Scale effects on the response of composite structures under impact loading

For several years, composite materials have taken a significant part in the realization of structures designed for transport (aeronautical, nautical, automotive. . .). In order to qualify the behavior of such structures, preliminary validation tests have to be done. These specific tests are often very expensive and difficult to set up, especially when the structure dimensions are large (fuselages of aircraft, ship hulls. . .). An alternative way is then to employ small-scale models. The use of these reduced scale structures requires the identification of similitude models allowing the extrapolation of the small-scale model behavior to the real structure. Although largely developed in the case of homogeneous materials, such similitude techniques are not clearly identified for composite materials taking into account the damage evolution during an impact. The purpose of this article is firstly to present existing similitude techniques making it possible to predict the composite structure behaviour from the knowledge of small-scale model response. Secondly, experiments were done on two scale of samples carried out by stratification of unidirectional carbon/epoxy plies. These results were finally compared with the analytical predictions of similitude laws currently used. The aim of this paper is to contribute to similitude laws development applied to composite structures. These laws permit to extrapolate the small-scale model behavior to the real scale one. Existing approaches have been established following two different methods. They are summarized in this paper and applied to impact loadings on two laminated plate scales. In order to complete data collected by ‘‘conventional’’ instrumentation (force transducer, displacement sensor, accelerometer.. .), optical device such as an high-velocity CCD camera, associated with optical techniques for the monitoring of markers, were used. These techniques make possible to compare displacement lines corresponding to each scale. It is shown that existing similitude laws, used for elastic materials, do not allow to simulate the behavior of the real scale when this one is damaged

Scale effects on the response of composite structures under impact loading

For several years, composite materials have taken a significant part in the realization of structures designed for transport (aeronautical, nautical, automotive. . .). In order to qualify the behavior of such structures, preliminary validation tests have to be done. These specific tests are often very expensive and difficult to set up, especially when the structure dimensions are large (fuselages of aircraft, ship hulls. . .). An alternative way is then to employ small-scale models. The use of these reduced scale structures requires the identification of similitude models allowing the extrapolation of the small-scale model behavior to the real structure. Although largely developed in the case of homogeneous materials, such similitude techniques are not clearly identified for composite materials taking into account the damage evolution during an impact. The purpose of this article is firstly to present existing similitude techniques making it possible to predict the composite structure behaviour from the knowledge of small-scale model response. Secondly, experiments were done on two scale of samples carried out by stratification of unidirectional carbon/epoxy plies. These results were finally compared with the analytical predictions of similitude laws currently used. The aim of this paper is to contribute to similitude laws development applied to composite structures. These laws permit to extrapolate the small-scale model behavior to the real scale one. Existing approaches have been established following two different methods. They are summarized in this paper and applied to impact loadings on two laminated plate scales. In order to complete data collected by ‘‘conventional’’ instrumentation (force transducer, displacement sensor, accelerometer.. .), optical device such as an high-velocity CCD camera, associated with optical techniques for the monitoring of markers, were used. These techniques make possible to compare displacement lines corresponding to each scale. It is shown that existing similitude laws, used for elastic materials, do not allow to simulate the behavior of the real scale when this one is damaged

Tolérance aux dommages par impacts de structures courbes composites : effets d'échelles

During its life, a composite structure can be damaged locally although this deterioration may not be visible to the naked eye. It is necessary, in the stage of design, to appreciate the criticality of such a damage. Nevertheless, tests needed to estimate this criticality are often very expensive, especially in the case of large-size parts. The use of small-scale models can be an alternative to this problem. That implies the use, even the development, of scaling methods making it possible to extrapolate the behaviour of the reduced structure to that of the real structure. The objective of this research work, realized in collaboration between the laboratory LAMEFIP-ENSAM-ParisTech and SNECMA Propulsion Solide SAFRAN Group, is to establish a methodology that would potentially free the compagny from a heavy experimental study on structures (filament wound vessels) at the real scale. From tests, carried out on structures at smaller scales and from the model established in this work, it will be possible to predict the residual behaviour under internal pressure of a structure at the real scale when it is damaged by impact. Curved panels were used for the establishment of this methodology since they are less expensive than filament wound vessels - even at reduced scale -. Initially, a damage tolerance study, consisting of damage initiation by impacts, ultrasonic and microscopic inspections of the damage and quasi-static tensile tests until rupture, was carried out on two sclaes of specimens, based on experimental designs. Through this study, it was possible to highlight the existence of many scale effects, mainly due to the thickness and the curvature of the specimens. Then, a progressive damage model of the layers and the interface was established and implemented in a finite element code and fine tuned with respect to the response surfaces experimentally determined. This numerical modeling establishes the methodology allowing the scaling of structures. Then, this methodology was applied to the case of filament wound vessels. Numerical results match with data of an experimental study carried out on scaled vessels and show the need for taking into account the damage of the interface for the prediction of the residual behaviour of pre-impacted curved structures.Lors du cycle de vie d'une structure composite, elle peut être endommagée localement sans pour autant que la détérioration ne soit visible à l'oeil nu. Il faut donc, dans l'étape de conception, apprécier la criticité d'un tel dommage. Néanmoins, les essais nécessaires à l'estimation de cette criticité sont souvent très onéreux, surtout lorsqu'il s'agit de pièces de grandes dimensions. L'emploi de modèles réduits constitue alors une alternative à cette problématique. Cela implique l'utilisation, voire l'élaboration, de méthodes de changement d'échelle permettant d'extrapoler le comportement de la structure réduite à celui de la structure réelle. L'objectif de ce travail de recherche, réalisé en collaboration entre le laboratoire LAMEFIP-ENSAM-ParisTech et Snecma Propulsion Solide-Groupe SAFRAN, est d'établir une méthodologie permettant à l'industriel de s'affranchir d'une lourde étude expérimentale sur structures (capacités bobinées) échelle 1:1. Ainsi, à partir d'une campagne d'essais effectuée sur des structures à des échelles inférieures et de par le modèle établi dans ce travail, il doit être possible de prédire la tenue résiduelle en pression interne d'une structure à l'échelle réelle lorsque celle-ci a subi un dommage par impact. Moins coûteuses que les capacités bobinées - même à échelle réduite -, des éprouvettes courbes ont été utilisées pour l'établissement de cette méthodologie. En premier lieu, une campagne de tolérance aux dommages - création des dommages par impacts, expertises ultrasonore et microscopique des dommages et essais de traction quasi-statiques jusqu'à rupture - a été effectuée sur deux échelles d'éprouvettes en s'appuyant notamment sur les plans d'expériences. Par cette étude, il est possible de mettre en évidence la présence de nombreux effets d'échelles dus notamment à l'épaisseur et à la courbure des éprouvettes. Puis, un modèle d'endommagement progressif du pli et de l'interface a été établi et implémenté dans un code de calcul par éléments finis pour enfin être recalé vis-à-vis des surfaces de réponses déterminées expérimentalement. Ce modèle numérique finalise la méthodologie proposée permettant le passage d'une échelle de structures à une autre. Cette démarche a alors été utilisée dans le cas de capacités bobinées. Les résultats numériques sont en bon accord avec les résultats issus de l'étude expérimentale réalisée sur une échelle de capacités et montrent la nécessité de prendre en compte le caractère endommageable de l'interface pour la prédiction du comportement résiduel de structures courbes pré-impactées