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

721

3

2.1. Inkjet Circuit Printing

The inkjet printed circuits employed during this work were designed in a CAD software (AutoCAD) and printed in a Fujifilm Dimatix DMP 2820 printer with a maximum printing area of 400 × 350 mm 2 . A silver nanoparticle suspension (Sigma Aldrich 736465) was employed to provide the electrical conductive properties by designing the appropriate printing cycle for the piezoelectric-driven nozzles which resulted in an appropriate drop formation and deposition on the non-conductive substrate (supplied by Lohmann Technologies Ltd.). The inkjet nozzles dispensed 2.4pL per cycle with an averaged 25V actuation which required several sequential runs to achieve good track resistance. The printing was performed on a Kapton substrate under a surface temperature at 60 ◦ to evaporate some of the liquid solvent. Once the printing was completed, the tracks were sintered at 135 ◦ to evaporate the remaining solvents and yield a uniform, conductive track. Previously, cure monitoring methodologies relying on inkjet printed circuits were performed with the printing operation occurring directly on the composite’s surface. However, this prevents the application of these procedures in large industrial structures given the printer’s limited area and the incapability to perform it on non-flat parallel surfaces. For this, robot arms can be employed for in situ creation of the circuits however, such solutions can be both costly and di ffi cult to employ. Hence, for monitoring the resin’s cure in large composite subcomponents and components, the inkjet printed layers should be transferred to the host structure from the polymeric substrate where circuits are directly printed. This procedure would permit the circuit to be manufactured in controlled environments and ensure its performance in large structures. In the interest of procedure duration, the bonding step can be performed concurrently with the track’s sintering since similar temperatures are applied.

Fig. 1: Inkjet Printed Circuit examples: (a) on the Kapton substrate; (b) bonded in a small composite part; (c) bonded in a flat CFRP coupon

3. Kapton cure test

The upscaling of the current procedure for monitoring the cure of thermoset films used bonded composite repairs requires determining the IDT configuration which yields highest sensitivity to the electrical particle’s migration den Otter (2002) when subjected to an alternated electric field. Additionally, the experimental acquisition parameters should be calibrated to ensure the curing is monitored accurately. The lowest system to which the current procedure can be reduced to corresponds to a single IDT printed on top of a non-conductive substrate. To determine the IDT’s sensitivity to the curing procedure, four individual IDTs were printed with similar track width but di ff erent spacing between the fingers to determine the optimal spacing without risking short-circuiting in the IDT due to ink spurging during print. Hence, 4 IDTs were designed with 1mm width and finger spacing of 4, 2, 1 and 0.5mm between the electrodes. After the tracks were sintered, soldering pads (by Tokyo Instruments) were bonded to the Kapton using superglue and the conductive tracks were connected to the pads using a silver paste (RS Pro 186-3600). To each IDT electrode pad, a single-core shielded wire was soldered with a BNC terminal on the other end for connecting to the impedance analyzer. The resin film was placed on top of the circuit followed by a layer of release film. The assembly was placed on top of a heated platen (at 120 ◦ C ), which was pressed down using a steel weight to ensure full connection between the film and the circuits. The results for the frequency-dependent imaginary component of the impedance for each IDT are given in Figure 2 for the measurements taken at di ff erent curing procedure stages (given by the figure’s colobar). The di ff erent finger