Francis Collombet

Impact behavior of laminated composites: Physical basis for finite element analysis☆

Abstract The aim of this paper is to present a method for taking into account, in numerical terms, the physics of damage events occurring during impact of composite structures and considering both the constraints of the explicit 3D finite-element analysis (especially the stability condition which links time and spatial discretisation) and the lack of local observations during impact. An explicit finiteelement code is used to model the dynamic characteristic of the impact loading. We present two approaches for the modeling of damage phenomena: (1) averaging models and the contact technique, (2) simple criteria for determining damage initiation and growth. The latter approach is applied here to the impact loading of symmetric 10-ply glass/epoxy 0/90 laminated plates. In this case matrix cracking controls delamination and hence is considered to be of primary importance.

EXPERIMENTAL CHARACTERIZATION OF QUASI STATIC AND SHOCK WAVE BEHAVIOR OF POROUS ALUMINUM

Experiments of quasi static hydrostatic and uniaxial strain compression, and of shock wave propagation performed on 9% and 17% porous aluminum are presented, analyzed, and compared. Quasi static experiments show the influence of coupling between void collapse and plasticity induced in the matrix on the material macroscopic behavior. The amount of pore compaction appears to be enhanced by the deviatoric stress component present in the uniaxial strain tests and not in the hydrostatic ones. The originality of the plate impact setup and its associated metrology [velocity interferometer system for any reflector (VISAR) interferometry and polyvinylidene fluoride (PVDF) piezoelectric gages] exhibits also the influence of these local physical mechanisms on shock wave propagation in porous aluminum. More, the variations observed between the rise times of shocks seem to point out a preponderance of the dynamic effects (inertia or strain rate) over the material behavior. We observe indeed that the higher the stress in the material, the shorter the shock rise time. This point is confirmed by comparing quasi static and dynamic responses of porous aluminum. Comparison of these experimental results to numerical simulations should be interesting to prove or not this hypothesis.

Damage criteria for the study of impacted composite laminates

Modelling the behaviour of laminated composite structures under low-velocity impact loading (i.e. m < 3kg, v < 6 m s−1) is closely linked to the study of the damage in the impacted structure. In this framework, the state of stress is multiaxial. Local observations are not easy to carry out, nevertheless the drop-weight set-up based on split Hopkinson pressure bar metrology gives the force/time and displacement/time relationships on the scale of the impactor composite-plate system (global scale). Having in mind that this kind of macroscopic measurement is not a direct representation of damage chronology on the local scale (material and layer scale), we have developed a hybrid approach allowing an efficient use of experimental results and, conversely, of numerical ones by identifying local damage criteria (global experimental measurements, local numerical identification). The last part discusses the methodology applied to symmetrical 10-ply glass-epoxy 090 laminates.

Effects of a dynamic confinement on the penetration resistance of ceramics against long rods

Adequate confinement of a ceramic block can lead to its impenetrability against long rod penetrators. New ballistic experiments (encapsulated rod experiments) enabling a pressurization of the front face of the ceramic block (dynamic confinement) have been performed and compared to results obtained from standard unconfined configurations (DOP tests). Impenetrability of the ceramic block is obtained with the encapsulated rod configuration. A modeling approach based on a description of the fragmentation process of the ceramic is proposed. In particular, effects of the void content of the fragmented ceramic on its shear resistance are taken into account. Comparisons between Eulerian computation and the experiments show that conditions for rod dwell are linked to immobilizing fragments of ceramic in front of the projectile.

Variability in Monolithic Composite Parts: From Data Collection to FE Analysis

A main challenge facing the structural applications of composite materials is related to the uncertainty in the material performance due to their inherent variabilities. Structural properties of composites are not only dependent on the manufacturing steps, but also on the constituent materials, reinforcement architecture and design choices. By introducing geometrical variabilities into finite element (FE) model through meta-models, the effect of variations in the composite structure can be studied at different scales. To assure that the FE results are in accordance with the real composite structure, the input parameters of the models must be in agreement with the actual material and its configuration in the structure. In this chapter, a methodology to study and introduce variabilities into a composite structure FE analysis is presented.

Assessments on the mechanical behaviour of a monolithic composite structure instrumented with a monitoring patch

In this paper, the monitoring patch is evaluated as an alternative instrumentation technique for aircraft-type composite structures, by means of the Multi-Instrumented Technological Evaluator. In this case, the goal is to evaluate the strength and failure modes of a carbon-epoxy composite plate with two drop-offs instrumented with a monitoring patch. With the aid of finite element models, the testing of the plate under combined loads is analysed to have a first numerical approach of its behaviour. Then, the experimental campaign is accomplished by testing the plate with multi-instrumentation devices and techniques such as strain gauges and digital image correlation. A correct calculation/test correlation is achieved by comparing the strain values calculated by the finite element model and the experimental strain data acquired by gauges and digital image correlation. The results confronted provide a first evidence to quantify the influence of the monitoring patch on the mechanical performance of the composite plate. Therefore, it could be employed in the near future as instrumentation technique on large composite structures.

Identification of a macroscopic anisotropic damage model using digital image correlation and the equilibrium gap method

The aim of the work is to demonstrate how an anisotropic damage model may be identified from full field measurements retrieved during a heterogeneous test. The example of a biaxial test performed on a 3D C / C composite is used. In a first step, the displacement fields measured by classical Digital Image Correlation are used as input data of a finite difference version of the Equilibrium Gap Method. A benefit from unloadings (assumed to be elastic) is shown to retrieve a damage law. In a second step, inelastic strains can be assessed from the total measured strain and the elastic estimated strains. The constitutive parameters relative to the inelastic part of the model are then identified.

Caractérisation de l'état initial de tubes composites. Maîtrise du placement des renforts et suivi de la cuisson par fibres optiques à réseaux de Bragg

In order to analyse the influence of the mesostructure of filament wound cylinders on their mechanical behaviour it is essential both to control the fabrication process and to use advanced measurement techniques, such as fiber optic Bragg gratings, to study their behaviour. In this research project advances in unit cell precision positioning are presented. This paper includes a description of the composite pipe, monitoring during fabrication, a pipe reinforcement architecture analysis system and a study of the influence of architecture on mechanical properties.

Delamination of Laminates under Impact Loading: Contact Technique and Constitutive Relation

During transverse low velocity impact of laminates, two kinds of damage - matrix cracking and delamination - are coupled and depend on the evolution of the loading versus time. The optimization of the behaviour of the composite structures in dynamic imposes the development of an hybrid approach synergy between experimental, theoretical and numerical aspects (Collombet & al, 1993). The aim of this paper is to model two kinds of contact, the impact loading on the macroscale (projectile-structure) and the delamination ou the mesoscale (between plies of different orientations).