Impacts on foam stabilised composite structures: experimental and numerical study

A dropweight tester is used to make low velocity tests on specific sandwich type structures. Sandwich are made of glass-epoxy skin and polyurethane foam core. The skins can be straight or little curved, and impact direction is the global skin direction. The aim of these tests is to study the initiation of rupture in such structures :local buckling of skin and foam core rupture. Experimental results are given. They show the evolution of buckling critical stress in the skin when impact velocity increases. The rupture mode in curved skin specimen is also studied : rupture is no more provoked by buckling. A numerical analysis is proposed to model the behaviour of the structure and the rupture initiation. Finally, a method is developed, in order to predict the propagation of skin debonding during impact : an element layer under the skin is damaged with a specific law to simulate debonding

Dynamic buckling of foam stabilised composite skin

Presented in the following pages is an experimental and numerical study of dynamic local buckling of skin on foam core. Impact tests on sandwich-type structures with skins stabilized by foam demonstrated that rupture appears by debonding of skins due to a local buckling phenomenon, and that the maximum stress in the skin, obtained at rupture, grows with the increase of the loading rate of the skin. A finite element analysis allows this phenomenon to be analyzed and understood, and a mass-spring-dashpot model is proposed to model the skin debonding initiation

Buckling of foam stabilised composite structures

An analytical modelling of the symmetrical wrinkling is proposed : from original assumptions on displacements within the core, and from an energy minimisation method, it is possible to predict critical loads and buckling modes better than traditional models do, and to distinguish the influence of each structure component. Compression tests were carried out on sandwich structures to validate the model. Little curved structures were also tested to estimate the influence of skin curvature on rupture and buckling mode. A finite elements analysis has been achieved in parallel : a fine modelling allows to find results close to experimental ones

Dissipation mechanisms identification of soft hollow particle-dampers in honeycomb structures for micro-vibrations environment

Particle dampers are enclosures partially filled with metallic or glass small spheres, attached to the vibrating structure. This paper deals with replacing hard classical particles by soft hollow ones to maximize damping and mass ratio. Hence, one aspect of this damping method is obtained by mixing the kinetic energy conversion of the structure into heat(frictional losses and collisions) and the elastic energy conversion into heat (visco-elastic deformation). This study is oriented toward experimental and theoretical investigations in order to distinguish the dissipation phenomena. The experimental approach first relies on identification and, then, on validation applied on composite aluminum honeycomb plates. Indeed, equivalent viscous damping is identified on small honeycomb samples; then cantilever honeycomb beams are filled with particles and studied. Theoretically, beyond the nonlinear dissipation by impact and friction, these particles add a visco-elastic behavior. The shapes of the hysteretic loops highlight that this behavior is predominant. Hence, oscillators are added in the FE model and permit to consider the effect of the particles. These kinds of particle dampers are highly nonlinear as a function of excitation frequency and amplitudes. The aim of this study is to provide a structural damping solution for space applications which require high pointing stability to enhance mission performances. In this perspective, damping of micro-vibrations was thought as a possible application; nevertheless it is shown that best efficiency is achieved in high frequency range

Bird strike shielding materials: development of a high velocity impact test platform

Commercial aircraft structures are exposed to bird strike events causing serious damages. Certification is thus required by regulation organizations and tests are performed using gas guns and birds of mass 1,8kg (JAR 25.631) (or 3,6kg for tail parts), which are launched at approximately 175m/s. In the design phase, modelling and simulation are rather used to assess and optimize the response of materials and structures under bird strike. However, it is difficult to correlate simulations with tests because of the very limited availability of test platforms for characterization and qualification from the smallest coupons to shielded structural component. The purpose of our work is to set up a test platform in close partnership with Institut Clément Ader (ICA), equipped with advanced metrology and combined with a virtual testing approach for correlation of tests and simulations up to 1m scale shielding concepts. New bird strike shielding materials could then be further developed at a lower cost in reduced time frames. Nonetheless the platform and virtual testing approach could be derived to other high velocity impacts (hail, engine debris, tire debris)

Flambement de structures composites stabilisées par une mousse

Cette étude a pour but d'améliorer la compréhension du phénomène de flambement local de peaux stabilisées par mousse, ainsi que la prédiction des forces et modes de flambage. Une campagne d'essais a été menée sur des éprouvettes de diverses géométries, en compression pure. Une analyse par éléments finis menée parallèlement permet, par une modélisation fine, de retrouver les valeurs expérimentales avec une bonne précision. Une modélisation analytique du wrinkling symétrique est également proposée. A partir d'hypothèses originales sur les déplacements au sein de la mousse, et d'une méthode de calcul fondée sur la minimisation des énergies, elle permet de prédire les forces critiques mieux que les modèles classiques, tout en distinguant l'influence de chaque constituant de la structure. Cette modélisation est également validée par une étude éléments finis, et donne des résultats plus précis que les modèles classiques

Experimental study of bird strike response of sandwich structures: overall trends

Nowadays sandwich structures are used as bird shields to protect aircraft nose bulkhead. They are usually made of aluminum honeycomb and sheets. In order to optimize mass, cost and efficiency of such bird shields, aircraft manufacturers want to explore new materials and designs. In this context, this work aims at exhibiting influences of material and design parameters on the bird strike response of sandwich structures. Based on previous finite element calculations that enabled to determine the most influential parameters, a design of experiment has been defined to span a parameter space in 4 dimensions, namely the thickness of the front skin, the thickness of the core, the yield stress of the front skin and the crushing stress of the core. 13 configurations of sandwich structures are then tested according to this design of experiment. These samples are 800 mm x 800 mm square sandwiches simply supported on a rigid square frame. The impactor is a 1.6 kg gelatin bird substitute, thrown at a speed of 160 m/s. Based on high speed camera image correlation, an original approach is developed to measure and quantify the responses of the samples. For each configuration, the full displacement field of the rear face of the sandwich is calculated during the impact event. The final shapes for both rear and front faces of the shield are also reconstructed, enabling to compare the behaviors of the different structures regarding bending, indentation and core crushing… The exhibited trends in terms of influences of material and design parameters open prospects for pre-selection of candidate materials and designs for bird shield applications which could lead to mass and cost reduction while satisfying aircraft design constraints (no failure, low rear face displacement…)

Tensile post-impact behaviour of thin carbon/epoxy and glass/epoxy hybrid woven laminates – Part II: Numerical study

International audienceThis article concerns the modelling of post-impact damage propagation in thin woven composite laminates. Simulations of low velocity impacts and post-impact quasi-static tension are performed on single-material and hybrid laminates. The modelling is based on the semi-continuous approach implemented into the explicit finite element code RADIOSS. The bundles are modelled with rod elements and a specific damageable shell element is used to stabilize this truss structure. Improvements are brought with the introduction of a compressive failure criterion for the rod elements and the development of a pseudo-plastic law with damaging for in in-plane shear. The results provided by the modelling well correlates the experimental observations in terms of damage propagation and load-displacement curves for all the configurations studied