Showing papers on "Fracture mechanics published in 1986"

Introduction to continuum damage mechanics

Abstract: 1 - Introduction.- 2 - Creep and Fracture under Uniaxial Stress.- 3 - Creep and Fracture under Multiaxial Stress.- 4 - Crack Growth under Creep Conditions.- 5 - Damage Model for Ductile Fracture.- 6 - Fatigue Damage.- References.- Notations.

Matrix fracture in fiber-reinforced ceramics

Abstract: A fiber-reinforced ceramic subject to tensile stress in the fiber direction can undergo extensive matrix cracking normal to the fibers, while the fibers remain intact. In this paper, the critical conditions for the onset of widespread matrix cracking are studied analytically on the basis of fracture mechanics theory. Two distinct situations concerning the fiber-matrix interface are contemplated : (i) unbonded fibers initially held in the matrix by thermal or other strain mismatches, but susceptible to frictional slip, and (ii) fibers that initially are weakly bonded to the matrix, but may be debonded by the stresses near the tip of an advancing matrix crack. The results generalize those of the Aveston-Cooper-Kelly theory for case (i). Optimal thermal strain mismatches for maximum cracking strength are studied, and theoretical results are compared with experimental data for a SiC fiber, lithium-alumina-silicate glass matrix composite.

An experimental investigation of drop deformation and breakup in steady, two-dimensional linear flows

Abstract: We consider the deformation and burst of small fluid droplets in steady linear, two-dimensional motions of a second immiscible fluid. Experiments using a computer-controlled, four-roll mill to investigate the effect of flow type are described, and the results compared with predictions of several available asymptotic deformation and burst theories, as well as numerical calculations. The comparison clarifies the range of validity of the theories, and demonstrates that they provide quite adequate predictions over a wide range of viscosity ratio, capillary number, and flow type.

On the Analysis and Design of the End Notched Flexure (ENF) Specimen for Mode II Testing

Abstract: The end notched flexure (ENF) specimen is examined as a candidate for measuring interlaminar fracture toughness in skew symmetric loading. A simple design study for sizing the ENF specimen to minimize geometric nonlinear response and to avoid nonlinear material behavior or flexural failure is presented. Results indicate that interlaminar shear effects may be significant for tough resin systems requiring large thickness-to-length geometries. The influences of interlaminar shear deformation and friction between the crack surfaces on the strain energy release rate are examined.

Tensile Tests and Failure Analysis of Concrete

Abstract: The tensile strength of concrete is receiving an increasing amount of attention since the loading capacity and durability of structures are being studied more thoroughly, and since nonlinear fracture mechanics and numerical methods require a complete stress deformation relation. The theoretical background of the application of the tensile strength of concrete in fracture analysis is given. Test results are presented that were obtained from deformation‐controlled uniaxial tensile tests under static and cyclic loading of lightweight and normalweight concrete. The results have been used to establish material models for finite element calculations. Numerical examples show the usefulness of these models for static and cyclic loading conditions.

Micromechanics of thermally induced cracking in three crustal rocks

Abstract: Samples of Westerly granite, Frederick diabase, and Oak Hall limestone were thermally cracked at room pressure to various peak temperatures. Scanning electon microscopy (SEM) was performed on ion-thinned samples. The thermally induced crack density is dependent upon the temperature, thermal expansion mismatch, thermal expansion anisotropy, initial crack porosity, and grain size. The mode of propagation is different for grain boundary and intragranular cracks. Crack densities in the granite were quantified using stereological techniques. The thermally induced crack surface area per unit volume apparently has a quadratic dependence on the temperature increase, a physical interpretation for which can be formulated on the basis of energetic balance. Fracture mechanics models are developed to interpret thermal cracking. The predictions concerning thermal crack initiation temperature and crack propagation and arrest behavior agree well with observations for the granite and the diabase. The model predicts significant thermal cracking for the Oak Hall limestone, which contradicts the SEM observation. A possible explanation for this discrepancy is that the internal stresses due to thermal expansion anisotropy are relaxed by plastic flow in this relatively fine grained limestone.

Mechanics of engineering materials

Abstract: Preface to second edition. Preface to first edition. Notation. 1. Statically Determinate Force Systems. 2. Statically Determinate Stress Systems. 3. Stress-Strain Relations. 4. Statically Indeterminate Stress Systems. 5. Torsion. 6. Bending Stress. 7. Bending: Slope and Deflection. 8. Statically Indeterminate Beams. 9. Energy Methods. 10. Buckling Instability. 11. Stress and Strain Transformations. 12. Yield Criteria and Stress Concentration. 13. Variation of Stress and Strain. 14. Application of the Equilibrium and Strain-Displacement. 15. Elementary Plasticity. 16. Thin Plates and Shells. 17. Finite Element Method. 18. Tension, Compression, Torsion and Hardness. 19. Fracture Mechanics. 20. Fatigue. 21. Creep and Viscoelasticity.

Crack growth resistance curves in strain-softening materials

Abstract: A theoretical model has been developed for the crack growth resistance ( K R ) curves in strain-softening materials with power law softening stress (σ)-crack opening displacement (δ) relationships. Both exact and approximate solution methods have been used to calculate K R curves for a fibre cement composite in a double-cantilever-beam (DCB) geometry. There is good agreement between these two solutions. When the crack growth is normalized with respect to the saturated softening (or fibre bridging) zone there is an almost unique K R curve which is independent of specimen geometry and initial crack length. The effects of the softening index ( n ) of the power law σ-δ relationship on the shapes, saturated softening zone lengths and plateau values of the K R curves are also studied.

Computational methods in the mechanics of fracture

Abstract: This volume not only covers the fundamental concepts of fracture mechanics, but also the computational methodologies necessary for practical engineering designs aimed at fracture control. It gives a concise summary of various fracture theories: linear elastic, elastic-plastic, and dynamic fracture mechanics of metals and composites. Novel numerical methods (finite element and boundary element) that enable the treatment of complicated engineering problems are emphasized. Examined are problems of linear elastic fracture of metallic and non-metallic composite materials, three-dimensional problems of surface flaws, elastic-plastic fracture, stable crack growth, and dynamic crack propagation. A comprehensive outline of the energetic approach and energy integrals on fracture mechanics is also given. Contents: Preface. Parts: I. Chapters: 1. Fracture: Mechanics or Art? (F. Erdogan). II. 2. Linear Elastic Fracture Mechanics (A.S. Kobayashi). 3. Elastic-Plastic Fracture (Quasi-Static) (S.N. Atluri and A.S. Kobayashi). 4. Dynamic Crack Propagation in Solids (L.B. Freund). 5. Energetic Approaches and Path-Independent Integrals in Fracture Mechanics (S.N. Atluri). III. 6.

Small Fatigue Cracks

Abstract: This book contains over 30 papers. Some of the titles are: Overview of the Small Crack Problem; Fatigue Crack Initiation along Slip Bands; Small Crack-Environment Interactions: The Hydrogen Embrittlement Perspective; High Resolution Techniques for the Study of Small Cracks; and Crack Inititiation and Growth Under Far-Field Cyclic Compression: Theory, Experiments and Applications.

Shear fracture tests of concrete

Abstract: Symmetrically notched beam specimens of concrete and mortar, loaded near the notches by concentrated forces that produce a concentrated shear force zone, are tested to failure. The cracks do not propagate from the notches in the direction normal to the maximum principal stress but in a direction in which shear stresses dominate. Thus, the failure is due essentially to shear fracture (Mode II). The crack propagation direction seems to be governed by maximum energy release rate. Tests of geometrically similar specimens yield maximum loads which agree with the recently established size effect law for blunt fracture, previously verified for tensile fracture (Mode I). This further implies that the energy required for crack growth increases with the crack extension from the notch. The R-curve that describes this increase is determined from the size effect. The size effect also yields the shear fracture energy, which is found to be about 25-times larger than that for Mode I and to agree with the value predicted by the crack band model. The fracture specimen is simple to use but not perfect for shear fracture because the deformation has a symmetric component with a non zero normal stress across the crack plane. Nevertheless, these disturbing effects appear to be unimportant. The results are of interest for certain types of structures subjected to blast, impact, earthquake, and concentrated loads.

A statistical model of brittle fracture by transgranular cleavage

Abstract: A MODEL for brittle fracture by transgranular cleavage cracking is presented based on the application of weakest link statistics to the critical microstructural fracture mechanisms. The model permits prediction of the macroscopic fracture toughness, K,,, in single phase microstructures containing a known distribution of particles, and defines the critical distance from the crack tip at which the initial cracking event is most probable. The model is developed for unstable fracture ahead of a sharp crack considering both linear elastic and nonlinear elastic ~‘elastjc~plastic”) crack tip stress fields. Predictions are evaluated by comparison with experimental results on the low temperature flow and fracture behavior of a low carbon mild steel with a simple ferrite/grain boundary carbide microstructure.

Failure of fibre-reinforced composites by pull-out fracture

Abstract: A simple model is proposed to predict the ultimate tensile strength of fibre-reinforced composites when the failure is governed by fibre debonding. The theoretical analysis is based on the concept of fracture mechanics where the debonded zone is considered as an interfacial crack. The analysis is first applied to the classical pull-out test in order to determine the specific work of interfacial cracking. Using this value, the uniaxial tensile strength of the composites can be predicted from an approximate, closed-form equation proposed here. The theoretically predicted results seem to compare favourably with experimental values for fibrere-inforced cement based composite.

Strain‐energy effects on dynamic fragmentation

Abstract: Grady’s model of the dynamic fragmentation process, in which the average fragment size is determined by balancing the local kinetic energy and the surface energy, is modified to include the stored elastic (strain) energy. The revised model predicts that the strain energy should dominate for brittle materials, with low fracture toughness and high fracture‐initiation stress. This conclusion is not borne out, however, by limited experimental data on brittle steels, even when the kinetic‐energy density is small compared with the strain‐energy density.

Nonlinear Fracture Properties from Size Effect Tests

Abstract: The previously derived size effect law for blunt fracture is exploited for determining the parameters of the R-curve of the crack band model, and of Hillerborg’s fictitious crack model. No measurements of the crack length or of the unloading compliance are needed. It suffices to measure only the maximum load values for a set of geometrically similar specimens of different sizes. The parameters of the size effect law can then be identified by linear regression. The inverse slope of the regression line yields the fracture energy. The regression also has a twofold benefit: it smoothes statistically scattered data, and it extends the range of the data, so that one can do with fewer tests. From the experimentally calibrated size effect law, the R-curve may then be obtained as the envelope of the family of fracture equilibrium curves for different specimen sizes. A simple algebraic formula for this envelope is presented. The size effect regression plot makes it also possible to determine crack band model parameters, particularly the fracture energy, the crack band width, and the strait-softening modulus. The same is made possible for Hillerborg’s model.

Multiaxial strain-softening of concrete

Abstract: The present paper deals with an experimental investigation of the strain-softening behaviour of concrete subjected to multiaxial loading conditions. A multiaxial apparatus for testing cubical specimens was developed and built at Eindhoven University of Technology. Servocontrols in combination with stiff loading frames allowed for measuring stable post-peak response. Loading was applied to the specimens using brush bearing platens. The failure modes of uni- and triaxially loaded specimens are discussed in the first part of this paper. Important in this context were the results of uniaxial compression tests on prisms with varying height. The experiments revealed a localised fracture mode in uniaxial compression. A constant fracture energy was measured, irrespective of the specimen’s size. Furthermore, from the multiaxial tests it was found that the slope of the descending branch is dependent on the applied symmetric or non-symmetric confinement. It was concluded that strain-softening is the response of the structure formed by the specimen and the complete loading system. In Part II [15] attention will be paid to the behaviour of this “structure” subjected to multiaxial load histories.

Determination of specimen-size independent fracture toughness of plain concrete

Abstract: Synopsis Simple analytical expressions are proposed for determining the critical stress intensity factor Kc, and the critical energy release rate Gc, of plain concretes in three-point bending. These expressions take into account the slow crack growth preceding fracture and the complex state of stress existing at a propagating crack front. The fracture toughness so determined is shown to be essentially independent of the test specimen dimensions and to depend only on the mix variables. It is also argued that unlike Kc, or Gc, the fracture energy GF of plain concretes is strongly dependent on the test specimen dimensions. It is therefore advisable to express the fracture toughness of plain concretes through the specimen-size independent parameters Kc, or Gc.

Fracture toughness and fracture energy of concrete

Abstract: Keywords: Beton ; Fracture ; Fissure Reference Record created on 2004-09-07, modified on 2016-08-08

Fatigue crack growth and fracture toughness behavior of an Al-Li-Cu alloy

Abstract: Slip behavior, fracture toughness, and fatigue thresholds of a high purity Al-Li-Cu alloy with Zr as a dispersoid forming element have been studied as a function of aging time. The fracture toughness variation with aging time has been related to the changes in slip planarity,i.e., slip band spacing and width. Although the current alloy exhibits planar slip for all aging conditions examined, the crack initiation toughness,Klc, compares favorably with those of 2XXX and 7XXX aluminum alloys. Near threshold fatigue crack growth results in air and vacuum suggest that irregularities in the crack profile and the fracture surfaces and slip reversibility are some of the major contributing factors to the crack growth resistance of this alloy.

Tensile Fracture of Laminates with Holes

Abstract: The methods currently used for predicting static fracture of notched composite laminates in tension, such as the point and average stress criteria and the inherent flaw criterion, are of semi-empirical nature and have limited applicability with respect to size and shape of the notch. In this paper, a damage zone analysis, based on more fundamental concepts, is used to predict fracture of laminates with circular holes of various radii, and oval and rectangular holes of various sizes. The damage is represented by a linear cohesive zone. Based on the two fundamental parameters unnotched tensile strength (σ0) and apparent fracture energy (G*c), this model excellently predicts the strength of notched laminates for a number of specimens tested.

Mechanics of shear rupture applied to earthquake zones

Abstract: The mechanics of shear slippage and rupture in rock masses are reviewed. The essential ideas in fracture mechanics are summarized emphasizing the interpretation and relation among the fracture parameters in shear cracks. The slip-weakening model is described. The general formulation of the problem of nonuniform slip distribution in a continuum is covered.

When does a crack grow under mode II conditions

Abstract: Even cracks which seem to be favourably oriented for mode II growth might extend in mode I by kinking. The conditions that favour mode I or mode II growth are examined. It is shown that a high confining pressure promotes mode II growth and so does a low ratio between the critical stress intensity factors of modes II and I. Quantitative results are given for different coefficients of friction and confining pressures.

Thermal-shock crack patterns explained by single and multiple crack propagation

Abstract: The phenomenon of thermal-shock cracking in ceramics is approached theoretically and experimentally. Sintered slabs made from a glass-quartz powder mix were quenched in water in order to generate crack patterns of various types, depending on the severity of the shock. The patterns serve as evidence for a scheme set up with the aim of explaining the variety of ways in which materials respond to thermal shock. The fracture-mechanical explanations are based on the time-dependent thermal load and on the concept of an energy release rate, with single and multiple crack propagation starting from randomly distributed initial flaws on the surface.