100 research outputs found
Active fibre-reinforced composites with embedded shape memory alloys
This dissertation concerns active fibre-reinforced composites with embedded
shape memory alloy wires. The structural application of active materials allows to develop adaptive structures which actively respond to changes in the
environment, such as morphing structures, self-healing structures and power
harvesting devices. In particular, shape memory alloy actuators integrated
within a composite actively control the structural shape or stiffness, thus influencing the composite static and dynamic properties. Envisaged applications
include, among others, the prevention of thermal buckling of the outer skin of
air vehicles, shape changes in panels for improved aerodynamic characteristics
and the deployment of large space structures.
The study and design of active composites is a complex and multidisciplinary topic, requiring in-depth understanding of both the coupled behaviour of
active materials and the interaction between the different composite constituents. Both fibre-reinforced composites and shape memory alloys are extremely
active research topics, whose modelling and experimental characterisation still
present a number of open problems. Thus, while this dissertation focuses on
active composites, some of the research results presented here can be usefully
applied to traditional fibre-reinforced composites or other shape memory alloy
applications.
The dissertation is composed of four chapters.
In the first chapter, active fibre-reinforced composites are introduced by
giving an overview of the most common choices available for the reinforcement, matrix and production process, together with a brief introduction and
classification of active materials.
The second chapter presents a number of original contributions regarding
the modelling of fibre-reinforced composites. Different two-dimensional laminate theories are derived from a parent three-dimensional theory, introducing
a procedure for the a posteriori reconstruction of transverse stresses along the
laminate thickness. Accurate through the thickness stresses are crucial for the
composite modelling as they are responsible for some common failure mechanisms. A new finite element based on the First-order Shear Deformation Theory and a hybrid stress approach is proposed for the numerical solution of the
two-dimensional laminate problem. The element is simple and computationally
efficient. The transverse stresses through the laminate thickness are reconstructed starting from a general finite element solution. A two stages procedure is
devised, based on Recovery by Compatibility in Patches and three-dimensional
equilibrium. Finally, the determination of the elastic parameters of laminated
structures via numerical-experimental Bayesian techniques is investigated. Two
different estimators are analysed and compared, leading to the definition of an
alternative procedure to improve convergence of the estimation process.
The third chapter focuses on shape memory alloys, describing their properties and applications. A number of constitutive models proposed in the literature, both one-dimensional and three-dimensional, are critically discussed and
compared, underlining their potential and limitations, which are mainly related
to the definition of the phase diagram and the choice of internal variables. Some
new experimental results on shape memory alloy material characterisation are
also presented. These experimental observations display some features of the
shape memory alloy behaviour which are generally not included in the current
models, thus some ideas are proposed for the development of a new constitutive
model.
The fourth chapter, finally, focuses on active composite plates with embedded shape memory alloy wires. A number of di®erent approaches can be used
to predict the behaviour of such structures, each model presenting different advantages and drawbacks related to complexity and versatility. A simple model
able to describe both shape and stiffness control configurations within the same
context is proposed and implemented. The model is then validated considering
the shape control configuration, which is the most sensitive to model parameters. The experimental work is divided in two parts. In the first part, an active
composite is built by gluing prestrained shape memory alloy wires on a carbon
fibre laminate strip. This structure is relatively simple to build, however it
is useful in order to experimentally demonstrate the feasibility of the concept
proposed in the first part of the chapter. In the second part, the making of
a fibre-reinforced composite with embedded shape memory alloy wires is investigated, considering different possible choices of materials and manufacturing
processes. Although a number of technological issues still need to be faced, the
experimental results allow to demonstrate the mechanism of shape control via
embedded shape memory alloy wires, while showing a good agreement with the
proposed model predictions
A hierarchy of models for the design of composite pressure vessels
International audienceThe mechanical response of pressure vessels to an applied internal pressure is essentially controlled by a few key parameters, related both to the overall geometry of the structure and to the orientations and thicknesses of the composite layers. The role of each parameter, and the way they interact to determine the structural response, can be apprehended at a very early design stage by using simple material and structural models, which enable to explore a wide range of designs with minimal computational cost. More complex models can then be called into play to predict the detailed structural response, including crucial information such as the burst pressure and failure mode. This paper discusses a hierarchy of models with increasing levels of details and complexity, which are useful to gain increasing insight on the pressure vessel response all along the design process
Interaction of inter- and intralaminar damage in scaled quasi-static indentation tests:Part 1 – Experiments
International audienceThe evaluation of the predictive capabilities of models proposed in the literature for laminated composites calls for experimental testing providing detailed results of both the global and local response in terms of degradation mechanisms, such as delamination, transverse cracking and fibre breaking. Scaled tests, in which one or more characteristic dimensions are modified, allow variation of the different mechanisms. In this paper, a unique series of scaled indentation tests are performed on quasi-isotropic composite plates, and a detailed assessment of the damage evolution is carried out through non-destructive techniques, including ultrasonic C-scan and X-ray Computed Tomography (CT). Four different configurations are tested, presenting changes in both in-plane dimensions and fully three dimensional scaled cases. The latter are performed with sublaminate and ply scaling to show the effect of ply thickness on response. A detailed set of results for both global behaviour and the damage evolution is provided to demonstrate the mechanisms controlling behaviour and to create a reference set of data for model validation. The scaling effects observed are also discussed making use of simplified analytical models
Thermo-oxidation behaviour of organic matrix composite materials at high temperatures
The present paper is a review of the main activities carried out within the context of the COMPTINN‟ program, a joint research project founded by a FUI program (Fonds Unifiés Interministériels) in which four research teams focused on the thermo-oxidation behaviour of HTS-TACTIX carbon-epoxy composite at „high‟ temperatures (120°C-180°C). The scientific aim of the COMPTINN‟ program was to better identify, with a multi-scale approach, the link between the physico-chemical mechanisms involved in thermo-oxidation phenomena, and to provide theoretical and numerical tools for predicting the mechanical behaviour of aged composite materials including damage onset and development
Stratégies d'amélioration de la tenue au délaminage des interfaces
International audienc
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