Tutor: Prof. Francesco Braghin
Università di Provenienza: Politecnico di Milano - Ingegneria Meccanica
Titolo della Tesi: An Impact Monitoring System for Aerospace Structures
An Impact Monitoring System for Aerospace Structures
The problem of foreign object impacts on flying structures is actual and intensifies with the adoption of composite materials; in fact, unexpected impacts could produce Barely Visible Impact Damages (BVID).
The dangerousness of this kind of damage lies in the fact that it could remain hidden in the structure, causing its continuous deterioration with performances reduction and increasing the risk for the common safety. Impact Monitoring is attracting the interest of the international academic community and companies, as an effective system able to deal with the problem (Fig.1 - Hailstorm damages on the nose of a civil aircraft, Fig.2 - Battle damages on the wing and the fuselage of a fighter aircraft).
Moreover, the Impact Monitoring system could be integrated with other on-board/monitoring systems, allowing the Condition Based Maintenance (CBM) implementation and bringing positive effects, in terms of costs and vehicle availability.
The main objective of the present work concerns the implementation of an Impact Monitoring system (Fig.3 - Representation of Impact generated strain waves propagation in the structure, Fig.4 - Conceptual operation and main outputs of the complete Impact Monitoring system), composed of two main parts:
- Passive Impact Monitoring
- Active Damage Monitoring
The former exploits sensors, i.e. Piezoelectric sensors (PZT) and Fiber Bragg Grating (FBG), and the strain waves generated by the impact event to its identification (i.e. impact detection, localization of its position and impact force reconstruction).
The latter exploits sensors and actuators, i.e. the PZT already installed on the structure, to generate and acquire ultrasonic waves, for the impact damage identification (i.e. detection of the damage presence and its localization).
The proposed solution for the Passive Impact Monitoring part is a combination of signal processing techniques and optimization tools, especially designed for the impact identification problem:
- Threshold method for impact detection (Fig.5 - Example of an acquired PZT signal).
- Genetic Algorithm for impact localization.
- Deconvolution technique for impact force reconstruction (Fig.6 - Comparison between recorded and reconstructed impact force).
Concerning the Active Damage Monitoring part, the proposed solution relies on the generation of ultrasonic Guided Waves, using PZT elements, and their acquisition by means of sensors. The impact damage identification is performed calculating Damage Indexes (DIs), able to give indications about the health state of the component (Fig.7 - Example of a DI colourmap for impact damage identification).
The designed methodologies were tested on Carbon Fiber Reinforce Polymer (CFRP) panels and Glass Fiber Reinforced Polymer (GFRP) panels representative of the real structure, considering impacts executed using impact hammers, falling masses and bullets, producing damages in terms of delamination and disbondings (Fig.8 - Example of a BVID measured with a Non-Destructive Technique).
The results obtained during the implementation of the Passive Impact Monitoring system, showed its capability:
- to properly detect the impact event.
- to localize the impact on the components with an error of only few millimeters.
- to reconstruct the impact force.
The results obtained during the implementation of the Active Damage Monitoring system, showed:
- the PZT actuator capabilities to generate Guided Waves.
- the DI effectiveness in assessing the structure health state.
Temperature, load conditions and multiple impacts could hamper the system performances, because of their influence on the dynamic behavior of the structure. Thus, further developments are required for applying the system to a full-scale component (Fig.9 - The author with the real structure: a 3.5m long CFRP wing-box portion connected to the test rig).
C. Sbarufatti, A. Beligni et al., Strain wave acquisition by a fiber optic coherent sensor for impact monitoring, Materials (Basel), vol. 10, no. 7, 2017.
A. Beligni, C. Sbarufatti, A. Gilioli, F. Cadini, M. Giglio, Robust identification of strain waves due to low-velocity impact with different impactor stiffness, Sensors (Switzerland), vol. 19, no. 6, 2019.
A. Beligni, C. Sbarufatti, A. Gilioli, M. Giglio, Robust identification of low velocity impact events under differentimpactor stiffness, Proceedings of the 26th European Safety and Reliability Conference, ESREL 2016, 2017.
A. Beligni, G. Musotto, C. Sbarufatti, K. Dragan, I. Di Luch, M. Ferrario, Impact induced strain wave detection using a robust feature extraction method, 9th European Workshop on Structural Health Monitoring, pp. 1–12, 2018.
A.Beligni, C. Sbarufatti, M. Giglio, An impact monitoring system for aeronautical structures, 10th European Workshop on Structural Health Monitoring, 2020.
The present thesis work has been developed within the European R&T project: SAMAS - EDA project n° B-1404ESM2-GP