Enrugamento de membranas anulares

No presente trabalho estuda-se a estabilidade de membranas axissimetricas homogeneas, isotrópicas e incompressíveis, sujeitas à deformaçóes finitas. Nestas membranas inicialmente planas, atuam cargas axiais distribuídas f e m.. As equaçóes de equilíbrio e condiçóes de contorno sao obtidas através do principio da energia potencial estacionária. Considera-se o material da membrana como sendo neo-Hookeano. Através da teoria de pequenas perturbacoes fez-se o estudo da estabilidade da membrana, comprovandese com um modelo experimental rudimentar.Peer Reviewe

INSTABILITIES OF INITIALLY STRESSED HYPERELASTIC CYLINDRICAL MEMBRANE AND SHELL UNDER INTERNAL PRESSURE

Summary This paper investigates the static non-linear behaviour and possible instabilities of cylindrical membranes and shells. A detailed experimental analysis was carried out. The structure was analysed using appropriate membrane or shell finite elements and the resulting non-linear equilibrium equations solved using the FE software ABAQUS. A detailed parametric numerical analysis was carried out and the influence of different types of local imperfections was also studied in detail

An autonomous infuser for a subcutaneous skin expander

Introduction: Skin expansion is a physiological process defined as the ability of human skin to increase its superficial area in response to a stress or given deformation. In reconstructive surgery, skin expanders are implanted beneath the skin and periodically infiltrated with a saline solution to provide an extra flap of skin. When the prescribed internal volume of the expander is reached, reconstructive surgery is performed. Methods: A semiautomatic and portable device was developed and built to facilitate a skin expansion procedure. The device aims to simplify the infiltration process, providing mobility and independence to the patient and assuring the physician of the infiltration quality and precision. The device also enables continuous expansion in hospitalized patients. Results: Using a code, the doctor accesses the menu of the device and sets the maximum pressure and/or value for each expander of the patient. The patient can control the infiltration velocity and reverse or stop the operation. All data are recorded on a simcard and include date, time, initial and final volumes, and initial and final pressures of each procedure for each expander. Conclusions: The device motorizes and optimizes the expansion, allowing the doctor to prescribe a maximum infiltration pressure or volume. All data are recorded to provide an important database of skin behavior related to sex, race, age, and expansion site

Influence of initial geometric imperfections on the stability of thick cylindrical shells under internal pressure

International audienceThis paper investigates numerically and experimentally the influence of initial geometric imperfections on the critical loads of initially stretched thick hyperelastic cylindrical shells under increasing uniform internal pressure. Imperfections in shells can have a global or local character. First, two types of local imperfections are considered: (1) a local axially symmetric imperfection in the form of a ring and (2) a small rectangular imperfection. The influence of the imperfection thickness, position and size are analysed in detail. Results show that the critical load decreases as the imperfections increase in size or thickness and as they move from the boundaries to the centre of the shell. The influence of multiple local imperfections is also studied in the present paper. Finally, the influence of global imperfections is considered with the imperfections described as a variation of the shell curvature in the axial direction. The results show that thick hyperelastic shells may be sensitive to local and global imperfections. In all cases the experimental results are in good agreement with the numerical ones, corroborating the conclusions

Finite deformations of an initially stressed cylindrical shell under internal pressure

International audienceThis paper investigates the large deformations of an extended thick cylindrical tube under internal pressure, with emphasis on the static nonlinear behavior and instabilities of the shell. Thick elastic tubes that undergo large elastic deformations under internal pressure can exhibit novel instabilities. After some deformation, part of the tube becomes highly deformed taking the form of a bulge, while the remainder appears almost unchanged. This local instability phenomenon corresponds to a limit point along the nonlinear equilibrium path. After the onset of these highly nonuniform deformations, the local bulge initially grows with a marked decrease in internal pressure while the rest of the tube unloads. First, a detailed experimental analysis is carried out involving different geometries and initial axial forces and the influence of the axial force and of the internal pressure on the critical pressure is investigated. The shell used in the experiments is composed of an isotropic, homogeneous and hyperelastic rubber, which is modeled as a Mooney–Rivlin incompressible material, described by two elastic constants. These constants are obtained by comparing the experimental and numerical solutions for the shell under axial tension. The governing shell equations are solved numerically using the finite-element method, using the program ABAQUS. The experimental results are, as shown in the paper, in satisfactory agreement with the numerical analysis

Relation Between Post-Augmentation Parenchymal Atrophy and Local Tissue Pressure

Background: Breast parenchyma interacts dynamically with an inserted implant, which may lead to local atrophy and sensory involvement, changes in vascular tissue and lactation, and volume reduction over time. The inversely proportional relationship between pressure and volume cannot be stated with certainty; that is, the larger implants having more local pressure would lead to compression, thus leading to atrophy of parenchyma more intensely compared with smaller implants. The objective of this study was to assess and list breast parenchyma volume changes with different pressure levels due to silicone implants of several sizes. Objectives: The authors sought to list the pressure exerted by silicone implants and the atrophy caused in the breast tissue. Methods: Thirty-six women were placed in 3 groups (n=12) and subjected to augmentation mammoplasty in the subglandular plane. The measurement of pressure in millimeters of mercury was conducted with help of molds with the same base and projection of implants introduced posteriorly. MRI was conducted in all participants in the preoperative period and at 6 and 12 months after surgery. Results: Twelve months after breast implant insertion, the groups had a significant glandular volume reduction (mean, 12.97% in the right breast and 12.42% in the left breast). There was a statistically significant difference in the proportions of volume reduction and the pressure levels measured. Conclusions: A reduction in breast volume was verified. This reduction was also related to the level of pressure exerted on the implant