Mode I translaminar fracture toughness of high performance laminated biocomposites reinforced by sisal fibers: Accurate measurement approach and lay-up effects

The present work performs a systematic experimental analysis of the translaminar fracture behavior of high performance biocomposites constituted by green epoxy reinforced by sisal fibers, by varying the main influence parameters as fiber concentration and lay-up. Despite the corrective function properly introduced to take into account the anisotropy as well as the use of the equivalent crack length, the study shows that the LEFM does not give accurate estimations of the fracture toughness, because the extension of the near tip damaged zone is higher than the singular dominated one. Accurate estimations can be obtained instead by the proposed modified area method that takes into account both the local damage and the fiber bridging that occurs during crack propagation, that lead to R-curves whose asymptotic values constitute the true fracture toughness of the biocomposites examined. The constancy of the damage mechanisms observed by varying the fiber concentration, allows the user to compute the fracture toughness of a generic laminate from the specific fracture energy of the unidirectional lamina. Finally, the relatively high fracture toughness of the examined laminates allows to state that they can advantageously replace not only other composites having lower toughness, but also metals as steel, aluminum and titanium.