Using X-Ray tomography to evaluate porosities in composite parts

National audienceIn aeronautics and automotive applications, manufacturing composite by consolidation is mainly used in order to answer current weigh reduction and optimization cost problematics. During the process, voids appear in the composite parts. An adequate process cycle must be selected to reach minimum porosity content within the laminate. These voids have influence on mechanical properties of composite parts. That's why, it is important to study voids in composite parts. Objective of this study is to develop a new way to study porosity in composite parts. This method permits not only to measure the voids content but also the voids size, location and morphology.The non-destructive technic consists on using X-ray beam to analyse the sample structure. The intensity of the attenuated X-ray is recorded from different angular position of the sample so as to get the spatial distribution of the absorption coefficient within the laminate by using a reconstruction algorithm. The sample is represented as voxels, which correspond individualy to the intensity of the material density (grayscale value). Objects, which have a low material density as voids, absorbs less X-ray than a higher material density as fibre and matrix. Consequently, in this case, porosities objet have the lowest attenuation coefficient with a grey value close to 0 (black color). On the contrary, fibre and matrix objects have a higher gray value (approching white color for the matrix one). All specimen were analysed with a Easytom 130 tomograph. Scan resolution, in terms of voxel size, reached for this study is 11.2 μm³ +/- 0.8 μm³. Reconstruted 3D images were analysed with post-processing software ImageJ and the plug-in 3D Objects Counter.To analyse voids within the laminates, it is necessary to separate it from matrix and carbon fibres into two groups, with segmentation method. The grey threshold, chosen for void segmentation, is taken at the first inflexion point of the hystogramm of the fibres/matrix image for all X-ray slices. The grey threshold must be determined for each X-ray slice to minimize errors. With this segmentation, voids location, size and morphology are analysed. Voids size permits to define micro and macro voids.This method was applied on carbon composite with different consolidation cycle in terms of consolidation pressure, processing temperature, consolidation time and vacuum levels