Dry Strength of Pelletized Spheres

Three limestone powders, differing only in specific surface area, were agglomerated in a balling drum to form pellets of various sizes. The pellets were tested for dry compressive strength at different rates of deformation. A limiting elastic energy criterion for failure was used to derive the relation (analogous to the classical Hertz relation for strength of an elastic isotropic sphere) between the pellet size, the total deformation, and the load at failure. The surface area of the powder, the porosity of the pellet, and its compressive strength are analytically related; the results are in good agreement with the experimental data.     

Strength Distributions of a Quench-Strengthened Aluminosilicate Ceramic

The quench strengthening of an aluminosilicate ceramic body was studied by measuring the strengths and determining the strength distributions, estimating the residual compressive stresses at the surface by measuring the bending moments, and directly observing the surface crack patterns of quench-strengthened specimens. When considered in combination, these related results enable the assessment of the individual contributions of the residual compressive surface stresses and the changes in the flaw populations to the quench strengthening phenomenon. The quench strengthening phenomenon is discussed as a combination of those features of the process and a general schematic diagram of the phenomenon is presented.     

Measurement of the Fracture Toughness of Ceramic Materials Using a Miniaturized Disk-Bend Test

The fracture toughness of several ceramic materials has been measured using a miniaturized disk-bend test apparatus and methodology based on small disk-shaped samples 3 mm in diameter. The method involves the Vickers indentation of specimens ranging in thickness from 300 to 700 μm, and testing them in a ring-on-ring bending mode. New experiments on a glass-ceramic (GC) and Si₃N₄ have been performed to demonstrate the validity of the technique, supplementing the original work on ZnS. The fracture resistances of these materials increase with increasing crack length ( R -curve behavior). The data are analyzed using a specific model for the relationship between fracture resistance and crack length; this model enables the R -curve behavior to be treated analytically, and the fracture resistance at "infinite" crack length to be evaluated using a straightforward graphical procedure. The resulting values of the fracture toughness for ZnS, GC, and Si₃N₄ are 0.74 ± 0.02, 2.18 ± 0.09, and 4.97 ± 0.07 MPa-m^1/2, respectively, which are all in very good agreement with values obtained from conventional fracture toughness tests on large specimens. The results verify the utility of the miniaturized diskbend method for measuring the fracture toughness of brittle materials.     

Fracture Toughness Measurement of Glass and Ceramic Materials Using Chevron-Notched Specimens

Fracture toughness of several different materials was measured using chevronnotched short bar and four-point bend specimens For glass-ceramic and ceramic samples both specimens gave valid results. Fracture toughness values measured with bend specimens are 5% to 10% higher compared to those of the short bar. Consistent results for glass could be obtained only with short bar specimens. The notch width of the bend specimen affected the stability of crack growth in glass samples. Fracture toughness values measured in the present study are in good agreement with those of the previous studies.     

Strength Degradation of Glass Resulting from Impact with Spheres

The nature and extent of degradation incurred by glass surfaces impacted with spheres of steel and tungsten carbide were studied. The residual strength after impact depends on the velocity, radius, and density of the projectile; on the toughness and (indirectly) the hardness of the target; and, to a lesser degree, on the preexisting mechanical condition of the surface. The damage morphology involves modification of the basic Hertzian cone crack pattern by median (radial) cracks and crushed glass at the impact site. The essential features of the degradation may be predicted by a theoretical analysis of residual strength as a function of impact velocity as derived from indentation fracture mechanics. This study accounts, in particular, for a threshold velocity for significant strength loss, above which further strength decrease is relatively slight. Small, but significant, discrepancies between observed and predicted degradation characteristics are attributed to the departure from ideal Hertzian fracture geometry and to the dynamic nature of the contact. However, it is suggested that quasi-statically based theory may be used for estimating the strength of structural ceramics in small-particle impact situations.     

提高绝缘瓷制品强度的工艺

研究了矿石组分分散性对瓷制品性能的影响。证明加入细分散组分(石英、长石)很有效，能提高机械强度(由60MPa提高到110MPa)，同时保持绝缘强度。     

Determination of Biaxial Compressive Strength of a Sintered Alumina Ceramic

The compressive strength of commercial high-density alumina was measured in several compressive biaxial stress states. The compressive strength was only slightly affected by the magnitude of the intermediate principal stress. The average value of the compressive strengths measured in all stress states, excluding the equibiaxial and the near-uniaxial compressive stress states, was 528 ksi. The average of the coefficients of variation was 3%. The tensile strength measured for the material indicates that the ratio of compressive strength to tensile strength is ∼ 18.     

Residual Strength of a Low‐Strength Ceramic with a Precrack Induced by Thermal Shock

A method of introducing a single sharp crack with controllable length and position in brittle materials by thermal shock is proposed. This method is simple to conduct and suitable for the precise testing of critical fracture parameters, with accurate values of fracture toughness of brittle ceramics, such as ZrB₂–SiC–graphite (ZSG), able to be obtained. Moreover, this provides an experiment foundation for the study of the relationship between mechanical properties and cracks: The effects of crack length and specimen thickness on the residual strength of ZSG were investigated here. Further comparison between the experimental data and the results of the extended finite‐element calculation was made. Through proper control over the thermal shock, a desired number of uniformly distributed and roughly parallel cracks can be obtained.     

Strength and Toughness of a Ceramic Reinforced with Metal Wires

Composites consisting of fine stainless steel fibers 6, 12, or 25 μm in diameter in a matrix of wustite were prepared by hot-pressing. The fracture stress (σ _F ) of unnotched beams and the critical stress-intensity factor ( K _1C) for crack initiation in notched beams were measured; both increased linearly with the volume fraction of fiber. These improvements were attributed partially to the action of yielding fibers which bridged the crack surfaces; plastic deformation of fibers at the crack tip was thought to play a role. Large increases (largest for the 25-μm fibers) in the total work of fracture (γ _F ) were attributed to the work done in fracturing crack-bridging fibers. For long cracks bridged by fibers, over a substantial fraction of the crack, the critical applied stress intensity factor for further crack propagation increased strongly with increasing bridged crack length. The form of this increase could be accounted for by considering the negative stress intensity factor resulting from the closure forces exerted on the crack by the bridging fibers.
Based on a thesis submitted by J. G. Zwissler for the Ph.D. degree at Northwestern University, Evanston, III., June 1976.
Supported by the U. S. Office of Scientific Research, Office of Aerospace Research, Grant No. AF–AFOSR–73–2431 and by a fellowship from the International Nickel Co.     

Effects of Specimen Size on Strength of Sintered Silicon Nitride

The strength of sintered silicon nitride was measured by three-point bending, four-point bending, and expanded-ring tests at room temperature. The data obtained were analyzed using Weibull statistics. The results indicate that effective volume is the major factor determining strength.     

Monolayer Packing of Submicrometer Ceramic Spheres by Employing a Langmuir–Blodgett Film of Dicarboxylic Acid

A closely packed monolayer of spherical SiO₂ or SiO₂–TiO₂ particles of submicrometer size has been fabricated on a silicon or glass substrate by employing a Langmuir–Blodgett (LB) film of 1,12-dodecanedicarboxylic acid, followed by dehydration–condensation reaction between carboxyl groups of the dicarboxylic acid and surface hydroxyl groups of both the substrate and the ceramic spheres. Deposition of the LB film on the substrate was essential for immobilization of ceramic spheres. The flexibility of an alkyl chain in the dicarboxylic acid may have resulted in better capturing of ceramic spheres compared with surface hydroxyl groups on the substrate. Two-dimensional packing of the ceramic spheres immobilized became dense with decreasing molecular area of the dicarboxylic acid in the LB film and the temperature of the LB medium (water), although partial three-dimensional attachment of ceramic spheres was observed, especially when an LB film of quite small molecular area was used. In addition, the amount of surface hydroxyl groups on the substrate was found to significantly affect the microstructure of the two-dimensionally packed ceramic spheres. The closest two-dimensional packing was observed on a glass substrate by employing an LB film with a molecular area of 0.03 nm²·molecule⁻¹ at 20°C.     

Specular-Gloss Measurement of Ceramic Materials

A standard specular-gloss method is required to achieve duplicability between laboratories for the wide range of gloss found in recently developed porcelain enamels. A decision was made to standardize on the gloss scale resulting from the geometry of the 45" gloss attachment of the Hunter multipurpose reflectometer. As the source and receiver apertures of this instrument were not readily measurable, a goniophotometer was adjusted so that its scale duplicated that of the Hunter instrument. The source and receiver apertures of the adjusted goniophotometer were measured and these were recommended to glossmeter manufacturers for inclusion in the design of their instruments. Significant improvement in the duplicability of specular-gloss measurements resulted, as evidenced by the measurement of 45" specular gloss of a large number of ceramic specimens having a wide variety of surface-appearance characteristics. The recommendations resulting from this investigation have been incorporated, with slight modification, into gloss tests for porcelain enamels proposed by the Porcelain Enamel Institute and the American Society for Testing Materials.     

Fracture Strength and Mirror Size in a Commercial Glass- Ceramic

Extensive observations on the strength-fracture mirror size relation for a commercial cordierite-based glass-ceramic indicate a systematic deviation from the expected behavior at high strength/small mirror sizes. Calculations of internal stresses from phase properties suggest that the observations result from direct effects of internal stresses on fracture-mirror formation or from indirect effects of internal stresses affecting the fracture toughness and hence fracture-mirror size.     

Measuring Hydro-Static Dependent Constitutive Behaviour of Adhesives Using a Bend Specimen

A simple bend test and associated equations that determine, simultaneously, both the tensile and compressive uniaxial stress-strain behaviour of a bulk adhesive are presented. From this, the ratio of the flow stress in compression to tension (S) can be found. Such data are required if a meaningful elastoplastic stress analysis of an adhesive joint is to be carried out. Experimental results for both the tensile and compressive stress-strain data are obtained for an epoxy specimen using this technique. The tensile data are compared with the results from uniaxial tensile tests on flat dumbell specimens of the same material and the good correlation that is found indicates that this is a reliable technique. Values of the ratio (S) for this adhesive are calculated as a function of both strain hardening and work hardening parameters. These results indicate that this technique complements standard test techniques and provides an elegant method not only of obtaining the ratio (S) but also of deriving uniaxial stress-strain curves.     

以氮化铝及氧化钇为原料制造的热压陶瓷材料强度性能的研究

研究了以氮化铝为原料并加入5％、10％和20％Y2O3(按质量计)制造的热压材料的微观及宏观性能。查明,抗折强度、维氏硬度、抗拉强度及抗裂性的数值说明可以采用上述复合材料作抗冲击的层状材料的工作层。     

Validity of Using Average Diameter for Determination of Tensile Strength and Weibull Modulus of Ceramic Filaments

Strengths and Weibull moduli for alumina/yttrium aluminum garnet eutectic (AYE) filaments and for Si-C-O (Nicalon ^TM) filaments were calculated using measured and average filament diameters. The strengths agreed closely. Thus an average filament diameter could be used instead of the measured filament diameter in calculating strengths. The Weibull modulus obtained from an average filament diameter approximates the Weibull modulus obtained using the measured filament diameter.     

Design of a Laminated Ceramic Composite for Improved Strength and Toughness

Adding aluminum titanate to alumina can result in dramatic improvements in toughness and R -curve properties. However, the improved toughness is offset by a significant reduction in strength at small flaw sizes. This problem can be overcome through the use of a laminated composite construction. By placing a thin layer of high-strength material on the surface of a high-toughness body, the toughness and flaw tolerance of the body material can be maintained without sacrificing small flaw strength. In this study, alumina + 20 vol% aluminum titanate (AAT20) was used for both the surface layer and the bulk material. The surface material was a homogeneous, fine-grained mixture of the two phases, while the bulk was an inhomogeneous mixture having a bimodal grain structure. In monolithic form, the homogeneous AAT20 displays a nearly P ^−1/3 indentation strength response, and the inhomogeneous material displays a flat strength response, indicative of R -curve behavior. The trilayer material shows a composite indentation strength response, with high strength throughout the entire range of starting flaw sizes. A method for predetermining the optimum surface layer thickness is presented. The processing and mechanical properties of these materials will be discussed.