Página 1 dos resultados de 11 itens digitais encontrados em 0.000 segundos

Sexual Morphogenesis in Achlya: Ultrastructural Basis for the Hormonal Induction of Antheridial Hyphae*

Mullins, J. Thomas; Ellis, E. Ann
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /04/1974 EN
Relevância na Pesquisa
26.21%
Male strains of the water mold Achlya ambisexualis produce antheridial hyphae in response to the steroid hormone antheridiol. The antheridial hypha is postulated to be initiated through a localized wall softening with the enzyme cellulase. Freeze-etch studies of hormone-treated hyphae were conducted to determine if aggregates of vesicles are induced at the location of new antheridial hyphae. Localized aggregates of vesicles were found in conjunction with areas of wall thinning. These data suggest that the processes of vesiculation and secretion provide a mechanism for concentration of cellulase at the site of initiation of antheridial hyphae.

Mechanical Properties of Murine and Porcine Ocular Tissues in Compression

Worthington, Kristan S.; Wiley, Luke A.; Bartlett, Alexandra M.; Stone, Edwin M.; Mullins, Robert F.; Salem, Aliasger K.; Guymon, C. Allan; Tucker, Budd A.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
EN
Relevância na Pesquisa
26.21%
Sub-retinal implantation of foreign materials is becoming an increasingly common feature of novel therapies for retinal dysfunction. The ultimate compatibility of implants depends not only on their in vitro chemical compatibility, but also on how well the mechanical properties of the material match those of the native tissue. In order to optimize the mechanical properties of retinal implants, the mechanical properties of the mammalian retina itself must be carefully characterized. In this study, the compressive moduli of eye tissues, especially the retina, were probed using a dynamic mechanical analysis instrument in static mode. The retinal compressive modulus was lower than that of the sclera or cornea, but higher than that of the RPE and choroid. Compressive modulus remained relatively stable with age. Conversely, apparent retinal softening occurred at an early age in mice with inherited retinal degeneration. Compressive modulus is an important consideration for the design of retinal implants. Polymer scaffolds with moduli that are substantially different than that of the native tissue in which they will ultimately reside will be less likely to aid in the differentiation and development of the appropriate cell types in vitro and will have reduced biocompatibility in vivo.

A study of the stretch-induced softening behavior of particle filled elastomers/

Greb, Andrea (Andrea L.)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 61 leaves
ENG
Relevância na Pesquisa
26.74%
Elastomers are able to undergo relatively large deformations in an elastic manner, which makes them the material of choice for a wide range of applications. In some cases, filler particles, such as carbon black, are added to the elastomer to alter the mechanical behavior when subjected to different loading conditions. When subjected to cyclic loading conditions, elastomers undergo stress-induced softening, known as the Mullins effect, and this softening behavior is influenced by the amount of filler particle present. The softening is considered to be an evolution of the soft and hard domain microstructure of the material, with the effective soft domain increasing with stretch. In this study, finite element analysis will be used to understand the softening behavior of particle reinforced elastomers. The softening behavior of the parent elastomer will be modeled using a constitutive model proposed by Qi and Boyce (2004). Nonlinear finite element analysis using the ABAQUS code was used to model elastomers with various volume fractions of filler particles, and the stress-strain behavior of the composite and evolution of the soft domain within the matrix is computed. The addition of filler particles was found to increase the overall stiffness of the elastomer...

A review on the Mullins effect

GILORMINI, Pierre; DIANI, Julie; FAYOLLE, Bruno
Fonte: Elsevier Publicador: Elsevier
EN_US
Relevância na Pesquisa
36.79%
The Mullins’ effect remains a major challenge in order to provide good mechanical modeling of the complex behavior of industrial rubber materials. It’s been forty years since Mullins [1] wrote his review on the phenomenon and still no general agreement has been found either on the physical source or on the mechanical modeling of this effect. Therefore, we reviewed the literature dedicated to this topic over the past six decades. We present the experimental evidences, which characterize the Mullins’ softening. The phenomenon is observed in filled and crystallizing rubbers. Then, the phenomenological models dedicated to fit the mechanical behavior of rubbers undergoing some Mullins’ softening are studied. To overcome the limit of a descriptive phenomenological modeling, several authors looked for a physical understanding of the phenomenon. Various theories have been exposed, but none of them has been supported unanimously. Nonetheless, these theories favor the emergence of physically based mechanical behavior laws. We tested some of these laws, which show little predictive abilities since the values of their parameters do not compare well with the physical quantities they are linked to.; Projet ANR jeunes chercheurs MELAC JC05_43403.

Constitutive modeling of the anisotropic behavior of Mullins softened filled rubbers

MERCKEL, Yannick; DIANI, Julie; BRIEU, Mathias; CAILLARD, julien
Fonte: Elsevier Publicador: Elsevier
EN
Relevância na Pesquisa
57.06%
Original constitutive modeling is proposed for filled rubber materials in order to capture the anisotropic softened behavior induced by general non-proportional pre-loading histo-ries. The hyperelastic framework is grounded on a thorough analysis of cyclic experimental data. The strain energy density is based on a directional approach. The model leans on the strain amplification factor concept applied over material directions according to the Mul-lins softening evolution. In order to provide a model versatile that applies for a wide range of materials, the proposed framework does not require to postulate the mathematical forms of the elementary directional strain energy density and of the Mullins softening evo-lution rule. A computational procedure is defined to build both functions incrementally from experimental data obtained during cyclic uniaxial tensile tests. Successful compari-sons between the model and the experiments demonstrate the model abilities. Moreover, the model is shown to accurately predict the non-proportional uniaxial stress-stretch responses for uniaxially and biaxially pre-stretched samples. Finally, the model is effi-ciently tested on several materials and proves to provide a quantitative estimate of the anisotropy induced by the Mullins softening for a wide range of filled rubbers.

Physical interpretation of the Mullins softening in a carbon-black filled SBR

DIAZ, Rodrigo; DIANI, Julie; GILORMINI, Pierre
Fonte: Elsevier Publicador: Elsevier
EN
Relevância na Pesquisa
67.03%
A 40 phr carbon-black filled styrene butadiene rubber has been submitted to several experiments in order to identify the physical damage responsible for the mechanical softening recorded upon first stretch. Damage in the rubber matrix was determined by swelling. The filler structure alteration was monitored by electrical conductivity measurements. Both damages are shown to be of minor importance compared to the substantial mechanical softening undergone by the material. Degradation at the rubber-filler interface may be recovered by exposing the material at high temperatures in vacuo. The chain mobility in such storage conditions promotes free chain adsorption at the filler surface. The existence of a layer of polymer whose movements are hindered adds to the filler reinforcement and its desorption creates Mullins softening.

Effect of the microstructure parameters on the Mullins softening in carbon-black filled SBRs

MERCKEL, Yannick; DIANI, Julie; BRIEU, Matthias; GILORMINI, Pierre; CAILLARD, Julien
Fonte: Wiley Publicador: Wiley
EN
Relevância na Pesquisa
67.15%
[The definitive version is available at www3.interscience.wiley.com]; A quantitative estimate of the Mullins softening is proposed and tested on various carbon-black filled styrene-butadiene rubbers. In order to model the behaviour of elastomeric materials, some constitutive equations reported in the literature are based on the account of a strain amplification factor, which evolves with the maximum strain history. The amplification factor is grounded on the representation of filled rubbers as heterogeneous materials made of hard rigid domains and soft deformable domains. In the present work, this factor is splitted into two parts with opposite effects that account for the Mullins softening and for the filler reinforcement, respectively. Evolutions of both parts are obtained through a direct analysis of cyclic uniaxial tensile tests performed on a series of materials. The Mullins softening part is shown to linearly depend on the filler volume fraction and on the maximum strain applied, when defined as the first invariant of the Hencky tensor. Its changes with the gum crosslink density parameter are insignificant. The reinforcement part of the amplification factor shows quadratic dependence on the filler volume fraction.; ANR MATETPRO AMUFISE

Charcaterization of the Mullins effect of carbon-black filled rubbers

MERCKEL, Yannick; DIANI, Julie; BRIEU, Matthias; GILORMINI, Pierre; CAILLARD, Julien
Fonte: American Chemical Society Publicador: American Chemical Society
EN
Relevância na Pesquisa
37.15%
Publisher version : http://rubberchemtechnol.org/doi/abs/10.5254/1.3592294?journalCode=rcat; Several carbon-black filled styrene-butadiene rubbers showed different sensibilities to the Mullins softening when submitted to cyclic uniaxial tension. In order to quantify this softening, a damage parameter was introduced. It is defined by using a classic damage approach and can be estimated by using either the strain amplification factor method or the tangent modulus at zero stress. The proposed parameter is used to study the effects of crosslink density and filler amount on the Mullins softening. The latter is shown to remain unaffected by a change of crosslink density and to increase with an increase of filler amount. The damage parameter exhibits mere linear dependences on the maximum Hencky strain applied and on the filler volume fraction. A simple linear expression is given finally to predict the Mullins softening of filled rubbers. The parameter also provides an objective analysis for the Mullins softening that supports comments on a better understanding of this effect.; ANR MATETPRO AMUFISE

On the thermodynamics of pseudo-elastic material models to reproduce the Mullins effect

Naumann, Christoph; Ihlemann, Jörn
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 04/08/2014
Relevância na Pesquisa
26.67%
This work focuses on the thermodynamics of pseudo-elastic models which represent the Mullins effect. Two established models are analyzed theoretically, their thermomechanical properties are derived, and certain critical points are identified. These findings are used to deduce an alternative approach to deviate pseudo-elasticity. This is achieved by defining a suitable free energy which imposes conditions on the stress tensor and the dissipation using the Clausius-Duhem inequality. The concept of pseudo-elasticity is then generalized to extend arbitrary thermomechanical, even inelastic, material models to allow for softening effects. Under weak assumptions on the softening function the thermomechanical consistency is shown.; Comment: 16 pages, 6 figures

Computational assessment of ballistic impact on a high strength structural steel/polyurea composite plate

El Sayed, T.; Mock, Willis, Jr; Mota, Alejandro; Fraternali, Fernando; Ortiz, Michael
Fonte: Springer-Verlag Publicador: Springer-Verlag
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em /03/2009
Relevância na Pesquisa
16.21%
Ballistic impact on a polyurea retrofitted high strength structural steel plate is simulated and validated. A soft material model for polyurea, which is capable of capturing complex mechanical behavior characterized by large strains, hysteresis, rate sensitivity, stress softening (Mullins effect), and deviatoric and volumetric plasticity, is calibrated against several uniaxial tension experiments and a three-dimensional release wave experiment to capture both the material point and bulk behaviors. A porous plasticity model is employed to model the high strength structural steel and localization elements are included to capture adiabatic shear bands and strain localization. The computational capabilities of these models are demonstrated by the prediction of the target plate displacement, which shows excellent agreement with experiments.

Modeling of Nonlinear Viscoelastic Solids with Damage Induced Anisotropy, Dissipative Rolling Contact Mechanics, and Synergistic Structural Composites

Zehil, Gerard-Philippe Guy May
Fonte: Universidade Duke Publicador: Universidade Duke
Tipo: Dissertação
Publicado em //2013
Relevância na Pesquisa
16.67%

The main objectives of this research are: (i) to elaborate a unified nonlinear viscoelastic model for rubber-like materials, in finite strain, accounting for material softening under deformation, and for damage induced anisotropy, (ii) to conceive, implement and test, simple, robust and efficient frictional rolling and sliding contact algorithms, in steady-state, as alternatives to existing, general purpose, contact solving strategies, (iii) to develop and verify high fidelity and computationally efficient modeling tools for isotropic and anisotropic viscoelastic objects in steady-state motion, (iv) to investigate, numerically and through experimentation, the influence of various material parameters, including material nonlinearities such at the Payne effect and the Mullins effect, as well as geometric parameters and contact surface conditions, on viscoelastic rolling resistance, and (iv) to explore, analytically and through experimentation, the conditions under which favorable mechanical synergies occur between material components and develop novel composites with improved structural performances.

A new constitutive model that unifies the behavioral characterizations of rubber-like materials in a broad range of loading regimes is proposed. The model reflects two fundamental aspects of rubber behavior in finite strain: (i) the Mullins effect...