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Caltech Authors (165.044 recursos)

Repository of works by Caltech published authors.

Group = GALCIT

Mostrando recursos 1 - 20 de 390

  1. Dynamic sliding of frictionally held bimaterial interfaces subjected to impact shear loading

    Lykotrafitis, G.; Rosakis, A. J.
    The fast frictional sliding along an incoherent interface of a bimaterial system composed of a Homalite and a steel plate is studied experimentally in a microsecond time-scale. The plates are held together by a static uniform compressive pre-stress while dynamic sliding is initiated by asymmetric impact. The full-field technique of dynamic photoelasticity is simultaneously used with a local technique of velocimetry based on laser interferometry. In the case where the impact loading is applied to the Homalite plate, a shear Mach line originates from a disturbance propagating along the interface supersonically with respect to the dilatational wave speed of Homalite...

  2. Diffusive molecular dynamics simulations of lithiation of silicon nanopillars

    Mendez, J. P.; Ponga, M.; Ortiz, M.
    We report diffusive molecular dynamics simulations concerned with the lithiation of Si nano-pillars, i. e., nano-sized Si rods held at both ends by rigid supports. The duration of the lithiation process is of the order of miliseconds, well outside the range of molecular dynamics but readily accessible to diffusive molecular dynamics. The simulations predict an alloy Li_(15)Si_4 at the fully lithiated phase, exceedingly large and transient volume increments up to 300% due to the weakening of Si-Si iterations, a crystalline-to-amorphous-to-lithiation phase transition governed by interface kinetics, high misfit strains and residual stresses resulting in surface cracks and severe structural degradation...

  3. Analysis of intersonic crack growth in unidirectional fiber-reinforced composites

    Huang, Y.; Wang, W.; Liu, C.; Rosakis, A. J.
    Recent experiments on dynamic fracture of unidirectional fiber-reinforced graphite/epoxy composite materials showed that, in Mode I, the crack tip velocity could never exceed the shear wave speed, while the crack tip velocity in Mode II not only exceeded the shear wave speed but also approached a stable velocity at which the crack grew for a substantial period of time in experiments. The experimentally obtained fringe patterns also clearly showed the existence of shear shock waves when the crack tip velocity exceeded the shear wave speed. In the present study, we have obtained the asymptotic fields near an intersonically propagating crack...

  4. Dynamic shear band propagation and micro-structure of adiabatic shear band

    Li, Shaofan; Liu, Wing-Kam; Qian, Dong; Guduru, Pradeep R.; Rosakis, Ares J.
    Meshfree Galerkin approximations in both two and three dimensions have been used in simulations of dynamic shear band propagation in an asymmetrically impact-loaded prenotched plate. Failure mode switching and failure mode transitions, which have been reported experimentally, are replicated in numerical computations. For intermediate impact speed (25m/s30m/s), the numerical results show that a dynamic shear band penetrates through the specimen without trace of cleavage-type...

  5. An adaptive non-conforming finite-element method for Reissner-Mindlin plates

    Carstensen, Carsten; Weinberg, Kerstin
    Adaptive algorithms are important tools for efficient finite-element mesh design. In this paper, an error controlled adaptive mesh-refining algorithm is proposed for a non-conforming low-order finite-element method for the Reissner–Mindlin plate model. The algorithm is controlled by a reliable and efficient residual-based a posteriori error estimate, which is robust with respect to the plate's thickness. Numerical evidence for this and the efficiency of the new algorithm is provided in the sense that non-optimal convergence rates are optimally improved in our numerical experiments.

  6. From atomistics to the continuum: a mesh-free quasicontinuum formulation based on local max-ent approximation schemes

    Kochmann, Dennis M.; Amelang, Jeffrey S.; Español, Malena I.; Ortiz, Michael
    A novel quasicontinuum formulation based on mesh-free local maximum-entropy approximation schemes is presented, whose accuracy (compared to full atomistic simulations) is tunable and, in particular, can be designed superior to conventional affine approximation schemes.

  7. Long-term atomistic simulation of hydrogen absorption in palladium nanocubes using a diffusive molecular dynamics method

    Sun, Xingsheng; Ariza, Pilar; Ortiz, Michael; Wang, Kevin G.
    Understanding the transport of hydrogen within metallic nanomaterials is crucial for the advancement of energy storage and the mitigation of hydrogen embrittlement. Using nanosized palladium particles as a model, recent experimental studies have revealed several interesting phenomena that occur over long time periods. The time scale of these phenomena is beyond the capability of established atomistic models such as molecular dynamics. In this work, we present the application of a new approach, referred to as diffusive molecular dynamics (DMD), to the simulation of long-term diffusive mass transport at the atomic scale. Specifically, we simulate the absorption of hydrogen by palladium...

  8. Design of ultra-thin shell structures in the stochastic post-buckling range using Bayesian machine learning and optimization

    Bessa, M. A.; Pellegrino, S.
    A data-driven computational framework combining Bayesian regression for imperfection-sensitive quantities of interest, uncertainty quantification and multi-objective optimization is developed for the design of complex structures. The framework is used to design ultra-thin carbon fiber deployable shells subjected to two bending conditions. Significant increases in the ultimate buckling loads are shown to be possible, with potential gains on the order of 100% as compared to a previously proposed design. The key to this result is the existence of a large load reserve capability after the initial bifurcation point and well into the post-buckling range that can be effectively explored by the...

  9. Impulsively-Generated Pressure Transients and Strains in a Cylindrical Fluid-Filled Tube Terminated by a Converging Section

    Veilleux, Jean-Christophe; Shepherd, Joseph E.
    The syringe in a subcutaneous autoinjector may be subjected to internal pressure transients due to the normal operation of the injection mechanism. These transients are similar to transients in fluid-filled pipelines observed during water hammer events. In this paper, the effect of an air gap in the syringe and a converging section are studied experimentally and numerically in a model system which consists of a fluid-filled metal tube that is impulsively loaded with a projectile to simulate the action of the autoinjector mechanism operation. The air between the buffer and the water results in a complex interaction between the projectile...

  10. A variational constitutive model for soft biological tissues

    El Sayed, Tamer; Mota, Alejandro; Fraternali, Fernando; Ortiz, Michael
    In this paper, a fully variational constitutive model of soft biological tissues is formulated in the finite strain regime. The model includes Ogden-type hyperelasticity, finite viscosity, deviatoric and volumetric plasticity, rate and microinertia effects. Variational updates are obtained via time discretization and pre-minimization of a suitable objective function with respect to internal variables. Genetic algorithms are used for model parameter identification due to their suitability for non-convex, high dimensional optimization problems. The material behavior predicted by the model is compared to available tests on swine and human brain tissue. The ability of the model to predict a wide range of...

  11. Localization analysis under dynamic loading

    Leroy, Y.; Ortiz, M.
    A finite element method proposed by Ortiz et al. (1987) is used to study shear band formation in rate dependent and rate independent pressure sensitive solids under dynamic loading. Under these conditions, shear bands are observed to propagate in an irregular fashion in time and space. In particular, the development of multiple shear bands appears to be a prevalent mechanism of deformation at sufficiently high impact velocities.

  12. C^0 finite element discretization of Kirchhoff's equations of thin plate bending

    Ortiz, M.; Morris, G. R.
    An alternative formulation of Kirchhoff's equations is given which is amenable to a standard C^0 finite element discretization. In this formulation, the potential energy of the plate is formulated entirely in terms of rotations, whereas the deflections are the outcome of a subsidiary problem. The nature of the resulting equations is such that C^0 interpolation can be used on both rotations and deflections. In particular, general classes of triangular and quadrilateral isoparametric elements can be used in conjunction with the method. Unlike other finite element methods which are based on three-dimensional or Mindlin formulations, the present approach deals directly with...

  13. Crack propagation in monolithic ceramics under mixed mode loading

    Ortiz, M.; Giannakopoulos, A. E.
    Finite element calculations are presented for a semi-infinite crack in a brittle solid undergoing microcracking normal to the maximum tensile direction. Microcracks are presumed stable and a saturation stage is postulated wherein the effective elastic moduli attain steady state values. Mode I, mode II and mixed mode loading conditions are investigated. In these two latter cases, the method of analysis employed allows for cracks to grow out of their initial planes. The mixed mode loading case investigated corresponds to taking equal values of the remote mode I and II stress intensity factors. Contrary to what is observed in the mode...

  14. A Physical Model for the Inelasticity of Concrete

    Ortiz, M.; Popov, E. P.
    A physical model for the inelasticity of concrete is proposed in this paper. The main effects are attributed to microcracking and softening elastoplastic coupling. The composite nature of concrete is seen to influence decisively the process of microcracking, resulting, for instance, in stable crack growth under uniaxial compression, but unstable crack growth under uniaxial tension. A model is proposed that relates the degradation of the elastic compliances of the material to the extent of microcracking, as described by a set of internal variables that represent the sizes of the microcracks oriented along some selected directions. A thermodynamic approach to the...

  15. A Statistical Theory of Polycrystalline Plasticity

    Ortiz, M.; Popov, E. P.
    The plasticity and viscoplasticity of polycrystalline materials are studied analytically in terms of lattice dislocations, with the principal effects attributed to non-extended obstacles. Non-equilibrium statistical mecha­nics is used to describe the evolution of the dislocation structures during loading and unloading processes. A plausible variation in the probability density function for mobile dislocations for such processes is suggested. The proposed material model is in good qualitative agreement with several observed phenomena that previously could not be quantified on the basis of the dislocation theory. Numerical examples illustrate the effect of the rate of loading, the variations in the recovery effect as...

  16. Plain concrete as a composite material

    Ortiz, Miguel; Popov, Egor P.
    The purpose of this paper is to study the consequences of the composite nature of concrete. A plausible energy balance equation is postulated and the Green-Rivlin invariance principle is applied to it to derive the linear and angular momentum balance laws. General constitutive equations are discussed with the aid of thermodynamic potentials and Coleman's method. The distribution of the applied stresses between mortar and aggregate is also studied in detail, showing for instance that substantial tensile lateral stresses may appear in mortar under uniaxial compressive loading. These results are used to derive a criterion for the onset of inelasticity in...

  17. Operator split methods for the numerical solution of the elastoplastic dynamic problem

    Ortiz, Miguel; Pinsky, Peter M.; Taylor, Robert L.
    The elastoplastic dynamic problem is first formulated in a form that facilitates the application of product formula techniques. The additive decomposition of the dynamic equations into elastic and plastic parts is taken as a basis for the definition of product algorithms that exploit such decomposition. In the context of a finite element discretization, these product algorithms entail, for every time step, the solution of an elastic problem followed by the application of plastic algorithms that operate on the stresses and internal variables at the integration points and bring in the plastic constitutive relations. Suitable plastic algorithms are discussed for the...

  18. Unconditionally stable element-by-element algorithms for dynamic problems

    Ortiz, Miguel; Pinsky, Peter M.; Taylor, Robert L.
    A collection of results is presented regarding the consistency, stability and accuracy of operator split methods and product formula algorithms for general nonlinear equations of evolution. These results are then applied to the structural dynamics problem. The basic idea is to exploit an element-by-element additive decomposition of a particular form of the discrete dynamic equations resulting from a finite element discretization. It is shown that such a particular form of the discrete dynamic equations is obtained when velocity and stress are taken as unknowns. By applying the general product formula technique to the element-by-element decomposition, unconditionally stable algorithms are obtained...

  19. Distortional Hardening Rules for Metal Plasticity

    Ortiz, Miguel; Popov, Egor P.
    A brief overview of the available experimental data regarding distortional hardening of metals is first presented. This material is subsequently used to motivate the need for accurate distortional hardening rules in computation. A general expression for the yield surface of a plastic material is proposed that includes the isotropic‐kinematic von Mises model as a particular case and that can be systematically used to incorporate distortional hardening features into the material modeling in a simple manner. This expression is complemented with suitable rate equations for the parameters involved. The proposed model is particularly convenient for computer implementation.

  20. Numerical integration of rate constitutive equations in finite deformation analysis

    Pinsky, Peter M.; Ortiz, Miguel; Pister, Karl S.
    In analysis of finite deformation problems the use of constitutive equations in rate form is often required. In a spatial setting, these equations may express a relationship between some objective rate of spatial stress tensor and the rate of deformation. Constitutive equations of this type characterize a variety of material models including hyperelasticity, hypoelasticity and elastoplasticity. Employing geometrical concepts, a family of unconditionally stable and incrementally objective algorithms is proposed for the integration of such rate constitutive equations. These algorithms, which are appropriate for finite deformation analysis, are applicable to any choice of stress rate and, in most cases, employ...

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