# Smart Patch for Structural Health Monitoring of Composite Repair
## 1 Introduction  
Repair of in-service composite parts plays an important role in the sustainability of composite airframes due to the recent increase in the utilization of composite structures in modern aircraft. In certain practical cases, the repair of composite structures according to the Structural Repair Manual (SRM) can reduce the cost of replacement without compromising the mechanical of composite structures [1; 2; 3]. However, the strength and durability of the repaired area still need to be considered since the stress transfer at the interface is under service load with different environmental conditions [4]. The mechanical properties of the exposed thermosetting adhesives and resin of bonded area will absorb the moisture, which will affect the durability [5; 6; 7; 8]. The defects will be also generated stress concentrations and initiate cracks when the repaired structure subjected to a cyclic loading [9; 10; 11]. In addition, failures may occur due to inconsistent processing methods which will degrade the mechanical properties between the repair patch and composite bondline [4; 12]. To meet the requirements of airworthiness certification, repair based on Structural Health Monitoring (SHM) system can be an effective way to monitor the integrity of the bonded repair patch for aircrafts [3; 8; 12; 13]. Generally, the smart patch consists of an array of actuators/sensors. Compared to traditional non-destructive inspection (NDI) techniques, SHM enables bondline inspection possible without destroying the repaired patch and can be used to monitor in real-time the bonding area of the repaired part [12; 14]. It has been shown that the smart patch can detect debonding, delamination of composite layers and damage growth in the repaired area [8]. Piezoelectric lead zirconate titanate (PZT) transducers and fibre Bragg gratings (FBG) are two main types of sensors commonly utilized for SHM applications. The advantages of PZT transducers include wide frequency range, low price and small size and goodcoupling capacity which are particularly suitable for embedding [15; 16]. In addition, they can simultaneously exhibit actuator/sensor behaviours, which allow for both passive and active detections [17; 18; 19]. While FBG sensors are super-light, small size, sensitive, lower power consumption, immune to electromagnetic interferences/corrosion and have high bandwidth and multiplexing sensors [15; 16; 20; 21; 22; 23]. By comparing PZT and FBG sensors, PZT transducers can capture the 'integrated' signals in the entire covered area, while FBG sensors are directional dependent [16]. In addition, PZT transducers are sensitive to both symmetric (S\({}_{0}\)) and anti-symmetric (A\({}_{0}\)) modes, while FBG sensors are less sensitive to the A\({}_{0}\) mode [16]. For damage localization, RAPID (reconstruction algorithm for probabilistic inspection of defects) [24; 25] and delay-and-sum (DAS) [26; 27; 28] algorithms are imaging methods and have been widely used. The RAPID algorithm does not need to acquire the information of wave modes and group velocity for ultrasonic guided waves (UGW) while the DAS algorithm only assumes one wave mode exists and little mode conversion exists when the wave interacts with damage. Many works were conducted on the SHM system for the smart repair patch. [PERSON] et al. [29] numerically and experimentally studied bondline monitoring using a composite repair patch with embedded FBG sensors. Both experimental and modelling results showed that the smart repair patch embedded with chirped FBG sensor can be used to monitor the initial disbond. [PERSON] et al. used surface-mounted PZT transducers [13] and hybrid system (surface-mounted PZT transducers + embedded FBG sensors) [12] to conduct bending fatigue and impact tests for the step-sand composite repair structures and used scaling subtraction method and RAPID to detect and locate these damages.