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

725

7

show a similar trend to those provided by the 0.5mm IDT in the previous test, and demonstrate a steep decrease in the Impedance as previously described. For the half covered IDT (IDT 2), the change in impedance during the resin cure is lower than the one obtained for the previous IDT 1. Finally, the fully covered IDT (IDT 3) shows no change in | Z ′′ max | values throughout the procedure since this circuit was not in contact with the adhesive. These results replicate the capability to detect manufacturing defects in the bondline given by the comparison between the di ff erent IDT results. These results reinforce those presented in Bekas et al. (2019) using a liquid adhesive for the lap joint, though only the gelation phase is observed corresponding to half the bell-shaped curve.

5. Composite coupon step-sand repair

The final step for validating the employment of the current methodology in large adhesively bonded composite re pairs comprises its application in patches with either scarf or step-sanded repair shapes. For this, a flat CFRP coupon was manufactured using Hexcel M21 prepreg with a quasi-isotropic stacking sequence [ ± 45 / 0 2 / 90 / 0] s with dimen sions of 250 × 250 mm 2 and a thickness of 2 . 208 mm . The step-sanded repair was manually performed using a Leslie Composite Repair kit (from GMI Aero) comprised of plastic guides and a pneumatic sander. The top five plies were removed in concentric circles in steps of 12mm width and a maximum removed perimeter with 121mm radius. The inkjet printed circuit consisted of similarly concentric wires with IDT fingers overlapping the step between the laminae and the tracks were extracted to allow for wire soldering and data acquisition,as seen in Figure 1. In scarf repaired plates bonded with inkjet printed circuits the conductive fibres can be insulated during the same procedure as the inkjet circuit bonding. However, in step-sanded repaired composites, the step introduces a small discontinuity which does not fully insulate the composite since the tracks connect to the exposed fibres on the step’s wall. Hence, the layers were insulated by spreading a thin layer of thermoset resin cured at 70 ◦ for 5 hours with subsequent bonding of the circuit through a thermoplastic film. The tracks are then connected to the conductive pads following the same procedure as previously reported and connecting the appropriate BNC terminated wires. The bonding operation is performed by increasing the temperature at 2 . 5 ◦ C / min up-to125 ◦ C and the temperature is stabilized during 20 minutes. Temperature monitoring is performed using a SHM data acquisition platform (de Sa´ Rodrigues et al. (2022)) by attaching 2 type-K thermocouples to the surface of the plate so the accurate temperature experienced by the composite is monitored. The BNC connectors correspondent to each IDT were sequentially connected to the impedance analyzer’s input port during the curing procedure.

Fig. 6: Cure Monitoring results for step-sanded CFRP coupon