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Brittle Fracture (brittle + fracture)
Selected AbstractsEffect of Decrease of Hydride-Induced Embrittlement in Nanocrystalline Titanium,ADVANCED ENGINEERING MATERIALS, Issue 8 2010M.A. Murzinova Abstract The room-temperature impact toughness, strength and ductility of nanocrystalline (NC) and microcrystalline (MC) titanium with hydrogen content ranging from 0.1 to 16,at.-% (0.002 to 0.450,wt.-%) are studied. NC titanium has higher strength and lower sensitivity to hydride-induced brittle fracture than the MC material. In contrast to MC titanium, the elongation and impact toughness in the NC material does not decrease dramatically with increasing hydrogen content. Moreover, the fracture toughness in hydrogenated NC condition is found to be higher than that in MC titanium. This unusual result may be associated with the precipitation of equiaxial nanoscale hydrides in the interior of ,-grains in the NC material, while platelet hydrides are formed in MC titanium. One can expect that the risk of hydride-induced embrittlement is lower in NC than in MC titanium, making the NC material attractive for potential application under conditions that may cause hydrogen saturation above the permissible level for MC titanium. [source] Unlubricated Wear Behavior of Ce-TZP/ Al2O3 Nanocomposites Against Bearing Steel,ADVANCED ENGINEERING MATERIALS, Issue 3 2005G. Yang Unlubricated friction and wear tests of Ce-TZP/Al2O3 nanocomposites against bearing steel were performed. In order to elucidate the wear mecahnism, the morphological investigation and phase structural analysis of worn surfaces were carried out by X-ray diffractometer and scanning electron microscope. Additionally, three kinds of wear mechanisms such as plastic deformation, adhesive wear and brittle fracture have been revealed. [source] Molecular dynamics simulation of crack tip blunting in opposing directions along a symmetrical tilt grain boundary of copper bicrystalFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 11 2007A. LUQUE ABSTRACT Mode I crack growth along some grain boundaries of copper embrittled by solute segregation shows strong anisotropy. For instance, growth along the direction on the symmetrical tilt boundary has been reported to occur by intergranular brittle fracture, whereas growth along the opposite sense occurs in a ductile manner. In this paper, we simulate such crack configurations using molecular dynamics (embedded atom method [EAM]) in 3-dimensional perfect bicrystalline samples of pure copper of the aforementioned orientation at room temperature. In both cases the response is ductile, crack opening taking place by dislocation emission from the crack tip. The critical stress intensity factors (SIFs) for dislocation emission have been calculated by matching the displacement fields of the atoms in the tip neighbourhood with the continuum elastic fields. They are of the same order of magnitude for both growth senses despite the different morphology of their respective blunted crack tips and of the patterns of dislocations constituting their plastic zones. Thus, it seems that, in agreement with published results of continuum crystalline plasticity for the same problem, the plastic anisotropy associated with the different orientation of the slip systems with respect to the crack cannot in this case explain the experimental behaviour observed with solute embrittled bicrystals. [source] Crack initiation in the brittle fracture of ferritic steelsFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 9-10 2006M. COATES ABSTRACT Fracture in many steels is thought to initiate from fractured carbides. It is often supposed that in pre-cracked specimens, many carbides fracture in the plastic zone of the pre-crack, and that eventually fracture propagates from one of these to cause fracture of the whole specimen. Sources of fracture initiation in steels were investigated using a modified A533B steel as a model material. Specimens were annealed to produce a distribution of micron-sized carbides in a ferrite matrix. Four-point bend tests were carried out in the temperature range 77,373K to determine the material's ductile brittle transition. Pre-cracked samples were loaded up to 90% of the fracture strength at temperatures on the lower shelf (163K) and at the mid point of the transition region (243K). The samples were then sectioned and polished to produce SEM and TEM samples containing the crack tip. Other samples were made of areas some distance from the crack tip and out of the plastic zone. An extensive search for fracture initiation sites found no evidence for fracture initiation originating from fractured carbides. [source] Physics-based GPS data inversion to estimate three-dimensional elastic and inelastic strain fieldsGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2010Akemi Noda SUMMARY The Earth's crust is macroscopically treated as a linear elastic body, but it includes a number of defects. The occurrence of inelastic deformation such as brittle fracture at the defects brings about elastic deformation in the surrounding regions. The crustal deformation observed through geodetic measurements is the sum of the inelastic deformation as source and the elastic deformation as effect. On such a basic idea, we created a theory of physics-based strain analysis with general source representation by moment tensor, and developed an inversion method to separately estimate 3-D elastic and inelastic strain fields from GPS data. In this method, first, the optimum distribution of moment density tensor is determined from observed GPS data by using Akaike's information criterion. Then, the elastic and inelastic strain fields are obtained from the optimum moment tensor distribution by theoretical computation and direct conversion with elastic compliance tensor, respectively. We applied the inversion method to GPS horizontal velocity data, and succeeded in separately estimating 3-D elastic and inelastic strain rate fields in the Niigata,Kobe transformation zone, central Japan. As for the surface patterns of total strain, the present results of 3-D physics-based inversion analysis accord with the previous results of 2-D geometric inversion analysis. From the 3-D patterns of the inverted elastic and inelastic strain fields, we revealed that the remarkable horizontal contraction in the Niigata,Kobe transformation zone is elastic and restricted near the surface, but the remarkable shear deformation is inelastic and extends over the upper crust. [source] Damage-viscoplastic consistency model for rock fracture in heterogeneous rocks under dynamic loadingINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 10 2010Timo Saksala Abstract This paper presents a damage-viscoplastic consistency model for numerical simulation of brittle fracture in heterogeneous rocks. The model is based on a combination of the recent viscoplastic consistency model by Wang and the isotropic damage concept with separate damage variables in tension and compression. This approach does not suffer from ill-posedness, caused by strain softening, of the underlying boundary/initial value problem since viscoplasticity provides the regularization by introducing a length scale effect under dynamic loading conditions. The model uses the Mohr,Coulomb yield criterion with the Rankine criterion as a tensile cut-off. The damage law in compression is calibrated via the degradation index concept of Fang and Harrison. Thereby, the model is able to capture the brittle-to-ductile transition occurring in confined compression at a certain level of confinement. The heterogeneity of rock is accounted for by the statistical approach based on the Weibull distribution. Numerical simulations of confined compression test in plane strain conditions demonstrate a good agreement with the experiments at both the material point and structural levels as the fracture modes are realistically predicted. Copyright © 2009 John Wiley & Sons, Ltd. [source] Regularized sequentially linear saw-tooth softening modelINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 7-8 2004Jan G. Rots Abstract After a brief discussion on crack models, it is demonstrated that cracking is often accompanied by snaps and jumps in the load,displacement response which complicate the analysis. This paper provides a solution by simplifying non-linear crack models into sequentially linear saw-tooth models, either saw-tooth tension-softening for unreinforced material or saw-tooth tension-stiffening for reinforced material. A linear analysis is performed, the most critical element is traced, the stiffness and strength of that element are reduced according to the saw-tooth curve, and the process is repeated. This approach circumvents the use of incremental,iterative procedures and negative stiffness moduli and is inherently stable. The main part of the paper is devoted to a regularization procedure that provides mesh-size objectivity of the saw-tooth model. The procedure is similar to the one commonly used in the smeared crack framework but, in addition, both the initial tensile strength and the ultimate strain are rescaled. In this way, the dissipated fracture energy is invariant with respect not only to the mesh size, but also to the number of saw-teeth adopted to discretize the softening branch. Finally, the potential of the model for large-scale fracture analysis is demonstrated. A masonry façade subjected to tunnelling induced settlements is analysed. The very sharp snap-backs associated with brittle fracture of the façade automatically emerge with sequentially linear analysis, whereas non-linear analysis of the façade using smeared or discrete crack models shows substantial difficulties despite the use of arc-length schemes. Copyright © 2004 John Wiley & Sons, Ltd. [source] A robust algorithm for configurational-force-driven brittle crack propagation with R-adaptive mesh alignmentINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 2 2007C. Miehe Abstract The paper considers a variational formulation of brittle fracture in elastic solids and proposes a numerical implementation by a finite element method. On the theoretical side, we outline a consistent thermodynamic framework for crack propagation in an elastic solid. It is shown that both the elastic equilibrium response as well as the local crack evolution follow in a natural format by exploitation of a global Clausius,Planck inequality in the sense of Coleman's method. Here, the canonical direction of the crack propagation associated with the classical Griffith criterion is the direction of the material configurational force which maximizes the local dissipation at the crack tip and minimizes the incremental energy release. On the numerical side, we exploit this variational structure in terms of crack-driving configurational forces. First, a standard finite element discretization in space yields a discrete formulation of the global dissipation in terms configurational nodal forces. As a consequence, the constitutive setting of crack propagation in the space-discretized finite element context is naturally related to discrete nodes of a typical finite element mesh. Next, consistent with the node-based setting, the discretization of the evolving crack discontinuity is performed by the doubling of critical nodes and interface segments of the mesh. Critical for the success of this procedure is its embedding into an r-adaptive crack-segment reorientation procedure with configurational-force-based directional indicator. Here, successive crack releases appear in discrete steps associated with the given space discretization. These are performed by a staggered loading,release algorithm of energy minimization at frozen crack state followed by the successive crack releases at frozen deformation. This constitutes a sequence of positive-definite discrete subproblems with successively decreasing overall stiffness, providing an extremely robust algorithmic setting in the postcritical range. We demonstrate the performance of the formulation by means of representative numerical simulations. Copyright © 2007 John Wiley & Sons, Ltd. [source] Cover Picture: Ductile-to-Brittle Transition in Nanocrystalline Metals (Adv. Mater.ADVANCED MATERIALS, Issue 16 200516/2005) Abstract Uniaxial tensile studies concerning electrodeposited nanocrystalline face-centered cubic Ni and Ni,Fe alloys are reported on p.,1969 by Ebrahimi and Li. The nanograined metals display a transition in the deformation mechanism at a critical grain size. The cover shows that their fracture surfaces exhibited a ductile-to-brittle transition from the "cup,cup" (intragranular, ductile failure, dislocation controlled) (top panels) to "cup,cone" (intergranular, brittle fracture, probably due to breaking of atomic bonds) (bottom panels) characteristics at room temperature across this critical grain size value. [source] Comparison of mechanical properties of PP/SEBS blends at intermediate and high strain rates with SiO2 nanoparticles vs.JOURNAL OF APPLIED POLYMER SCIENCE, Issue 2 2008CaCO3 fillers Abstract The present article focuses on the effect of two types of inorganic fillers (SiO2 and CaCO3) on the mechanical properties of PP/SEBS blend. The nominal particle diameters of SiO2 and CaCO3 are 7 nm and 1 ,m, respectively. The studied blend ratios were PP/SEBS/SiO2 (CaCO3) = 75/22/3 and 73/21/6 vol %. The morphology of polymer blends was observed and the distributions of the SEBS, SiO2, and CaCO3 particles were analyzed by transmission electron microscopy (TEM). Tensile tests were conducted at nominal strain rates from 3 × 10,1 to 102 s,1. The apparent elastic modulus has the local strain-rate dependency caused by SiO2 nanoparticles around SEBS particles in the blend of PP/SEBS/SiO2. The yield stress has weak dependency of morphology. The absorbed strain energy has strong dependency of the location of SiO2 nanoparticle or CaCO3 fillers and SEBS particle in the morphology. It is considered that such morphology, in which inorganic nanoparticles are located around SEBS particles, can prevent the brittle fracture while the increased local strain rate can enhance the apparent elastic modulus of the blend at the high strain rate. On the basis of the results of this study, the location and size of inorganic nanoparticles are the most important parameters to increase the elastic modulus without decreasing the material ductility of the blend at both low and high strain rates. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source] GRINDING SPRAY-DRIED MILK POWDER NEAR the GLASS TRANSITION TEMPERATUREJOURNAL OF FOOD PROCESS ENGINEERING, Issue 2 2003GREGORY R. ZIEGLER ABSTRACT The fine grinding of chocolate is typically accomplished on five-roll mills. Chocolate manufacturers consider milk powder, a component of milk chocolate, difficult to grind. Spray-dried milk powders comprise a glassy lactose matrix in which fat globules, air vacuoles and protein are entrapped. the glassy-rubbery transition in commercial milk powders usually lies between 60,70C, depending on the moisture content. A mixture of 60% wt/wt commercial whole milk powder, Tg, 60C, and 40% wt/wt cocoa butter was ground in a three-roll refiner at temperatures of 40, 50, 60, 70 and 75C. Below Tg the particles exhibited brittle fracture, while above Tg plastic deformation was evident and particles became highly asymmetric. the amount of fat liberated from the lactose matrix, so-called free fat, particle density, and mean particle size increased with grinding temperature. However, the Casson yield value and plastic viscosity of finished "white chocolate" coatings, manufactured to a constant free fat content, increased with grinding temperature, suggesting an influence of particle shape on flow behavior. [source] The Relationship Between Multiple Scratch Tests and Wear Behavior of Hot-Pressed Silicon Nitride Ceramics with Various Rare-Earth Additive SystemsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2008Hideki Hyuga The wear behavior of Si3N4 ceramics sintered with various rare earth additives was studied for nonlubricated sliding under different conditions, and scratch tests carried out in an attempt to correlate the wear behavior. When multiple scratch testing is used the results can be used to indicate the initial wear behavior under fracture-dominated wear of the materials. The additive system used in the sintering of the Si3N4 ceramics affected the specific wear rate under nonlubricated sliding conditions, and under high load conditions, where fracture is dominant, the specific wear rate was shown to increase in samples sintered with lutetium as a consequence of a strong bonding strength between the grains and grain boundary resulting in a higher degree of brittle fracture. [source] Large-scale specimen testing on friction and wear of pure and internally lubricated cast polyamidesLUBRICATION SCIENCE, Issue 3 2006Pieter SamynArticle first published online: 4 JUL 200 Abstract Due to the casting process for nylons, their composition can easily be modified to cover a wide range of mechanical properties and applications, especially as large wear surfaces in, for example, crane guidances. Presently, selection tests for working conditions up to 40MPa are presented on pure Na-catalysed polyamides, oil-filled polyamides with homogeneous oil dispersions and holes in the surface containing oil lubricant and two types of thermoplastic solid-lubricated polyamides. Pure polyamides are, however, prone to high and unstable sliding at pressures as low as 10MPa with brittle fracture and lumpy transfer. Oil lubrication is not able to remove the sliding instabilities as oil supply to the sliding interface is controlled by migration effects that are restricted by deformation and thermal softening or melting of the polyamide matrix. Although friction and wear are lower and more stable for samples with oil supplied through lubricating holes, additional running-in phenomena are attributed to a relatively thick transfer film that is brittle and easily peels off. A continuous thick molten film or island-like deposition occurs on the polyamide surface. Solid lubricants are able to stabilize friction and lower wear down to the formation of a thin and coherent transfer film. However, increasing the amount of lubricants induces lower mechanical properties and higher deformation of the test samples. The differences in transfer behaviour are discussed with reference to optical microscopy and calculations of bulk and flash temperatures. Copyright © 2006 John Wiley & Sons, Ltd. [source] Tailoring mechanical properties of nano-structured Eurofer 97 steel for fusion applicationsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 5 2010M. Kozikowski Abstract EUROFER 97 steel is a candidate structural material for future fusion reactors and Test Blanket Modules (TBMs). In the present work microstructure of Eurofer 97 was modified by hydrostatic extrusion in multi-step process with total true strain exceeding 3. TEM observations showed that HE causes significant grain refinement from about 400 to 80 nm. This is accompanied by improvement of the tensile mechanical properties and microhardness. On the other hand, there is a clear decrease in the resistance to brittle fracture as measured in the Charpy impact tests. In order to improve strength/ductility/fracture toughness balance, the extruded samples were annealed for 1 hour at temperature range of 473-1073K. The results obtained for samples after post-extrusion annealing are discussed in terms of mechanical properties of Eurofer 97 steel. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Acrylonitrile-butadiene-styrene nanocomposites filled with nanosized aluminaPOLYMER COMPOSITES, Issue 5 2008Kamal K. Kar A polymer nanocomposite was produced by acrylonitrile-butadiene-styrene (ABS) and ,-alumina was prepared through sol-gel process using aluminum nitrate and citric acid. The particle size was analyzed by X-ray diffraction and scanning electron microscopy (SEM) studies. The nanocomposites were characterized through tensile strength, Young's modulus, strain% at break, flexural strength, flexural modulus, and impact strength. The ABS/Al2O3 nanocomposites are found to have slightly higher Young's modulus, but lower tensile strength, strain% at break, flexural and impact strength than the virgin ABS. But its flexural modulus increases with increasing Al2O3 content in ABS matrix. The d-spacing was calculated in nanocomposites to evaluate the interaction between Al2O3 and ABS. The particle distributions in nanocomposites were studied by SEM. The fractured surfaces of tensile test samples were also examined through SEM and show that the ductile fracture of ABS is converted to brittle fracture with addition of Al2O3. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers [source] Numerical modeling of brittle fracture in porous CFC-materialsPROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2006Romana Piat Both energy and stress criteria are necessary conditions for fracture but neither the one nor the other is sufficient. A combination of these criteria is proposed in [1]. This combined criterion is used for numerical simulation of crack propagation by the 4-point bending test in porous materials. Examples of such materials are carbon-carbon composites (CFC) [2, 3]. Micrographs of the cross-sections of these materials are used for FEM modeling of the crack propagation on the basis of the proposed criterion. Results of the numerical modeling are compared with experimental results. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] A Finite Element Approach for the Simulation of Quasi-Brittle FracturePROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2005Oliver Hilgert In the context of a strong discontinuity approach, we propose a finite element formulation with an embedded displacement discontinuity. The basic assumption of the proposed approach is the additive split of the total displacement field in a continuous and a discontinuous part. An arbitrary crack splits the linear triangular finite element into two parts, namely a triangular and a quadrilateral part. The discontinuous part of the displacement field in the quadrilateral portion is approximated using linear shape functions. For these purposes, the quadrilateral portion is divided into two triangular parts which is in this way similar to the approach proposed in [5]. In contrast, the discretisation is different compared to formulations proposed in [1] and [3], where the discontinuous part of the displacement field is approximated using bilinear shape functions. The basic theory of the underlying finite element formulation and a cohesive interface model to simulate brittle fracture are presented. By means of representative numerical examples differences and similarities of the present formulation and the formulations proposed in [1] and [3] are highlighted. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] A Novel Quantum/Classical Hybrid Simulation TechniqueCHEMPHYSCHEM, Issue 9 2005Mike C. Payne Prof. A successful scheme: The authors, hybrid modelling scheme can link one or more molecular-mechanics-based simulation techniques to one or more quantum mechanical atomistic simulation techniques in a seamless manner. This scheme is tested by studying the failure of a silicon nanobar under tensile stress and allows the study of brittle fracture in silicon (see simulation snapshot of the opening (111) crack system). [source] Existence and convergence for quasi-static evolution in brittle fractureCOMMUNICATIONS ON PURE & APPLIED MATHEMATICS, Issue 10 2003Gilles A. Francfort This paper investigates the mathematical well-posedness of the variational model of quasi-static growth for a brittle crack proposed by Francfort and Marigo in [15]. The starting point is a time discretized version of that evolution which results in a sequence of minimization problems of Mumford and Shah type functionals. The natural weak setting is that of special functions of bounded variation, and the main difficulty in showing existence of the time-continuous quasi-static growth is to pass to the limit as the time-discretization step tends to 0. This is performed with the help of a jump transfer theorem which permits, under weak convergence assumptions for a sequence {un} of SBV-functions to its BV-limit u, to transfer the part of the jump set of any test field that lies in the jump set of u onto that of the converging sequence {un}. In particular, it is shown that the notion of minimizer of a Mumford and Shah type functional for its own jump set is stable under weak convergence assumptions. Furthermore, our analysis justifies numerical methods used for computing the time-continuous quasi-static evolution. © 2003 Wiley Periodicals, Inc. [source] | |||||||||||||||||||