Experimental characterization of short flax fiber mat composites: tensile and flexural properties and damage analysis using acoustic emission

In this work, tensile and flexural tests are realized on composites reinforced with short flax fibers mats produced by a papermaking process. Plates are molded with different fiber volume contents (V-f), and to support the analysis, acoustic emission (AE) is coupled to test samples to follow the evolution of different damage modes using a multivariable analysis to classify the acoustic events. It is shown that the tensile and flexural properties increase with V-f up to a critical value of about 40%, above which they start to decrease. The contribution of each damage mode in the global failure of the composites is calculated, and their effect in the evolution of mechanical properties is discussed. The results show that compared to the tensile tests, AE events of flexural tests appear at much higher strains, with considerably lower cumulated energies, reflecting the low level of AE events attributed to matrix microcracking. The AE analysis also reveals a clear domination of fiber-matrix friction and fiber pullout mode of fracture, raising the importance of the adhesion of flax fibers-epoxy matrix. The decrease in Young's modulus and strength at V-f above 40% is in a large measure explained by a poor fiber-matrix adhesion.