equiaxed grain morphology. It was hypothesized that the high strength of Si₃N₄ hot-pressed from α-phase powder results from its high fracture energy, which is attributed to the elongated grains. High-strength Si₃N₄ has directional properties caused, in part, by the elongated grain structure, which is oriented preferentially with respect to the hot-pressing direction.     

Fracto-emission during Fracture of Engineering Ceramics

An experimental study of the emission of fracto-electrons from engineering ceramics, such as Al₂O₃, ZrO₂, Al₂O₃/ZrO₂, and Si₃N₄, has been made under ambient atmospheric conditions when they were fractured with an indenter mounted on the arm of a Charpy impact machine or by a power press machine. In these experiments, the indenter, made of a hard electro-conductive material, acted as an electrode to collect the emitted electrons; the collected electrons were fed into a high-sensitivity charge amplifier. The results obtained showed that, when the test specimen fractured, fracto-electrons were emitted. This was confirmed by the simultaneous appearance of the electrical and acoustical signals. The total charge collected varied in the range of 10⁻¹¹–10⁻⁹ C, depending on the fracture mode, fracture load, and the nature of the gaseous environment. The fracture surfaces of the specimens were found to be electrified, reaching potentials of the order of a few hundred volts. We believe that the origin of the fracto-emission of electrons is the separation of the charges on the fracturing surfaces, which leads to the formation of an electric field and the liberation of exoelectrons; these electrons are of sufficiently high energies to cause the ionization of the ambient gaseous atoms/molecules.     

Fracture Toughness and Spalling Behavior of High-Al2O3 Refractories

The fracture energies and spalling resistance of high-Al₂O₃ refractories were studied. The fracture energies, γ _WOF and γ _NBT , were measured by the work-of-fracture and the notched-beam-test methods, respectively. Spalling resistance, as measured by the relative strength retained in a water quench, correlated well with the thermal-stress resistance parameter applicable to stable crack propagation under conditions of thermal shock, (γ _WOF /α² E ₀). Many of the refractories exhibited high ratios of γ_WOF to γ_NBT; such high ratios were shown analytically to maximize the parameter ( R ¹¹¹¹= E _0γWOF/S₁²) which describes the resistance to catastrophic spalling. The increase of crack length with increasing quenching temperature difference (Δ T ) was somewhat less than that predicted theoretically; the discrepancy was attributed to an increase of crack density with Δ T . In general, the results show that fracture energy is important in establishing the spalling resistance of high-Al₂O₃ refractories.     

Site-Dependent Study of the Optical Properties of Eu3+ in Pure and Chlorine-Doped Fluorozirconate Glasses

For the present study, the emission spectra and lifetimes of Eu³⁺ in fluorozirconate and fluorochlorozirconate glasses were measured site dependently using the fluorescence line narrowing (FLN) technique. At high excitation energies, two different types of Eu³⁺ sites can be distinguished in fluorozirconate glass. The introduction of chloride into this glass caused a significant effect at low excitation energy.     

Fracture of Glass in Vacuum

The fracture of 6 glasses was studied in vacuum, <10⁻⁴ torr (10⁻² N/m²), as a function of temperature from 25° to 775°C. Subcritical crack growth was observed in 4 of the glasses. Activation energies for crack motion ranged from 60 to 176 kcal/mol. The glasses which did not exhibit slow crack growth were "anomalous" glasses with abnormal thermal and elastic properties. Critical stress intensity factors for these 2 glasses increased ∼10% as the temperature increased to ∼600°C. It is felt that subcritical crack growth is not the result of alkali-ion diffusion or viscous flow but rather of a thermally activated growth process which depends on the crack-tip structure in the glass. A narrow cohesive region at the crack tip favors subcritical crack growth, whereas a wide region favors abrupt fracture.     

Design and Fracture of Layered Al2O3/TZ3Y Composites Produced by Electrophoretic Deposition

Alumina/yttria-stablized tetragonal zirconia (Al₂O₃/TZ3Y) multilayer composites with strong interfaces and containing residual stresses were produced by electrophoretic deposition. As-synthesized and Vickers-indented samples with different layering designs have been tested in bending (up to 1300°C) to experimentally define conditions for crack deflection and flaw tolerance. The compressive residual stress in the Al₂O₃ layers (ς_r) is a function of layer thickness ( t ). It was found that the parameter ς_r² t is an effective indicator of the fracture behavior, as predicted by strain energy release calculations. With decreasing ς_r² t , the fracture followed a sequence from spontaneous delamination, multistage fracture with extensive crack deflection, to catastrophic failure with, and finally without, deflection steps. Decrease of ς_r with increasing test temperature causes changes in fracture behavior which correspond to the room-temperature transitions of ς_r² t .     

Thermodilatometric Characterization for Devitrification of a Micaceous Dental Glass-Ceramic

The devitrification of an uncerammed micaceous glass-ceramic used for dental applications was studied by thermodilatometry and compared with the kinetics of mica crystallization studied by differential thermal analysis. The thermodilatometry plots and their derivative plots revealed thermally impeded processes, namely, structural relaxation, glass softening and nucleation, and crystallization, and they were characterized by glass-transition temperature, glass-softening temperatures, and crystallization temperature. The heating-rate dependence for these characteristic temperatures was used to determine the activation energy for structural relaxation of 338 kJ·mol^-1, the activation energy for viscous flow of 276 kJ·mol^-1, and the apparent activation energy for crystallization of 286 kJ·mol^-1 by one model and 342 kJ·mol^-1 by another model. The similar magnitudes for these activation energies suggested the mechanisms for different thermal processes involved analogous molecular motions. Furthermore, the activation energy for crystallization of the micaceous phase of the glass-ceramic could be estimated from thermodilatometry plots, because it was comparable in magnitude to that obtained from the widely used nonisothermal differential thermal analysis method. Finally, the dilatometry and scanning electron microscopy studies strengthened the earlier opinion that the devitrification of the base glass is a single-step process, that is, without occurrence of an intermediate phase before the formation of the final crystalline phase.     

Fracture Toughness of a Beta-SiC Whisker

Tensile fracture from an internal defect allowed measurement of the fracture toughness, K_Ic³ and fracture surface energy, y, for a single-crystal beta-SiC whisker with (111) crystallographic orientation in the tensile direction. Values of K_Ic=3.23 MPa.m^1/2 and γ=9.06 J/m² were obtained.     

Eletrical Properties of Glasses in the System PbO-Al2O3-B2O3-SiO2

Glasses in the system Pb0–Al₂O₃-B₂O₃-SiO₂ are chemically stable over a wide composition range and have very desirable electrical characteristics such as high electrical resistivities and activation energies for conduction. Variations in these electrical properties were studied as a function of composition changes within the system, the object being to identify the role of the constituent oxides in achieving the highest activation energy and resistivity values consistent with moderate preparation temperatures. Measurements were made in the temperature range 25° to 400°C on carefully prepared glass disks in which the individual oxide components or different oxide ratios such as PbO/SiO₂, Al₂O₃/SiO₂, and B_sO₃/SiO₁ were systematically varied. The activation energy and resistivity values obtained ranged from 1.2 to 1.6 ev and 10° to 10¹⁴ ohm-cm, with dielectric constants ranging from 9 to 19 and densities from 4.30 to 4.50 g/cmY. Indications were that, for the composition range studied, the behavior manifested was basically that of the binary PbO-SO₂ glass with additions of Al₂O₃ or B₂O₃, even in small concentrations, sharply increasing the activation energy for conduction while lowering the density.     

Linear and Nonlinear Optical Properties of TeO2 Glass

A thin plate of TeO₂ glass of 5.0 × 4.0 × 0.25 mm³ size, which was large enough for various optical measurements, was obtained by a rapid quenching method. The linear refractive index was measured as a function of wavelength from 486.1 to 1000 nm. The refractive index at 486.1 nm was as high as 2.239. The optical energy band gap was estimated as 3.37 eV from the optical absorption spectrum. The third-order nonlinear optical susceptibility, χ⁽³⁾, was determined by the third-harmonic generation (THG) method. The χ⁽³⁾ value was as high as 1.4 × 10⁻¹² esu, about 50 times as large as that of SiO₂ glass. The results are discussed based on Lines' model in which an influence of cationic empty d -orbitals on the nonlinear properties was taken into account.     

Oxidation States of Chromium Dissolved in Glass Determined by X-ray Photoelectron Spectroscopy

The oxidation states of chromium dissolved in binary sodium silicate glasses were analyzed by X-ray photoelectron spectroscopy (XPS). The measured "equilibrium" Cr⁶⁺ fractions in glasses with varying Na₂O contents are shown to be in good agreement with results obtained by other techniques reported in the literature. XPS was then used to analyze the interface of a sodium silicate glass/chromium alloy (Inconel 718) seal. It is shown that Cr²⁺ and Cr³⁺ were the major chromium species dissolved in the glass interfacial region.     

Porous Alumina Coating with Tailored Fracture Resistance for Alumina Composites

A porous Al₂O₃ coating for Al₂O₃ composites was prepared by aerosol-spray deposition of submicrometer-sized Al₂O₃ powder. A model composite specimen was hot-pressed to change the coating's porosity and, thereby, change the interphase fracture resistance. The mixed-mode fracture resistance of the interphase ranged from 0.5 to 14.8 J/m². The interphase fracture was characterized using electron and acoustic microscopy. Finite-element analysis (FEA) showed that the testing method possessed a short transient behavior and was immune to asymmetrical cracks. This approach provided a fundamental investigation of the relationships among interphase microstructure, processing, and fracture resistance. The results also provided a detailed test of the He-Hutchinson criterion for crack deflection.     

Mirror Constant for Tyranno® Silicon-Titanium-Carbon-Oxygen Fibers Measured in Situ in a Three-Dimensional Woven Silicon Carbide/Silicon Carbide Composite

The strength, S , of ceramic and glass fibers often can be estimated from fractographic investigation using the fracture mirror radius, r _m, and the relationship S = A _m/( r _m)^1/2, where A _mis the "mirror constant." The present work estimates the value of A _mfor Tyranno^® Si-Ti-C-O fibers in situ in a three-dimensional woven SiC/SiC-based composite to be 2.50 ± 0.09 MPa·m^1/2. This value is within the range of 2–2.51 MPa·m^1/2 previously obtained for nominally similar Nicalon^® Si-C-O fibers.