Strength Of Brittle Materials Research Articles

Torsional Strength of Glass under Hydrostatic Pressure

This paper presents an investigation on the effect of hydrostatic pressure on the strength of brittle materials. Pyrex glass was taken as the test material. Hollow and solid cylinders of Pyrex glass, naked or coated with thin rubber films, were twisted under hydrostatic pressures up to 1000kg/cm2.The torsional strength of the coated specimens increased with the superimposed hydrostatic pressure, while that of the naked ones exhibited no significant change. It was observed that fracture started from a surface flaw.The effect of hydrostatic pressure on the torsional strength of glass may be explained using the concept of critical stress intensity factor for a part-through surface crack.

The strength of brittle materials containing second phase dispersions

Abstract Cracks in brittle materials can be impeded by obstacles in the form of second phase dispersions. The crack tends to bow out between obstacles forming secondary semi-elliptical flaws. The strength is determined by the stress to propagate these secondary cracks. Stress calculations show that this depends on the ratios of the obstacle dimensions and the obstacle spacing and, except for large relative obstacle spacings, the stress is larger than the stress to extend the primary crack. Second phase dispersions will usually increase strength, therefore, through an effect equivalent to the ‘line tension effect’ observed for dislocation motion. A comparison of the calculated ‘line tension effect’ with observed increases in strength in brittle matrix composites shows that line tension is the major contribution to the strength increase for brittle obstacles, but only a minor contribution for fibre composites, and probably a minor contribution for ductile obstacles.

A new indirect tension test for concrete and other brittle materials

A new indirect test for determining the tensile strength of brittle materials like concrete is presented. The method consists of testing frame-type specimens between the compression platens of a testing machine, the geometry of the frame being such that one of its members is in tension. Frames having three different sizes are tested. Photoelastic investigation has been conducted to determine the tensile stresses in the frame. Tests have been made on cement mortar and concrete frames and the frame size appropriate for determining the tensile strength has been indicated. The tensile strength determined from the frame specimens has been compared with the cylinder splitting strength.

Some problems relating to the heat resistance of a continuous (solid) cylinder

1. We have considered the special aspects involved in determining heat resistance by using samples in the form of solid discs and cylinders. 2. We have obtained equations for calculating the mode factor and a quantity allowing for the form of the stress distribution in the Weibull equations in the case of the sharp heating of a solid disc or cylinder. 3. We have considered the special features involved in comparing the strength and heat resistance of materials differing substantially in homogeneity. 4. We have compared the results of calculations based on the proposed equations with exper imental data secured in annular samples of various sizes. The experimental results relating to the determination of heat resistance may most conveniently be expressed in the form of a criterion which allows for the statistical character of the strength of brittle materials.

The effect of small porosity on the strength of brittle ceramic materials

1. The dependence of the strength of a brittle material on porosity has been determined analytically for materials with small porosity and spherical pores. 2. It is shown that the nature of the effect of porosity on strength depends to a considerable extent on the parameter m of the strength uniformity of the material. Materials having large values of m weaken in regions of small porosity much more than do materials with a high degree of inhomogeneity.

Model for Strength of Brittle Materials Built up of Particles Joined at Points of Contact

Theoretical analysis of the strength of materials consisting of particles joined along contact areas small in relation to particle size resulted in the model where p denotes strength, D particle size, γ surface energy, d linear extent of the contact area, E modulus of elasticity for the particles, and θ porosity. The first factor has the same form as the Griffith model except that particle size is substituted for crack size; it shows that strength increases inversely with the square root of particle size. The model can be used as a guide in the design of materials such as mortars with low cement content, sand-lime bricks, soil stabilizers, and sintered products. The strengths of mortars made from finely ground quartz and cement and cured at high temperatures were 4 to 40 times higher than those of normal mortars with the same cement/water ratio.

Internal strain energy and the strength of brittle materials

The theory of Clarke for the formation of grain boundary cracks in anisotropic polycrystalline materials, is re-examined in the light of recent experimental data. The theory predicts correctly the conditions for the formation of grain boundary cracks of length similar to a grain dimension. However, the theory cannot be used to explain the experimentally observed strength/grain size and strength/irradiation dose relationships, for example for BeO. The theory supposes that the process controlling catastrophic fracture is the growth of a crack from a grain boundary pore with an energy absorption rate corresponding to the grain boundary surface energy of ∼103 erg/cm2. In practice, the process controlling catastrophic fracture is the subsequent growth of a crack from a grain dimension, with a higher energy absorption rate corresponding to an effective surface energy of ∼104 erg/cm2.

Influence of size and shape on the tensile strength of brittle materials

A simple power law is shown to relate the tensile strength in brittle materials to the volume of material subjected to 95% or more of the maximum stress.

A stress-wave technique for determining the tensile strength of brittle materials

A stress-wave technique for determining the dynamic tensile strength of brittle materials is discussed. The results of experiments with bar specimens of aluminum oxide are presented. Experimental evidence to verify the reliability of the state of dynamic tensile stress resulting from the reflection of a longitudinal stress wave from a free boundary is included. Values of dynamic tensile strength of aluminum oxide are tabulated and compared with values obtained using the same material and two different static methods.

The effect of high hydrostatic pressures on the cohesive strength of brittle materials

Experiments have been conducted to determine the fracture threshold for cast iron and steel when subjected to hydrostatic pressures up to 11300 kg/cm2. General agreement with a maximum elastic strain theory of failure is found.

The effect of the loading rate on the strength characteristics of a number of cermets

1. The use of the above-described devices, wire sensors, and appropriate apparatus enabled us to trace the effect of the loading rate on the strength of brittle materials in a wide rate of rates (maximum and minimum rates differed by about six orders, 10−3–103 kg/mm2· sec). 2. These investigations demonstrated that, with increasing loading rates the strength characteristics of the investigated brittle cermets increased (they increased by 10–20% for the investigated range of rates). 3. The strength of the porous cermets is virtually independent of the loading rate within the investigated range.

Estimate of the strength of cermets in a complex stressed state

The strength of brittle materials of the cermet type can be expressed by the product of two functions, one of which is a function of the tensor invariants of the given material, or of the class of materials, and defines the limiting surface in terms of the principal axes, whereas the second, being a function of the volume of the deformed material and containing some constants, bears a statistical character.

On the theory of the indentation test for the measurement of tensile strength of brittle materials

A theoretical solution has been obtained for the state of stress in a rectangular plate under a pair of symmetrically placed rigid indenters. The stress distributions along the two central axes have been calculated for a square plate assuming the pressure distribution under the indenters as uniform, parabolic and one resulting from `constant displacement' on a semiinfinite boundary, for different ratios of indenter-width to side of square. The results are compared with those of photoelastic analysis of Berenbaum and Brodie and the validity of the solution is discussed. The solution has been extended to orthotropic materials and numerical results for one type of coal are given.

Approximate theoretical solutions to the problem of an elastic square with rigid indenters

A theoretical analysis is attempted of the `indenter' method of measuring the tensile strength of brittle materials. In this method a slab of the material, square in shape and of uniform thickness, is squeezed on opposite edges by a pair of symmetrically placed, rigid indenters, until a tensile crack develops along the axis of the indenters. Four approximate solutions for elastic stresses in the square are obtained and compared with a photoelastic analysis which gave tensile stress at the centre as a function of the ratio of indenter width to side of square. One solution is in good agreement with experiment. The inadequacy of the other three shows how careful it is necessary to be when approximating to boundary conditions, a consideration that applies equally to experiments with models.

Reply to "Measurement of the tensile strength of brittle materials"

Reply to "Measurement of the tensile strength of brittle materials"

Measurement of the tensile strength of brittle materials

Measurement of the tensile strength of brittle materials

Mechanism of Brittle Rupture

THE low strength of brittle materials has been ascribed by A. A. Griffith1 to the presence of discontinuities which have the properties of small cracks in these materials. Since this theory was published, however, several investigations have established the fact that glass exhibits the phenomenon of delayed rupture to a marked degree, and this is not easily explicable by Griffith's theory alone. The most recent of these investigations, by F. W. Preston2 in the United States, reports the variation in strength of glass when loads are applied for periods of 10–2 sec. up to 105 sec., the breaking stress falling from 14·3 kgm./mm.2 for 10–2 sec. duration of load to 4·47 kgm./mm.2 for 105 sec. duration.