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The translaminar fracture behaviors of partially different unidirectional composite systems, constituted by the same carbon fibers but different (thermoset vs. thermoplastic) matrices, were characterized by means of compact tension fracture tests. The resulting crack resistance curves (R-curves) and fracture surfaces, were studied in detail and found to be rather different between those material systems, in spite of the same reinforcing fibers at similar volume fractions. In the attempt to justify this difference, the effects of the underlying micromechanisms were evaluated by employing experimental micromechanical measurements of the fracture characteristics of fibers, matrices and fiber/matrix interfaces. By means of a thorough analysis and quantification of the micromechanisms that contribute to the work of fracture, it was possible to decompose the translaminar fracture toughness of the composites into different contributions. Independently of the material considered, fibre bundle pull-out was found to be the mechanism that dissipates the highest amount of energy. Different patterns of bundle pull-out in different material systems were found to be the result of different outcomes from the competition of fracture micromechanims, and to be responsible for the differences between the translaminar fracture energies of both material systems. Moreover, it was realized that the energy dissipated in bundle pullout, hence also the overall measured translaminar fracture toughness are strongly governed by in-situ effects, i.e. effects of specimen lamination features.