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Fibre reinforced polyurethane sandwich structures: characterisation, optimization and modelling.

The materials under consideration in the present research are a glass fibre reinforced PU-foam sandwich structure and its constituents. These constituents are: a flexible open cell PU-foam, a rigid closed cell PU-foam and a continuous glass fibre mat. 
The general goal of this research is to build different tools to analyse and predict the mechanical properties of the constituents and the final sandwich structure. Therefore, this research combines experimental work with numerical modelling techniques.
The first part of this dissertation discusses the open cell PU-foam. Although the open cell foam is not the main load carrying material in the sandwich panel, characterizing and modelling it is of great importance in other research areas (e.g. surface functionalization or pressure drop calculations). The cellular structure of this foam is characterized by means of optical microscopy, SEM and CT. Based on the outcome of the experimental observations, the Kelvin cell and Weaire-Phelan structure were selected as RVE for the FE-modelling. Up to now, the use of the latter structure to build a FE-model has not been reported in literature.
In correspondence to real foams, the material distribution inthe cell edges of the RVE based FE-models is completely governed by a minimization of the surface energy. The influence of the cell size, solidPU material stiffness, relative density and shape anisotropy on the mechanical properties of the open cell foam, is investigated by means of these models. The Weaire-Phelan based FE-model proved to represent and predict the mechanical properties of the open cell foam in a better way than the standard Kelvin cell based FE-model. In order to decrease the large spread on the available data regarding the solid PU stiffness, specialattention is given to this parameter.
The skins of the investigated sandwich structures consist of a GF/PU-foam composite. Because the skinsare formed in situ, they cannot be characterized in advance which hinders a pre-production prediction of the mechanical properties of the sandwich panel. Therefore, X-ray CT-imaging combined with image processing algorithms are used in the current study to determine the skin thickness and fibre orientation distribution function. This allows to calculate thestiffness of the skins based on the rules of mixture and the Mori-Tanaka inclusion model. The resulting data were successfully validated by experimental work.
In the final part of this research the knowledge on the stiffness of PU-foam core and GF/PU-foam composite skin is joined into a simple calculation tool to predict the bending stiffness of the sandwich panels. A comparison of the calculated values to experimental stiffness data, measured on industrially produced plated, revealed an accuracy of +/-10%. Moreover, the influence of different parameters like the weight and orientation of the applied fibre reinforcement mats is indicated by this tool and allows to identify directions for future material developments.
Date:8 Sep 2009 →  1 Jul 2014
Keywords:Fibre reinforced
Disciplines:Other engineering and technology
Project type:PhD project