Thermo oxidative aging of polymers and polymer‐matrix composites studied with cyclic indentation

The growing use of polymer-matrix composites in aircraft structures leads to the necessity of understanding the degradation phenomena due to their exposure to high temperatures. For T The aging of three epoxy resins (PR520, Tactix and RTM6) in pure state and reinforced with carbon fibers has been characterized by Ultra-Micro Indentation on force-controlled Fischerscope H100C equipment. These materials were aged at 150°C, in air at atmospheric pressure up to 1000 hours and under pure oxygen environment at a pressure of 2 bar, to accelerate the oxidative aging. Please click Additional Files below to see the full abstract

Cyclic indentation test to characterize viscoelastic behavior of polymers

The Oliver & Pharr analysis of indentation data is generally acknowledged and used by research labs and industries to characterize the mechanical behaviour of metals at small scales. It is based on the Sneddon’s contact model of a rigid indenter penetrating an elasto-plastic matter and is perfectly suitable for these materials. However, when the nanoindentation technique is used to study polymer behaviour, there is no general agreement on the procedure and analysis method to use. It is clear that getting the elastic and plastic parameters from the fitting of a single unloading is insufficient to fully characterize the time-dependent behaviour of these materials. The indentation methods applied on polymers are usually inspired by the well-known macroscopic mechanical tests and implemented with the nanoindentation equipment. For instance, a creep test is reproduced through nanoindentation at constant force, revealing a logarithmic behaviour similar to macroscopic one. When small oscillations are superposed on the static nanoindentation loading, a dynamical loss and storage moduli can be calculated by an analogy to the macroscopic DMA technique. The direct analogy between macroscopic static and dynamic behaviour, and microscale contact behaviour might be questionable due to many factors. Nevertheless, these methods are the only procedures available today to access the local mechanical properties of polymers with nanoindentation. In this work, we suggest a new cyclic indentation method and material behaviour analysis inspired by a macroscopic tension fatigue test. Our test consists of up to1200 loading-unloading cycles after an initial holding period at low load to correct for a thermal drift. Each hysteresis loop is analyzed to obtain its area, the secant rigidity and mean (ratcheting) displacement similarly to the area, secant modulus and ratcheting strain characterizing the macroscopic test (see Figure 1). The frequency and load ratio effects on these three parameters are also studied. The cyclic indentation test is performed at two glasses, a titanium alloy and several polymers to make sure that the observed time-dependency is due to the material behaviour and not to the indentation technique itself. The results obtained on a high density polyethylene are discussed and compared with the macroscopic tension fatigue behaviour demonstrated by this material previously [1]. Please click Additional Files below to see the full abstract

Modèle géométrique de frottement entre matériaux composites

Dans cette étude, on propose un modèle géométrique de frottement sous faible charge normale entre deux matériaux composites. Le contact est présenté comme une multitude de microcontacts dont la nature peut être de trois types : fibre-fibre, fibre-matrice ou matrice-matrice. La force de frottement interfaciale est la somme des forces locales de frottement en ces microcontacts. Le calcul montre que le coefficient de frottement hybride ne dépend pas de l'orientation de fibres mais seulement de leur fraction surfacique. Ce modèle est testé expérimentalement

Dissipation interfaciale et volumique induite par le frottement dans les matériaux composites

An investigation of interfacial and bulk friction-induced dissipation in model epoxy-based composite materials reinforced with carbon fillers, as well as pure epoxy, is a challenge of this PhD thesis. While the interfacial dissipation depends mostly on surface properties of very thin material layer, the bulk dissipation involves high volume deformations. Firstly, an experimental friction investigation on carbon fibre- and carbon nanopearl-reinforcedepoxies of different filler volume friction under soft tribological conditions is carried out. In order to understand the results, a generalized frictional law for interfacial friction between two composites is proposed. It is based on Bowden and Tabor theory applied to multimaterial contact and requires a contact in-plane geometry parameter and local friction coefficients. Depending on applied assumption, effective shear stress or effective hardness for all composite phases, it results in direct or inverse proportionality frictional law. These analytical results complete and explain the experimentally obtained tendencies: the direct law should be applied to the composite/epoxy contact, while the inverse law is valid for the composite/composite contact. The second part of the experimental work deals with pure epoxy and carbon fibre-reinforced epoxy under severe tribological conditions. It aims to investigate bulk frictional dissipation and associated wear. A two-scale approach is established, which consists in calculation of macro parameters, such as wear rate, dissipated frictional energy, friction coefficient and relative contact temperature rise, and their coupling with damaged surface observations. This approach allows us to distinguish several wear modes, as two-body and three-body abrasion, adhesion, fatigue and thermal effects, and to associate their appearance and evolution to macro parameters. In contrast to pure epoxy, carbon fibre-reinforced epoxy tends to be more wear resistant.L’enjeu de ce travail concerne la dissipation interfaciale et volumique induite par le frottement dans les matériaux composites modèles á base d’époxy renforcée par du carbone, ainsi que dans l’époxy pure. Alors que la dissipation interfaciale dépend surtout des propriétés d’une couche mince de surface, la dissipation volumique induit des déformations de volume importantes. Dans un premier temps, nous avons effectué une étude expérimentale de frottement sur les résines époxy renforcées par des fibres et des nanoperles de carbone en différentes concentrations sous des conditions tribologiques faibles. Afin de comprendre les résultats obtenus, une loi de frottement généralisée pour le frottement interfacial entre deux composites a été proposée. Basée sur la théorie de Bowden et Tabor et appliquée au contact des composites, elle requiert un paramètre géométrique de contact et des coefficients de frottement locaux. En fonction de l’hypothèse appliquée, la contrainte de cisaillement ou la dureté effective pour toutes les phases du composite, elle découle sur une loi de proportionnalité directe ou inverse. Ces résultats analytiques complètent et expliquent les tendances obtenues expérimentalement : la loi directe doit être appliquée pour le contact composite/époxy, tandis que la loi inverse est valide pour le contact composite/composite. La deuxième partie du travail expérimental est consacrée á l’étude sur l’époxy pure et celle renforcée par des fibres de carbone sous des conditions tribologiques plus sévères. Son objectif est d’étudier la dissipation volumique et l’usure associée. Dans ce cadre, une approche multi-échelle est établie, qui consiste d’abord á calculer les paramètres macroscopiques, comme le taux d’usure, l’énergie dissipée par le frottement, le coefficient de frottement et l’augmentation de la température. Ces paramètres sont ensuite couplés avec les observations des surfaces endommagées. Cette approche nous permet de distinguer plusieurs régimes d’usure, i.e. l’abrasion á deux et trois corps, l’adhésion, la fatigue et les effets thermiques, et associer leur apparence et leur évolution avec les paramètres macroscopiques. Contrairement á l’époxy pure, le composite renforcé par des fibres de carbone s’avère être plus résistant á l’usure

Unexpected Frictional Behavior of Laser-Textured Hydrophobic Surfaces

Hydrophobic surfaces can allow a liquid to slip over the surface and can thus reduce friction in lubricated contact working in a full film regime. Theory supports that the amount of slip can be increased if super-hydrophobic surfaces that are composed of a textured low surface energy material are used. In this work, polytetrafluoroethylene (PTFE) polymer samples were textured with a femto second laser to create super-hydrophobic surfaces by machining a hexagonal network of small circular holes with 10 and 20 μm lattice sides. The frictional behavior of these surfaces was compared to the smooth PTFE samples. Surprisingly, the textured surfaces revealed higher friction coefficients than the smooth surfaces. This higher friction can be explained by a change of wetting regime due to high pressure in fluid and a possible generation of vortices in the cavities

Friction of Fibrous Tows

No abstract available

Real Area of Contact in Carbon Fabric Forming

An experimental rig and image analysis method have been developed to measure the real area of contact between carbon fibre fabric and a smooth tool. By using as the tool surface a flat glass slide with an optical coating, and viewing the contact through the slide, the contact area is clearly visible at a fibre scale. Straightforward image analysis is used to estimate the length of fibre contact. A microcontact density is defined, normalising the measured fibre contact length by the value for an idealised packing arrangement of close-packed parallel fibres. The method is applied to a 2×2 twill weave carbon fabric and a 12k carbon fibre single tow. Both for the woven fabric and the single tow, the microcontact density is significantly below the idealised value of 100%. The contact density is sensitive to contact pressure and has a similar value of between 4 and 8 % for the two materials considered at the contact pressure of a few kPa appropriate to dry pre-forming operations

Friction Between a Dry Fibre Bundle and Rough or Smooth Surfaces

No abstract available

Experimental Observations on the Friction of Textile Fibres Relevant to Carbon Fibre Composite Forming

No abstract available

Dissipation interfaciale et volumique induite par le frottement dans les matériaux composites

L enjeu de ce travail concerne la dissipation interfaciale et volumique induite par le frottement dans les matériaux composites modèles á base d époxy renforcée par du carbone, ainsi que dans l époxy pure. Alors que la dissipation interfaciale dépend surtout des propriétés d une couche mince de surface, la dissipation volumique induit des déformations de volume importantes. Dans un premier temps, nous avons effectué une étude expérimentale de frottement sur les résines époxy renforcées par des fibres et des nanoperles de carbone en différentes concentrations sous des conditions tribologiques faibles. Afin de comprendre les résultats obtenus, une loi de frottement généralisée pour le frottement interfacial entre deux composites a été proposée. Basée sur la théorie de Bowden et Tabor et appliquée au contact des composites, elle requiert un paramètre géométrique de contact et des coefficients de frottement locaux. En fonction de l hypothèse appliquée, la contrainte de cisaillement ou la dureté effective pour toutes les phases du composite, elle découle sur une loi de proportionnalité directe ou inverse. Ces résultats analytiques complètent et expliquent les tendances obtenues expérimentalement : la loi directe doit être appliquée pour le contact composite/époxy, tandis que la loi inverse est valide pour le contact composite/composite. La deuxième partie du travail expérimental est consacrée á l étude sur l époxy pure et celle renforcée par des fibres de carbone sous des conditions tribologiques plus sévères. Son objectif est d étudier la dissipation volumique et l usure associée. Dans ce cadre, une approche multi-échelle est établie, qui consiste d abord á calculer les paramètres macroscopiques, comme le taux d usure, l énergie dissipée par le frottement, le coefficient de frottement et l augmentation de la température. Ces paramètres sont ensuite couplés avec les observations des surfaces endommagées. Cette approche nous permet de distinguer plusieurs régimes d usure, i.e. l abrasion á deux et trois corps, l adhésion, la fatigue et les effets thermiques, et associer leur apparence et leur évolution avec les paramètres macroscopiques. Contrairement á l époxy pure, le composite renforcé par des fibres de carbone s avère être plus résistant á l usure.An investigation of interfacial and bulk friction-induced dissipation in model epoxy-based composite materials reinforced with carbon fillers, as well as pure epoxy, is a challenge of this PhD thesis. While the interfacial dissipation depends mostly on surface properties of very thin material layer, the bulk dissipation involves high volume deformations. Firstly, an experimental friction investigation on carbon fibre- and carbon nanopearl-reinforcedepoxies of different filler volume friction under soft tribological conditions is carried out. In order to understand the results, a generalized frictional law for interfacial friction between two composites is proposed. It is based on Bowden and Tabor theory applied to multimaterial contact and requires a contact in-plane geometry parameter and local friction coefficients. Depending on applied assumption, effective shear stress or effective hardness for all composite phases, it results in direct or inverse proportionality frictional law. These analytical results complete and explain the experimentally obtained tendencies: the direct law should be applied to the composite/epoxy contact, while the inverse law is valid for the composite/composite contact. The second part of the experimental work deals with pure epoxy and carbon fibre-reinforced epoxy under severe tribological conditions. It aims to investigate bulk frictional dissipation and associated wear. A two-scale approach is established, which consists in calculation of macro parameters, such as wear rate, dissipated frictional energy, friction coefficient and relative contact temperature rise, and their coupling with damaged surface observations. This approach allows us to distinguish several wear modes, as two-body and three-body abrasion, adhesion, fatigue and thermal effects, and to associate their appearance and evolution to macro parameters. In contrast to pure epoxy, carbon fibre-reinforced epoxy tends to be more wear resistant.LYON-Ecole Centrale (690812301) / SudocSudocFranceF