Showing papers in "Journal of Materials Science in 1979"

Residual stress effects in sharp contact cracking

Abstract: A study is made of residual stress effects in the mechanics of median fracture in sharp indenter contact. Starting with a simplistic treatment of the elastic-plastic indentation field, the problem is conveniently resolved into two separable parts, involving reversible (elastic) and irreversible (residual) components. The assumption of geometrical similarity in the residual field about the deformation zone, later backed up by stress birefringence measurements, leads to a stress intensity factor for median crack propagation containing the elastic and residual parts as the sum of two terms. The resulting formulation for equilibrium fracture shows some differences in the crack response during the loading and unloading half-cycles. By imposing certain stress states on the specimen surface during indentation the residual component of the field may actually cause the median crack to continue in downward extension as the indenter is withdrawn, a response which is especially amenable to experimental investigation. Direct observations of median crack evolution in soda-lime glass confirm this and other essential predictions of the fracture mechanics theory. The contribution of the residual component to the crack growth is found to be by no means secondary in importance to that of the elastic component.

Determination of the plastic behaviour of solid polymers at constant true strain rate

Abstract: The methods of conventional tensile testing as applied to solid polymers are compared and reviewed critically. Experiments were performed using these techniques, and it is shown that large variations in local strain rate occur while necking and cold-drawing take place. A new tensile testing method is described in which the samples are tested atconstant local true strain rate. This technique is based on the use of a diameter transducer, an exponential voltage generator and a closed-loop testing machine. Flow curves for poly(vinyl chloride) and high density polyethylene were determined at room temperature over the strain rate range of 10−1 to 10−4 sec−1. It is shown that the flow behaviour of these two polymers can be approximated by the constitutive relation:\(\sigma = K \cdot \exp [(\gamma _ \in /2) \in ^2 ] \cdot \dot \in ^m\), whereK andγ∈ are constants andm, the rate sensitivity, is in the range 0.02 to 0.06. It is concluded that the positive curvature of the log σ flow curve is responsible for the stabilization of flow localization associated with cold drawing, and that the rate sensitivity plays a much smaller role.

Reaction-bonded silicon nitride: its formation and properties

Abstract: The theme of the review is the construction of a model embracing the mechanism of formation of reaction-bonded silicon nitride, the development of microstructure and mechanical properties. Possible nitridation reactions are discussed, with emphasis on kinetics and on phase composition and microstructure of the reaction product. The influence of Fe, a common impurity in silicon powders, and of H2, as an additive to the nitriding atmosphere, is considered in some detail. The optical, electrical and thermal properties are briefly discussed and areas for further research and development studies identified.

On estimating the Weibull modulus for a brittle material

Abstract: Common methods of estimating the Weibull modulus are surveyed. Computer simulation is used to obtain the statistical properties of different estimators. Most estimators are shown to be biased and their respective adjustment factors, for a range of experimentally feasible sample sizes, are given.

Liquid metal embrittlement

Abstract: Liquid metal embrittlement is the reduction in the elongation to failure that can be produced when normally ductile solid metals are stressed while in contact with a liquid metal This review describes its principal characteristics and the several models which have been advanced in attempts to explain the occurrence and different features of the process Comparison between theory and experiment indicates that many, but not all, of its aspects are consistent with a mechanism which operates by reducing the fracture surface energy of the solid metal Literature reports show that liquid metal embrittlement can occur with a very wide range of material combinations, and while most of the data refer to laboratory studies, it is clear that the phenomenon is also of technological significance as a potential cause of plant damage

Monolithic glass formation by chemical polymerization

Abstract: Historically, glasses have been formed by solidification of oxide melts from elevated temperatures. Recently it has been demonstrated that monolithic oxide glasses can be formed by chemical polymerization at low temperatures. By the use of this technique, high-temperature reactions such as crystallization, phase separation, etc., which restrict glass formation in certain systems and regions, can be largely avoided. Thus, the technique not only permits investigation of glass structure from a fundamentally different point of view, but also allows the formation of new glasses which can not be formed by thermal means.

Martensitic transformations in Ti-Mo alloys

Abstract: A detailed investigation has been made of the structure of alloys of the Ti-Mo system containing up to 10wt% Mo, water-quenched from theβ-phase region. With increase in molybdenum content, the martensite structure changes from hexagonal (α′) to orthorhombic (α″) at ∼4 wt% Mo, and at 10 wt% Mo, the structure is completely retained β. For alloy compositions <4 wt% Mo, there is a diffusional component in the transformation of the β-phase at the quench rates employed. There is a transition, with increase in molybdenum content, in morphology (from massive to acicular) and in substructure (from dislocations to twins). However, the transitions in crystallography, morphology and sub-structure are not directly related to one another except for an abrupt loss of dislocation substructure at theα′/α″ transition. The α toα″ crystallographic transition has the characteristics of a second order transformation, and evidence has been obtained of the existence of a spinodal within the metastable orthorhombic system. The orthorhombic martensites of Ti-6 and 8 wt% Mo decompose during quenching producing a fine modulated structure within the martensite plates, consistent with a proposed spinodal mode of decomposition.

Micromechanics of crack nucleation during indentations

Abstract: An analysis for the nucleation of microcracks from the inhomogeneous flow lines in soda-lime glass under Vickers indentations is considered. The minimum loads for crack nucleation are shown to depend on the hardness,H, and the critical stress intensity factor,K IC. Unlike the Lawn and Evans analysis, the present model does not require the presence of any fortuitous flaws of critical dimensions in the material, since the flaws are nucleated by the deformation in the deformed zone.

Synthesis and fabrication of β-tricalcium phosphate (whitlockite) ceramics for potential prosthetic applications

Abstract: A novel process is described for preparing dense, polycrystalline tricalcium phosphates. Singleβ-phase compositional integrity is achieved by introducing catalytic amounts of sulphate ion and this pore free material has close to theoretical density. Preliminary mechanical properties include a compression strength of 687 MN m−2 and a tensile strength of 154 MN m−2. The relationship between processing variables and phase composition, microstructure, strength and translucence is described. The material has promise for bone implant applications.

The thermal conductivity of carbon fibre-reinforced composites

Abstract: Measurements of the thermal conductivity between approximately 80 and 270 K of a series of unidirectional and bidirectional specimens of epoxy resin DX210/BF3400 reinforced with Morganite high modulus (HMS) and high strength (HTS) carbon fibres are reported for in-plane and out-of-plane directions. The main features of the results conform with expectations based upon known structural properties of the fibres and predictions based upon current theoretical models. Employing the results for the composites in association with results for the pure resin, the account concludes with an assessment of some of the heat transmission characteristics of the fibres.

Self-diffusion of14C in polycrystalline β-SiC

Abstract: The14C self-diffusion coefficients for both lattice (D lc * ) and grain boundary (D bc * ) transport in high purity CVDβ-SiC are reported for the range 2128 to 2374 K. The Suzuoka analysis technique revealed thatD bc * is 105 to 106 faster thanD bc * ; the respective equations are given by $$\begin{gathered} D_{I c}^* = (2.62 \pm 1.83) \times 10^8 exp\left\{ { - \frac{{(8.72 \pm 0.14)eV/atom}}{{kT}}} \right\}cm^2 sec^{ - 1} \hfill \\ D_{b c}^* = (4.44 \pm 2.03) \times 10^7 exp\left\{ { - \frac{{(5.84 \pm 0.09)eV/atom}}{{kT}}} \right\}cm^2 sec^{ - 1} \hfill \\ \end{gathered} $$ A vacancy mechanism is assumed to be operative for lattice transport. From the standpoint of crystallography and energetics, reasons are given in support of a path of transport which involves an initial jump to a vacant tetrahedral site succeeded by a jump to a normally occupied C vacancy.

The chemistry of dicalcium silicate mineral

Abstract: Dicalcium silicate is of vital importance in several fields of silicate science. It exists in several polymorphic forms, of which one (theγ-form) is stable at room temperature without any stabilizer. Theβ-form is commonly found in ordinary portland cement (OPC) in association with stabilizing ions. Stabilization of other forms,α,α′L,α′H andα′m for structural and other studies have been reported. Theoretical structural analysis using topology has been reported to be of value in understanding the stabilization process of the polymorphs. The conversion ofβ→γ form is at times a problem in the cement industry, in addition to the formation of unwanted compounds, such as spurrite. Theγ-form is low in hydraulic properties but in the presence of impurities such as excess CaO over the stoichiometric ratio, shows fairly high hydraulic properties. Of the other phases, the hydraulic properties of the a forms are quite encouraging but the choice of stabilizers etc. plays a dominant role. Correlation of hydraulicity with structural properties such as crystal defects, etc., has been reported but satisfactory explanation is yet to come. The hydration products ofβ-C2S are quite similar to those of C3S but the kinetics are fairly slow. In the presence of active silica, and at elevated temperatures, even theγ-form hydrates at a faster rate. The influence of chemical accelerators on the hydration of C2S at room temperature is well studied and NaF is found to be one of the best accelerators. The formation of ″reactive″β-C2S by different preparative methods shows a quite interesting trend for potential manufacture of low-temperature inorganic cement or OPC with low C3S; even utilization of low-grade limestone could be possible. The role of C2S in the hydration of aluminous cements is being increasingly recognized and, in fact, a newer class of cements called alumina-belite cement, etc., are being developed in which C2S is purposely maintained as a major phase.

Experimental and theoretical investigations on morphological changes of γ′ precipitates in Ni -Al single crystals during uniaxial stress-annealing

Abstract: Influences of the external tension and compression on the morphological changes of γ′ precipitates during coarsening in an Ni-15 at. % Al single crystal were experimentally investigated, and theoretical evalutions on the energetically favourable shape and its orientation were also derived based upon the anisotropic elasticity theory. The shape transformation with growth in size is experimentally found to follow the sequence: cuboid→rod→ plate, under all annealing conditions, i.e. compression-annealing, tension-annealing and external stress-free annealing. However, the external stresses give preferential orientation to such microstructures. The tension of the [001] direction aligns the rods and plates along the tension axis, while the compression aligns them on a plane perpendicular to the compression axis. Theoretical evaluations based upon the anisotropic elasticity for the energetically favourable shapes and orientations of γ′ are consistent with experimental results except in the appearance of rods after compression-annealing. This discrepancy is understood by a conception that the shape transformation is influenced not only by energetics in the elastic strain and interfacial energies, but also by elastic interaction energy.

Fracture of a brittle particulate composite

Abstract: Previous models for crack-particle interactions in brittle composites are modified to account for penetrable obstacles, obstacle shape and secondary crack interactions. The modified model is applied to a glass-unbonded nickel sphere composite system, the experimental aspects of which were summarized in Part 1. Increases in fracture energy are explained in terms of local crack blunting. It is shown that these results fall, as expected, between those for an entirely sharp crack front and an entirely blunt one.

Low temperature synthesis of a monolithic silica glass by the pyrolysis of a silica gel

Abstract: The reaction process in the pyrolysis of silica gel has been investigated as the basic study on the low temperature synthesis of monolithic glass from a metal alkoxide. A large volume change which may cause stress-induced fracture of a gel occurred in the following process stages: (a) the decompostion of residual organic compounds into carbon dioxide (300 to 500° C); (b) small pore collapse (400 to 500° C); (c) larger pore collapse (700 to 900° C). A fracture-free monolithic silica glass was successfully prepared from a dry silica gel formed by the hydrolysis of silicon methoxide by careful heat-treatment. The properties of the synthetic silica glass were similar to those of commercial vitreous silica.

X-ray photoelectron spectroscopy studies of polymer surfaces: Part 3 Flame treatment of polyethylene

Abstract: X-ray photoelectron spectroscopy showed that a normal flame treatment caused a high level of oxidation in low-density polyethylene. 0.02% of the antioxidant 2,6-ditertbuty-p-cresol did not reduce the degree of oxidation or the level of adhesion in contrast to the extrusion of low-density polyethylene. It is estimated that the depth of oxidation is between 40 and 90 Å which is much less than for a moderate chromic acid treatment or with extrusion. There were no significant changes in the XP-spectra or adhesion levels of flame treated samples after 12 months.

Fracture of a brittle particulate composite: Part 1 Experimental aspects

Abstract: A study was made of the fracture process in a model brittle composite containing nickel spheres in a glass matrix. The macroscopic fracture characteristics of the system were determined by fracture surface energy, fracture strength and elastic modulus measurements. The microstructures of the composites were defined using quantitative microscopy and the fracture process was studied by the technique of ultrasonic fractography. This procedure traced changes in the crack front configuration and local crack velocity as the crack interacted with the array of nickel particles.

Zirconia electrolyte cells

Abstract: The electrical conductivity of zirconia electrolyte cells containing Al2O3 and TiO2 sintering aids made by conventional cold pressing and sintering procedures, and also by the tape process with co-fired electrodes, are compared with the conductivities of undoped material

Thermophysical properties of oil shales

Abstract: Recent developments in the characterization of the thermophysical properties of various types of oil shales are reviewed. Changes in the thermal, mechanical and electrical properties of these technologically important materials are discussed, with temperature and organic content as the experimental variables. Structural models are presented to aid in predicting the variation of thermophysical parameters with organic content in the shale. Comparison of calculated results with experimental data are shown with thermal diffusivity as a representative parameter. Areas where further research of a fundamental nature would be of particular relevance are also highlighted in the review.

Influence of water content on the rates of crystal nucleation and growth in lithia-silica and soda-lime-silica glasses

Abstract: Two sets of glasses were studied with compositions close to Li2O·2SiO2 and Na2O·2CaO·3SiO2, and with water contents ranging from 0.019 to 0.136 wt% and 0.007 to 0.040 wt%, respectively. The crystal nucleation and growth rates increased markedly with increase in water content, whereas the viscosities of the glasses decreased. For the lithia glasses, increases in nucleation rates at various temperatures closely corresponded to reductions in viscosity, indicating that the main effect of water was to lower the kinetic barrier to nucleation (ΔGD), rather than to alter the thermodynamic barrier to nucleation (W*). For the soda-lime glasses, ΔGD was also lowered by water content but additional effects due to differences in base compositions were observed. The kinetic barriers to growth were lowered by water content for both sets of glasses, increases in growth rates corresponding closely to reductions in the viscosities. It is suggested that the large effects of water on nucleation and growth may be due to an increase in the oxygen ion diffusion coefficient. In the soda-lime glasses addition of sodium fluoride produced similar effects to the addition of water. Liquidus temperature measurements, and the results of DTA, optical microscopy and electron microscopy are also reported.

The crack-initiation threshold in ceramic materials subject to elastic/plastic indentation

Abstract: The threshold for indentation cracking is established for a range of ceramic materials, using the techniques of scanning electron microscopy and acoustic emission. It is found that by taking into account indentation plasticity, currant theories may be successfully combined to predict threshold indentation loads and crack sizes. Threshold cracking is seen to relate to radial rather than median cracking.

Cone cracks around Vickers indentations in fused silica glass

Abstract: Surface and subsurface deformation and cracking around Vickers identations in fused silica have been studied. The indentations were sectioned by making the Vickers indents on and near the tip of a pre-existing crack. The characteristic median and lateral cracks around the impression and shallow cracks within the surface of the indentation were observed on the specimen surface. The subsurface deformation showed compacted or densified zones, devoid of any flow line rosettes. The dominant cracks, similar to the Hertzian cone cracks observed around purely elastic spherical indentations, occurred outside the compacted zones. These cone cracks make angles of 30 to 40° with the specimen surface. Multiple cone cracks with shallower angles often formed outside the major cone cracks. It has been suggested that the expansion of the boundary of the compacted zone as the indenter load is increased can cause median cracks during loading while the mismatch of strain at this boundary may give rise to lateral cracks during unloading.

The quantitative calculation of SiC polytypes from measurements of X-ray diffraction peak intensities

Abstract: An experimental determination on powder mixtures of SiC-3C and 6H polytypes using an X-ray goniometer system showed the possibility of quantitative determination of polytype fraction directly from peak intensities. In combination with calculated X-ray intensities of 15R and 4H polytype, the method yields a simple equation system for the relative quantities of SiC polytypes 15R, 6H, 4H and 3C in polycrystalline samples and powder mixtures.

Tensile strength of composites with brittle reaction zones at interfaces

Abstract: A theoretical model of the longitudinal strength of brittle fibre-reinforced composites with brittle reaction zones was presented for both cases of strongly and weakly bonded fibre/brittle zone interfaces. First, on the basis of the fracture mechanics, a model of the strength of the fibres coated with strongly adhering brittle zones was proposed as a function of the thickness of the brittle zones. Next, the conditions under which debonding occurs at the interfaces were investigated with the aid of the shear lag analysis proposed by Cox. The theoretical model was then examined using composites with strongly and weakly bonded interfaces. The proposed model agreed fairly well with the experimental results. Finally, the permissible thickness of the brittle zone below which no reduction in fibre strength appears was calculated, using the proposed theory, under the condition of strong interfacial bonding, for carbon, boron and silicon carbide fibres which are of practical use. The calculated values of the thickness were smaller than 1 μm.

Effect of low-temperature annealing and deformation on the structure of metallic glasses by X-ray diffraction

Abstract: The changes in the atomic structure of amorphous Pd-Si and Ni-P alloys due to low-temperature annealing, cold-rolling and isothermal creep have been studied by the conventional X-ray diffraction. The present results on the effect of low-temperature annealing were consistent with those of amorphous Fe40Ni40P14B6 alloy studied by the energy dispersive X-ray diffraction method. In addition, the present results have indicated that the effect of cold-rolling causes small changes in the structure of amorphous Pd80Si20 alloy which are qualitatively different from the structural relaxation, and the effect of annealing plays a significant contribution in the structural change during an isothermal creep test.

The debonding and pull-out of ductile wires from a brittle matrix

Abstract: An experimental investigation into the debonding and pull-out of nickel wires from epoxy resin and cement paste matrices has been carried out. Above a critical embedded length both the debonding and pull-out stresses attain limiting values. A theory based on the model of a yielded zone travelling up the wire behind a debonding front was shown to describe the observed dependence of the limiting debonding stress on the yield stress, diameter and surface roughness of the wire. Pull-out behaviour subsequent to debonding was explained using this model in terms of an unyielded plug at the end of the wire. Orientation of the wire to the loading direction was found to raise the limiting debonding and pull-out stresses due to enhanced friction at the wire exit point.

Oxidative stabilization of acrylic fibres: Part 3 Morphology of polyacrylonitrile

Abstract: A new model for the structure of oriented acrylic fibres is presented. The polyacrylonitrile molecules (or the acrylic sequences in a co-polymer) are suggested to form two distinct regions within a fibril: amorphous (disordered) and partially ordered. In the partially ordered regions, the polymer molecules assume a contorted helical shape to form rods with a diameter averaging about 6.0 Å in which the nitrile units are oriented at various angles to the rod axis, but are spaced irregularly on or near the surface of the rod. The nitrile groups of adjacent rods can interpenetrate to form dipole pairs. The rods are ordered into a liquid crystal-type array, giving in some cases a lamellar-like texture oriented perpendicular to the fibril axis, with the ordered lamellae regions interspersed with amorphous regions. Evidence for the structure is obtained from transmission electron microscopy observations, a transient peak observed in small-angle X-ray scattering when fibres are thermally treated, as well as wide-angle X-ray diffraction patterns. The proposed model is consistent with the absence of a periodic repeat unit along the chain direction, with the h k 0 reflections seen in wide-angle X-ray and electron diffraction, with the spherulitic morphology reported in some studies, and with the platelike morphologies obtained under some conditions of precipitation from dilute solution.