The work focuses on the mechanical strength of multifunctional composite structures submitted to low energy impact. The material is defined for aeronautical applications as multilayers of Glass fiber composite with flat electrical cables layers embedded.

The main question is the damage involved by the low energy impact, and its influence on the electrical cables layer. The sensitivity of the material to low energy impact leads to failure with no external signs of damage and insulation degradation between adjacent embedded conductors.

Experimental campaign by drop tower test are carried out for energy level between 10 and 25 Joules. An hemispherical impact tool (10Kg drop weight) is used for punctual impact on the top of a clamped specimens with or without (similar blank) embedded electrical cable layers. The comparison of the dynamic contact force history curves shows a much more « ductile behaviour » with lower peak contact force and higher impact durations (up to 5ms) for specimen with embedded cable layers. The non-linearity is mostly associated to both sliding and inter-laminar damage among the embedded cables.

To get a better understanding of these local damage mechanisms, several specific 3 point bend tests are conducted. These tests are performed on similar blank and multifunctional test specimens. The impact energies are chosen based on the damage threshold, which is considered as impact energy at which first ply failure occurs on the back face of test specimens around 12 J. With the help of Digital Image Correlations analysis, a single tensile and interlaminar shear strain gradient close to the interface of the electric cable can be detected. Different coupled damage mechanisms like internal fibre failure and inter-laminar damage (debondings) are also identified.

In an effort to reproduce the damage observed experimentally, numerical finite element modelling is achieved. It is based on composite progressive damage law coupled with Hashin failure criterion for each composite ply. The embedded cables are given a linear anisotropic behaviour and modelled similarly to a composite ply. Adhesive elements are used to model delamination damage and the sliding between ply at the interfaces. Models for blank and multifunctional material show similar evolutions as compared to the experimental data, as well as the distribution of tensile strains, damage initiation and failure progression across the stacking sequence of plies.

This study aims at identifying some mechanical criteria for geometry improvement of functional structures embedding electrical functionalities (electromagnetic compatibility, data & power transmission, etc.).