Sensitivity to crack propagation in flax fiber-reinforced polypropylene (PP) composites has been examined on SEN-T specimens. The sound waves that arise due to loading have been investigated with the acoustic emission method. Linear functional relationship has been revealed between the number of acoustic events that occur during loading and the fracture toughness. It has also been observed that increase in moisture content influences the mechanical properties unfavorably because water entering the fiber–matrix interface during swelling reduces the interaction. The size of the damage zone has been determined, and it proved to be smaller than in case of glass and carbon fiber-reinforced systems.
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