Structural health monitoring of complex composite structures under environmental and operational conditions
Creators: name: De Sá Rodrigues, Francisco
Publisher: name: Imperial College London
Year: 2023

description: The thesis addresses a critical aspect of aircraft safety and maintenance by proposing advanced Struc- tural Health Monitoring (SHM) techniques and software for detecting and localizing Barely Visible Impact Damage (BVID) in composite aircraft structures. This work contributes to enhancing the over- all safety and reliability of aircraft by identifying potential structural issues early on and facilitating timely maintenance and repairs. The primary methodology proposed in this research involves utilizing ultrasonic guided waves to identify and pinpoint BVID generated by low-velocity transverse impacts, such as those resulting from tool drops, debris, and hail during both flight and runway activities. The SHM techniques and acquisition software developed are validated on sub-component structures that are representative of composite fuselage panels. Aircraft components are also subjected to numerous environmental and operational conditions which influence the propagation of guided waves and affect the detection capabilities of BVID in the structure. These conditions have been investigated in small scale coupons or structures. However, the higher variability and complexities associated to large and more complex structures under real operational conditions cannot be fully verified by simply repli- cating these conditions for simpler structures. This research focused on addressing these challenges in structures of varying complexities by using a Bayesian framework for compensating these effects and introduce statistical variability to enhance the accuracy of the compensation factors. Therefore, the focus here is to develop damage detection methodologies using guided wave signals which address challenges like the presence of environmental and operational effects such as temperature on complex build-up structures as well as including uncertainty associated with the large scale structures. The developed methodologies have been verified for detecting various impact scenarios and locations in large curved subcomponent panels. Finally, the impact of including different variations of operational effects in the damage detection threshold is quantified by computing the minimum detectable dam- age size by the SHM network. Post impact, the presence of BVID associated with delaminations and skin/stringer bondline debondings can compromise the structural integrity due to subsequent static and fatigue loading, leading to increases in damage sizes which can severely impact the aircraft section’s residual strength. The component requires adequate replacement or local repair for which bonded patch repairs offer solutions with minimal disturbance to aerodynamic airflow and avoid stress concentrations due to drilling of holes in composite. An embedded inkjet printed solution is devel- oped for monitoring the repaired section’s bondline cure and provides both impact damage detection and severity assessment in the patch area, further reducing the associated downtime and financial implications to airline operators whilst ensuring structural integrity.