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Caltech Authors (161.377 recursos)

Repository of works by Caltech published authors.

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Mostrando recursos 1 - 20 de 287

  1. Computational modelling of impact damage in brittle materials

    Camacho, G. T.; Ortiz, M.
    A Lagrangian finite element method of fracture and fragmentation in brittle materials is developed. A cohesive-law fracture model is used to propagate multiple cracks along arbitrary paths. In axisymmetric calculations, radial cracking is accounted for through a continuum damage model. An explicit contact/friction algorithm is used to treat the multi-body dynamics which inevitably ensues after fragmentation. Rate-dependent plasticity, heat conduction and thermal coupling are also accounted for in calculations. The properties and predictive ability of the model are exhibited in two case studies: spall tests and dynamic crack propagation in a double cantilever beam specimen. As an example of application...

  2. Ductile fracture by vacancy condensation in f.c.c. single crystals

    Cuitiño, A. M.; Ortiz, M.
    We explore the feasibility of vacancy condensation as the void-nucleating mechanism underlying ductile fracture by void growth and coalescence in single crystals at room temperature. Vacancies are presumed to be primarily generated by the dragging of intersection jogs. The equations governing the rate of growth of voids by vacancy condensation are derived. These equations are used to follow the evolution of vacancy concentrations and void sizes in the Wang and Anderson [Acta metall. 39, 779 (1991)] [1] Σ9 test. We find that, when pipe diffusions are taken into account, the time required for the nucleation of a macroscopic void in...

  3. Three-dimensional crack-tip fields in four-point-bending copper single-crystal specimens

    Cuitiño, A. M.; Ortiz, M.
    The three-dimensional near-tip fields in copper single crystals loaded in four-point bending are characterized numerically. For comparison purposes, the corresponding plane-strain fields are also computed numerically and their asymptotic behavior determined semi-analytically. On the basis of these analyses, we investigate: (i) the dependence of the fields on the hardening law; (ii) the degree of correlation between surface and interior fields in finite specimens; and (iii) the degree of correlation between plane-strain and three-dimensional fields. In particular, we endeavor to ascertain the extent to which surface observations of near-tip fields in single crystals, such as those obtained by Moire interferometry, are...

  4. The Two-Dimensional Structure of Dynamic Boundary Layers and Shear Bands in Thermoviscoplastic Solids

    Gioia, G.; Ortiz, M.
    A general boundary layer theory for thermoviscoplastic solids which accounts for inertia, rate sensitivity, hardening, thermal coupling, heat convection and conduction, and thermal softening is developed. In many applications of interest, the boundary layer equations can be considerably simplified by recourse to similarity methods, which facilitates the determination of steady-state and transient fully non-linear two-dimensional solutions. A simple analysis of the asymptotic behavior of the steady-state solutions leads to a classification of stable and unstable regimes. Under adiabatic conditions, the resulting material stability criterion coincides with that previously derived by Molinari and Clifton [(1987) Analytical characterization of shear localization in...

  5. Finite‐element modeling of dry sliding wear in metals

    Molinari, J. F.; Ortiz, M.; Radovitzky, R.; Repetto, E. A.
    This paper is concerned with the calibration and validation of a finite‐element model of dry sliding wear in metals. The model is formulated within a Lagrangian framework capable of accounting for large plastic deformations and history‐dependent material behavior. We resort to continuous adaptive meshing as a means of eliminating deformation‐induced element distortion, and of resolving fine features of the wear process such as contact boundary layers. Particular attention is devoted to a generalization of Archard’s law in which the hardness of the soft material is allowed to be a function of temperature. This dependence of hardness on temperature provides a...

  6. A theory of subgrain dislocation structures

    Ortiz, M.; Repetto, E. A.; Stainier, L.
    We develop a micromechanical theory of dislocation structures and finite deformation single crystal plasticity based on the direct generation of deformation microstructures and the computation of the attendant effective behavior. Specifically, we aim at describing the lamellar dislocation structures which develop at large strains under monotonic loading. These microstructures are regarded as instances of sequential lamination and treated accordingly. The present approach is based on the explicit construction of microstructures by recursive lamination and their subsequent equilibration in order to relax the incremental constitutive description of the material. The microstructures are permitted to evolve in complexity and fineness with increasing...

  7. Large-eddy simulation of flow over a grooved cylinder up to transcritical Reynolds numbers

    Cheng, W.; Pullin, D. I.; Samtaney, R.
    We report wall-resolved large-eddy simulation (LES) of flow over a grooved cylinder up to the transcritical regime. The stretched-vortex subgrid-scale model is embedded in a general fourth-order finite-difference code discretization on a curvilinear mesh. In the present study grooves are equally distributed around the circumference of the cylinder, each of sinusoidal shape with height ε, invariant in the spanwise direction. Based on the two parameters, ε/D and the Reynolds number Re_D = U_∞D/ν where U_∞ is the free-stream velocity, D the diameter of the cylinder and ν the kinematic viscosity, two main sets of simulations are described. The first set...

  8. Large-eddy simulation of flow over a grooved cylinder up to transcritical Reynolds numbers

    Cheng, W.; Pullin, D. I.; Samtaney, R.
    We report wall-resolved large-eddy simulation (LES) of flow over a grooved cylinder up to the transcritical regime. The stretched-vortex subgrid-scale model is embedded in a general fourth-order finite-difference code discretization on a curvilinear mesh. In the present study grooves are equally distributed around the circumference of the cylinder, each of sinusoidal shape with height ε, invariant in the spanwise direction. Based on the two parameters, ε/D and the Reynolds number Re_D = U_∞D/ν where U_∞ is the free-stream velocity, D the diameter of the cylinder and ν the kinematic viscosity, two main sets of simulations are described. The first set...

  9. Performance of the star‐shaped flyer in the study of brittle materials: Three dimensional computer simulations and experimental observations

    Espinosa, H. D.; Raiser, G.; Clifton, R. J.; Ortiz, M.
    A three dimensional finite element computer simulation has been performed to assess the effects of release waves in normal impact soft‐recovery experiments when a star‐shaped flyer plate is used. Their effects on the monitored velocity‐time profiles have been identified and their implications in the interpretation of wave spreading and spall signal events highlighted. The calculation shows that the star‐shaped flyer plate indeed minimizes the magnitude of edge effects. The major perturbation to the one‐dimensional response within the central region of the target plate results from spherical waves emanating from the corners of the star‐shaped plate. Experimental evidence of the development...

  10. Mode mixity effects on crack tip deformation in ductile single crystals

    Mohan, R.; Ortiz, M.; Shih, C. F.
    Crack tip deformation and stress fields in ductile single crystals, under mixed mode loading conditions, are examined within the framework of a geometrically rigorous formulation of crystalline plasticity. The theory accounts for finite deformations and finite lattice rotations, as well as for the full three-dimensional crystallographic geometry of the crystal. An experimentally based self-hardening rule exhibiting an initial stage of rapid hardening followed by a saturation stage is used in the analysis. For the orientation of an f.c.c. crystal considered in this study, the geometric nature of slip gives rise to competing deformation modes. Our studies reveal that mode mixity...

  11. An analysis of cracks in ductile single crystals—II. Mode I loading

    Mohan, R.; Ortiz, M.; Shih, C. F.
    A geometrically rigorous formulation of crystalline plasticity is used to analyze the crack-tip deformation and stress fields in ductile single crystals subjected to mode I loading. The theory accounts for finite deformations and finite lattice rotations, as well as for the full three-dimensional crystallographic geometry of the crystal. An experimentally based self-hardening rule exhibiting an initial stage of rapid hardening followed by a saturation stage is also adopted. The problem of a stationary semi-infinite crack in FCC and BCC crystals is considered. As regards the dominant modes of deformation, the results are in partial agreement with earlier analytical and numerical...

  12. An analysis of cracks in ductile single crystals—I. Anti-plane shear

    Mohan, R.; Ortiz, M.; Shih, C. F.
    The problem of a stationary mathematically sharp semi-infinite crack in an FCC crystal is considered. We adopt a geometrically rigorous formulation of crystalline plasticity accounting for finite deformations and finite lattice rotations, as well as for the full three-dimensional crystallographic geometry of the crystal. A comparison of results with earlier small-strain solutions reveals some notable differences. These include the expected development of finite deformations and rotations near the crack tip, but also discrepancies such as a considerable spread of the plastic zones. In addition, nearly self-similar, square-root singular fields are obtained within the portion of the plastic zone where the...

  13. Influence of Cracking Direction on Interfacial Fracture in Bicrystals With Symmetric Tilt Boundary

    Mohan, R.; Ortiz, M.; Shih, C. F.
    Recent experiments by Wang (1990) on copper bicrystals with a [110] symmetric tilt of 38.9 degrees have shown that the mode of fracture of these bicrystals, i.e., whether fracture is of a ductile or brittle nature, depends on the direction of cracking. An analysis of this effect within the framework of continuum crystal plasticity is presented. The formulation accounts for finite deformations and finite lattice rotations, as well as for the full three-dimensional collection of slip systems in FCC crystals. Our results indicate that, whereas the level of stress ahead of the crack tip is similar for the ductile and...

  14. Effect of Strain Hardening and Rate Sensitivity on the Dynamic Growth of a Void in a Plastic Material

    Ortiz, M.; Molinari, A.
    The problem studied in this paper concerns the dynamic expansion of a spherical void in an unbounded solid under the action of remote hydrostatic tension. The void is assumed to remain spherical throughout the deformation and the matrix to be incompressible. The effects of inertia, strain hardening, and rate sensitivity on the short and long-term behavior of the void, as well as on its response to ramp loading, are investigated in detail.

  15. Stability of solids with interfaces

    Suo, Z.; Ortiz, M.; Needleman, A.

  16. The Influence of Grain Size on the Toughness of Monolithic Ceramics

    Bower, A. F.; Ortiz, M.
    Experiments have shown that there may be an optimal grain size which maximizes the toughness of polycrystalline ceramics. In this paper, we attempt to develop a theoretical model which can predict the effect of grain size on the toughness of ceramics. We assume that three principal mechanisms affect the toughness of the material: distributed microcracking; crack trapping by tough grains; and frictional energy dissipation as grains are pulled out in the wake of the crack. The grain size influences these mechanisms in several ways. The energy dissipated due to frictional crack bridging increases with the size of the bridging grains,...

  17. An Analysis of Crack Trapping by Residual Stresses in Brittle Solids

    Bower, A. F.; Ortiz, M.
    The residual stress distribution in a brittle polycrystalline solid may have a significant influence on its toughness. Grains in a state of residual compression are less likely to be fractured by a growing crack and may trap the crack front or be left behind as bridging particles (Evans et al., 1977). This paper estimates the toughness enhancement due to intergranular residual stresses, using a three-dimensional model. The residual stress is approximated as a doubly sinusoidal distribution acting perpendicular to the plane of an initially straight semi-infinite crack. An incremental perturbation method developed by Bower and Ortiz (1990) for solving three-dimensional...

  18. Statistical Properties of Residual Stresses and Intergranular Fracture in Ceramic Materials

    Ortiz, M.; Suresh, S.
    The problem addressed in this paper concerns the statistical characterization of the state of residual stress generated in polycrystalline ceramics during cooling from the fabrication temperature. Detailed finite element simulations are carried out for an ensemble of large numbers of randomly oriented, planar hexagonal grains with elastic and thermal expansion anisotropy, and brittle grain interfaces. The calculations show that the distribution of normal and shear tractions induced by thermal contraction mismatch among grains is gaussian and that these tractions are statistically independent random variables. Although the gaussian nature of the distributions remains unaffected by the introduction of elastic anisotropy, the...

  19. A variational boundary integral method for the analysis of 3-D cracks of arbitrary geometry modelled as continuous distributions of dislocation loops

    Xu, G.; Ortiz, M.
    A finite element methodology for analysing propagating cracks of arbitrary three-dimensional geometry is developed. By representing the opening displacements of the crack as a distribution of dislocation loops and minimizing the corresponding potential energy of the solid, the kernels of the governing integral equations have mild singularities of the type 1/R. A simple quadrature scheme then suffices to compute all the element arrays accurately. Because of the variational basis of the method, the resulting system of equations is symmetric. By employing six-noded triangular elements and displacing midside nodes to quarter-point positions, the opening profile near the front is endowed with...

  20. Modelling and simulation of high-speed machining

    Marusich, T. D.; Ortiz, M.
    A Lagrangian finite element model of orthogonal high-speed machining is developed. Continuous remeshing and adaptive meshing are the principal tools which we employ for sidestepping the difficulties associated with deformation-induced element distortion, and for resolving fine-scale features in the solution. The model accounts for dynamic effects, heat conduction, mesh-on-mesh contact with friction, and full thermo-mechanical coupling. In addition, a fracture model has been implemented which allows for arbitrary crack initiation and propagation in the regime of shear localized chips. The model correctly exhibits the observed transition from continuous to segmented chips with increasing tool speed.

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