Fracture and fracture toughness of stoichiometric MgAl2O4 crystals at room temperature

The fracture toughness and path of stoichiometric spinel (MgAl₂O₄) crystals were determined at 22 °C for key low-index planes by double cantilever beam, as well as fractography of flexure specimens failing from either machining or indentation flaws. These results are compared with other single and polycrystalline MgAl₂O₄ fracture toughness values measured by various techniques, as well as single crystal versus polycrystal results for other materials. Evaluation of experimental and theoretical results shows (1) the fracture toughness of the spinel {110} plane is only a limited amount (e.g. 6%) higher than for the {100} plane (1.2 MPa m^1/2), (2) fractography of machining flaw fracture origins was the most effective source of K _IC results, and (3) caution must be used in applying fracture toughness techniques to single crystals. Cautions include accounting for possible effects of elastic anisotropy (especially for double cantilever beam and probably double torsion tests), the nature of failure-initiating flaws (especially for notch-beam tests), and the frequent lack of symmetric plastic deformation and fracture (especially for indentation techniques).Retired.     

Developments in highly toughened CeO2-Y2O3-ZrO2 ceramic system

A wet-chemical approach has been applied to derive fine powders with various ceria and yttria compositions in the CeO₂-Y₂O₃ ZrO₂ ceramic system by the co-precipitation method. The characteristics of the as-derived powders have been evaluated through differential thermal analysis, thermogravimetric analysis, BET surface-area analysis, and inductively coupled plasma technique. The hardness and fracture toughness of the as-sintered specimens were evaluated by the indentation method. A highly toughened ceramic withK _IC>25 MPa m^1/2 was achieved with the composition 5.5 mol% CeO₂-2 mol% YO_1.5-ZrO₂. The relationship between the mechanical properties and the compositions of stabilizers, CeO₂ and Y₂O₃ is discussed with respect to the degree of tetragonal to monoclinic transformation as well as the grain size of the as-sintered ceramics.     

Mechanical properties and fracture behaviour of ZrO2-Y2O3 ceramics

Stabilized ZrO₂-Y₂O₃ ceramics have been prepared with varying grain sizes and microstructures with the help of different preparation techniques. Bi₂O₃ has been added as a sinter aid to some of the samples. This results in a certain amount of a zirconia-rich second phase. For Bi₂O₃-free samples the fracture toughness (K _lc), and therefore the fracture energy, increases with decreasing grain size. A linear relation with the inverse square root of the average grain size is found. The highest value ofK _lc amounts to 4.1 MPa m^1/2. Fracture toughness values of 1.9±0.2 MPa m^1/2 are measured for Bi₂O₃ containing materials. The fracture surfaces are intergrannular for Bi₂O₃-containing and transgranular for Bi₂O₃-free samples, respectively.     

The fracture behaviour of two Co-Mo-Cr-Si wear resistant alloys (“Tribaloys”)

The room temperature modulus of rupture (MOR) and the plane strain fracture toughness (K _IC) of two Co-Mo-Cr-Si wear resistant alloys in various microstructure conditions have been determined. The modulus of rupture varied from 304 to 927 MN m^–2 and as such was more structure sensitive thanK _IC, which was in the range 15 to 26 MN m^–3/2 for all conditions of the alloys. The best combination of room-temperature properties was associated with an f c c solid solution whereas h c p solid solution with a Widmanstätten precipitate produced the poorest properties. The industrial significance of these results are discussed.     

Mechanical and elastic properties of transparent nanocrystalline TeO2-based glass-ceramics

Mechanical and elastic properties of transparent TeO₂-based glass-ceramics (15K₂O · 15Nb₂O₅ · 70TeO₂) consisting of nanocrystalline particles (each particle size: 40–50 nm) and showing optical second harmonic generation were evaluated by means of usual Vickers indentation and nanoindentation tests. The precursor glass has Vickers hardness H _v of 2.9 GPa, Young's modulus E of 54.7 GPa, the fracture toughness K _c of 0.25 MPam^1/2 and Poisson's ratio of 0.24. The transparent nanocrystalline glass-ceramic heat-treated at 420°C for 1 h has H _v = 3.8 GPa, E = 75.9 GPa and K _c = 0.34 MPam^1/2, and the opaque glass-ceramic heat-treated at 475°C for 1 h has H _v = 4.5 GPa, E = 82.9 GPa and K _c = 0.68 MPam^1/2, demonstrating that poor mechanical and elastic properties of the precursor TeO₂-based glass are improved through sufficient crystallization. The fracture surface energy, brittleness and elastic recoveries (about 44%) after unloading (the maximum load: 30 mN) of transparent nanocrystalline glass-ceramics are almost the same as those of the precursor glass, implying that the interaction among nanocrystalline particles is not so strong.     

Fabrication and properties of SiC fibre-reinforced Li2O·Al2O3·6SiO2 glass-ceramic composites

Ta₂O₅, Nb₂O₅ and TiO₂ were used separately as additives to a Li₂O·Al₂O₃·6SiO₂ glass-ceramic composition, to act as nucleating dopants and to aid the formation of an interfacial carbide layer (TaC and NbC) between the fibre and matrix in SiC fibre uniaxially reinforced glass-ceramic composites, The composites exhibited high modulus of rupture (>800 MPa) and fracture toughness (K _IC > 15 MPam^1/2). The interfacial amorphous carbon rich layer and carbide layer were responsible for lowered interfacial shear strength but permitted high composite fracture toughness. The composite with the TiO₂ additive in the matrix showed a lower flexural strength (<500MPa) and a smaller K _IC (-11 MPam^1/2) which resulted from the high interfacial shear strength between the SiC fibre and the matrix due to the formation of the interfacial TiC layer.     

Evaluation of elastic/mechanical properties of some glasses and nanocrystallized glass by cube resonance and nanoindentation methods

Elastic and mechanical properties of 10La₂O₃·30Bi₂O₃·60B₂O₃ (LaBiB) glass, 15K₂O·15Nb₂O₅·68TeO₂·2MoO₃ (KNbTeMo) glass and a transparent KNbTeMo nanocrystallized (particle size: ∼40 nm) glass were examined using cube resonance and nanoindentation methods. The values of Poisson’s ratio, Young’s modulus (E), Debye temperature (θ_D), fractal bond connectivity, Martens hardness, indentation hardness, indentation Young’s modulus, elastic recovery, Vickers hardness, fracture toughness (K_c) and brittleness for the samples were evaluated, and the relation with the structure and nanocrystallization were clarified. LaBiB glass containing high oxygen-coordinated La³⁺ ions and two-dimensional BO₃ structural units shows excellent properties of E=90.6 GPa, θ_D=404 K and K_c=0.72 MPa m^1/2 and a high resistance against deformation during Vickers indentation. KNbTeMo glass with the three-dimensional network structure and consisting of weak Te-O bonds has small values of E=51.4 GPa and K_c=0.29 MPa m^1/2. It was demonstrated that the elastic and mechanical properties of KNbTeMo precursor glass are largely improved by nanocrystallization, e.g., E=69.7 GPa and K_c=0.32 MPa m^1/2. The nanocrystallization also induces a high resistance against deformation during Vickers indentation.     

Hertzian indentation of thorium dioxide,ThO2

The Hertzian indentation technique was used to study the fracture properties of ThO₂ and to measure the fracture surface energy, , of sintered ThO₂. Optical microscopy and acoustic emission were employed to detect ring crack formation. Perfect cracks were always formed and no indication of permanent plastic deformation was observed. From the observed crack behaviour, a fracture surface energy, , of 2.5±0.2 J m^–2 at room temperature and a fracture toughness, K _Ic, of 1.07 MN m^–3/2 were deduced.     

Processing and mechanical properties of biocompatible Al2O3 platelet-reinforced TiO2

The mechanical behaviour of Al₂O₃ platelet-reinforced TiO₂ bioceramics produced by hot-pressing has been investigated. The variation of the elastic constants, fracture strength and fracture toughness with the volume fraction of platelet content was studied. The addition of platelets did not affect the critical flaw size of the composites. This fact, and the good matrix/platelet interfacial bond resulted in a simultaneous increase of the fracture strength and toughness. The mechanical properties increased from K_Ic=2.4 MPa m^1/2 and ₀=215 MPa for pure TiO₂ to K_Ic=3.3 MPa m^1/2 and ₀=265 MPa for a 30 vol% platelet-containing composite. The indentation technique demonstrated the anisotropic behaviour of the fracture toughness in the composites due to platelet orientation during hot-pressing. Load transfer was identified as the main reinforcing mechanism and the toughening effect could be assessed by a load transfer-based model equation. Fracture surface analysis showed mainly intercrystalline fracture for the TiO₀ matrix, whilst with the composites, fracture became more transcrystalline with increasing platelet content.     

Sintering ofβ-sialon with 5mol% Y2O3-ZrO2 additives

The sintering behaviour ofβ-Sialon composition powders with 5 mol% Y₂O₃-ZrO₂ additives at 1750°C for 1.5 h in nitrogen or argon atmospheres was studied.β-Sialon composition powders could be pressureless-sintered to about 93% theoretical density by the addition of 5 wt% 5 mol% Y₂O₃-ZrO₂. By HIPing the pressureless-sintered bodies the density was increased to higher than 98% theoretical density, and uniform submicrometre ZrO₂ particles were homogeneously dispersed in theβ-Sialon matrix, resulting in an increase of fracture toughness,K _1c, from 5.1 to about 5.7 MN m^−1.5. Increasing the amount of tetragonal ZrO₂ transformable to monoclinic phase in theβ-Sialon matrix increasedK _1c.     

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%.     

Bruchmechanische Charakterisierung des Vergütungsstahls 30 CrNiMo 8 nach verschiedenen Anlaßbehandlungen

Fracture Mechanics Characterisation of the Q & T Steel 30 CrNiMo 8 after Different Heat Treatments The investigations show that the fracture toughness K_Ic of the Q & T steel 30 CrNiMo 8 for large structural members is more sensitive to a change of the tempering treatment than the Charpy energy A_v. For tempering temperatures between 200 and 250°C the fracture toughness K_Ic shows maximum values with K_Ic ? 87 MN/m^3/2, R_m = 1800 MN/m² and R_p0,2 = 1400 MN/m² at room temperature. With these values this steel may be an inexpensive alternative to the maraging steels, for example to 18 Ni 300 (X 2 NiCoMo 18 9 5) with K_Ic = 76 MN/m^3/2 at R_m = 2100 MN/m² and R_p0.2 = 2000 MN/m² (aged 4 h 480°C). Elastic-plastic fracture toughness values K_Ji were determined at onset of stable crack growth with specimens that fail after more or less large plastic deformations because the thickness of specimen is too small or test temperature and tempering temperature, respectively are too high. These values are essentially independent of specimen size, if some minimum size conditions are fulfilled, and they are in agreement with the linear elastic fracture toughness K_Ic. But the test of more especially large linear elastic failing specimens may clarify the uncertainty whether this procedure delivers conservative results in all cases.     

Synthesis,microstructure, and mechanical properties of boron suboxide materials doped with chromium boride

Improvement on the densification and fracture toughness of ceramic materials based on boron suboxide (B₆O) has been of great importance. The mechanical properties of B₆O with and without chromium boride addition, hot pressed at a temperature range of 1850–1900°C and pressures of 50–80?MPa for 20 minutes, were investigated. The relative density, phase relationship, microstructures and mechanical properties of the processed ceramics were examined. More than 96% of the theoretical density was obtained for both ceramic systems. A good combination of mechanical properties was obtained with the B₆O-CrB₂ material (H_V?=?32.1?GPa, K_IC?=?4.5?MPa?·?m^0.5) compared to the pure B₆O material (H_V?=?30.5?GPa, K_IC?=?brittle). The addition of 1.7?wt.% CrB₂ resulted in a pronounced improvement in both the hardness and fracture toughness values. Crack bridging and deflection are some of the toughening mechanisms liable for the enhanced fracture toughness of the sintered ceramic materials.     

Indentation testing of nuclear-waste glasses

The fracture properties of several nuclear-waste glasses were determined by indentation techniques. The fracture toughness,K _Ic, was calculated from the measurement of radial cracks around Vickers diamond indentations as a function of applied load, and the results agree quite satisfactorily with values obtained by the Hertzian indentation technique. The fracture toughness of the waste glasses containing simulated fission products ranged from 0.9 to 1.1 MN m^–3/2 in air, with slightly higher values measured in dry nitrogen. The hardness was also obtained from the Vickers indentations and the ratioH/E was determined from the elastic recovery of Knoop diamond indentations. The values ofE deduced fromH andH/E were within 15% of values measured by ultrasonic tests. The results along with the limitations of the different techniques are discussed in detail.Supported by the US Department of Energy.     

Hardness and elastic properties of Bi2O3-based glasses

The hardness and elastic properties of 20PbO · xBi₂O₃ · (80 – x)B₂O₃ glasses with x = 20–60 were evaluated through usual Vickers indentation and nanoindentation tests. The glass transition temperature (T _g = 295–421°C), Vickers hardness (H _v = 2.9–4.5 GPa), true hardness (H = 1.5–3.8 GPa) and Young's modulus (E = 24.4–72.6 GPa) decreased monotonously with increasing Bi₂O₃ content. This compositional trend demonstrates that the strength of Bi–O chemical bonds in these glasses is considerably weak compared with B–O bonds and plastic deformations under indentation loading occur easily. The elastic recovery after unloading was about 45% for the glasses with x = 20–50, and the Poisson's ratio was 0.27 for the glass with x = 20. The fracture toughness was evaluated to be 0.37–0.88 MPam^1/2 from the values of H _v and E, and it was proposed that not only weak Bi–O bonds but also boron coordination polyhedra (BO₃ or BO₄) and their arrangements affect on crack formation. From the temperature dependence of Vickers hardness up to the glass transition region, it was suggested that the glasses with high Bi₂O₃ contents belong to the category to fragile glass-forming liquids.     

Effects of composition on selected physical properties of SiO2-K2O-Na2O glasses

Physical properties were used to characterize a range of five, three-component glass formulations (SiO₂-K₂O-Na₂O) synthesized from spray-dried precursor powders in terms of their final bulk chemical compositions. Five specimens per composition were produced by mixing dry glass powder with methyl alcohol to form a slurry, then shaping the slurry in a cylindrical mould, using conventional methods. Specimens were fired under vacuum. Optimum firing conditions for each glass were determined by selecting holding times which produced the maximum Young's modulus. Fracture toughness, K _IC, values ranged from 0.92–0.99 MPa m^1/2 and Young's modulus, E, values ranged from 45.09–58.06 GPa. Linear regression analyses showed significant correlations between chemical composition and fusion temperature (n=5; P<0.001), opacity (n=5; P <0.01), specific gravity (n=5; P<0.02), dynamic Young's modulus (n=5; P<0.01) and fracture toughness (n=30; P<0.001). The selected physical properties were found to be very sensitive to small variations in chemical composition.     

Effect of annealing on the fracture behavior of La0.58Sr0.4Co0.8Fe0.2O3‐δ, a potential energy material for oxygen separation

The perovskite material La_0.58Sr_0.4Co_0.8Fe_0.2O_3‐δ, offers high oxygen permeability at elevated temperature and is considered as a potential material for oxygen separation membranes. It can enhance the efficiency of oxy‐fuel combustion at high temperatures (> 800 °C) and hence due to the high reliability demands, required by the long term operation at elevated temperatures, it requires a thorough investigation from the view point of structural stability. Aiming towards long term stability, the present work is a detailed and systematic study on the effect of annealing on the mechanical behavior of dense La_0.58Sr_0.4Co_0.8Fe_0.2O_3‐δ. The study reveals that the indentation fracture toughness of the material increases with increase in annealing temperature. In most of the indentation loads, the subsurface crack profile was Palmqvist in nature with low value of the ratio of crack length versus indentation size (c/a). A consistent pattern of variation of c/a and indentation fracture toughness (K_IC) at all indentation loads was observed. Systematic drop in c/a and subsequent increase in fracture toughness in the as prepared test pieces has been attributed to residual stress accumulation during preparation.     

Fracture toughness KIC analysis of Co-doped 7YSZ

Abstract

Co-doped samples of 7YSZ with Yb³⁺, Ce⁴⁺ and Nb⁵⁺ having high porosity are subject to Vickers hardness testing. Fracture toughness K_IC values are obtained by measuring linear and non-linear crack geometries. Three separate means are used to calculate the fracture toughness and to investigate the associated trends. It is confirmed that high amounts of retained tetragonal zirconia improve fracture toughness, while elevated amounts of monoclinic zirconia lower overall fracture toughness. The experimental trend for increasing K_IC is Nb:7YSZ<Yb:7YSZ<7YSZ<Ce:7YSZ; however, this trend is qualitative as the Young’s modulus values for different samples are corrected for porosity using an equation that does not generally apply to indentation techniques.     

Carbon nanotubes: do they toughen brittle matrices?

The development of a model CNT-brittle matrix composite system, based on SiO₂ glass containing well-dispersed CNTs at up to 15 wt%, allows a direct assessment of the effect of the nanoscale filler on fracture toughness (K _IC). Samples were prepared by colloidal heterocoagulation followed by spark plasma sintering. Detailed K _IC measurements, using both indentation and notched beam techniques, show a linear improvement with CNT content, with up to a twofold increase of fracture toughness at maximum loading. The results from the two methods used in this study show equivalent trends but differing absolute values; the relative merits of these two approaches to measuring nanocomposite toughness are compared. Possible toughening mechanisms associated with CNT pull-out, crack bridging, and crack deflection are identified, and discussed quantitatively, drawing on conventional short-fibre composite theory and the potential effects of scaling fibre diameter.     

Microhardness and brittle fracture of garnet single crystals

Hardness and fracture toughness were measured using the Vickers microhardness test in the low load range from 25 to 100 g near to the fracture threshold for near-perfect single crystals of garnets. The influence of crystal growth parameters, calcium impurity content and crystallographic orientation of Gd₃Ga₅O₁₂ (GGG) and Ca₃Ga₂Ge₃O₁₂ (CaGeGG) samples was investigated. Fracture starts with radial cracking from indent corners followed by lateral fracture of two distinct modes. The mean hardness of [111] oriented GGG isH=13 GN m^–2, for [111] oriented CaGeGG it is 12 GN m^–2, the average fracture toughness beingK _c=1.2 and 0.8 MN m^–3/2, respectively for the two crystals. Impurity doping slightly increases the strength of the material. Among the investigated crystals (111) faces are the least strong, the (100) face has maximumH andK _c values for CaGeGG. The constraint factor,, and yield stress,Y, were deduced from the measured hardness data, giving=2.2 andY about 7 GN m^–2.