In Situ Estimation of the Thickness of Oxidized Layer in Composite Structures by Means of Ultrasonic Guided Waves

Under the exposition to air rich in oxygen, oxidation leads to a non-negligible thickness of oxidized layer (TOL) which appears initially on the free surfaces of composite structures. This oxidized layer has modified mechanical properties in comparison with the virgin ones and is progressing during time within the composite thickness as evidence of diffusion-limited oxidation. Being able to estimate in situ the TOL of composite structures under service is thus of great importance for safety and maintenance purposes. Ultrasonic guided waves (UGW) propagating in aged composites will have different velocities in comparison with UGW propagating in virgin composites. Taking benefits of this physical properties linking TOL thickness to UGW velocities, a strategy allowing to estimate in situ the TOL of composite materials is here proposed. As a first step towards its implementation, this strategy is here validated numerically using semi-analytical models allowing to predict UGW velocities in composites as a function of TOL. This model allows to demonstrate the sensitivity of the various UGW propagating modes to TOL thickness for signals representative of SHM applications. As a next step, the proposed approach will be used experimentally for the evaluation and tracking of the in situ growth of TOL. Such a tool can be deployed in situ and help to follow the health state of composite structures in practical applications.