Showing papers on "Fracture (geology) published in 1979"

Hardness, Toughness, and Brittleness: An Indentation Analysis

Abstract: The ratio H/Kc, wjere His hardness (resistance to deformation) and Kc. is toughness (resistance to fracture), is proposed as an index of brittleness. Indentation mechanics provides the scientific basis for this proposal. The analysis, developed in terms of a model contact system, indicates that all materials are more susceptible to deformation in small-scale loading events and to fracture in large-scale events. By normalizing the characteristic dimensions of the two competing processes and the contact load in terms of appropriate functions of H and Kc a universal deformation/fracture diagram can be constructed. From this diagram the mechanical response of any material of known hardness and toughness may be predicted for any prospective in-service contact loading conditions. The concept offers a simple approach to materials classification for design purposes.

Mechanical Behaviour of Ceramics

Abstract: Preface 1. Background to ceramics 2. Elastic behaviour 3. Fundamentals of fracture chemicals 4. Geometrical aspects of dislocations 5. The flow stress and plastic flow 6. The fracture strength of ceramics 7. Impact resistance and toughness 8. Thermal stresses and fracture in ceramics 9. Engineering design data References Index.

Indentation deformation/fracture of normal and anomalous glasses

Abstract: Vickers deformation/fracture indentations have been investigated in six silicate glasses. The characteristic damage patterns fall into two distinct groups, according to whether the glass shows “normal” or “anomalous” mechanical behaviour. Observations of the damage morphology during and after contact, of the scales of the deformation and fracture zones, and of the residual stress intensity about the impressions, all point to a basic difference in the local stress/strain micromechanics. This difference is discussed in relation to the factors which control the brittleness of glass.

Analysis of surface cracks in finite plates under tension or bending loads

Abstract: Stress-intensity factors calculated with a three-dimensional, finite-element analysis for shallow and deep semielliptical surface cracks in finite elastic isotropic plates subjected to tension or bending loads are presented. A wide range of configuration parameters was investigated. The ratio of crack depth to plate thickness ranged from 0.2 to 0.8 and the ratio of crack depth to crack length ranged from 0.2 to 2.0. The effects of plate width on stress-intensity variations along the crack front was also investigated. A wide-range equation for stress-intensity factors along the crack front as a function of crack depth, crack length, plate thickness, and plate width was developed for tension and bending loads. The equation was used to predict patterns of surface-crack growth under tension or bending fatigue loads. A modified form of the equation was also used to correlate surface-crack fracture data for a brittle epoxy material within + or - 10 percent for a wide range of crack shapes and crack sizes.

Effect of microstructure on cleavage fracture toughness of quenched and tempered steels

Abstract: The cleavage fracture of quenched and tempered steels at a sharp crack is seen to involve a statistical competition between different sized crack nuclei in the rapidly changing stress gradient ahead of the crack tip. A procedure is proposed, based on this statistical fracture model, whereby it is possible to estimate the cleavage fracture toughness of a steel containing spheroidal carbide particles from a knowledge of the carbide particle radius distribution. Predictions so made are seen to be in good agreement with experimentally determined fracture toughness values over a range of different temperatures and microstructures.

Critical fracture stress and fracture strain models for the prediction of lower and upper shelf toughness in nuclear pressure vessel steels

Abstract: Critical fracture stress and stress modified fracture strain models are utilized to describe the variation of lower and upper shelf fracture toughness with temperature and strain rate for two alloy steels used in the manufacture of nuclear pressure vessels, namely SA533B-1 (HSST Plate 02) and SA302B (Surveillance correlation heat). Both steels have been well characterized with regard to static and dynamic fracture toughness over a wide range of temperatures (−190 to 200°C), although validJIc measurements at upper shelf temperatures are still somewhat scarce. The present work utilizes simple models for the relevant fracture micromechanisms and local failure criteria to predict these variations in toughness from uniaxial tensile properties. Procedures are discussed for modelling the influence of neutron fluence on toughness in irradiated steel, and predictions are derived for the effect of increasing fluence on the variation of lower shelf fracture toughness with temperature in SA533B-1.

Dynamic photoelastic studies of fracture

Abstract: Dynamic characterization of brittle fracture is possible by relating the instantaneous stress-intensity factorK(t) to the velocity of propagation of the crack. High-speed photographic systems are employed with photoelastic methods to obtain a sequence of isochromatic-fringe patterns representing the state of stress associated with the propagating crack. Methods for determiningK(t) from these isochromatic patterns are reviewed.

Method for monitoring the recovery of minerals from shallow geological formations

Abstract: In the process of recovering minerals, such as hydrocarbons from shallow tar sands, and the like, one method is to create a shallow horizontal fracture within the tar sand. The size and shape of the perimeter is determined, and one or more secondary boreholes are drilled to the formation inside of the perimeter of the fracture. Pressurized air is injected into the fracture and the hydrocarbons are ignited. After a selected period of burning is carried out, the radius to the burn front, surrounding the recovery area, from which the hydrocarbons have been removed, is determined by placing a source of seismic waves at a selected point over the fracture, and a long array of seismic sensors colinear with the source. The source is energized for different conditions of pressurization in the formation and fracture. The reflected seismic waves are converted to electrical signals, which are compared for all sensors. By comparison of the character and/or energy of the reflections, the geometry of the burn zone can be determined.

The influence of the lath morphology on the yield stress and transition temperature of martensitic- bainitic steels

Abstract: A decrease in the packet size or lath width of bainitic-martensitic steels produces a simultaneous increase of toughness and yield stress. Specific interpretations are necessary to give the actual relationships since classical theories of the grain size effect cannot be directly applied to both of these microstructural parameters. To obtain these specific interpretations, a detailed analysis of lath orientations inside a packet is necessary. This analysis reveals that assumptions such as all laths in any one packet are similarly oriented are unfounded and that in fact a packet contains many high angle lath boundaries which are given by laths adopting different Kurjumov-Sachs orientation variants during the γ → α transformation. The yield stress then depends on the average lath “diameter” which is a function of lath width and length, the latter dimension being related to the packet diameter. A Petch agreement is not found, rather the yield stress is found to be related to the reciprocal of the average lath diameter. A theoretical analysis shows that for very fine grain sizes, as encountered in bainites and martensites, macroscopically heterogeneous deformation (a necessary condition leading to the Petch formulation) tends to vanish and that for macroscopically homogeneous deformation the yield stress is expected to be related to the reciprocal of the grain diameter. The fracture transition temperature is determined by the particular fracture characteristics of these steels. It is possible to explain the fracture of bainite and lath martensite without recourse to concepts such as “effective grain size” or “covariant packet size”. In spite of the different lath orientations inside a packet, a brittle crack may adopt an average, approximately straight fracture direction across a packet by following a particular group of different fracture planes that are separated by low angle boundaries. At a packet boundary the crack must find another group of fracture planes, which will impose an important deviation of the crack. At the transition temperature, the controlling event in the fracture sequence is this crack deviation, which imposes an energy requirement for the crack to undergo high angle deviations across the first laths that are adjacent to the packet boundary, until the new average fracture direction is found. Using this model the transition temperature can be related to a logarithmic function of the product of the packet diameter and lath width.

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.

A Comparison of the Theories for Predicting Width and Extent of Vertical Hydraulically Induced Fractures

Abstract: Prediction of fracture dimensions during propagation of a hydraulically induced fracture for well stimulation is essential for the design of a stimulation treatment. During the past decade much effort has been spent on the development of a suitable theory for this purpose. Since neither the length nor the width of a hydraulically induced fracture can be measured in situ during a field treatment, this is primarily a mental exercise in applied mechanics. The main measurable quantities that are directly related to the fracture propagation process are the total volume of fracturing fluid injected into the reservoir and the time required to accomplish this. Not surprisingly, various authors have arrived at different theories, depending on the assumed conditions prevailing downhole. In this paper, the assumptions underlying the various theories currently in use for the prediction of fracture dimensions, viz., those of Perkins and Kern, of Nordgren, of Geertsma and De Klerk and of Daneshy, are compared. Rather than take issue for one particular theory, which appeared impossible because none of the theories is perfect, the paper shows what the various theories have in common, where and why they differ from each other and what the practical consequences are in case ofmore » application to treatment design. 12 references, 10 figures, 2 tables.« less

On the calibration of the electrical potential technique for monitoring crack growth using finite element methods

Abstract: Finite element analysis procedures are utilized to provide theoretical calibration curves for the electrical potential crack-monitoring system as applied to single-edge-notch (SEN) and compact tension (CT) fracture specimens. The results are compared to existing calibrations for such test piece geometries derived using experimental, electrical analog and analytical (conformal mapping) procedures.

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.

Dynamic fracture strength of rock

Abstract: Dynamic fracture strengths of a representative suite of crustal rocks have been determined. The rocks investigated included silicates and carbonates of varying grain size and porosity, oil shale with several kerogen contents and orientations, and fused silica. With the exception of fused silica, fracture strengths ranged from a few tens of MPa to slightly over 100 MPa. Rate dependence of fracture stress and fracture energy appears to be important in determining the dynamic strength of rock.

Dilatancy anisotropy and the response of rock to large cyclic loads

Abstract: An experimental study of cyclic fatigue of granite and diabase in triaxial compression was conducted. It was found that dilatancy progressively developed during cyclic loading of these rocks with a form in time very similar to a creep curve. Dilatancy was in all cases highly anisotropic. This anisotropy was controlled by the crack microstructure in the granite and by the residual stress in the diabase. The strike of the fracture plane was found typically to form normal to the axis of maximum dilatancy. Increasing dilatancy during cyclic loading was found to be caused by progressive dilatant creep plus additional damage produced by the cycling itself. The former leads to a pronounced loading rate effect on fatigue, the latter to a complex effect of cycle amplitude on fatigue. No stabilization of fatigue by pressure was found for confining pressures up to 300 MPa. The total dilatant strain at fracture was found to increase strongly with peak stress and decrease with increasing loading rate. Three types of cracking are shown to result in dilatancy: type 1, stress-induced cracking; type 2, stress corrosion cracking; and type 3, fatigue cracking. Rock fracture is sensitive to which type is prevalent.

Some Effects of Cavities on the Fracture of Ceramics: I, Cylindrical Cavities

Abstract: Fracture probabilities associated with spherical cavities were analyzed by combining principles of fracture mechanics and fracture statistics. The analysis is based on the theory that fracture occurs from a distribution of cracks located at the cavity surfaces. It predicts effects on strength of cavity size and cavity volume content (porosity) that depend sensitively on the flaw population in relation to the cavity size distribution. The theory has limit solutions that coincide with several models of fracture derived for porous ceramics. The predicted effects of pore size on strength are compared with some available data.

Application of fractography to core and outcrop fracture investigations

Abstract: Purpose of this paper is to introduce geologists to the principles of fractography, especially those principles that govern the formation of fracture surface structures commonly observed in rocks. A knowledge of the inception mechanics governing the formation of a fracture's tendential and transient structures should provide geologists with a method to distinguish natural from coring-induced and handling-induced fractures in oriented core samples, and show how coring-induced fractures may be assisted in their formation by stresses that can be attributed to the drilling process. 118 figures.