Combining short flax fiber mats and unidirectional flax yarns for composite applications: Effect of short flax fibers on biaxial mechanical properties and damage behaviour

Short flax fibers mats, unidirectional (UD) layers of flax yarns, and an assembly of these two, are used as reinforcements for an epoxy matrix composite. The tensile and flexural properties of the three different composites, in both longitudinal and transverse directions, were studied in order to highlight the effect of mat surface density, using two stacking sequences. A slight decrease of the measured longitudinal properties is observed when low levels of short fibers content are used. This is compensated by a significant decrease of the material variability, a significant increase of the transverse properties, and a favorable effect on the fracture behaviour of the tested specimens. Damage evolution is monitored using acoustic emission (AE) and the use of multivariable analysis of the collected data has given a clear description of the different damage modes, their evolution and their contribution to overall composite failure. Different damage behaviours are observed for the mats and UD samples. Most importantly their combination into a single reinforcement has a positive effect on damage type and evolution. The presence of short fibers in the composite reduces the crack propagation along the fibers/matrix interface, which reduces splitting and transforms the fracture surface from a serrated fracture surface to a flat and confined fracture surface.