Search results for "gradient"

A theoretical link between gradient and nonlocal elasticity models, including higher order boundary conditions

2013

The paper presents a recently developed rational derivation of the strain gradient elasticity model from the nonlocal (or integral) model. This kind of derivations are generally recovered just by an expansion into a Taylor series of the nonlocal strain field up to a certain order, and then operating the integration (or averaging) over the spatial interaction domain. The latter procedure is fully consistent when the analysis is performed over an unbounded domain, but when a classical bounded domain is analyzed it lacks in reproducing the so-called higher-order boundary conditions. In the present contributions the complete derivation is achieved employing an extended version of the Principle …

Energy-Residual-Based approach to gradient plasticity

2008

The “energy-residual-based approach” mentioned in the title consists in a thermodynamically consistent procedure for the formulation of a phenomenological plasticity model of either strain gradient, or nonlocal (integral) type. The authors have developed this procedure on the last ten years. It seem therefore appropriate to present an update of this theory at this forum. For brevity we shell limit ourselves to strain gradient plasticity.

A link between the residual-based gradient plasticity theory and the analogous theories based on the virtual work principle

2009

A link is shown to exist between the so-called residual-based strain gradient plasticity theory and the analogous theories based on the (extended) virtual work principle (VWP). To this aim, the former theory is reformulated and cast in a residual-free form, whereby the insulation condition and the (nonlocal) Clausius–Duhem inequality, on which the theory is grounded, are substituted with equivalent residual-free ingredients, namely the energy balance condition and the residual-free form of the Clausius–Duhem inequality. The equivalence of the residual-free formulation to the original one is shown, also in their ability to cope with energetic size effects and interfacial energy ones. It emer…

Unified thermodynamic framework for nonlocal/gradient continuum theories

2003

Abstract A thermodynamic framework, equipped with the concept of nonlocality (energy) residual, is utilized to address nonlocal/gradient internal variable material models. A unified procedure is provided for either nonlocal and gradient materials, which makes it possible to determine the thermodynamic restrictions upon the constitutive equations, and in particular the pertinent state equations, the consistent form of the dissipation power and the constitutive expression of the nonlocality residual. Additionally, for gradient models, the associated nonstandard boundary conditions are derived, pointing out their basically constitutive nature and their substantial differences from the standard…

PLLA Scaffold with Gradient pore size in microphysiological tissue system bioreactor for Osteochondral regeneration

2015

Cartilage and bone tissues in the joints are intimately linked and form the osteochondral unit. A better understanding of both disease and regenerative processes of bone and cartilage requires the study of both tissues together, as part of the osteochondral unit to account for their mutual interactions. However, the production of scaffolds for osteochondral tissue regeneration is a challenging task, since scaffolds must mimic the differents morphologies of cartilage and bone. Thermally Induced Phase Separation (TIPS) is one of the most adaptable techniques to produce porous scaffold for Tissue Engineering applications. A wide range of morphologies in terms of both pore size and distribution…

An Innovative Method to Produce Scaffolds with a Pore Size Gradient for Tissue Engineering

2015

Green Synthesis, Molecular Characterization and Associative Behavior of Some Gemini Surfactants without a Spacer Group.

2013

A series of new gemini surfactants without a spacer group, disodium 2,3-dialkyl-1,2,3,4-butanetetracarboxylates, were synthesized in a green chemistry context minimizing the use of organic solvents and applying microwaves (MW) when activation energy was required. Once the desired architecture was confirmed by means of the nuclear magnetic resonance technique (1H-NMR, 1H-1H COSY) for all the studied surfactants, the critical micellization concentration was determined by conductance measurements. The diffusion coefficient of micelles formed by the four compounds was characterized using pulsed field gradient (PFG)-NMR. Diffusion coefficients were found to be dependent on the concentration and …

Conjugate Gradient Method for Brain Magnetic Resonance Images Segmentation

2018

Part 8: Pattern Recognition and Image Processing; International audience; Image segmentation is the process of partitioning the image into regions of interest in order to provide a meaningful representation of information. Nowadays, segmentation has become a necessity in many practical medical imaging methods as locating tumors and diseases. Hidden Markov Random Field model is one of several techniques used in image segmentation. It provides an elegant way to model the segmentation process. This modeling leads to the minimization of an objective function. Conjugate Gradient algorithm (CG) is one of the best known optimization techniques. This paper proposes the use of the nonlinear Conjugat…

Isolation and Characterization of Epidermal DNA and RNA from Guinea Pig Skin

1971

DNA and RNA were isolated from mammalian epidermis in a relatively small scale procedure. The high purity and native state of the DNA isolated is reflected by its molar absorptivity E (P), its thermal hyperchromicity and its hyperchromicity upon DNase treatment and by its sedimentation profile as well as by its profile in a cesium chloride density gradient. The very low content of protein and RNA, as well as the data of DNA determination, indicate that this method permits the isolation of a highly purified product. This is further substantiated by the determination of UV absorption spectra and by analysis of the base composition.The mammalian skin DNA showed the following properties: Mol. w…

Hellinger-Reissner variational principle for stress gradient elastic bodies with embedded coherent interfaces

2017

An Hellinger-Reissner (H-R) variational principle is proposed for stress gradient elasticity material models. Stress gradient elasticity is an emerging branch of non-simple constitutive elastic models where the infinitesimal strain tensor is linearly related to the Cauchy stress tensor and to its Laplacian. The H-R principle here proposed is particularized for a solid composed by several sub-domains connected by coherent interfaces, that is interfaces across the which both displacement and traction vectors are continuous. In view of possible stress-based finite element applications, a reduced form of the H-R principle is also proposed in which the field linear momentum balance equations are…