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.