Étude expérimentale du comportement dynamique des matériaux

Experimental characterisation of materials under dynamic loading. This paper is specially dealing with dynamic testing and in particular the experimental characterisation of materials under dynamic loading. We shall focus on the SHPB (Split Hopkinson Pressure Bar) system because of its frequent use in a range of strain rates from 50/s to 5000/s which corresponds to the strain rates involved in most of civil industrial applications. The limitations of this system and its special advantages will be recalled. Some typical results obtained for various materials and small structures will be shown.     

Modélisation du comportement et de la rupture des matériaux sous sollicitations dynamiques

Modelling deformation and failure of materials under dynamic loading conditions. There is an increasing demand for thermomechanical computations of structures under dynamic loading conditions for which the current methods are often unsatisfactory. A simple analysis quickly shows that different dynamic regimes can be distinguished depending on the magnitude of inertia effects. Furthermore, it is shown that the space scaling becomes time dependent. Experimental, theoretical and numerical tools are still evolving and have not yet reached a sufficient degree of maturity. This is due to the inherent complexity of dynamic problems such as wave propagation, heating and thermomechanical coupling, rate-dependent phenomenology, space and time dependent characteristic scales. An overview of the current knowledge allows one to identify missing developments in experiments, constitutive models including degradation and failure effects, as well as computational strategies. It is shown that the state of the art is very disparate when considering different material classes such as polymers, composites and foams for which few studies are conducted. For metals, concrete, ceramics and glasses some results are available but more advanced studies are still needed. Some future directions of research are sketched and they constitute the scope of the newly created working group common to the MECAMAT and DYMAT societies, both members of the French Society of Mechanics. Its first meeting was held April 4–5, 2000, in Poitiers.     

Un dispositif pour la caractérisation des propriétés amortissantes de matériaux viscoélastiques non-linéaires en cisaillement

Viscoelastic materials are widely used in passive antivibration mounting design. In order to optimize their use, designers have to take into account the non-linearities caused by the large strain amplitudes these systems are subjected to. In this paper we present an apparatus designed to characterize the mechanical behavior of some viscoelastic materials used for this application. Simple shear deformation under sinusoidal load is used. The results are displayed as an extension of the complex moduli linear concept. The highly non-linear behaviour of the samples is underlined. A large scale of frequency and strain amplitude can be studied using this apparatus, without any restrictive assumption regarding the material model. This apparatus was designed in order to validate the design of an aerospace antivibration equipment.     

Problématique de la conception d'un siège ferroviaire en matériaux composites

Technical and economical constraints of composite material train seats design. The aim of this work is to design a frame of metro seat in composite materials and to establish a design methodology, in order to reduce the weight of structure and the manufacturing cost, respecting the specifications that impose technical and economical constraints. A global analysis was done on all parameters related to the design of the structure: sizing, assembly, manufacturing process and choice of composite materials to use. The retained solution permits reducing the manufacturing cost with regard to the metallic seat, by reducing the number of components to assembly.     

Un nouvel outil pour l'étude des interactions tôle–outil–lubrificant: Le test en cadence

In order to obtain a formed part, free from any defect such as necking, failure, galling marks or bad surface appearance, it is increasingly important to control the deep interactions between the steel sheet, the lubricant and the tooling. Another fundamental requirement for the industry is an ever growing productivity which implies a better stability of the process, also allowing an increase in the number of parts drawn per minute. Recently, RDCS has developed a new friction set-up able to study the behaviour of the steel sheet, coated or not, during its short contact with the tooling in the presence of a lubricant. Series of hundreds of samples are successively drawn. First results are interpreted.     

Comportement sous ultra-vide de matériaux auto-lubrifiants chargés en fluorure de graphite (The ultra-high-vacuum behaviour of graphite fluoride filled self-lubricating materials)

Self-lubricating materials were obtained when graphite fluoride, (CF_x)_n, was introduced into a polyimide matrix. Tests have been undertaken to evaluate the wear rate and coefficient of friction of these materials using stainless steel as a counterface, with medium loads of 1–10 N and speeds of 0.4 and 0.8 m s^?1. In ambient air, the specific wear rate was found to be very low being about 0.7 × 10^?7 mm³ N^?1 m^?1, and in ultra-high vacuum (10^?8–10^?9 Torr) the wear rate still remained low. Thus materials containing (CF_x)_n appear to behave quite differently in vacuum in comparison to graphite powder or carbon fibre-filled polymers, for which specific wear rates 100 and 400 times greater, respectively, have been observed.     

Dégradation du comportement magnétique des tôles lors de leur mise en œuvre industrielle : mise en évidence expérimentale et modélisation

Degradation of the magnetic properties of electrical steels after industrial processing: experimental and numerical magneto-mechanical analyses. The magnetic properties of electrical steels strongly depend on their mechanical state. Hence, the final performances of electric machines are altered by the material degradation occurring during the manufacturing process. In the present study, we analyse the effect of assembling stresses on one hand and of punching on the other hand, on the global behaviour of the stator of an electrical machine. Stators of electrical machines were used to design and build simple devices in order to stress out the importance of these phenomena: the local degradation of the material induces a degradation of the global behaviour of the stator, and an increase of magnetic losses in particular. Simplified coupled finite element analyses are presented and a good agreement between simulated and experimental results is found.