PSI - Issue 52

Francisco de Sá Rodrigues et al. / Procedia Structural Integrity 52 (2024) 719–729 F. de Sa´ Rodrigues et al. / Structural Integrity Procedia 00 (2023) 000–000

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currently employed which are not optimal in multi-layered materials given the introduction of damage-prone areas and local disturbance of incoming airflow and subsequent aerodynamic loads. Structural Health Monitoring (SHM) techniques were developed to shift the current maintenance paradigm to opti mize the design of new aircraft structures by providing in situ assessment of structural integrity based on health indica tors from sensors. Among these methods, Impedance Spectroscopy measurements characterize a circuit’s frequency dependent electrical conductivity. This technique has provided reliable indicators of bonding quality in piezoelectric sensors Yue et al. (2018), of damage presence in the vicinity of piezoelectric sensors Sharif et al. (2015) and cure monitoring evaluation through conductive IDTs. Thermoset adhesives employed during the bonded repair of composite laminates are cured alongside the composite material for attachment to the host structure. Curing of the thermoset adhesive encompasses the assembly of processes which result in the gelation of the material through cross-linking of polymeric chains. The behaviour of a thermoset resin when subjected to an alternated-current electrical excitation is described by a three-fold phenomena (Skordos and Partridge (2004); Mijovic´, Jovan te al. (1993)): (a) dipole polarization; (b) migrating charge conduction, and (c) electrode polarization. The first two are sensitive to the adhesive’s curing stage and can provide useful information through the impedance since the resin’s devitrification and vitrification stages yield substantially di ff erent dielectric responses Bekas et al. (2016). Additive manufacturing solutions relying on drop-on-demand methods such as inkjet printed circuits have been demonstrated to be applicable in the context of SHM by providing lightweight, thin conductive tracks and cus tomised multi-functional sensors (Yue et al. (2021); de Sa´ Rodrigues et al. (2022, 2023)). Additionally, dielectric based methodologies relying on inkjet printed circuits have been employed to provide information regarding the resin curing stages in bonded single lap joints Bekas et al. (2019) and scarf repairs in composites Bekas et al. (2019). The upscaling of impedance spectroscopy techniques relying on embedded inkjet printed circuits in composite materials still requires improvements given the incapability to directly inkjet print on top of the host substrate. Hence, a new procedure should be developed for attaching the circuit to the host structure in order to enable its application in larger industrial structures. Moreover, these methodologies must validated in industrial-representative environments to tackle the large amount of uncertainty sources (temperature, pressure, environmental noise) which require addressing. In this work, a procedure is reported for monitoring the cure of bonded composite patches with accurate portrayal of gelation time and damage detection capabilities. Firstly, the experimental properties for the cure monitoring are defined by printing inkjet circuits on top of non-conductive Kapton substrates to determine the IDT design which pro vides higher sensitivity to the cure monitoring. Subsequently, the IDT transposition to composite structures without directly printing onto the structure is detailed and cure monitoring of a single lap joint is performed with the informa tion collected at the single IDT level. Finally, the upscaling of the current system to a composite step-sanded repair is performed by bonding the inkjet circuits on the composite with the introduction of minimal thickness and consequent monitoring of the bonded repair with posterior detection of impact damages and severity increase through an in-house developed LabVIEW platform. The curing cycle of a polymeric material results in chemical changes in the microstructure which result, not only in enhancements to the mechanical performance, but also to changes in the electric properties during the curing, where ionic particle transmission occurs once the polymer is subjected to dipolar orientation. Under the application of a time-varying voltage V ( t ), an equivalent time-varying electric field E ( t ) is generated between the two poles (dielectric sample). The dielectric principle under which the curing procedure will be monitored relies on the same procedure as a classic parallel-plate capacitor where the electric fields displaces the conductive particles from an equilibrium state to a polarized state (von Hippel and Morgan (1955)). To monitor the progress in particle migration, an Impedance Analyzer was employed which consists of a single input system capable of multiple parameter measurement and subsequent output. Here, the system’s output consisted of the impedance magnitude, | Z | andphase, θ , where the Impedance’s imaginary component is computed as | Z | · sin ( θ ). For performing the adhesive bondings throughout this work, an AF 163 2K (supplied by 3M) was used with an adaptable curing cycle which was chosen as 60 minutes at a temperature of 120 ◦ C . 2. Experimental Procedure