Life prediction for carbon fibre filament wound composite structures

International audienceThe processes governing the failure of filament wound composite structures have been examined. It is shown that the fibres controlling the failure of such a structure, when it is internally pressurised, can be considered to be subjected only to tensile loads. A multi-scale model has been developed which considers the effects on the scale of the elastic fibres and includes the effects of the viscoelastic matrix as well as debonding around fibre failures. The intact fibres neighbouring fibre breaks are subjected to an increase in stress and a higher probability of failure than elsewhere in the composite. During a monotonic failure test the initially random fibre failures are seen to begin to coalesce in a way governed by the stochastic nature of the fibre breaks and this eventually leads to failure. Under prolonged loading the relaxation of the matrix around fibre breaks causes the overloads in the neighbouring fibres to evolve and to induce delayed fibre breaks which can eventually lead to instability in the structure. The model allows these processes to be taken into account while calculating the behaviour of the whole composite structure

Three-dimensional analysis of load transfer micro-mechanisms in fibre/matrix composites

International audienceThis study gives a detailed analysis of load distributions around fibre breaks in a composite. In contrast to other studies reported in the literature, the analysis considers different configurations of composite damage from the failure of a few to the failure of many fibres. The model considers three types of matrix behaviours (elastic, elastic–plastic and viscoelastic) with or without debonding at the broken fibre/matrix interface. In this way, the usual limitations of the finite element approach are overcome so as to take into account the number and interactions of broken fibres whilst maintaining an evaluation of the various fields (stresses in particular)

Building body identities - exploring the world of female bodybuilders

This thesis explores how female bodybuilders seek to develop and maintain a viable sense of self despite being stigmatized by the gendered foundations of what Erving Goffman (1983) refers to as the 'interaction order'; the unavoidable presentational context in which identities are forged during the course of social life. Placed in the context of an overview of the historical treatment of women's bodies, and a concern with the development of bodybuilding as a specific form of body modification, the research draws upon a unique two year ethnographic study based in the South of England, complemented by interviews with twenty-six female bodybuilders, all of whom live in the U.K. By mapping these extraordinary women's lives, the research illuminates the pivotal spaces and essential lived experiences that make up the female bodybuilder. Whilst the women appear to be embarking on an 'empowering' radical body project for themselves, the consequences of their activity remains culturally ambivalent. This research exposes the 'Janus-faced' nature of female bodybuilding, exploring the ways in which the women negotiate, accommodate and resist pressures to engage in more orthodox and feminine activities and appearances

Mechanical behaviour of polyethylene terephthalate & polyethylene naphthalate fibres under cyclic loading

International audiencePolyethylene naphthalate (PEN) fibres possess a higher initial stiffness than that of polyethylene terephthalate (PET) fibres and this makes them an attractive competitor for use in mooring ropes and other applications for which a low compliance would be an advantage. The two types of fibres have been characterised and compared in tension, creep and fatigue and found to behave in very similar ways. Failure of both fibres results in similar fracture morphologies although under high cyclic loading a new failure process has been observed for the PEN fibres which combines step by step crack propagation and final failure normal to the fibre axis. In the light of this observation, similar fracture behaviour has also been identified in PET fibres and which, until now had been overlooked. The loading criteria for fatigue failure are similar for both fibres and it has been shown that, for a given maximum cyclic load, lifetime is raised if the minimum cyclic load is increased

Characterisation and modelling of aging of composites

International audienceThe aim of this study was to better understand the aging of glass fibre-epoxy composites exposed to humid conditions and loading so as to predict its effects on the lifetimes of composite structures. Water diffusion in the material was initially determined by gravimetric methods under different conditions of relative humidity (r.h.) conditions. A fickian model of diffusion could describe the results obtained. The specimens, saturated at different levels, were mechanically characterised and tensile strengths and shear moduli were seen to decrease with water uptake. The effects of matrix cracking of the laminate on water absorption and its mechanical properties have also been studied. Differences between reversible and irreversible changes in properties were revealed and analysed in detail. A predictive model has been proposed by considering different sections throughout the thickness of the material. As a first step in modelling the diffusion process, the non-uniform water distribution across the composite for any conditions (temperature, humidity, aging time) are determined. The resulting mechanical properties of the material, as a function of the absorbed water concentration, are determined in each point. The model which is proposed enables the global behaviour of composite to be determined, at all stages of water absorption and matrix cracking, by calculating behaviour in each section of the composite through its thickness

Une méthode FE2 simplifiée appliquée à la modélisation de l'éclatement des réservoirs composites hautes pressions

International audienceOn propose dans cette étude la modélisation des phénomènes physiques à l'origine et induits par celui de rupture de fibre au sein d'un composite unidirectionnel. Cette modélisation, faite à l'échelle microscopique, permet de construire une base de données qui va décrire cette échelle au sein d'un procédé multiéchelles FE2 simplifié. Ce procédé est ensuite utilisé pour réaliser le dimensionnement à l'éclatement d'un réservoir composite bobiné

Dimensionnement de structures composites par une méthode FE2 simplifiée. Application au calcul de réservoirs composites

National audienceOn propose dans cette étude la modélisation des phénomènes physiques à l'origine et induits par celui de rupture de fibre au sein d'un composite unidirectionnel. Cette modélisation, faite à l'échelle microscopique, permet de construire une base de données qui va décrire cette échelle au sein d'un procédé multiéchelles FE2 simplifié. Ce procédé est ensuite utilisé pour réaliser le dimensionnement à l'éclatement d'un réservoir composite bobiné

Détection et discrimination par émission acoustique des endommagements dans les composites. Comparaison expérience / modélisation

International audienceOn présente dans cette étude la discrimination des phénomènes de ruptures de fibre, de microfissuration intralaminaire, de macrofissuration intralaminaire ainsi que du microdélaminage au sein des composites stratifiés d'unidirectionnels orientés, dans le cadre d'une méthode de détection des endommagements basée sur la technique de l'Emission Acoustique. Ces résultats expérimentaux sont confrontés aux prévisions issues d'un modèle de comportement de matériaux composites unidirectionnels, basés sur la physique des phénomènes à l'échelle des constituants

Rupture de fibre : comparaison entre un modèle écrit a l'échelle microstructural et une détection du phénomène par tomographie très haute résolution

International audienceOn présente dans cette étude la discrimination des phénomènes de ruptures de fibre, de microfissuration intralaminaire, de macrofissuration intralaminaire ainsi que du microdélaminage au sein des composites stratifiés d'unidirectionnels orientés, dans le cadre d'une méthode de détection des endommagements basée sur la technique de l'Emission Acoustique. Ces résultats expérimentaux sont confrontés aux prévisions issues d'un modèle de comportement de matériaux composites unidirectionnels, basés sur la physique des phénomènes à l'échelle des constituants

Micro-Raman Study of the Fatigue and Fracture Behaviour of single PA66 Fibres. Comparison with single PET and PP Fibres

International audienceThe micro/nano structural evolution of PA66, PP, PBO and PET (both drawn and precursor) fibres, before and after tensile loading, fatigue and ultimately, failure has been studied by Raman microspectroscopy using two probes. The Raman analysis has used low wavenumber collective modes at <150 cm-1 as representative of the crystalline and amorphous chains and stretching and bending modes, as representative of the C-C bond local behaviour. Wavenumber and bandwith distribution across fibre/precursors diameters revealed different types of skin/core heterogeneities. The in situ analysis at different strain levels showed that amorphous zones in the fibres accommodate the stress differently. Slight tensile straining is observed for crystalline PA 66 chains, and compression for PP and PET chains. Post mortem analysis of a series of fibres failed in fatigue showed that the amorphous domains were highly stressed during failure and a resulting residual stress could be measured