**Francesca Lionetto \* , Francesco Montagna and Alfonso Ma**ff**ezzoli**

Department of Engineering for Innovation, University of Salento, Via Arnesano, 73100 Lecce, Italy; francesco.montagna@unisalento.it (F.M.); alfonso.maffezzoli@unisalento.it (A.M.)

**\*** Correspondence: francesca.lionetto@unisalento.it; Tel.: +39-08-3229-7326

Received: 20 May 2020; Accepted: 10 June 2020; Published: 12 June 2020

**Abstract:** Out-of-plane permeability of reinforcement preforms is of crucial importance in the infusion of large and thick composite panels, but so far, there are no standard experimental methods for its determination. In this work, an experimental set-up for the measurement of unsaturated through thickness permeability based on the ultrasonic wave propagation in pulse echo mode is presented. 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 experiment. The set-up was tested on three thick carbon fiber preforms, obtained by stacking thermal bonding of balanced or unidirectional plies either by automated fiber placement either by hand lay-up of unidirectional plies. The ultrasonic data were used to calculate unsaturated out-of-plane permeability using Darcy's law. The permeability results were compared with saturated out-of-plane permeability, determined by a traditional gravimetric method, and validated by some analytical models. The results demonstrated the feasibility and potential of the proposed set-up for permeability measurements thanks to its noninvasive character and the one-side access.

**Keywords:** ultrasound; time of flight; reinforcement; resin transfer molding (RTM); permeability; liquid composite molding; material characterization; composite manufacturing
