Sylvain Drapier

Finite-element investigation of the compressive strength of non-crimp-fabric-based composites

The compressive strength of composites based on non-crimped fabric (NCF) under axial loading has been investigated. The objectives are firstly to give insight into the mechanisms leading to failure, !and secondly to provide guidelines for the fabric structure to optimise the NCF properties. The modelling approach is at the blanket scale where the main microstructural parameters can be accounted for based on experimental characterisation of the NCF composites (A.J. Miller, School of Industrial and Manufacturing Science, 1996). Using a two-dimensional repeating finite-element model through the thickness of a biaxial fabric, we relate the NCF compressive properties to the geometrical and mechanical characteristics of the constituents. It is established that mesobuckling of the 0°-oriented tows leads to an overall shear instability of the model that can be associated with the experimental occurrence of failure. The coupling of tow crimp and material properties, especially the resin mechanical characteristics, is shown to very much modify the NCF compressive strength. The 90° tow shape is also relevant as it defines the support provided against the 0° tow plastic mesobu!ckling. Finally, it is seen that in order to improve the NCF compressive strength, the stitching tension during the manufacturing stage should be kept low so that the tows can spread. Thus, mesobuckling is delayed owing to the improvement in 90° tow support and the reduction in 0° tow crimp. These guidelines are consistent with compressive strength measurements carried out on NCF made during manufacturing trials (R. Backhouse, Eng D. Thesis, 1998). Improvement of the resin characteristics is also shown to bring about substantial improvement in predicted NCF compressive strength.

Influence of the stitching density on the transverse permeability of non-crimped new concept (NC2) multiaxial reinforcements : measurements and predictions.

New manufacturing processes arise for polymer-based composites which involve resin infusion through dry pre-forms. Modelling approaches of these processes require to assess new physical characteristics of the materials. In this paper, the transverse permeability of new multiaxial stitched materials, referred to as NC2, is investigated. First, in the framework of Darcys flows, this permeability is assessed for various biaxial NC2 using a specific device. Through this approach it is shown that the transverse permeability depends strongly on the stitching density. Then, the results from a simplified FE study carried out at the blanket scale are shown to correlate quite well the experimental measurements and evidences established in the first part.

A finite-element investigation of the interlaminar shear behaviour of non-crimp-fabric-based composites

Abstract In this paper the interlaminar shear behaviour of non-crimped-fabric-based composites is investigated by using a finite-element approach. It is intended to provide an understanding of the basic mechanisms which control the NCF behaviour, together with manufacturing guidelines for the fabric structure to optimise the NCF properties. The present approach is based on a bi-dimensional mesoscopic model of a biaxial blanket developed in a previous study devoted to the compressive strength of NCF (Drapier, S, Winsom MR. Finite element investigation of the compressive strength of non-crimp fabric based composites. Composites Science and Technology, 1999;59:1287–97). This through-thickness repeating cell is completed with the proper boundary conditions representative of interlaminar shear loading. It is established that the NCF ILS behaviour is controlled by the development of high shear-strain concentrations induced by the combination of mechanical and mesoscopic geometrical characteristics. These shear-strain concentrations are very likely to lead to local damage that could affect the NCF load-bearing capacity. It is mainly the resin shear behaviour, and to a lesser extent the tow size, which are shown to control the ILS behaviour. Also, the presence of resin layers which can form between the tows during the manufacturing process increases these strain concentrations. From a manufacturing point of view, it comes out from this study that the resin should have high shear stiffness and yield stress, and that tow bunching should be prevented by limiting both the stitching tension and the size of the tows used. Finally, the formation of resin layers should be limited as far as possible.

Nonlinear interaction of geometrical and material properties in sandwich beam instabilities

The first part of this paper is dedicated to the analytical and numerical characterization of local and global sandwich beam instabilities in a perfect linear framework. Analytical loads are extracted from an original unified model and used to understand in depth, through a parametric study, the role played by each geometrical and material parameter in the development of global as well as local instabilities. Also, the effects of the combinations of these characteristics is used to draw precious design indications. A low CPU time-consuming simplified model is then built and assessed. Critical loads and wavelengths computed from this model are shown to correlate very well with analytical predictions. It is established that this first approach is essential in order to lead to more detailed investigations in a numerical nonlinear framework which is the aim of the second part. The first geometrical nonlinear investigations in which linear elastic materials are considered permit to isolate sandwich configurations developing super- or sub-critical post-buckling behaviours. As a general trend, unstable behaviours are rather related to the occurrence of geometrical localizations along the beam. This is illustrated by the drastic effects of the so-called interactive buckling onto the whole stiffness of the sandwich beam. Moreover, it is shown that sandwiches are very sensitive towards imperfection sizes and forms. Eventually, an elastoplastic constitutive law is introduced for the core. It is demonstrated that plastic flow and strain localization in the core, combined with the occurrence of instabilities, are associated with a drastic drop in the global beam stiffness and with a strong decrease of the maximum limit load for some cases. The phenomenon of shear crimping is also observed which can be assimilated to a post-bifurcated development of the global buckling mode.

A structural approach of plastic microbuckling in long fibre composites: comparison with theoretical and experimental results

Abstract The aim of this paper is to compare some predictions obtained from a structural plastic microbuckling model presented in detail by Drapier et al. (1999) , with theoretical and experimental results from the literature. After a short presentation of this model, it is established that with our approach it is possible to find the elastic modes determined by Drapier et al. (1996) on the composite microstructure. The plastic instability mechanism is then investigated and its understanding is refined. Some simulations are carried out varying the fibre initial imperfection, and the results are detailed and compared with predictions from a kink-band model (Budiansky and Fleck, 1993) . Compared to the present knowledge, the understanding of the influence of the imperfection shape and of its distribution across the ply thickness is improved and new results are exposed. Validation of the present approach is completed by comparing the influence of both matrix and fibre behaviours as predicted by Budiansky and Fleck (1993) with the ones obtained from our numerical tool. Results demonstrate the influence of the change in the matrix tangent stiffness. Secondly, we have quantified the effects of the applied loading, thickness and stacking sequence on the compressive strength of laminates. Numerical predictions provide new results that yield a proper justification of the very high compressive strength measured with bending tests. These predictions also fit well experimental measurements from the literature showing the effects of the thickness (Wisnom, 1992) and of the stacking sequence (Grandsire-Vincon, 1993) on the compressive strength. For the first time, the effect of the gradient of loading across the laminate thickness is predicted. Results are shown to correlate well with experimental results from Wisnom et al. (1997) .

First applications of a novel unified model for global and local buckling of sandwich columns

Due to the intrinsic heterogeneity of sandwich structures, phenomena at various scales can co-exist in these layered-like assembly of thick-soft and thin-stiff materials. Especially under in-plane compression loadings, geometrical instabilities can occur at both global (structure) and local (skins) scales. Therefore, the in-plane compressive response of sandwich structures is of major concern in designing structural applications. In the present paper, the first applications of a novel unified model for sandwiches are presented, with closed-form solutions for both global and local buckling. For the perfect structure, analytical critical loads are extracted for a simply supported beam, through the calculation of two eigenvalues leading to three buckling modes: it appears that the eigenvalue associated with the antisymmetrical mode can correspond to the occurrence of either global or local (wrinkling) buckling. These global and local loads from the present unified model are shown to compare very well with the predictions given by the most complete specific models from the literature. Moreover, it is shown that conversely to the classical models, our approach yields critical loads that depend only on rigorous well-founded mechanical hypotheses. The simple but general analytical expressions from the unified model permit to select quickly configurations against local and global buckling. In this simplified framework, conclusions can be drawn from this unified model capable of properly predicting the phenomena at both scales. This simplified study is essential in getting an insight in the role played by each geometrical and material parameter, the combination of which is of importance for subsequent non-linear interactive post-buckling analyses (Leotoing et al., 2001).

Mixed experimental and numerical approach for characterizing the biomechanical response of the human leg under elastic compression.

Elastic compression is the process of applying an elastic garment around the leg, supposedly for enhancing the venous flow. However, the response of internal tissues to the external pressure is still partially unknown. In order to improve the scientific knowledge about this topic, a slice of a human leg wearing an elastic garment is modeled by the finite-element method. The elastic properties of the tissues inside the leg are identified thanks to a dedicated approach based on image processing. After calibrating the model with magnetic resonance imaging scans of a volunteer, the pressure transmitted through the internal tissues of the leg is computed. Discrepancies of more than 35% are found from one location to another, showing that the same compression garment cannot be applied for treating deficiencies of the deep venous system or deficiencies of the large superficial veins. Moreover, it is shown that the internal morphology of the human leg plays an important role. Accordingly, the approach presented in this paper may provide useful information for adapting compression garments to the specificity of each patient.

Structure effect and microbuckling

Abstract At the present time, it is clearly established that the compressive failure in the direction of fibers of a unidirectional ply is not intrinsic to the material but depends on the structure (stacking sequence, thickness of the ply and loading). This dependence can be explained by the appearance of fiber microbuckling. The purpose of this work is the ‘exact’ calculation of the microbuckling modes which may appear in the laminate. Unlike the local models from the literature, the complete microstructure of a laminate is represented and the modes are sought in the form of harmonics in the direction of fibers. The following parametric study clearly shows the effect of the ply thickness, the boundary conditions, the loading, the stacking sequence and the presence of damage, on the mode and on the critical strain of microbuckling. Moreover, these results lead to a very reliable homogenized model which gives the possibility of taking into account the effect of fiber alignment defects and the appearance of plasticity inside the matrix.

Towards a numerical model of the compressive strength for long fibre composites

The compressive failure of long fibre composites is tackled as a structural instability that is initiated by plastic microbuckling. A homogenised model is proposed to take into account the fibre initial waviness, the non-linear matrix behaviour and some structural effects. The problem is formulated at the ply scale by using a two-scale displacement field and a specific finite element is developed to reduce the extent of computations. This numerical tool permits us to rapidly determine the failure of various unidirectional ply configurations by use of a maximum load criterion. The results establish the influence of some structural parameters on the compressive failure strains, as has been observed in related experiments. They also explain the high compressive strength achieved under flexural loading.

An Experimental Assessment of the Saturated Transverse Permeability of Non-crimped New Concept (NC2) Multiaxial Fabrics

Over the past few years some manufacturing processes involving resin impregnation in dry preforms prior to cure, and more specifically resin infusion across the fabric thickness (the so-called Resin Infusion Processes) have been developed. In these increasingly used processes, the transverse permeability of the fabric controls both manufacturing cycles and dimensions, and consequently the mechanical properties of the final composites. A big effort has been reaized to obtain reliable experimental techniques for measuring the transverse permeability. Since the distinction between saturated and effective permeabilities is still being discussed, and the flow-front is hardly defined in such small dimensions (of the order of 1 mm), our measurements on some so-called Non-Crimped New Concept (NC2) are carried out for saturated flow, i.e., the fiber network is completely impregnated right from the onset of the experiment. In our case [1], these measures rely on the measurements of the pressure drop induced by the flow of a controlled fluid across the fabric, easily converted into transverse permeability through Darcy’s law. One of the main results of these measurements is that the low NC2 transverse permeability depends on the face of the fabric receiving the fluid. This original behavior may, in turn, be translated into manufacturing guidelines to achieve optimal processing configuration. An attempt to relate this differential to the stitching hole surface is proposed which appears to explain some minor phenomena. On the contrary, the 3D shape of the stitching hole reconstruction from polishings could yield some appropriate explanation.