Modelos de material para espumas poliméricas aplicadas a estruturas aeronáuticas em material compósito sanduíche

Abstract

Aircraft structures are mostly made of composite material in order to achieve the specifications of a project. Among these structures one highlights the sandwich structure. The usage of this structure requires extensive studies on new materials as well as on the application of these very materials. A special attention for the core's material of these structures is needed because it is in fact a cellular structure, as the polymeric foams. This dissertation seeks to concatenate the literature and practice, studying the calibration of material models to describe the mechanical behavior of polymeric foams, as well as to analyse their potentials and limitations. These foams are cellular structures whose failure mechanisms comprise micro and macro responses. The identification and quantification of these behaviors can be done through micro-mechanical or phenomenological (macro-mechanical) material models along with experimental tests and analyses of both the cellular material and the structure in which this material is used. Each approach, micro or macro, has advantages and disadvantages that in the present work are discussed for the studied material (PVC, polyvinyl-chloride, rigid closed-cell polymeric foam with a density of 60kg/m³). A series of experimental tests based on standard procedures are carried out and the data collected are compared with data obtained simultaneously through an image correlation technique. All the experimental information are confronted and associated to the failure mechanisms of the polymeric foam. Finally, the experimental data are used for the identification of material models parameters, currently available in the commercial finite elements software-ABAQUS. With the material models calibrated, the present work investigates the representativeness and the limitations of these very models when applied to aircraft structures submitted to monotonic or not localized loads. One has observed that there is a strong dependence of the foam's macroscopic response with its cellular structure when it is submitted to localized and/or non-monotonic loads. Moreover, the usage of simplified material models, and/or with some implementation hypotheses, renders doubtful results when these models are applied to cellular materials with complex responses (micro-mechanical mechanisms, anisotropy, viscosity, etc.). Nevertheless, the present work shows that a strategic calibration taking into account the implementation hypotheses and the limitations of the material model, yields good macroscopic results that are strongly influenced by the micromechanical failure mechanisms.

Desenvolvimento De Uma Metodologia Para Caracterização Tridimensional De Espumas De Poliuretano a Base De Poliols De Origem Vegetal

“…Quando o volume da mistura reativa é menor que o volume das bolhas que expandiram, a região ou área de contato, que forma uma estrutura triangular e que existe entre as bolhas adjacentes, recebe o nome de bordas (ou ligamentos) de P lateau como indicado na Figura 2.4. O fluído é transportado das membranas para as bordas formando células poliédricas irregulares (31,32,33,34,35).…”

“…Sem a dissolução do ar a espuma não poderia ser criada. No decorrer do tempo e continuando com o processo de agitação, a substância que antes era líquida torna-se densa (ou cremoso), em resposta à dissolução dos reagentes (33,34).…”

“…Ocorre também a liberação de calor (processo exotérmico) e de gás carbônico (Figura 2.6(e), (f)) (33,34).…”

“…III Estabilidade da bolha: Nesta fase, as bolhas que eram esféricas viram poliédricas, aumentam o seu tamanho e passam a tocar-se, neste ponto inicia a solidificação da matriz polimérica para manter a geometria da estrutura celular formada (Figura 2.6(g) e (h)) (33,34).…”

“…Figura 2.10 Alongamento das faces de uma espuma de células fechadas: (a) células original, (b) tração e (c) compressão [Adaptada] (31). Figura 2.11 Curva típica tensão-deformação para materiais celulares submetidos a compressão (33,34). Figura 2.12 Esquema simplificado de um microtomógrafo (6).…”