Showing papers on "Fracture toughness published in 1997"

Fracture and Size Effect in Concrete and Other Quasibrittle Materials

Abstract: Why Fracture Mechanics? Historical Perspective Reasons for Fracture Mechanics Approach Sources of Size Effect on Structural Strength Quantification of Fracture Mechanics Size Effect Experimental Evidence for Size Effect Essentials of LEFM Energy Release Rate and Fracture Energy LEFM and Stress Intensity Factor Size Effect in Plasticity and in LEFM Determination of LEFM Parameters Setting Up Solutions from Closed-Form Expressions Approximate Energy-Based Methods Numerical and Experimental Procedures to Obtain KI and G Experimental Determination of KIc and Gf Calculation of Displacements from KI-Expressions Advanced Aspects of LEFM Complex Variable Formulation of Plane Elasticity Problems Plane Crack Problems and Westergaard's Stress Function The General Near Tip Fields Path-Independent Contour Integrals Mixed Mode Fracture Criteria Equivalent Elastic Cracks and R-Curves Variability of Apparent Fracture Toughness for Concrete Types of Fracture Behavior and Nonlinear Zone The Equivalent Elastic Crack Concept Fracture Toughness Determination Based on Equivalent Crack Concepts Two Parameter Model of Jenq and Shah R-Curves Stability Analysis in the R-Curve Approach Determination of Fracture Properties from Size Effect Size Effect in Equivalent Elastic Crack Approximations Size Effect Law in Relation to Fracture Characteristics Size Effect Method: Detailed Experimental Procedures Determination of R-Curve from Size Effect Cohesive Crack Models Basic Concepts in Cohesive Crack Model Cohesive Crack Models Applied to Concrete Experimental Determination of Cohesive Crack Properties Pseudo-Boundary-Integral Methods for Mode I Crack Growth Boundary-Integral Methods for Mode I Crack Growth Crack Band Models and Smeared Cracking Strain Localization in the Series Coupling Model Localization of Strain in a Softening Bar Basic Concepts in Crack Band Models Uniaxial Softening Models Simple Triaxial Strain-Softening Models for Smeared Cracking Crack Band Models and Smeared Cracking Comparison of Crack Band and Cohesive Crack Approaches Advanced Size Effect Analysis Size Effect Law Refinements Size Effect in Notched Structures Based on Cohesive Crack Models Size Effect on the Modulus of Rupture of Concrete Compressing Splitting Tests of Tensile Strength Compression Failure Due to Propagation of Splitting Crack Band Scaling of Fracture of Sea Ice Brittleness and Size Effect in Structural Design General Aspects of Size Effect and Brittleness in Concrete Structures Diagonal Shear Failure of Beams Fracturing Truss Model for Shear Failure of Beams Reinforced Beams in Flexure and Minimum Reinforcement Other Structures Effect of Time, Environment, and Fatigue Phenomenology of Time-Dependent Fracture Activation Energy Theory and Rate Processes Some Applications of the Rate Process Theory to Concrete Fracture Linear Viscoelastic Fracture Mechanics Rate-Dependent R-Curve Model with Creep Time-Dependent Cohesive Crack and Crack Band Models Introduction to Fatigue Fracture and Its Size Dependence Statistical Theory of Size Effect and Fracture Process Review of Classical Weibull Theory Statistical Size Effect Due to Random Strength Basic Criticisms of Classical Weibull-Type Approach Handling of Stress Singularity in Weibull-Type Approach Approximate Equations for Statistical Size Effect Another View: Crack Growth in an Elastic Random Medium Fractal Approach to Fracture and Size Effect Nonlocal Continuum Modeling of Damage Localization Basic Concepts in Nonlocal Approaches Triaxial Nonlocal Models and Applications Nonlocal Model Based on Micromechanics of Crack Interactions Material Models for Damage and Failure Microplane Model Calibration by Test Data, Verification, and Properties of Microplane Model Nonlocal Adaptation of Microplane Model or Other Constitutive Models Particle and Lattice Models Tangential Stiffness Tensor via Solution of a Body with Many Growing Cracks References Index

Fracture toughness and fatigue-crack propagation in a Zr–Ti–Ni–Cu–Be bulk metallic glass

Abstract: The recent development of metallic alloy systems which can be processed with an amorphous structure over large dimensions, specifically to form metallic glasses at low cooling rates (similar to 10 K/s), has permitted novel measurements of important mechanical properties. These include, for example, fatigue-crack growth and fracture toughness behavior, representing the conditions governing the subcritical and critical propagation of cracks in these structures. In the present study, bulk plates of a Zr41.2Ti13.8Cu12.5Ni10Be22.5 alloy, machined into 7 mm wide, 38 mm thick compact-tension specimens and fatigue precracked following standard procedures, revealed fracture toughnesses in the fully amorphous structure of K(lc)similar to 55 MPa root m, i.e., comparable with that of a high-strength steel or aluminum ahoy. However, partial and full crystallization, e.g., following thermal exposure at 633 K or more, was found to result in a drastic reduction in fracture toughness to similar to 1 MPa root m, i.e., comparable with silica glass. The fully amorphous alloy was also found to be susceptible to fatigue-crack growth under cyclic loading, with growth-rate properties comparable to that of ductile crystalline metallic alloys, such as high-strength steels or aluminum alloys; no such fatigue was seen in the partially or fully crystallized alloys which behaved like very brittle ceramics. Possible micromechanical mechanisms for such behavior are discussed.

Mechanical properties of steel fiber-reinforced, high-strength, lightweight concrete

Abstract: This paper presents basic information on the mechanical properties of steel fiber-reinforced, high-strength, lightweight concrete with compressive and flexural strengths up to 85.4 MPa and 11.8 MPa, respectively. The influence of steel fiber on modulus of elasticity and Poisson's ratio of concrete are investigated, and flexural fracture toughness is calculated. Test results show that the effect of fiber volume fraction (Vf) and aspect ratio (lfdf) on flexural strength and fracture toughness is extremely prominent, compressive strength is only slightly improved, and tensile/compressive strength ratio is obviously enhanced. It is observed that the flexural deflection corresponded to ultimate load increased with the increase of Vf and lfdf, and due to fiber arresting cracking, the shape of the descending branch of load-deflection tends towards gently.

Crack bridging in fiber reinforced cementitious composites with slip-hardening interfaces

Abstract: A new crack bridging model accounting for slip-hardening interfacial shear stress is derived for randomly oriented discontinuous flexible fibers in cement-based composites, based on a micromechanics analysis of single fiber pull-out. The complete composite bridging stress versus crack opening curve (σB − δ relation) and associated fracture energy are theoretically determined. A micromechanics-based criterion which governs the existence of post-debonding rising branch of the σB − δ curve is obtained. Implications of the present model on various composite properties, including uniaxial tensile strength, flexural strength, ductility and critical fiber volume fraction for strain-hardening, are discussed together with an example of a 2% polyethylene fiber reinforced cement composite. It is found that the present model can very well describe the slip-hardening behavior during fiber pull-out which originates from fiber surface abrasion at fiber/matrix interface. In addition, the new model predicts accurately the enhanced toughness in terms of both ultimate tensile strain and fracture energy of the composite and resolves the deficiency of constant interface shear stress model in predicting the crack opening and ultimate strain, which are critical for material design of pseudo strain hardening engineered cementitious composites (ECCs).

Normal/shear cracking model: application to discrete crack analysis

Abstract: A simple but general model for normal/shear cracking in quasi-brittle materials is presented. It is defined in terms of the normal and shear stresses on the average plane of the crack and the corresponding normal and shear relative displacements. A crack surface in stress space determines crack initiation under pure tension, shear-tension, or shear-compression loading. Two independent fracture energy parameters are used: the classical Mode I fracture energy and the asymptotic Mode II fracture energy under very high shear compression and no dilatancy. The cracking model proposed can be implemented in two ways: directly as constitutive law of an interface element in the context of discrete crack analysis, or as the law of a generic cracking plane in a multicrack formulation in the context of smeared crack analysis. In this paper, the first approach is presented and examples are given of numerical constitutive testing and verification with experimental data.

Normal/Shear Cracking Model: Application to Discrete Crack Analysis

Abstract: A simple but general model for normal/shear cracking in quasi-brittle materials is presented. It is defined in terms of the normal and shear stresses on the average plane of the crack and the corresponding normal and shear relative displacements. A crack surface in stress space determines crack initiation under pure tension, shear-tension, or shear-compression loading. Two independent fracture energy parameters are used: the classical Mode I fracture energy GfI, and the asymptotic Mode II fracture energy GfIIa under very high shear-compression and no dilatancy. The cracking model proposed can be implemented in two ways: directly as the constitutive law of an interface element in the context of discrete crack analysis, or as the law of a generic cracking plane in a multicrack formulation in the context of smeared crack analysis. In this paper, the first approach is presented and examples are given of numerical constitutive testing and verification with experimental data.

Steady-state crack growth and work of fracture for solids characterized by strain gradient plasticity

Abstract: Mode I steady-state crack growth is analysed under plane strain conditions in small scale yielding. The elastic-plastic solid is characterized by a generalization of JZ flow theory which accounts for the influence of the gradients of plastic strains on hardening. The constitutive model involves one new parameter, a material length 1, specifying the scale of nonuniform deformation at which hardening elevation owing to strain gradients becomes important. Gradients of plastic strain at a sharp crack tip result in a substantial increase in tractions ahead of the tip. This has important consequences for crack growth in materials that fail by decohesion or cleavage at the atomic scale. The new constitutive law is used in conjunction with a model which represents the fracture process by an embedded traction-separation relation applied on the plane ahead of the crack tip. The ratio of the macroscopic work of fracture to the work of the fracture process is calculated as a function of the parameters characterizing the fracture process and the solid, with particular emphasis on the role of 1. 0 1997 Elsevier Science Ltd

Application of a J-Q Model for Fracture in the Ductile-Brittle Transition

Abstract: A model to predict cleavage failure of precracked bodies in the transition region for steels was recently proposed by the author. It is based on the concept that the stress-controlled fracture of a weak link triggers the failure of the entire body. The stress that triggers fracture is predicted by a numerical crack-tip stress analysis. The model uses toughness measured at one condition as input to predict toughness at another. For example, toughness measured at one temperature can be used to predict toughness at another temperature, or toughness measured from one geometry can be used to predict toughness for another geometry. In this paper the model is applied to predict transition toughness for some cases where the toughness is known so that the predictions from the model can be evaluated. The results show that the predictions have the same trends as many of the measured transition toughness results. The model is also applied to several component-type geometries to show that it can be used to transfer laboratory results to structural component models.

A review on particulate-reinforced titanium matrix composites

Abstract: The current status of particulate-reinforced titanium matrix composites is reviewed. The different types of reinforcements being used, together with the alternative processing routes, are described. The mechanical properties of these composites are influenced by a wide range of factors. Particulate reinforcements can affect properties by enhancing modulus and strength or by refining the grain size. Mechanical properties such as elastic modulus, low and high temperature strength, fracture toughness and compressive creep are discussed as functions of reinforcement size/shape and volume fraction. The review concludes by underlining the importance of further research in some critical areas to fully realise the industrial potential of these composites.

Double cemented carbide composites

Abstract: Double cemented carbide composites comprise a plurality of first regions and a second ductile phase that separate the first regions from each other. Each first region comprises a composite of grains and a first ductile phase bonding the grains. The grains are selected from the group of carbides consisting of W, Ti, Mo, Nb, V, Hf, Ta, and Cr carbides. The first ductile phase is selected from the group consisting of Co, Ni, Fe, alloys thereof, and alloys with materials selected from the group consisting of C, B, Cr, Si, and Mn. A preferred first region comprises tungsten carbide grains that are cemented with a cobalt first binder phase and which are in the form of substantially spherical pellets. The second ductile phase is selected from the group consisting of Co, Ni, Fe, W, Mo, Ti, Ta, V, Nb, alloys thereof, and alloys with materials selected from the group consisting of C, B, Cr, and Mn. A preferred second ductile phase is cobalt. Additionally, additives such as those selected from the group consisting of carbides, nitrides, and borides can be added to the second ductile phase to provide improved properties of wear resistance. The composites are prepared by combining hard phase particles formed from the grains and first ductile phase, with the second ductile phase material under conditions of pressure and heat, and have improved properties of fracture toughness and equal or better wear resistance when compared to conventional cemented tungsten carbide materials.

Comparison of the fracture and failure behavior of injection‐molded α‐ and β‐polypropylene in high‐speed three‐point bending tests

Abstract: The fracture and failure mode of α- and β-isotactic polypropylene (α-iPP and β-iPP, respectively) were studied in high speed (1 m/s) three-point bending tests on notched bars cut from injection-molded dumbbell specimens and compared. The fracture response of the notched Charpy-type specimens at room temperature (RT) and −40°C, respectively, was described by terms of the linear elastic fracture mechanics (LEFM), namely fracture toughness (Kc) and fracture energy (Gc). Kc values of both iPP modifications were similar, while Gc values of the β-iPP were approximately twofold of the reference α-iPP irrespective of the test temperature. It was demonstrated that β-iPP failed in a ductile and brittle-microductile manner at RT and −40°C, respectively. By contrast, brittle fracture dominated in α-iPP at both testing temperatures. Based on the fracture surface appearance, it was supposed that β-to-α (βα) transformation occurred in β-iPP. The superior fracture energy of β-iPP to α-iPP was attributed to a combined effect of the following terms: morphology, mechanical damping, and phase transformation. Results indicate that their relative contribution is a function of the test temperature. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 2057–2066, 1997

On the role of microcracks in the dynamic fracture of brittle materials

Abstract: The dynamic fracture behavior of brittle materials is investigated. The morphology of the fracture surface is examined in detail in four polymers: polymethylmethacrylate, Solithane-113, Homalite-100 and poly-carbonate. The fracture surface markings are examined to determine the micromechanisms of fracture. This examination reveals clearly that the operative micromechanism that governs dynamic fracture in brittle materials is the nucleation, growth and coalescence of microcracks. Following a quantitative characterization of the microcracking patterns, a very simple nucleation and growth model is then put forward. Imposing nucleation and growth criteria based on the experimental observations, the simulation recreates the experimental observations, not only of the microscope surface features, but also of the macroscopic behavior such as the constancy of the crack speed.

Polyetherimide-modified epoxy networks: Influence of cure conditions on morphology and mechanical properties

Abstract: The morphologies and mechanical properties of thermoplastic-modified epoxy networks generated through the reaction-induced phase separation procedure were studied as a function of isothermal cure conditions. The selected model system was diglycidyl ether of bisphenol A cured with 4,4′-methylenebis [3-chloro,2,6-diethylaniline] in the presence of a nonfunctionalized polyetherimide. Appropriate precuring and postcuring schedules were selected. The precure temperature had a strong effect on final morphologies because it affected the viscosity of the system at the cloud point and the extent of the separation process. The morphologies generated are discussed in connection with phase separation mechanisms. The ratio of the height of the loss peaks corresponding to each phase was an appropriate parameter to qualitatively predict the shape of morphology and to determine if the system was phase-inverted or not. The fracture toughness, KIc was significantly improved only when bicontinuous or inverted structures were generated, resulting from the plastic drawing of the thermoplastic-rich phase. Before phase inversion, KIc was hardly higher than that of the neat matrix due to poor interfacial adhesion. Nevertheless, the thermoplastic-rich particles constitute obstacles to the propagation of the crack and contribute to the toughening of the material, measured through impact resistance measurements. The observation of fracture surfaces revealed the occurrence of microcracking and crack-pinning. Strain recovery experiments showed that particle-induced shear yielding of the matrix was present as well. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 2433–2445, 1997

Effect of Carbide Grain Size on the Sliding and Abrasive Wear Behavior of Thermally Sprayed WC-Co Coatings

Abstract: The carbide size and cobalt content of thermally sprayed tungsten carbide/cobalt coatings (WC-Co) can influence their microstructure, fracture strength, friction response and abrasion resistance. In this paper, these properties have been determined for one commercially available and three experimental WC-17 wt.% Co thermally sprayed coatings. The experimental coatings were processed from starting powders containing median carbide size distributions of 1.2, 3.8 and 7.9μm, respectively. All the coatings were produced using a high velocity oxy-fuel (HVOF) spray process. The present results indicate that coatings with a higher percentage of finer carbide size distribution in the starting powder display a higher degree of decomposition of the WC phase to W2C phase and, consequently, display lower fracture toughness and abrasion resistance values. Unidirectional, unlubricated sliding wear tests did not reveal major differences in the sliding wear response of the coatings as a function of carbide size. The micro...

Fracture toughness of type 304 and 316 stainless steels and their welds

Abstract: Fracture toughness data for type 304 and 316 stainless steels and their welds are reviewed. The material and operational parameters evaluated in this paper include: heat to heat variability; weld process variations; welding induced, heat affected zones; crack orientation; cold work; monotonic and cyclic prestrain; long term thermal aging; neutron irradiation; temperature; and loading rates. Statistical analyses of literature data are provided to establish minimum expected fracture toughness values for use in fracture mechanics design evaluations. Guidance is also provided concerning the potential for component failure and when fracture mechanics assessments are required to guard against unstable fracture. Macroscopic fracture toughness properties are correlated with key microstructural features and operative fracture mechanisms.

Hydroxyapatite/Hydroxyapatite‐Whisker Composites without Sintering Additives: Mechanical Properties and Microstructural Evolution

Abstract: Hydroxyapatite/0%-30% hydroxyapatite-whisker (HAp/0%-30%HAp(w)) composites have been fabricated by pressureless sintering, hot pressing, and hot isostatic pressing (HIP). Composites that were HIPed at 1000°-1100°C (2 h, 190 MPa) exhibited the best properties: relative densities of 97.0-99.5%, fracture toughness of 1.4-2.0 MPa·m1/2 (as compared with 1.0 MPam1/2 for the nonreinforced HAp matrix). Compressive pre-stressing and crack deflection contributed mostly to the increase of fracture toughness. Depending on processing conditions, grain growth in the HAp matrix and/or Rayleigh instability of the HAp whiskers were probably responsible for microstructural changes in the composites. The HAp/HAp(w) composites exhibited improved toughness, attaining the lower fracture-toughness limit of bone without a decrease of bioactivity and biocompatibility.

Dynamics and Instabilities of Planar Tensile Cracks in Heterogeneous Media

Abstract: The dynamics of tensile crack fronts restricted to advance in a plane are studied. In an ideal linear elastic medium, a propagating mode along the crack front with a velocity slightly less than the Rayleigh wave velocity, is found to exist. But the dependence of the effective fracture toughness $\ensuremath{\gamma}(v)$ on the crack velocity is shown to destabilize the crack front if $\frac{d\ensuremath{\gamma}}{\mathrm{dv}}l0$. Short wavelength radiation due to weak random heterogeneities tends to lead to this instability at low velocities. The implications of these results for the crack dynamics are discussed.

Microstructural Development of Silicon Carbide Containing Large Seed Grains

Abstract: Fine ({approximately}0.1 {micro}m) {beta}-SiC powders, with 3.3 wt% large ({approximately}0.44 {micro}m) {alpha}-SiC or {beta}-SiC particles (seeds) added, were hot-pressed at 1,750 C and then annealed at 1,850 C to enhance grain growth. Microstructural development during annealing was investigated using image analysis. The introduction of larger seeds into {beta}-SiC accelerated the grain growth of elongated large grains during annealing, in which no appreciable {beta}{yields}{alpha} phase transformation occurred. The growth of matrix grains in materials with {beta}-SiC seeds was slower than that in materials with {alpha}-SiC seeds. The material with {beta}-SiC seeds, which was annealed at 1,850 C for 4 h, had a bimodal microstructure of small matrix grains and large elongated grains. In contrast, the material with {alpha}-SiC seeds, also annealed at 1,850 C for 4 h, had a uniform microstructure consisting of elongated grains. The fracture toughnesses of the annealed materials with {alpha}-SiC and {beta}-SiC seeds were 5.5 and 5.4 MPa{center_dot}m{sup 1/2}, respectively. Such results suggested that further optimization of microstructure should be possible with {beta}-SiC seeds, because of the remnant driving force for grain growth caused by the bimodal microstructure.

Fracture initiation at sharp notches under mode I, mode II, and mild mixed mode loading

Abstract: In the context of linear elasticity, a stress singularity of the type Knrδ(δ<0) may exist at sharp re-entrant corners, with an intensity Kn In general the order of the stress singularity δ and the stress intensity differ for symmetric (mode I) and antisymmetric (mode II) loading Under general mixed-mode loadings, the magnitudes of the mode I and II intensities fully characterize the stress state in the region of the corner A failure criterion based on critical values of these intensities may be appropriate in situations where the region around the corner dominated by the singular fields is large compared to intrinsic flaw sizes, inelastic zones, and fracture process zone sizes We determined the mode I and II stress intensities for notched mode I tensile specimens and notched mode II flexure specimens using a combination of the Williams (1952) asymptotic method, dimensional considerations, and detailed finite element analysis We carried out a companion experimental study to extract critical values of the mode I and II stress intensities for a series of notched polymethyl methacrylate (PMMA) tensile and flexure specimens with notch angles of 90- The data show that excellent failure correlation is obtained, in both mode I and II loading, through the use of a single parameter, the critical stress intensity We then analyzed and tested a series of T-shaped structures containing 90- corners The applied tensile loading results in mixed-mode loading of the 90- corners Failure of the specimens is brittle and can be well-correlated with a critical mode I stress intensity criterion using the results of the notched mode I tensile tests This is attributed to large difference in the strength of the stress singularities in modes I and II: δ= -04555 and -00915 for modes I and II for a 90- notch As a result, the mode I loading dominates the failure process for the 90- corner in the T-structure

Microstructural Evolution and Mechanical Properties of Si3N4 with Yb2O3 as a Sintering Additive

Abstract: Ytterbium oxide (Yb2O3) was used as a sintering aid to enhance the mechanical properties of silicon nitride (Si3N4) ceramics. The amount of Yb2O3 had significant effects on microstructural evolution and the composition of secondary phases at the grain boundary. When the Yb2O3 added was less than 8 wt%, small homogeneous grains were formed. At the grain boundary, crystalline Yb2Si2O7 was formed along with a glassy phase. As the amounts of Yb2O3 were increased to higher than 8 wt%, large elongated grains were developed in the fine matrix. In those cases, the grain boundary crystalline phase was changed from Yb2Si2O7 to Yb4Si2O7N2. Mechanical properties were influenced by these changes in microstructure and grain boundary phase. The fracture toughness increased with the Yb2O3 content up to 8 wt% and decreased slightly thereafter. The increase in fracture toughness was apparently due to the formation of the large elongated grains. When more than 8 wt% of Yb2O3 was added, interfacial debonding energy between the elongated grains and grain boundary phase became too large, resulting in a decrease in the fracture toughness. The room-temperature flexural strength was not significantly affected by the Yb2O3 content or the microstructure, other than in the case of 2 wt% addition. The high-temperature strength in nitrogen, however, increased steadily with Yb2O3 content. The highest strength, 870 MPa at 1400°C, was observed when 16 wt% of Yb2O3 was added. The increase in the high-temperature strength with Yb2O3 content was attributed to the formation of crystalline Yb4Si2O7N2 phase at the grain boundary.

A molecular dynamics investigation of rapid fracture mechanics

Abstract: Dynamic fracture is investigated for two-dimensional notched solids under tension using million atom systems. Brittle material and ductile material are modeled through the choice of interatomic potential functions which are Lennard-Jones and embedded-atom potentials, respectively. Numerical calculations are carried out on the IBN SP parallel computer and molecular dynamics is implemented using a spatial-decomposition algorithm. Many recent laboratory findings occur in our simulation experiments. A detailed comparison between laboratory and computer experiments is presented, and microscopic processes are identified. For rapid brittle fracture, the dynamic instability of the crack growth is observed as the crack velocity approaches one-third of the Rayleigh wave speed. At higher crack velocity, the crack either follows a wavy path or branches and the anisotropy due to the large deformation at the crack tip plays the governing role in determining the crack path. Limited comparison of rapid brittle fracture process with the rapid ductile fracture process is made.

On Modulus and Fracture Toughness of Rigid Particulate Filled High Density Polyethylene

Abstract: A Kaolin-filled, high-density polyethylene (HDPE) system was used to illustrate the influence of particulates on modulus and toughness of the bulk material. A variation of filler content, particulate size and coupling quality for two HDPE-matrix systems with different viscosity led to a strong dependency of elastic modulus and fracture toughness under various testing conditions, e.g. static loading, fatigue and impact. A stiffness improvement with increasing filler content was achieved by all coupling qualities. The developed Kaolin reinforcement of HDPE with optimised coupling offers an improvement of the stiffness and toughness under all investigated loading conditions. The degree of improvement depends on the particulate size and matrix viscosity. The energy dissipation mechanisms were investigated by fractographic analysis.

Crack Front Propagation and Fracture in a Graphite Sheet: A Molecular-Dynamics Study on Parallel Computers

Abstract: Crack propagation in a graphite sheet is investigated with million atom molecular-dynamics simulations based on Brenner{close_quote}s reactive empirical bond-order potential. For certain crystalline orientations, multiple crack branches with nearly equal spacing sprout as the crack tip reaches a critical speed of 0.6V{sub R}, where V{sub R} is the Rayleigh wave speed. This results in a fracture surface with secondary branches and overhangs. Within the same branch the crack-front profile is characterized by a roughness exponent, {alpha}=0.41{plus_minus}0.05. However, for interbranch fracture surface profiles the return probability yields {alpha}=0.71{plus_minus}0.10. Fracture toughness is estimated from Griffith analysis and local-stress distributions. {copyright} {ital 1997} {ital The American Physical Society}