Effet de l’endommagement transverse sur la résistance en compression sens fibre d’un composite carbone/époxy

National audienceL'étude expérimentale proposée analyse l'influence de l'endommagement transverse sur la résistance en compression sens fibre pour des composites tissés carbone/époxy. Afin d'explorer plusieurs méthodes expérimentales, l'étude porte d'abord sur des essais de flexion pure où le comportement dans la zone comprimée est exploré pour des éprouvettes saines. Ensuite un essai de compression pure sur des tubes préalablement endommagés en torsion est proposée. Les essais sont suivis par corrélation d'image afin de déterminer les effets de structure nuisant à la caractérisation du matériau. On montrera finalement que l'endommagement transverse influence la tenue mécanique en compression

A Virtual Testing Approach for Laminated Composites Based on Micromechanics

International audienceThe chapter deals with a crucial question for the design of composite structures: how can one predict the evolution of damage up to and including final fracture? Virtual testing, whose goal is to drastically reduce the huge number of industrial tests involved in current characterization procedures, constitutes one of today’s main industrial challenges. In this work, one revisits our multiscale modeling answer through its practical aspects. Some complements regarding identification, kinking, and crack initiation are also given. Finally, the current capabilities and limits of this approach are discussed, as well as the computational challenges that are inherent to “Virtual Structural Testing.

The Flexible Initiation Test (FIT): A new experimental test to characterize fracture initiation in mode I at the free edge of bonded assemblies

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Load identification for full-field reconstruction: applications to plates under tension loads

International audienceThe full-field reconstruction method presented here is suitable for real-time structural monitoring purposes, such as improving the performances of structures. The aim is to characterize mechanical fields and boundary conditions while the structure is in service, using just a few on-line measurements. This characterization is an ill-posed inverse problem, the solution of which requires making some prior assumptions. The structures studied are assumed to be in steady-state configurations and to show linear mechanical behavior, and the loading zones are assumed not to be overstressed; the mechanical fields are monitored only inside the structures. The full-field identification is regularized using the boundary conditions, which are identified from strain measurements, and the mechanical fields are then reconstructed from these identified boundary conditions. On the basis of Saint-Venant's principle, the boundary conditions are approximated with a few parameters, the basis functions of which are obtained from the projections of Trefftz-like solutions onto the boundaries of the structures. These approximate boundary conditions are linked to the global mechanical responses of the structures. Lastly, this full-field reconstruction method is applied to plate-like structures with holes subjected to in-plane loads. The results show that the load identification procedure efficiently regularizes the full-field reconstruction method, and that this method is suitable for structural monitoring purposes. The sensitivity of this method to errors is similar to that of the load identification procedure, and the maximum errors in the solutions are located at the boundaries

DAMAGE EVOLUTION UNDER LONG-TERM, HIGH AND CONSTANT LOADING OF A CARBON/EPOXY LAMINATE

International audienceWhen a composite material of carbon fiber and epoxy resin is subjected to mechanical loading, different phenomena can lead to changes in its properties or even irreversible damage (propagation of cracks, debonding between folds, fiber breakage). The objective of this work is to study, under high loads and long periods, the propagation of transverse microcracks (diffuse damage) until the fracture of the laminate under matrix modes (instability). Since long term tests are necessary to characterize the damage evolution as well as the creep and stress relaxation phenomena, a testing machine was carried out. Optimal orientations ([45°,-45°]ns and [60°,-60°]ns) and specimen shape have been defined to study this phenomena. Experimental results are presented here and it is observed an evolution of the matrix damage under constant loads. This evolution is compared to the evolution of the matrix damage in the case of cyclic loads (fatigue) which in this latter case is faster

Caractérisation non destructive de l’anisotropie d’un matériau composite à l’aide des ondes ultrasonores transverses

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Fatigue of laminated composite structures with stress concentrations

International audienceA model defined at the ply scale to predict the failure of laminated composites for static and fatigue loadings has been proposed. The model describes the loss of strength in the fibre direction for high levels of transverse damage. This phenomenon can be observed in a 0° tube by applying a cyclic torsion loading up to a high level of damage followed by a tensile test. The model includes an original approach based on a Fracture Characteristic Volume (FCV) to predict the fibre failure of laminated structures with stress concentrations. The FCV is a cylinder defined at the ply scale where the average stress is calculated and compared to the maximal strength of the material. The fibre failure model and an application to open hole plates loaded in fatigue are presented in this paper