Nowadays aircraft structural health monitoring (SHM) systems have been actively implemented to reduce the occurrence of aircraft accidents and to effectively replace the conventional aircraft maintenance and management systems, which result in excessive maintenance works and low aircraft availability. SHM allows for the real-time monitoring of the structural condition using various sensors installed in the major parts of aircraft structures along with diagnosis and assessment of structural integrity by analyzing the acquired condition data. However, previous studies suffered difficulties and limitations with regard to the applicability for in-situ monitoring of large structures such as aircrafts. The monitoring techniques in the previous studies require large number of sensors causing complications in wiring and post-processing of the acquired data. Electrical strain gauge sensors could suffer wiring, measurement, and corrosion problems for monitoring of aircraft wing structures. Optical fiber sensors require a complex sensing configuration with heavy equipment and relatively high-cost measurement system compared with the electrical strain gauge method.
Therefore, to remedy such disadvantages of the conventional methods, FBG-based monitoring techniques are presented in this study. This study surpasses the limitations of conventional studies on the impact localization and in-flight strain measurement of aircraft wing structures that have mostly been done in lab-scale with electrical sensors or complex optical fiber sensors. In this study, a real aircraft wing structure is considered for the implementation and verification of each monitoring technique. This study aims to develop and evaluate real-time monitoring techniques for aircraft wing structures which effectively replace conventional monitoring techniques.
For the research on the in-flight strain measurement of aircraft wing structures, an FBG-based cost-effective measurement system was considered for measuring the strain responses of the flying platform. A low-speed and compact size FBG interrogator was adopted and tested on the ground to evaluate its measurement performance. Practical installation methods for the embedment of FBG sensors into the wing structure were proposed. Finally, the low-speed interrogator and FBG sensors were implemented on a real aircraft wing structure to evaluate the potential applicability of the proposed method. The FBG sensors were embedded into the testbed aircraft wing structure during the wing manufacturing process to minimize the effect of sensor implementation on the flight condition and sensor handling. The measurement devices for data acquisition, processing, and storage were installed in the testbed aircraft. Ground tests were conducted to verify the proposed sensor installation methods and measurement performance of the FBG interrogator. Finally, dozens of actual flight tests were conducted using the testbed aircraft to evaluate the applicability of the proposed method under flight conditions. The measured in-flight strain data were accumulated and compared with corresponding flight parameters in detail to show its reliability and relevance. Furthermore, the flight load on the main wing was successfully estimated using the in-flight strain responses through the ground load calibration test.
For the research on the impact localization of aircraft wing structures, a high-speed FBG measurement system was considered to identify impact sources on aircraft wing structures. An FBG-based impact localization algorithm using the cross-correlation method was proposed. The proposed method was applied to a stiffened composite panel in which four multiplexed FBG sensors were attached. Verification tests were conducted on the composite panel using the proposed localization method. The effect of signal normalization methods and sensor numbers on the impact localization performance was examined. Finally, the proposed impact localization method was applied to the testbed aircraft wing structure to evaluate its feasibility and potential applicability on real wing structures. Effects of sensor configurations and grid sizes on the impact localization performance were examined to find out how effectively the impact source could be localized on aircraft wing structures with less training points and sensors.
In this study, in-situ monitoring techniques for impact localization and in-flight strain measurement of aircraft wing structures were developed. The developed monitoring techniques were evaluated through the testbed aircraft to show its applicability and performance for real aircraft wing structures. Through the study on the in-flight strain measurement of aircraft wing structures, the low-speed FBG interrogator and embedded FBG sensors provided reliable in-flight strain data from the testbed aircraft wing structure for a long period of time. A total of 74 flight tests were conducted and the measured in-flight data showed reasonable responses for the corresponding loading conditions of the main wings at various maneuvers. Also, the flight load on the wing structure was successfully estimated from the in-flight strain responses. Through the study on the impact localization of aircraft wing structures, the impact localization algorithm based on the cross-correlation was proposed. The proposed method successfully identified the impact source on complex composite structures with high localization performance. The location of 30 impact sources on the testbed aircraft wing structure was successfully estimated using a single FBG sensor with the maximum error of 56.59 mm and average error of 17.87 mm through the proposed method. The developed monitoring techniques could easily solve certain issues that may occur in the actual application of monitoring systems for in-situ aircraft wing structures, and the monitoring techniques are applicable for other vehicles or structures requiring real-time monitoring of structural conditions. Through this study, it is expected that the developed monitoring techniques will be helpful in enhancing efficiency, safety, and reliability of aircraft operations in the near future.