Mixed-Mode Fracture in Soda-Lime Glass

Soda-lime-silica glass was fractured under combined mode I and mode II loading from flaws produced by hardness indentations. Critical stress intensities calculated from K₁ and K₁₁ combined using four analyses, are compared to K_IC measured by a fracture mechanics technique and to values of K_IC determined by measuring fracture-mirror size. The comparison showed that K_IC calculated using a noncoplanar strain-energy release-rate analysis gave the best agreement with values obtained by fracture mechanics techniques over the widest range of crack orientations. K_ICcalculated from mirror sizes was constant regardless of the orientation of the original flaw.     

Controlled Surface Flaws in Hot-Pressed Si3N4

Surface flaws of controlled size and shape were produced in high-strength hot-pressed Si₃N₄ with a Knoop microhardness indenter. Fracture was initiated at a single suitably oriented flaw on the tensile surface of a 4-point-bend specimen, with attendant reduction in the measured magnitude and scatter of the fracture strength. The stress required to propagate the controlled flaw was used to calculate the critical stress-intensity factor, K _IC, from standard fracture-mechanics formulas for semielliptical surface flaws in bending. After the bend specimen had been annealed, the room-temperature K _IC values for HS-130 Si₃N₄ increased to a level consistent with values obtained from conventional fracture-mechanics tests. It was postulated that annealing reduces the residual stresses produced by the microhardness indentation. The presence of residual stresses may account for the low K _IC, values. Elevated-temperature K_IC values for HS-130 Si₃N₄ were consistent with double-torsion data. Controlled flaws in HS-130 Si₃N₄ exhibited slow crack growth at high temperatures.     

Effect of Subcritical Crack Growth on Fracture Toughness and Work-of-Fracture Tests Using Chevron-Notched Specimens

A correlation between the plane strain stress intensity factor K_I , load, and crack extension has been analyzed for constant displacement and constant loading rate experiments, using chevron-notched, four-point-bend specimens. It is assumed that at the beginning of the experiment the chevron triangle tip is not ideally sharp. As loading continues, the crack initially moves with velocity v_t at K_I equal to a threshold value K_t . Maximum crack velocity is reached at K_I= K_IC , the fracture toughness. Depending on the type of material tested, a specific displacement or loading rate must be used to correlate the maximum load with K_Ic . An error in K_IC calculation is estimated if different displacement rates are applied. Repeated loading-unloading work-of-fracture (WOF) experiments generate values related to the resistance of the material to fracture initiation, K_t , only when the crack length approaches 100% of the specimen width. Values related to material's fracture toughness, K_IC are not generated in WOF tests.     

Effect of Surface Flaw Size on Fracture Strength of Alumina Ceramics

Semielliptical surface flaws of different sizes were introduced into Al₂O₃ by Knoop microhardness indentation. The specimens were fractured by four-point bending and the profiles of the indentation flaws were determined by observing the fracture surfaces with a scanning electron microscope. The relation between the indentation flaw size and the fracture strength could be well explained by applying the fracture-mechanics analysis for semielliptical surface flaw in bending. The calculated values of the as-indented critical stress intensity factor, K_IC, were lower than previously reported presumably because of the influence of the residual stresses produced by the indenter.     

Mixed-Mode Fracture from Controlled Surface Flaws in Hot-Pressed Si3N4

The room-temperature mixed-mode fracture of commercial hot-pressed Si₃N₄ was examined using controlled surface flaws in 4-point bending, oriented at various angles 6 with respect to the outer fiber tensile stress direction. Catastrophic fracture paths were non-coplanar with the initial flaw plane, and the stress intensity factor ratio K_I/K_IC was < 1 for fracture in modes II and III. A non-coplanar maximum strain-energy release rate fracture criterion best described mixed-mode fracture.     

Effects of Composition and Environment on the Fracture of Fluoride Glasses

Indentation-crack-length procedures were used to determine the critical fracture toughness (K_IC) and the sensitivity to environmentally enhanced crack growth in heavy-metal fluoride glasses of varying chemical composition. The data show that while K_IC was more or less invariant with composition, some glasses were more susceptible to subcritical crack growth than others. The results are interpreted in terms of existing crack-growth models. A technique for predicting K_IC from fundamental atomic bonding and crystal-structure data for these glasses is discussed.     

Effect of Loading Rate on Fracture Toughness of SiC at High Temperatures

Loading rate sensitivity effects at high temperatures are brought about by a viscous metallic Si phase within an SiC mircostructure. The cross-head speeds were 0.024 and 1260 mm/min. At 0.024 mm/min and 1200°C, K_IC increases up to ∼12 MN/m^3/2, whereas at 1260 mm/min, K_IC remains nearly unaffected, comparable with the room temperature value of ∼4 MN/m^3/2. The effect contradicts the normal behavior where K_IC increases with increasing cross-head speed. Fracture toughness data should therefore be determined as a Junction of loading rate, to account for viscous reactions at certain temperatures.     

Controlled Surface Flaw-Initiated Fracture in Reaction-Densified Silicon Carbide

Controlled surface flaws were produced in commercial reaction-densified SiC by Knoop microhardness indentation. The flaws themselves could not be observed easily, thus an etching technique was used to delineate their semielliptical shape, thereby enabling calculation of the critical stress- intensity factor K _IC at room temperature. Room-temperature fracture was insensitive to annealing environment (air or vacuum), flaw "healing" being observed at ≫1000°C. The variation in fracture stress of indented specimens with temperature showed 3 distinct regions of behavior which were interpreted in terms of residual stress relief, flaw healing, and Si-SiC bond weakening.     

Temperature Dependence of KIC for High-Pressure Hot-Pressed Si3N4 Without Additives

Temperature dependence of K_IC values for Si₃N₄ bodies sintered at high pressures without additives was studied from room temperature to 1400°C. Little change in K_K was found in this range.     

Fabrication and Mechanical Properties of Titanium Boride Ceramics

Dense TiB ceramics (99.6% of theoretical) with a grain size of ∼5 µm have been fabricated by reaction hot pressing of TiB₂ and titanium for 2 h at 1900°C and 28.5 MPa. The TiB ceramics exhibit a fracture toughness ( K_IC ) of 4.5 MPa·m^1/2 and a bending strength (sigma_b) of 360 MPa. Electrical resistivity (rho) is 3.4 × 10^-7 Omega·m at room temperature.     

Valid KIC Determination via In-Test Subcritical Precracking of Chevron-Notched Bend Bars

The initiation of the crack in a chevronnotched bend bar was studied. Ideal testing conditions depend strongly on specimen preparation and material; valid K_IC values do not always result. An in-test precracking technique was developed for ceramics at room and elevated temperatures. A yttria-partially-stabilized zirconia ceramic was tested, and the results showed that ideal testing conditions were obtained, which guaranteed valid K_IC determination.     

Temperature Dependence and Fracture Toughness and Elastic Moduli of a Waste Glass

The temperature dependence of elastic moduli, of the crack lengths formed by Vickers indentations, and of the hardness of the nuclear waste borosilicate glass GP 98/12 has been measured up to 300°C. The temperature dependence of the fracture toughness K_IC exceeds that predicted by the variations of Young's modulus E and hardness H with temperature.     

Joining of Alumina Ceramic by Inducing Localized Reducing Conditions

Alumina ceramics were joined by hot-pressing with polystyrene or mica sheets at temperatures of 1250° or 1350°C. The polystyrene decomposed during hot-pressing, creating reducing conditions in the joints which enhanced interfacial diffusion and bonding. Measured fracture toughness and flexural strength of such joints approached values for monolithic alumina. Joints prepared under similar conditions but without polystyrene showed much lower K_IC and strength values.     

Fracture Stress as Related to Flaw and Fracture Mirror Sizes

Fractographically observed critical-flaw boundaries in strength-tested specimens of 2 polycrystalline ceramics were used in calculating critical stress-intensity factors ( K_IC )- Each ceramic exhibited a K_IC which had little or no dependence on critical-flaw characteristics and which agreed with the value obtained from independent determinations on artificially precracked specimens. Analyses both of fracture-mirror sizes and of water-enhanced subcritical crack growth data supported the evidence that K_1C is a material constant. The fracture-mirror analysis further indicated that the parameter A/K_IC , where A is the fracture-mirror constant, is a dimensionless, material-independent constant.     

Mechanical Behavior of MoSi2 Reinforced–Si3N4Matrix Composites

The mechanical behavior of MoSi₂ reinforced–Si₃N₄ matrix composites was investigated as a function of MoSi₂ phase content, MoSi₂ phase size, and amount of MgO densification aid for the Si₃N₄ phase. Coarse-phase MoSi₂-Si₃N₄ composites exhibited higher room-temperature fracture toughness than fine-phase composites, reaching values >8 MP·am^1/2. Composite fracture toughness levels increased at elevated temperature. Fine-phase composites were stronger and more creep resistant than coarse phase composites. Room-temperature strengths >1000 MPa and impression creep rates of ∼10⁻⁸ s⁻¹ at 1200°C were observed. Increased MgO levels generally were deleterious to MoSi₂-Si₃N₄ mechanical properties. Internal stresses due to MoSi₂ and Si₃N₄ thermal expansion coefficient mismatch appeared to contribute to fracture toughening in MoSi₂-Si₃N₄ composites.     

Structural and Optical Properties of Sol–Gel-Derived Au/BaTiO3 Nanocomposite Thin Films

Au/BaTiO₃ nanocomposite thin films with different Au concentrations were prepared by a sol–gel process. The films were characterized using X-ray diffraction, thermal analyses, X-ray absorption spectroscopy, UV-vis spectroscopy, and transmission electron microscopy. The effects of Au concentration and annealing temperature on the structural and optical properties of composite films were investigated. Gold doping lowered the crystallization temperature of as-synthesized amorphous BaTiO₃ and enhanced its crystallinity in post-deposition annealing. The Au–BaTiO₃ interface was also investigated and no alloying occurred between Au and BaTiO₃. The evolution of Au surface plasmon resonance spectra with increasing annealing temperature was observed in the 10 mol% Au/BaTiO₃ thin films. The variations of band-gap energy for Au/BaTiO₃ films were also discussed.     

Crack Branching in Transformation-Toughened Zirconia

Crack propagation and branching were investigated in calcia partially stabilized zirconia aged at 1300°C for various times. The crack-branching radii were measured and used to calculate the apparent stress intensity factors at crack branching ( K₈ ) which increase with increasing aging time. The K_B/K_IC ratios increased with aging time with a maximum of 5.3. This maximum ratio is much greater than the ratios previously observed for ceramics.     

Transformation Toughening in Composites Containing Dicalcium Silicate

By taking into account solid-state Compatibility relations in the system CaO-SiO₂-ZrO₂, a series of dense, tough CaZrO₃-β-Ca₂SiO₄ composites were obtained. The increase in K_IC and σ_f values observed is interpreted in terms of the β-Ca₂SiO₄→γ-Ca₂SiO₄ polymorphic transformation.     

Processing Temperature Effects on Molybdenum Disilicide

A series of MoSi₂ compacts were fabricated at increasing hot-pressing temperatures to achieve different grain sizes. The materials were evaluated by Vickers indentation fracture to determine room-temperature fracture toughness, hardness, and fracture mode. From 1500° to 1800°C, MoSi₂ had a constant 67% transgranular fracture and linearly increasing grain size from 14 to 21 μm. Above 1800°C, the fracture percentage increased rapidly to 97% transgranular at 1920°C (32-μm grain size). Fracture toughness and hardness decreased slightly with increasing temperature. MoSi₂ processed at 1600°C had the highest fracture toughness and hardness values of 3.6 MPa.m^1/2 and 9.9 GPa, respectively. The effects of SiO₂ formation from oxygen impurities in the MoSi₂ starting powders and MoSi₂–Mo₅Si₃ eutectic liquid formation were studied.     

Influence of Interfacial Roughness on Fiber Sliding in Oxide Composites with La-Monazite Interphases

Room-temperature debonding and sliding of fibers coated with La-monazite is assessed using a composite with a polycrystalline alumina matrix and fibers of several different single crystal (mullite and sapphire) and directionally solidified eutectic (Al₂O₃/Y₃Al₅O₁₂ and Al₂O₃/Y-ZrO₂) compositions. These fibers provide a range of residual stresses and interfacial roughnesses. Sliding occurred over a debond crack at the fiber-coating interface when the sliding displacement and surface roughness were relatively small. At large sliding displacements with relatively rough interfaces, the monazite coatings were deformed extensively by fracture, dislocations, and occasional twinning, whereas the fibers were undamaged. Dense, fine-grained areas (10 nm grain size) resembling recrystallized microstructures were also observed in the most heavily deformed regions of the coatings. Frictional heating during sliding is assessed. Potential mechanisms for forming such microstructures at low temperature are discussed, and a parallel is drawn with the known resistance of monazite to radiation damage. The ability of La-monazite to undergo both debonding and plastic deformation relatively easily at low temperatures may enable its use as a composite interface.