Steven Marguet

Modeling of Nomex honeycomb cores, linear and nonlinear behaviours

The purpose of this study is to develop tools dedicated to the design of sandwich panels involving Nomex® honeycomb cores. Special attention is paid to the ability to perform full three dimensional calculations up to failure of such structures. In the first part, the determination of effective elastic properties of Nomex® honeycomb cores is carried out thanks to strain based periodic homogenization technique. Using an equivalence in energy between a real honeycomb and a fictitious continuous medium, it becomes possible to evaluate the elastic behavior of Nomex® cores, starting from the knowledge of the behavior of the constitutive paper. Next, relying on experimental observations, the strengths of Nomex® honeycomb cores are evaluated with a linear Eulers buckling analysis. Results are compared with data coming from manufacturers, and give satisfaction. In order to carry out these two first studies, the NidaCore software has been developed using the finite element code Cast3M from CEA. The last part deals with the modeling of the nonlinear compressive response of Nomex® cores. A model based on the thermodynamics of irreversible process is proposed and the identification technique detailed. Good agreement between experimental data and computed values is obtained.

A Rate Dependent Constitutive Model for Carbon-Fiber Reinforced Plastic Woven Fabrics

This paper deals with the modelling until rupture of composite structures made of carbon-fiber/epoxy-resin woven fabrics submitted to dynamic loadings. The model is built at the mesoscale of the elementary ply. It takes into account the slightly nonlinear brittle behavior of the fibers under tensile solicitations, their nonlinear behavior in compression as well as the strongly nonlinear and irreversible behavior of the ply in shear. Strain rate effects are also introduced and special attention is paid to the objectivity of the model in the context of finite element calculation. Therefore the choice of a delayed damage mesomodel coupled with viscoplasticity is made. In order to identify the values of the parameters of the model, an optimization procedure based on a gradient-free direct search method has been developed. As a logical procedure to this study, the models ability to avoid strain localization and mesh dependence is then checked on simple uniaxial examples. The last part of this paper is devoted to structural calculation. The results of the simulations of both the impact on composite plate and the crushing of thin-walled tube demonstrate the capability of the model to reproduce observed physical phenomena.

Numerical modelling of Nomex® honeycomb cores : Failure and effective elastic properties

The aim of the present study is to propose and develop the numerical determination of the effective stress-strain behaviour of Nomex® honeycomb cores made from aramid paper material. This study highlights the determination of the hexagonal and rectangular over-expanded core materials. These honeycombs are extensively used in the manufacturing of aeronautic structures and of oceanic multihull sailing race boats. These sandwich structures are made of carbon-fiber epoxy-matrix composite laminate skins and Nomex® cores. The understanding of the behaviour and eventually failure of honeycombs are extremely important for the design of these engineering composite sandwich structures. Honeycomb cores predictions are directly related to the structural integrity and safety requirements of the entire composite structure. Since the pioneering work numerous studies on the effective properties of cellular sandwich cores have been published [1]–[2]. In the past, core behaviours were studied under strength of material assumptions. In this context a software dedicated to Nomex® honeycombs was developed at the Laboratory in order to predict the failure conditions of these cores [2]. Our software NidaCore has been developed to determine the three dimensional mechanical core properties. The elastic mechanical properties have been determined by a three-dimensional Finite Element model that involves periodic homogenization techniques. For the homogenisation of the honeycomb microstructure, a strain energy-based concept is used which assumes macroscopic mechanical equivalence of a Representative Volume Element for the given microstructure with a similar homogeneous volume element. The software has been developed using the Finite Element structure analysis program Cast3M-CEA. Numerical predictions are compared with the mechanical properties given by the Euro-Composites company. The present study confirms that for honeycombs under consideration the Representative Volume Element symmetries lead to orthotropic homogenized mechanical properties. The key point of the modeling is that the RVE buckling modes conduct to determine the ultimate stresses of the homogenized core. Based on buckling modes, numerical analysis reproduces the ultimate stresses experimentally observed on standard test methods. This approach strengthens by experimental results leads to a failure criteria based on the mechanical understanding of local damage effect. In order to go further, the skin effect on the core properties is discussed for T700/M10 carbon-fiber epoxymatrix cross-ply and angle-ply laminates skins. The honeycomb mechanical properties and ultimate stresses are used to model three dimensional reinforcements that we used for the study of the Oceanic sailboat structures.

A Rate Dependent Constitutive Model for Carbon-Fibre/Epoxy-Matrix Woven Fabrics Submitted to Dynamic Loadings

This paper deals with the modelling until rupture of composite structures made of carbon/epoxy woven fabrics and submitted to dynamic loadings.