# Smart Patch for Structural Health Monitoring of Composite Repair
## 1 Introduction
In addition, they [3] also proposed a so-called Minimal Intersection Score (MIS) algorithm to detect and locate the damage to the smart repair patch under different environmental conditions. [PERSON] et al. [30] numerically and experimentally used a smart patch with phased-array PZT transducers to monitor the artificial disbond. The Teflon tape was inserted in the edge between the patch and host composite structure during manufacturing. The results showed that disbond can be detected by using the electro-mechanical impedance (EMI) method. [PERSON] et al. [31] used the smart patch combined with the SMART layer (Stanford Multi-Actuator-Receiver Transduction Layer) system to monitor the curing progress and disbond by using the active sensing SHM technique. Their results showed that the combined SMART layer system for the smart repair patch can be used to monitor the curing progress and integrity of the bonding quality of the composite repair structure. Later, [PERSON]. [PERSON] et al. [32] used inkjet-printed interdigital sensors to monitor the bondline integrity of bonded composite joints by using the EMI method. [PERSON] et al. [33] used carbon nanotube (CNT) based adhesive films to investigate the crack sensing capabilities of adhesive film for composite bonded repair. They demonstrated that the use of CNT adhesive films for the bonded repair composite structures did not affect their mechanical performance and the electometrical results showed it can be used for SHM purposes. Furthermore, [PERSON]. [PERSON] et al. developed the SHM layer and renamed it as diagnostic film [34]. This layer had been shown to reduce the thickness of integrated layers (25.4 \(\upmu\)m) [35] by 50% compared to that of the SMART layer (50.8 \(\upmu\)m) [36]. They found this diagnostic film was effective for thin composites and can be used under extreme environmental and operational conditions. In the authors' previous work, a novel embedding technique using this diagnostic film with PZT transducers based on the edge cut-out method for the composite structure was developed [35]. This technique allowed edge trimming possible without damaging the printed circuits, which met industrial requirements for the next higher assembly. In addition, the EMI properties and sensing performance of embedded PZT transducers remain stable up to 1 million loading cycles under fatigue tests, and reductions of tensile and compressive modulus for the composite coupons remain acceptable for the worst-case scenario [37]. Earlier, [PERSON]. [PERSON] et al. [38] reported that the SMART layer did not meet the operational and environmental conditions of regional aircraft as related to low/high temperatures changes under cyclic loading, while diagnostic film utilizing inkjet-printed technology performed well in the tests [39]. In addition, the SMART layer was the most fragile before and during the bonding procedures [38]. This paper is the first application and assessment of the embedded PZT transducers for detection of the damage along the bonding and on the surface of a composite repair patch. In previous work, PZT transducers were installed on the surface of the host structure to detect damage to the repair patch. This approach as shown in [3; 13] required the placement of many sensors surrounding the entire patch and the guided waves had to travel the entire length of the patch to detect possible damage. There was also scattering from the edge to the patch since the guided wave was generated from the host structure and not internally to the patch as with the approach proposed in this paper. Furthermore, the use of a hybrid PZT-FBG acquisition system has already been reported in [12]. Hence, a smart repair patch with developed embedded diagnostic film and PZT transducers is the most innovative because it reduces the number of sensors. In this paper, the _edge cut-out method_ is applied to manufacture a smart repair patch combined with an embedded diagnostic layer and PZT transducers.