A Characterisation of the Damage Process under Buckling Load in Composite Reinforced by Flax Fibres

The purpose of this work was to analyse the damage process resulting from buckling load applied on composites reinforced by flax fibre. Continuous buckling tests were performed on specimens until cracks appeared on their outer face. These tests were monitored with an acoustic emission system. The high sensitivity of this method allows the detection of any process or mechanism generating sound waves. Moreover, this technic has the advantage of not causing contact in the deformed zone and, thus, to overcome the parasitic damage that may result from the stress concentrations in these areas. A multiparametric analysis was used to identify the acoustic signatures corresponding to each damage mechanism involved in the materials and then to follow their evolution in order to identify the most critical mechanisms leading to the final breakage of the material. The presence of these damage mechanisms were confirmed post-test by microscopic observations. Three orientations of laminate specimens (0°, 90° and 45°), relative to flax fabric architecture, were tested in order to characterise and highlight their own damage process. Similarities and differences were observed among these mechanisms. We deduced that the high porosity rate found in our composites were the result of manufacturing parameters. Architecture and properties of the flax fabric influenced negatively the mechanical properties later by accentuating the gap between theoretical and practical values (17% to 22.4%) and by accelerating the development of certain damage, such as matrix cracking, where the acoustic hit density was superior to 70% and the fibre/matrix decohesion which occurred very early.