Modeling the nonlinear viscoelastic behavior of polyurea using a distortion modified free volume approach

The pressure-dependent behavior of polyurea was examined under monotonic loading in the confined compression configuration. Additional data from Arcan shear and uniaxial compression was used to respectively complete parameter selection for the linear and nonlinear behavior and then validate it. The bulk and shear relaxation behavior were both pressure dependent. Under ramp loadings, the shear and tensile responses were quite nonlinearly viscoelastic.     

On the dynamics of localization and fragmentation—III. Effect of cladding with a polymer

In this series of papers, we investigate the mechanics and physics of necking and fragmentation in ductile materials. The behavior of ductile metals at strain rates of about 4,000–15,000 per second is considered. The expanding ring experiment is used as the vehicle for examining the material behavior in this range of strain rates. In Part I, the details of the experiment and the experimental observations on Al 6061-O were reported. Statistics of necking and fragmentation were evaluated and the process was modeled through the idea of the Mott release waves both from necking and fragmentation. Finally, it was shown that the strain in the ring never exceeded the necking strain in regions that strained uniformly. In Part II, we addressed the issues of strain hardening, ductility, geometry and size. Specifically, we examined different materials—Al 1100-H14, and Cu 101—and concluded that geometric constraint influences the strain at onset of localization significantly. The time taken for the localization to propagate across the cross-section and begin to unload its neighborhood was shown to control the amount of strain that can be experienced by the material; this also influences the statistics of localization and fragmentation. In the present paper, Part III, we examine the influence of compliant polymeric claddings on the localization and fragmentation response of metallic materials. Thin aluminum rings were coated with a layer of polyurea, with the thickness being an important parameter in the study. The onset of necking localization is shown not to be influenced by the coating; however, the propagation of unloading or release waves is shown to be significantly impeded by the cladding and therefore, further straining and fragmentation of the rings is affected. This result is of great importance in determining the impact resistance of elastomer-clad metallic structures. In future contributions as part of this sequel, we will explore the effect the development of localization and fragmentation in tubes and sheets where the geometric constraint can be varied over an even larger range.     

A non-obtrusive user interface for increasing social awareness on the World Wide Web

Arguing for the need of increasing social awareness on the World Wide Web, we describe a user interface based on the metaphor of windows bridging electronic and physical spaces. We present a system that, with the aim of making online activity perceptible in the physical world, makes it possible tohear people visiting one's website. The system takes advantage of theseamless andcontinuous network connection offered by handheld Web-appliances such as personal digital assistants.     

On the Use of Laminated Beams for the Determination of Pure and Mixed-Mode Fracture Properties of Structural Adhesives

The use of cracked laminated beam specimens is proposed for determining fracture properties of structural adhesives. Slight variations in specimen geometry and loading are used to produce pure mode I and II conditions as well as an intermediate mode-mix. Bounds are established for proper use of the specimens and mixed-mode fracture criteria are examined.     

Experimental myocardial infarct in the pig: effect of temporary ligation and revascularization on the ST segment in the epicardial ECG and on the incidence of ventricular fibrillation

A short period of myocardial ischemia induced by temporary ligation of 2 min duration prior to occlusion of the left anterior descending branch of the left coronary artery (LAD) in the pig does not have a beneficial effect on epicardial ST-segment alterations or on the incidence of ventricular fibrillation. One-stage ligation therefore seems to be the procedure of choice in occluding the porcine LAD. In 14 of 24 animals an isolated alternans of the ST-T segment was observed during the first few minutes of occlusion. Reperfusion after 60 min caused temporary increase of the ST-segment alterations, in contrast to the findings in the dog, where revascularization causes prompt return of the ST-segment to the pre-occlusion level. In addition, ectopic rhythms were frequently observed after removal of ligation.     

A Distortion-Modified Free Volume Theory for Nonlinear Viscoelastic Behavior

Many polymeric materials, including structural adhesives, exhibit anonlinear viscoelastic response. The nonlinear theory of Knauss and Emri(Polym. Engrg. Sci. 27, 1987, 87–100) is based on the Doolittle conceptthat the free volume controls the mobility of polymer molecules and,thus, the inherent time scale of the material. It then follows thatfactors such as temperature and moisture, which change the free volume,will influence the time scale. Furthermore, stress-induced dilatationwill also affect the free volume and, hence, the time scale. However,during this investigation, dilatational effects alone were found to beinsufficient for describing the response of near pure shear tests of abisphenol A epoxy with amido amine hardener. Thus, the free volumeapproach presented here has been modified to include distortionaleffects in the inherent time scale of the material. The same was foundto be true for a urethane adhesive.The small strain viscoelastic responses of the two materials havebeen determined from master curves of uniaxial and bulk creep testing atvarious temperatures. The nonlinear free volume model, modified toinclude distortional effects in the reduced time, was incorporated inthe ABAQUS finite element code via a user-defined material subroutine.For the epoxy, validation of the modified theory (a strain-basedformulation of free volume) has been achieved through good agreementbetween the computational and experimental results of butterfly-shapedArcan specimens subjected to loadings ranging from near pure shear toshear with various amounts of superposed tension and compression. Inaddition to predicting the response under a variety of multiaxial stressstates, the modified free volume theory also accurately predicts theformation and growth of shear banding, or regions of highly localizeddeformation, which have been found to occur upon continued loading ofthe epoxy. The urethane did not appear to exhibit any localizeddeformation over the range of temperatures at which it was tested.As a result, a stress-based modified free volume approach was requiredto model its multiaxial and temperature-dependent behavior. Althoughfree volume was the unifying parameter for the two materials, the needfor a stress-based and strain-based formulation of the free volume forthe urethane and epoxy, respectively, could not be reconciled at thistime.     

A digital photography framework enabling affective awareness in home communication

By transforming the personal computer into a communication appliance, the Internet has initiated the true home computing revolution. As a result, Computer Mediated Communication (CMC) technologies are increasingly used in domestic settings, and are changing the way people keep in touch with their relatives and friends. This article first looks at how CMC tools are currently used in the home, and points at some of their benefits and limitations. Most of these tools supportexplicit interpersonal communication, by providing a new medium for sustaining conversations. The need for tools supportingimplicit interaction between users, in more natural and effottless ways, is then argued for. The idea of affective awareness is introduced as a general sense of being in touch with one's family and friends. Finally, the KAN-G framework, which enables affective awareness through the exchange of digital photographs, is described. Various components, which make the capture, distribution, observation and annotation of snapshots easy and effortless, are discussed.     

Controlled delivery of lipophilic agents to cell cultures for in vitro toxicity and biocompatibility assays

Synopsis
In this report, we present a novel method for delivering lipophilic compounds to cell cultures. The delivery system is based on a nanoemulsion stabilized by phospholipids. These nanoemulsions are well tolerated by cell cultures, such as TK6 lymphoblastoid cells and can be used to deliver defined amounts of encapsulated lipophilic compounds into cells.
We measured the growth inhibition of TK6 lymphoblastoid cells caused by different oils, UV-filters and fragrances to determine the biocompatibility or the toxicity of these compounds in simple cell culture experiments. Our data show that the applied nanoemulsion technology is also very suitable to study biological effects of the UV-A-irradiated compounds in cell culture assays.
Résumé
Dans ce compte-rendu, nous présentons une nouvelle méthode pour dispenser des composés lipophiles à des cultures de cellules. Le système de répartition est fondé sur une nanoémulsion stabilisée par des phospholipides. Ces nanoémulsions sont bien tolérées par les cultures cellulaires, comme les cellules lymphoblastoides TK6 et peuvent etre employées pour délivrer des quantités précises de composés lipophiles encapsulés aux cellules.
Nous avons mesuré l'inhibition de la croissance des cellules lymphoblastoides TK6 provoquée par diverses huiles, des filtres UV et des parfums pour déterminer la biocompatibilité ou la toxicité de ces composés par des essais sur de simples cultures de cellules. Nos données montrent que l'utilisation de la technologie des nanoémulsions convient également à l'étude des effects biologiques des composés irradiés par des UVA dans les cultures cellulaires.