# Smart Patch for Structural Health Monitoring of Composite Repair
## 2 Experimental Setup  
Unidirectional carbon fibre prepregs Hexply(r) IM7/8552 were used in this experiment. For fabrication of the composite repair patch, the quasi-isotropic stacking sequence [(0\({}^{\circ}\)/+45\({}^{\circ}\)/\(-\)45\({}^{\circ}\)/+90\({}^{\circ}\))2]s was used for the lay-up and the thickness of the repair patch was about 2 mm after curing. The KAPTON(r) film (DuPontLM HM, melting temperature 400 \({}^{\circ}\)C) with a thickness of 25.4 \(\upmu\)m was used as a diagnostic film and DuraActTM PZT transducers (P-876.K025 - PIC255, Curie temperature: 350 \({}^{\circ}\)C) were used during the manufacturing. Dimatrix printer (DMP-2580) was used to print the circuits on the KAPTON(r) film. In addition, thermoplastic film (TPU-Pontocal AG) was used to pre-bond the PZT transducer to KAPTON(r) film for diagnostic film preparation. Furthermore, resin film Hexply(r) M56 was used during embedding to bond the surface and increase the bonding properties between the prepreg, KAPTON(r) films and PZT transducers. The preparation of the diagnostic film, sensor installation and details of the novel cut-out method for the embedding were reported in the author's previous work [35]. PZT transducers needed to be pre-bonded to the printed diagnostic film using two layers of thermoplastic films for preparing the printed diagnostic film to ensure there is no delamination between the printed diagnostic film and transducers. Figure 1a shows the general drawing of the smart repair patch with the configuration of PZT transducers and Figure 1b shows the schematic of the embedding procedure during lay-up. For the embedding, a release film was placed in the middle of the cut-out area shown in Figure 1b to prevent the bonding of the diagnostic film to the prepregs within the cut-out area. Then the resin film and a blank diagnostic film were applied on the surface of the bottom prepregs respectively to prevent the short-circuited of printed circuits due to the conductivity of carbon fibre prepregs, followed by the resin film, the prepared KAPTON(r) film, resin film and main upper prepregs. After that, the exposed area of prepared KAPTON(r) films was applied on the surface of the main upper prepregs and the release film placed on the cut-out area was removed. Finally, cut-out prepregs were applied to the designated position. After lay-up and bagging, the patch was cured at 180 \({}^{\circ}\)C in the autoclave and the curing cycle was set up according to the prepregs' datasheet. After curing and trimming, a connector (RS 514-4408, operating temperature range: \(-40\)\({}^{\circ}\)C to \(+85\)\({}^{\circ}\)C) was mounted on the surface of the repair patch and bonded by a super glue (RS 473-445, operating temperature range: \(-50\)\({}^{\circ}\)C to \(+80\)\({}^{\circ}\)C) to connect circuits and embedded PZT transducers by conductive epoxy adhesive resin/hardener (RS 186-3616). After the fabrication, the trimmed repair patch is shown in Figure 2b. For manufacturing the host composite structure, a quasi-isotropic stacking sequence (\(0^{\circ}\)/\(+45^{\circ}\)/\(-45^{\circ}\)/\(+90^{\circ}\))\({}_{48}\) was used for the layup and the size was 300 mm \(\times\) 260 mm, and the thickness of the host structure was about 4 mm after curing. After manufacturing, the EMI method was used to evaluate the bonding properties between the PZT transducer and the repair patch. The EMI method can be used to evaluate local damage severities, transducers' fractures, mechanical/electrical properties for the degradation and integrity of bonding properties [35]. According to the EMI method, the imaginary part of admittance at a low-frequency range will make the mechanical impedance of the composite structure close to zero and only the mechanical impedance of the PZT transducer is considered [41]. Therefore, any slope change in the imaginary part of  
Figure 1: Schematics of (**a**) general drawing of repair patch and (**b**) embedding procedure during layup. Figure 2: Schematics of (**a**) embedding procedure and (**b**) trimmed smart repair patch.