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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The results obtained during the adhesive curing section of the repair patch are presented in Figure 6. These highlight the consistency in the cure monitoring results between similar IDTs, positioned in each laminae. Given the di ff erent track lengths at each step, the starting | Z ′′ max | value can be di ff erent which would skew a direct comparison between the results for all IDTs. Therefore, the imaginary impedance results were adimensionalized by the initially measured | Z ′′ max | value at each IDT. The early portion of the results, prior to the changes in viscosity, highlights the robustness of the current approach to environmental conditions and the success in isolating the circuit to the heating blanket’s resonance e ff ect. Furthermore, the consistency between the di ff erent IDT results for the minimum impedance value indicate (considering the time interval between each acquisition), the achievement of a uniform bondline given the results obtained for the single-lap joint. To improve the current procedure, a multiple input-output instrument should be developed to synchronously obtain results from all IDTs. The final bonded assemblies of the structures discussed throughout this work are presented in Figure 7.

Fig. 7: Final Cured results: (a) on the Kapton substrate; (b) bonded single lapt joint; (c) step-sanded repaired coupon; (d) step-sanded repaired large flat sti ff enedpanel

One of the concerns when proposing the embedding of any layers in the composite’s mesoscale involves the gener ation of damage prone areas Bekas et al. (2019). Therefore, for quantifying the additional thickness introduced in the composite by the inkjet printed circuits and the isolation layers, microscope sections were manufactured by cutting samples similar to those created for the bonded-single lap joint test but without removing the Kapton layer after bond ing the circuits so as to establish an identifiable boundary layer on the microscope images. The results obtained by measuring the thicknesses of the embedded circuit yield a thickness between 20 − 30 µ m for each layer associated to the inkjet circuit with a thickness between 2 − 3 µ m . Therefore, the final assembly introduces a thickness of 55 − 60 µ m , considerably lower than a CFRP lamina’s thickness (184 µ m ). The embedding of electric circuits in the composite’s mesoscale can also be employed for SHM capabilities after the curing process has terminated which is associated to damage detection and growth monitoring capabilities pro vided by measurements of each track’s resistance since the generation of damage in the composite patch will break the inkjet track and, consequently, the conductivity between the track’s terminal. To evaluate the current system’s damage detection capabilities, the composite plate is installed in an Instron drop-tower machine where a hemispherical-tip weight is dropped with a 30J energy. The tracks resistance is measured using an Agilent 34972A unit by connecting each single wire terminal (after removing the initial soldered ones) to the unit’s multiplexer and the measurements were performed through an in-house LabVIEW code based on the Agilent 34970 Data Acquisition driver. The results obtained after the impact test are illustrated in Figure 9 and location shown for the respective result. The first three impacts were performed on location 1, where after each impact a significant resistance increase was obtained. These results show not only the capabilities for detecting the final ultimate impact damage which cuts the tracks connectivity entirely, both in-situ and intermittently, but also the monitoring of sequential damages (increase in severity) which increase the tracks’ resistance significantly to the pristine state, which replicates the real conditions during aircraft operations. The final third impact in location 1 permanently destroys the track conductivity by causing a failure at patch, adhesive and laminate level, identified by fibre breakages in the patch’s surface and on the laminates reverse side. The results from the impacts performed in location 2 reinforce those obtained from location 1 and the capabilities of the circuit for monitoring increases in damage severity, with the final impact resulting in a growth which encloses 5.1. Transverse Impact Damage Detection