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Kumbakonam R. Rajagopal

Bio: Kumbakonam R. Rajagopal is an academic researcher from Texas A&M University. The author has contributed to research in topics: Constitutive equation & Viscoelasticity. The author has an hindex of 77, co-authored 659 publications receiving 23443 citations. Previous affiliations of Kumbakonam R. Rajagopal include Kent State University & University of Wisconsin-Madison.


Papers
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Journal ArticleDOI
TL;DR: The coupling between the response of the biodegradable stent and the arterial wall is examined in order to support the design of biodesgradable polymeric stents by considering reasonably realistic geometrical and material models.

6 citations

Journal ArticleDOI
TL;DR: In this article, the velocity of a lamina in between two parallel plates containing a fluid of second grade was determined by solving the mixed initial-boundary value problem using Laplace transform.
Abstract: The falling of a lamina in between two parallel plates containing a fluid of second grade is studied. The velocity of the lamina and the fluid are determined by solving the mixed initial—boundary value problem using Laplace transform. Explicit exact solutions are obtained for the velocity of the lamina and the fluid. Next, the falling of a cylinder in a tube containing a fluid of second grade is analyzed using Laplace transform, and once again exact solutions are found.

6 citations

Journal ArticleDOI
TL;DR: In this article, the effects of the anisotropy of the material and the pre-stretching on the process of diffusion were studied and the non-linear equations governing the diffusion through the shell were solved numerically.
Abstract: The problem of radial diffusion of a fluid through a transversely isotropic non-linearly elastic thick spherical shell is studied. The anisotropic shell is also pre-stretched radially. The non-linear equations governing the diffusion through the shell are solved numerically. The effects of the anisotropy of the material and the pre-stretching on the process of diffusion are studied.

6 citations

Journal ArticleDOI
TL;DR: In this article, a new constitutive approach to describing the response of bodies, both solid and fluid, that can only undergo isochoric motions in isothermal processes but which can undergo non-isochoric motion in arbitrary processes is discussed.
Abstract: In this short note, we discuss a new constitutive approach to describing the response of bodies, both solid and fluid, that can only undergo isochoric motions in isothermal processes but which can undergo non-isochoric motions in arbitrary processes. Within this new framework, one finds that conditions that were perceived as constraints on the response of the body now arise naturally within the frame work of the constitutive definition of these bodies. For instance, a central approximation in fluid mechanics that is of great utility in the analysis of fluid flow problems in geophysics and astrophysics is that due to Oberbeck (Ann Phys Chem 1:271–292, 1879; Uber die bewengungsercheinungen der Atmosphare, Sitz Ber K Preuss Akad Miss, pp 383–395, 1129–1138, 1888) and Boussinesq (Theorie Analytique de la Chaleur. Gauthier-Villas, Paris, 1903) for describing the flow of fluids, which can undergo only isochoric motions in isothermal processes but which are otherwise capable of non-isochoric motions. A similar demand can be made concerning the response of solid bodies wherein one could carry out an approximation similar to that of the Oberbeck–Boussinesq equations, and such an approach might be of great value in the study of technologically relevant problems.

6 citations

Journal ArticleDOI
TL;DR: In this article, a mixture model for curing and interphase evolution is presented that is based on a consistent thermodynamic theory for multi-constituent materials, cast in a stabilized finite element method that is developed employing variational multi-scale ideas for edge-based stabilization and consistent tying of the constituents at domain boundaries.
Abstract: Chemical reactions at bimaterial interfaces during manufacturing of fiber–matrix systems result in an interphase that plays a dominant role in the response of the composite when subjected to mechanical loads. An accurate modeling of the degree of cure in the interfacial region, because of its effect on the evolving properties of the interphase material, is critical to determining the coupled chemo-mechanical interphase stresses that influence the structural integrity of the composite and its fatigue life. A mixture model for curing and interphase evolution is presented that is based on a consistent thermodynamic theory for multi-constituent materials. The mixture model is cast in a stabilized finite element method that is developed employing variational multi-scale ideas for edge-based stabilization and consistent tying of the constituents at the domain boundaries. The ensuing computational method accounts for curing and interphase chemical reactions for the evolution of the density and material modulus of the constituents that have a direct effect on the interfacial stiffness and strength. Several test cases are presented to show the range of applicability of the model and the method.

6 citations


Cited by
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Journal ArticleDOI
TL;DR: In this paper, the authors developed a constitutive law for the description of the (passive) mechanical response of arterial tissue, where the artery is modeled as a thick-walled nonlinearly elastic circular cylindrical tube consisting of two layers corresponding to the media and adventitia.
Abstract: In this paper we develop a new constitutive law for the description of the (passive) mechanical response of arterial tissue. The artery is modeled as a thick-walled nonlinearly elastic circular cylindrical tube consisting of two layers corresponding to the media and adventitia (the solid mechanically relevant layers in healthy tissue). Each layer is treated as a fiber-reinforced material with the fibers corresponding to the collagenous component of the material and symmetrically disposed with respect to the cylinder axis. The resulting constitutive law is orthotropic in each layer. Fiber orientations obtained from a statistical analysis of histological sections from each arterial layer are used. A specific form of the law, which requires only three material parameters for each layer, is used to study the response of an artery under combined axial extension, inflation and torsion. The characteristic and very important residual stress in an artery in vitro is accounted for by assuming that the natural (unstressed and unstrained) configuration of the material corresponds to an open sector of a tube, which is then closed by an initial bending to form a load-free, but stressed, circular cylindrical configuration prior to application of the extension, inflation and torsion. The effect of residual stress on the stress distribution through the deformed arterial wall in the physiological state is examined. The model is fitted to available data on arteries and its predictions are assessed for the considered combined loadings. It is explained how the new model is designed to avoid certain mechanical, mathematical and computational deficiencies evident in currently available phenomenological models. A critical review of these models is provided by way of background to the development of the new model.

2,887 citations

Journal ArticleDOI
TL;DR: In this paper, a simple model that satisfies most of these criteria uses depth-averaged equations of motion patterned after those of the Savage-Hutter theory for gravity-driven flow of dry granular masses but generalized to include the effects of viscous pore fluid with varying pressure.
Abstract: Recent advances in theory and experimen- tation motivate a thorough reassessment of the physics of debris flows. Analyses of flows of dry, granular solids and solid-fluid mixtures provide a foundation for a com- prehensive debris flow theory, and experiments provide data that reveal the strengths and limitations of theoret- ical models. Both debris flow materials and dry granular materials can sustain shear stresses while remaining stat- ic; both can deform in a slow, tranquil mode character- ized by enduring, frictional grain contacts; and both can flow in a more rapid, agitated mode characterized by brief, inelastic grain collisions. In debris flows, however, pore fluid that is highly viscous and nearly incompress- ible, composed of water with suspended silt and clay, can strongly mediate intergranular friction and collisions. Grain friction, grain collisions, and viscous fluid flow may transfer significant momentum simultaneously. Both the vibrational kinetic energy of solid grains (mea- sured by a quantity termed the granular temperature) and the pressure of the intervening pore fluid facilitate motion of grains past one another, thereby enhancing debris flow mobility. Granular temperature arises from conversion of flow translational energy to grain vibra- tional energy, a process that depends on shear rates, grain properties, boundary conditions, and the ambient fluid viscosity and pressure. Pore fluid pressures that exceed static equilibrium pressures result from local or global debris contraction. Like larger, natural debris flows, experimental debris flows of ;10 m 3 of poorly sorted, water-saturated sediment invariably move as an unsteady surge or series of surges. Measurements at the base of experimental flows show that coarse-grained surge fronts have little or no pore fluid pressure. In contrast, finer-grained, thoroughly saturated debris be- hind surge fronts is nearly liquefied by high pore pres- sure, which persists owing to the great compressibility and moderate permeability of the debris. Realistic mod- els of debris flows therefore require equations that sim- ulate inertial motion of surges in which high-resistance fronts dominated by solid forces impede the motion of low-resistance tails more strongly influenced by fluid forces. Furthermore, because debris flows characteristi- cally originate as nearly rigid sediment masses, trans- form at least partly to liquefied flows, and then trans- form again to nearly rigid deposits, acceptable models must simulate an evolution of material behavior without invoking preternatural changes in material properties. A simple model that satisfies most of these criteria uses depth-averaged equations of motion patterned after those of the Savage-Hutter theory for gravity-driven flow of dry granular masses but generalized to include the effects of viscous pore fluid with varying pressure. These equations can describe a spectrum of debris flow behav- iors intermediate between those of wet rock avalanches and sediment-laden water floods. With appropriate pore pressure distributions the equations yield numerical so- lutions that successfully predict unsteady, nonuniform motion of experimental debris flows.

2,426 citations

01 Mar 1987
TL;DR: The variable-order Adams method (SIVA/DIVA) package as discussed by the authors is a collection of subroutines for solution of non-stiff ODEs.
Abstract: Initial-value ordinary differential equation solution via variable order Adams method (SIVA/DIVA) package is collection of subroutines for solution of nonstiff ordinary differential equations. There are versions for single-precision and double-precision arithmetic. Requires fewer evaluations of derivatives than other variable-order Adams predictor/ corrector methods. Option for direct integration of second-order equations makes integration of trajectory problems significantly more efficient. Written in FORTRAN 77.

1,955 citations

Journal ArticleDOI
TL;DR: A structural continuum framework that is able to represent the dispersion of the collagen fibre orientation is developed and allows the development of a new hyperelastic free-energy function that is particularly suited for representing the anisotropic elastic properties of adventitial and intimal layers of arterial walls.
Abstract: Constitutive relations are fundamental to the solution of problems in continuum mechanics, and are required in the study of, for example, mechanically dominated clinical interventions involving soft biological tissues. Structural continuum constitutive models of arterial layers integrate information about the tissue morphology and therefore allow investigation of the interrelation between structure and function in response to mechanical loading. Collagen fibres are key ingredients in the structure of arteries. In the media (the middle layer of the artery wall) they are arranged in two helically distributed families with a small pitch and very little dispersion in their orientation (i.e. they are aligned quite close to the circumferential direction). By contrast, in the adventitial and intimal layers, the orientation of the collagen fibres is dispersed, as shown by polarized light microscopy of stained arterial tissue. As a result, continuum models that do not account for the dispersion are not able to capture accurately the stress–strain response of these layers. The purpose of this paper, therefore, is to develop a structural continuum framework that is able to represent the dispersion of the collagen fibre orientation. This then allows the development of a new hyperelastic free-energy function that is particularly suited for representing the anisotropic elastic properties of adventitial and intimal layers of arterial walls, and is a generalization of the fibre-reinforced structural model introduced by Holzapfel & Gasser (Holzapfel & Gasser 2001 Comput. Meth. Appl. Mech. Eng. 190, 4379–4403) and Holzapfel et al. (Holzapfel et al. 2000 J. Elast. 61, 1–48). The model incorporates an additional scalar structure parameter that characterizes the dispersed collagen orientation. An efficient finite element implementation of the model is then presented and numerical examples show that the dispersion of the orientation of collagen fibres in the adventitia of human iliac arteries has a significant effect on their mechanical response.

1,905 citations