**4. Conclusions**

An experimental set-up for the measurement of unsaturated through thickness permeability based on the ultrasonic wave propagation in pulse echo mode was proposed. A single ultrasonic transducer, working both as emitter and receiver of ultrasonic waves, was used to monitor the through thickness flow front during a vacuum assisted resin infusion set-up. The new measurement method was tested on three thick carbon fiber preforms, obtained by stacking and vacuum bagging or by automated fiber placement of unidirectional plies. The ultrasonic data were used to calculate unsaturated out-of-plane permeability applying Darcy's law. The permeability results were compared with saturated out-of-plane permeability, determined by a traditional gravimetric method.

For each preform, as the fiber volume fraction increased, the permeability values decreased, due to the increased fiber compaction that lead to a slowdown of the fluid flow. Moreover, at the same fiber volume fraction and for the same preform material, the saturated out-of-plane permeabilities were higher than the unsaturated out-of-plane permeabilities.

Finally, the permeability data were validated by two analytical models: Berdichevsky and Cai model and Gebart model. The best agreement was obtained using Berdichevsky and Cai model assuming a hexagonal fiber arrangement.

**Author Contributions:** Conceptualization, F.L. and A.M.; Methodology, F.L. and A.M.; Experimental activity, F.L. and F.M.; Data Curation, F.L. and F.M.; Writing, Review and Editing, F.L. and A.M.; Supervision, A.M.; Funding Acquisition, A.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** The research leading to these results was partially funded from by the Italian Minister for University and Research (MIUR) through the PON research project SPIA.

**Acknowledgments:** The authors kindly acknowledge Silvio Pappadà from CETMA, Giuseppe Totaro from Leonardo S.p.A. and Giuseppe Barile from Novotech Aerospace Advanced Technology srl for providing the three different carbon fiber preforms. Riccardo Dell'Anna and Alessia Peluso are kindly acknowledged for the assistance in the ultrasonic set-up and permeability measurements.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
