3D Full Field Displacement and Strain Measurements at the Microscale in Fiber Reinforced Composites Under Transverse Load Using Digital Image Correlation

RESUME La rentabilite de l’industrie aerospatiale est fortement liee a une reduction de la consommation de carburant et de la masse des structures utilisees tout en maintenant un niveau de performance et de securite similaire. Pour atteindre ces objectifs, les materiaux composites a renfort fibreux (CRF) sont de plus en plus utilises dans cette industrie. Ces materiaux offrent une resistance specifique elevee, resistent a la corrosion, sont legers par rapport a leurs proprietes mecaniques et ont de bonnes proprietes de resistance a la fatigue. Ces materiaux sont de plus en plus utilises mais sont aussi consideres comme une des causes de plusieurs catastrophes aeriennes. Les capacites de predictions de l’etat d’endommagement et de rupture de pieces faites en CRF sont encore limitees, tel que demontre par les deux editions terminees du World Wide Failure Exercise. L’endommagement dans les CRF est caracterise par une multitude d’evenements microscopiques qui se developpent puis se regroupent graduellement jusqu’a former un large reseau de micro fissures a travers le materiau. A l’echelle de la fibre, le type d’endommagement le plus critique pour des CRF unidirectionnels est la decohesion inter-faciale entre les fibres et la matrice d’apres la litterature. Ce mecanisme commence avec une decohesion inter-faciale en Mode I entre la fibre et la matrice, la fissure inter-faciale se propage ensuite autour de la fibre dans et hors plan. L’interface entre la fibre et la matrice ne peut alors plus transferer de contraintes correctement, ce qui entraine une augmentation locale de la contrainte autour de la fibre en decohesion. Une des fibres environnantes va ensuite a son tour avoir une decohesion inter-faciale qui se produit du a l’augmentation de contrainte, et ainsi de suite pour toutes les fibres environnantes. La rupture de l’echantillon se produit eventuellement lorsque les fissures de decohesion commencent a croitre dans la matrice, se regroupent et forment un large reseau de fissures qui grandit a travers tout le specimen. Des observations de decohesion inter-faciale sont disponibles dans la litterature, mais il n’y a pas encore de modele pour ce mecanisme qui est generalement accepte. La decohesion inter-faciale implique une croissance de la fissure en Mode I, en Mode II et en mode mixte. D’autres mecanismes s’ajoutent aussi, tels que la friction entre la fibre et la matrice, les contraintes residuelles dues a la cuisson de la matrice et le retrait chimique de la matrice durant la cuisson. Cette combinaison de mecanismes qui participent a la decohesion inter-faciale en font un mecanisme d’endommagement complexe. Des donnees experimentales additionnelles, telles que les champs de deplacement ou de deformation in-situ, permettraient de fournir une comprehension plus complete de la decohesion inter-faciale pour differents types de fibres.----------ABSTRACT This thesis aimed at experimentally investigate damage initiation and growth of FRCs under transverse loading at the fiber level, provide in plane and out of plane full field measurements and crack area measurements for different single fiber composites and for a bundle of carbon fibers. Firstly, a single fiber composite specimen is designed and manufactured in such a way that a large fiber, approximately 1 mm in diameter, is under transverse loading during a tensile test. Four specimens were manufactured out of fibers having strong adhesive bonding with epoxies and no adhesive bonding with epoxies combined with an epoxy and a modified epoxy. A stereoscopic Digital Image Correlation (DIC) setup is then used to track 3D displacements and compute in plane strains for a fiber’s free surface and its vicinity. The experimental results showed that inter-facial debonding happened in three steps, an inter-facial crack opened under Mode I at the fiber / matrix interface at the location where an out of plane displacement difference between the fiber and matrix was the highest and where "y was maximum for all specimens. The inter-facial debonding crack then grew under mixed mode around the fiber while it kept protruding out of the matrix. Finally, specimen failure occurred differently for the specimens with no adhesive bonding compared to the strongly bonded ones. For the ones without adhesive bonding, specimen failure was caused by a crack growing under Mode I in the matrix where fibers were horizontally compressed and large out of plane deformation was experienced. Strongly bonded specimens’ failure was also caused by a Mode I crack growing in the matrix but located where the tension is maximal within the inter-facial crack’s free surface. The complete experimental results, containing the stereoscopic full field displacement and strain results for each test and timestep, were provided in a data package for further analysis, or benchmarking of simulation results. DIC provided quantitative information about displacement and strain fields, however, the method has limitations in the vicinity of cracks. In addition, DIC did not provide any quantitative information about cracks themselves. A method using the raw images from the experiment and the DIC results was developed to combine both results and accurately determine the crack area, crack path in the reference coordinates and the exact applied stress on the specimen for the crack to grow through a certain area. This method was applied to the previously mentioned experiments. Results showed that inter-facial crack initiation happens in fact at the same strain value applied on the specimen, whether the fiber has strong or weak adhesive bonding.