Fracture Resistance of Residually-Stressed Ceramic Laminated Structures

We have studied the effect of residual stresses on fracture resistance and crack arrest behavior of asymmetric ceramic laminated Si ₃ N ₄/Si ₃ N ₄–TiN structures. Using the compliance method, we assessed the technique of R-curve construction for laminar composites. For laminar structures with layers varying by their elastic characteristics we developed an analytical method for calculation of fracture resistance – crack length dependence. The method applicability is verified by calculation of stress intensity factors for laminar specimens with an edge crack. The calculated results are compared to the experimental data.     

Micro/nanoscale mechanical characterization and in situ observation of cracking of laminated Si3N4/BN composites

Micro/nanoscale mechanical characterization of laminated Si₃N₄/BN composites was carried out by nanoindentation techniques. A custom-designed micro mechanical tester was integrated with an optical microscope and an atomic force microscope to perform in situ three-point bending tests on notched Si₃N₄/BN composite bend specimens where the crack initiation and propagation were imaged simultaneously with the optical microscope and atomic force microscope during bending loading. The whole fracture process was in situ captured. It was found that crack deflection was initiated/induced by the pre-existing microvoids and microcracks in BN interfacial layers. New fracture mechanisms were proposed to provide guidelines for the design of biomimetic nacre-like composites.     

Fracture toughness of Si3N4 measured with short bar chevron-notched specimens

The short bar chevron-notched specimen was used to measure the plane strain fracture toughness of hot-pressed Si₃N₄. Specimen proportions and chevron-notch angle were varied, thereby varying the amount of crack extension to maximum load (upon which K_ic was based). The measured toughness (4.68 ± 0.19 MN m^3/2) was independent of these variations, inferring that the material has a flat crack growth resistance curve.Nomenclature a crack length - a _A crack length at arrest of unstable crack advance - a ₁ length of chevron notch at specimen surface (distance from line of load application to point of chevron emergence at specimen surface) - a ₀ initial crack length (distance from line of load application to tip of chevron) - a _R crack length at ending of stable crack extension (conversely, crack length at onset of abrupt, unstable crack advance) - B specimen thickness - H specimen half-height - K _1A stress intensity factor at arrest of unstable crack advance - K _IR stress intensity factor at end of stable crack extension (crack growth resistance) - K _IC plane strain fracture toughness - P _max maximum applied load in fracture toughness test - W specimen width - Y ^* dimensionless stress intensity factor coefficient for chevron-notched specimen - Y ^* _m minimum value ofY ^* as a function of - a/W - ₀ a ₀/W - ₁ a ₁/W     

Effects of pores on shear bands in metallic glasses: A molecular dynamics study

The effects of nanoscale pores on the strength and ductility of porous Cu₄₆Zr₅₄ metallic glasses during nanoindentation and uniaxial compression tests are modelled and investigated using molecular dynamics (MD) simulations. In the MD simulations, atomistic amorphous samples were digitally prepared through fast quenching from the liquid states of copper and zirconium alloy. In both of the nanoindentation and uniaxial compression simulations, shear transformation zones and shear bands are observed through the local deviatoric shear strains in the samples. The results show that the existence of pores causes strain concentrations and greatly promotes the initialization and propagation of shear bands. Importantly, only pores reaching critical size can effectively facilitate the formation of multiple shear bands. It is also observed that hardening occurs through pore annihilation and the shear band stops in porous metallic glasses.     

Microstructural modification to improve mechanical properties of a 70%Si3N4-30%BAS self-reinforced ceramic composite

Different microstructures of the 70%Si₃N₄-30%BAS self-reinforced composite, fine, coarse and bimodal, are obtained by pressureless sintering at 1920°C. Flexural strength, fracture toughness and crack-growth resistance (R-Curve) behavior of each microstructure are characterized by three-point bending, indentation and modified compact tension methods respectively, at room temperature. The crack deflection, whisker bridging and pullout are considered as major toughening mechanisms in the composite. It is found that coarsening -Si₃N₄ whiskers of this composite can improve the toughness/fracture resistance but deteriorate the strength. When limited large abnormally grown whiskers are introduced into the microstructure, the composite shows an improved toughness/fracture resistance behavior and concurrently sustains a high strength.     

Fracture energy of Si3N4

The fracture energy of Si₃N₄ made by hot pressing, reaction sintering, and chemical vapour deposition (CVD) was studied. Extrapolation of fracture energies to zero additive or porosity levels, as well as analysis of CVD Si₃N₄ all indicate an intrinsic fracture energy of 20–30J m^–2. Higher fracture energies in dense bodies with increasing additive content, or in some more porous bodies (relative to expected porosity dependence) are associated with crack branching. In dense bodies such branching may arise due to micro-cracking from combined effects of crack tip stresses and mismatch stresses due to differences in properties, especially thermal expansion, between Si₃N₄ and the additive or its reaction products. In porous bodies such branching appears to be due to spatial distribution of pores.     

Effects of rate,temperature and absorption of organic solvents on the fracture of plain and glass-filled polystyrene

The rate/temperature dependent fracture behaviour of plain and glass-filled polystyrene has been investigated over the crack speed (a) range of 10^–6 to 10^–2 m sec^–1 and in the temperature (T) range of 296 to 363 K. TheK _c (a, T) relationships obtained, whereK _c is the stress intensity factor at fracture, are shown to follow those given by the Williams/Marshall relaxation crack growth model and the toughness-biased rate theory. Crack propagation in both materials is shown to be controlled by a-relaxation molecular process associated with crazing. Crack instabilities observed in plain polystyrene are analysed successfully in terms of isothermal-adiabatic transitions at the crack tip. Fracture initiation experiments are also conducted in which the effects of organic liquids on the fracture resistances of both plain/glass-filled polystyrene have been determined. Good correlations betweenK _i ² (K _i being the crack initiation stress intensity factor) and _s, solvent solubility parameter, of various liquid environments have been obtained, which give a minimumK _i ² value at _{s p}, where _p is the solubility parameter of the polymer. For a given temperature, liquid environment and crack speed, the glass-filled polystyrene is shown to possess greater resistances to crack propagation than plain polystyrene.     

Nanoscale fracture analysis by atomic force microscopy of EPDM rubber due to high-pressure hydrogen decompression

The relationship between internal fracture due to high-pressure hydrogen decompression and microstructure of ethylene–propylene–diene–methylene linkage (EPDM) rubber was investigated by atomic force microscopy (AFM). Nanoscale line-like structures were observed in an unexposed specimen, and their number and length increased with hydrogen exposure. This result implies that the structure of the unfilled EPDM rubber is inhomogeneous at a nanoscale level, and nanoscale fracture caused by the bubbles that are formed from dissolved hydrogen molecules after decompression occurs even though no cracks are observed by optical microscopy. Since this nanoscale fracture occurred at a threshold tearing energy lower than that obtained from static crack growth tests of macroscopic cracks (T _s,th), it is supposed that nanoscale structures that fractured at a lower threshold tearing energy (T _nano,th) than T _s,th existed in the rubber matrix, and these low-strength structures were the origin of the nanoscale fracture. From these results, it is inferred that the fracture of the EPDM rubber by high-pressure hydrogen decompression consists of two fracture processes that differ in terms of size scale, i.e., bubble formation at a submicrometer level and crack initiation at a micrometer level. The hydrogen pressures at bubble formation and crack initiation were also estimated by assuming two threshold tearing energies, T _nano,th for the bubble formation and T _s,th for the crack initiation, in terms of fracture mechanics. As a result, the experimental hydrogen pressures were successfully estimated.     

Fracture surface analysis of ceramics

Flaw size, c, fracture mirror boundaries, r, fracture stress, , and critical fracture energy were measured for glasses, glass ceramics, and single and polycrystalline ceramics. The relationship r ^1/2 = constant was verified for all these materials. The mirror constants, A, in these materials were shown to be directly proportional to the average critical stress intensity factor for crack propagation, K _IC. Based on the A — K _IC relationship, the outer mirror to flaw size ratio is shown to scatter about a value of 131. Thus, the mirror constants were used to predict critical flaw sizes in these materials. The observed flaw sizes in most cases correlated well with those calculated. The cases in which poorer correlation was obtained are those in which flaw sizes were smaller than the grain size, flaws were pores or surrounded by porous regions, or where severe microcracking existed. It is shown that the elastic modulus is proportional to the mirror constant and probably to the critical fracture energy, but that the latter is highly dependent on local microstructure. The smaller inner to outer mirror ratios for polycrystalline ceramics over glasses is attributed to the difference in available paths for crack propagation.     

Influence of dissolution rate on crack growth and fatigue of Na2O-Al2O3-B2O3 SiO2 glasses

Fracture toughness, macroscopic crack growth and dynamic fatigue of glasses of the system 15Na₂O-4Al₂O₃-xB₂O₃-(81-x)SiO₂ are studied. The fracture toughness as a function of B₂O₃ content correlates to the dependence of elastic modulus which has a maximum between 20 and 30 mol%. The shape of the crack growth curve changes characteristically. Region 1 of the curves normalized toK _Ic is shifted to higher crack growth velocities (smallerK _I/K _Ic values, respectively) for increasing B203 content. The rise of velocity correlates approximately to the dissolution rate of the glasses in water. The determination of the slope in region 1 is problematic, particularly for poorly resistant glasses. The slopen of the crack growth velocity fitted by the power law decreases above 20 mol% B₂O₃ in correspondence with the results of the dynamic fatigue (23 >n > 7). A clear fatigue limit at one fifth of the inert strength occurs in glasses with B₂O₃ content above 20 mol%.     

Strength and fracture surface energy of phase-separated glasses

Flexural strength and fracture surface energy were determined for lead borate glasses whose compositions lie in the immiscible region of the PbO-B₂O₃ system. The microstructural characterization indicated that the glasses are typical particulate composites which consist of two immiscible phases. For the glasses whose microstructure consists of PbO-rich particles/B₂O₃-rich matrix (B₂O₃-rich side of the miscibility gap), the fracture surface energy was found to decrease with increasing second-phase particles. To explain this behaviour, a crack propagation model in a brittle composite containing penetrable particles was proposed. On the other hand, for the glasses whose microstructure consists of B₂O₃-rich particles/PbO-rich matrix (PbO-rich side of the miscibility gap), an increase in fracture surface energy with volume fraction of dispersed particles was observed. This phenomenon could be best explained by Lange-Evans theory of fracture in brittle composites containing impenetrable particles. It was concluded that, when the critical crack size in a non-dispersed host glass is much larger than the particle size, the crack size in particulate composites is invariant with microstructure and also that the variation of strength results entirely from the variation of fracture toughness.     

Electrical properties and infrared spectra of TeO2-Fe2O3 glasses

The d.c. and a.c. electrical properties on TeO₂-Fe₂O₃ glasses with various compositions of PbO, B₂O₃ and SiO₂ were studied over a frequency range of 10²–10⁵ Hz and in the temperature range 300–500 K. The a.c. conductivity is proportional tow ⁿ, and the conduction mechanism is due to an electronic hopping process. The effects of composition and temperature on the dielectric constant and loss factor (tan ) were studied. The infrared absorption spectra of these glasses reveal that the addition of PbO, B₂O₃ and SiO₂ of these glasses does not introduce any new absorption band in the infrared spectrum of TeO₂-Fe₂O₃ glasses. These results prove the distribution of TeO₄ polyhedra which determines the network and basic oscillation of the building units in the tellurite glasses.     

A comparative study of “gels” and oxide mixtures as starting materials for the nucleation and crystallization of silicate glasses

Nucleation and crystallization behaviour of glasses in SiO₂-La₂O₃, SiO₂-La₂O₃-Al₂O₃ and SiO₂-La₂O₃-ZrO₂ systems were investigated using glasses prepared by the fusion of gels and the mixtures of oxides in solar/image furnace. Two methods for the preparation of multicomponent homogeneous non-crystalline products in the form of gels were developed. The phase separation, devitrification and micro-hardness of the above glasses were investigated in relation to the starting materials and the composition. The results show that the glasses made from gels are more homogeneous than those made from oxide mixtures. The phase separation characteristics of glasses made from gels are markedly different from those of glasses made from a mixture of oxides. The addition of Al₂O₃ to the binary SiO₂-La₂O₃ glasses improves the homogeneity but reduces the micro-hardness and the devitrification tendency, whereas the addition of ZrO₂ causes a considerable increase in micro-hardness and enhances the devitrification. The rates of nucleation and crystallization of glasses of different compositions made from gels are much higher than those made from the mixture of oxides. The formation of the high temperature crystal form, (-La₂Si₂O₇ is more evident with the crystallization of gel-glasses. When the rate of nucleation is low, (in the case of glasses from the mixture of oxides), the curve representing the relation between the micro-hardness and the time of heat-treatment shows a distinct minimum, whereas this minimum is not obtained with the gel-glasses. With most of the gel-glasses, the micro-hardness rises very sharply with the length of heat-treatment. The curve showing the relation between the micro-hardness and the volume fraction of the dispersed crystalline phase also gives a distinct minimum which can be explained on the basis of the fracture mechanism consisting of the processes of crack nucleation and of crack propagation around the dispersed crystalline particles.     

Correlation between the structure and glass transition temperature of potassium,magnesium and barium tellurite glasses

The Mössbauer spectrum of tellurite glasses, containing 5 mol% Fe₂O₃ as a probe, consists of a paramagnetic quadrupole doublet with an isomer shift of 0.39 ± 0.01 mmsec^–1. This indicates that Fe³⁺ ions are present at substitutional sites of Te⁴⁺ ions constituting distorted TeO₄ trigonal bipyramids, each of which has one oxygen vacancy at an equatorial site. On increasing the K₂O content from O to 35 mol%, the quadrupole splitting () for potassium tellurite glasses decreases continuously from 0.76 to 0.44 mm sec^–1. On the other hand, for magnesium and barium tellurite glasses increases with increasing MgO and BaO content, respectively. When the alkali or alkaline earth oxide contents are the same as each other, increases in proportion to the ionic potential (Z/r) of the alkali or alkaline earth metal ion. These results suggest that the glass matrices of alkaliv and alkaline earth tellurite glasses are continuously changed into a chain and a three-dimensional network structure, respectively. Differential thermal analysis studies reveal that there exists a linear relationship between the glass transition temperatureT _g and the quadrupole splitting, indicating thatT _g is primarily determined by the magnitude of the distortion of TeO₄ trigonal bipyramids. This relationship is also applicable to several oxide glasses.     

Analysis of Si3N4+β-Si3N4 whisker ceramics

Dense Si₃N₄+-Si₃N₄ whisker composite ceramics were fabricated by hot pressing powder-whisker mixtures. Addition of -Si₃N₄ whiskers had no significant influence on the densification behaviour for up to 20 wt% addition. Light microscopy and scanning and transmission electron microscopy were used to study their microstructure and fracture behaviour. An increase in fracture toughness was observed for -Si₃N₄ whisker additions of up to 10 %. The main toughening mechanisms observed were crack deflection, crack branching, whisker-matrix debonding and whisker pull-out.     

Nb添加对Ti基非晶合金腐蚀及力学性能的影响

为研究微量添加Nb元素对Ti_(40)Zr_(10)Cu_(34-x)Pd_(14)Sn_2Nb_x(x为原子数分数,x=0、1%、3%、5%)非晶合金的耐腐蚀性能及力学性能的影响,本文利用动态极化曲线,分析了在0.144 mol/L的NaCl溶液及0.2 mol/L的PBS溶液中非晶合金Ti_(40)Zr_(10)Cu_(34-x)Pd_(14)Sn_2Nb_x(x=0、1%、3%、5%)的电化学性能,并通过材料拉伸试验研究了非晶合金块状样品的室温压缩性能.结果表明:在0.144 mol/L的NaCl溶液中,非晶合金样品在阳极区出现了自发钝化的特征,钝化电流密度在10~(-7)～10~(-8) A/cm~2,钝化电流密度随着Nb的添加略有降低,且点蚀电位随Nb原子数分数的增加分别为200、340、400和490 mV,说明微量添加Nb元素能有效提高Ti基非晶合金的耐点蚀能力,即在0.144 mol/L的NaCl溶液中Ti基非晶合金的耐腐蚀性随着Nb含量的增加而增强;在0.2 mol/L的PBS溶液中,因磷酸根离子的缓蚀作用,Nb添加导致的成分变化对非晶合金的腐蚀行为影响不大;此外,添加了原子数分数为1%及3%Nb的非晶合金,其压缩强度及塑性变形能力变化不大,但添加5%Nb的非晶合金因较大体积分数纳米晶的存在导致其室温断裂强度及塑性变形能力有明显下降.     

FATIGUE CRACK GROWTH IN SEVERAL CERAMIC MATERIALS

Fatigue crack growth tests of five ceramic materials were carried out in order to investigate the general characteristics of cyclic and static fatigue crack growth in ceramics. A cyclic effect was found in Si₃N₄, Al₂O₃ and TiB₂, where the main fracture mechanism was intergranular separation, and stress shielding due to grain bridging occurred near the crack tip. On the other hand, no cyclic effect was observed in Sic and ZrO₂, where the main fracture mechanism was transgranular fracture. The fatigue crack growth curves reduced to a unique curve by using the parameter K_up/E, regardless of the kind of ceramics studied.     

Effect of water vapor on the mechanical behaviors of hot isostatically pressed silicon nitride containing Y2O3

The effect of water vapor on the mechanical behavior of Si₃N₄ ceramics was studied. Strength measurements by flexural dynamic fatigue tests were made at temperatures from 1038°C to 1350°C and at actuator speeds of 8.4 × 10^–2 and 8.4 × 10^–5 mm/s (200 and 0.2 MPa/s). Step stress rupture tests were also performed at 1288°C and 1150°C. Water vapor had a beneficial effect on the flexural strength due to flaw healing, and/or blunting mechanisms. Dynamic fatigue results demonstrated that the beneficial effects of water vapor on the strength increases as temperature increases and/or loading rate decreases. Time-to-failure was always longer in wet air during step stress rupture testing. Creep crack growth by formation and coalescence of cavities ahead of the crack tip generated from the oxidation pits or subsurface pores were the primary mechanism for slow crack growth for NT 164 Si₃N₄.     

Low-Temperature Internal Friction and Thermal Conductivity of Plastically Deformed, High-Purity Monocrystalline Niobium

The low-temperature internal friction Q ^–1 and thermal conductivity of plastically deformed, high-purity niobium monocrystals have been investigated and compared with measurements on an amorphous SiO₂ (a-SiO₂) specimen. After plastic deformation at intermediate temperatures, an approximately temperature independent internal friction Q ^–1 was observed with a magnitude comparable to that of the a-SiO₂ specimen. Plastic deformation at low temperatures leads to an internal friction Q ^–1 with a considerably smaller magnitude. In the temperature range between about 0.3 and 1.5K, the lattice thermal conductivity k of the deformed specimens decreases with increasing deformation. It is, however, nearly independent of the amount of deformation at the lowest temperatures investigated. In this temperature regime, the lattice thermal conductivity of the specimens varies proportional to T ³ and has a magnitude as would be expected for an undeformed sample. Additional heat release experiments on an undeformed sample clearly show no long-time energy relaxation effects. We conclude that the defects introduced by plastic deformation cannot be described with the tunneling model which had been proposed to describe the low temperature elastic and thermal properties of amorphous solids. The phonon scattering mechanisms observed in deformed niobium are tentatively related to the dynamic interaction of phonons with geometrical kinks in dislocations.     

Non-adiabatic polaron hopping conduction in semiconducting V2O5-Bi2O3 oxide glasses doped with BaTiO3

BaTiO₃-doped (5–40 wt %) 90V₂O₅-10Bi₂O₃ (VB) glasses have been prepared by a quick quenching technique. The d.c. electrical conductivities, _d.c., of these glasses have been reported in the temperature range 80–450 K. The electrical conductivity of these glasses, which arises due to the presence of V⁴⁺ and V⁵⁺ ions, has been analysed in the light of the small-polaron hopping conduction mechanism. The adiabatic hopping conduction valid for the undoped VB glasses (with 80–95 mol % V₂O₅), in the high-temperature region, is changed to a non-adiabatic hopping mechanism in the BaTiO₃-doped VB glasses. At lower temperatures, however, a variable range hopping (VRH) mechanism dominates the conduction mechanism in both the glass systems. Such a change-over from adiabatic to non-adiabatic conduction mechanism is a new feature in transition metal oxide glasses. Various parameters, such as density of states at the Fermi level N(E_F), electron wave-function decay constant, , polaron radius, r _p, and its effective mass, m _p ^* , etc., have been obtained for all the glass samples from a critical analysis of the electrical conductivity data satisfying the theory of polaron hopping conduction.