In-service structural health monitoring of composite aircraft structures plays a key role in the assessment of their performance and integrity. In recent years, Fibre Optic Sensors (FOS) have proved to be a potentially excellent technique for real-time in-situ monitoring of these structures due to their numerous advantages, such as immunity to electromagnetic interference, small size, light weight, durability, and high bandwidth, which allows a great number of sensors to operate in the same system, and the possibility to be integrated within the material. However, more effort is still needed to bring the technology to a fully mature readiness level. In this paper, recent research and applications in structural health monitoring of composite aircraft structures using FOS have been critically reviewed, considering both the multi-point and distributed sensing techniques.

Fiber optic sensors represent one of the most promising technologies for the monitoring of various engineering structures. A major challenge in the field is to analyze and predict the strain transfer to the fiber core reliably. Many authors developed analytical models of a coated optical fiber, assuming null strain at the ends of the bonding length. However, this configuration only partially reflects real experimental setups in which the cable structure can be more complex and the strains do not drastically reduce to zero. In this study, a novel strain transfer model for surface-bonded sensing cables with multilayered structure was developed. The analytical model was validated both experimentally and numerically, considering two surface-mounted cable prototypes with three different bonding lengths and five load cases. The results demonstrated the capability of the model to predict the strain profile and, differently from the available strain transfer models, that the strain values at the extremities of the bonded fiber length are not null.

The successful implementation of Structural Health Monitoring (SHM) systems is confined to the capability of evaluating their performance, reliability, and durability. Although there are many SHM techniques capable of detecting, locating and quantifying damage in several types of structures, their certification process is still limited. Despite the effort of academia and industry in defining methodologies for the performance assessment of such systems in recent years, many challenges remain to be solved. Methodologies used in Non-Destructive Evaluation (NDE) have been taken as a starting point to develop the required metrics for SHM, such as Probability of Detection (POD) curves. However, the transposition of such methodologies to SHM is anything but straightforward because additional factors should be considered. The time dependency of the data, the larger amount of variability sources and the complexity of the structures to be monitored exacerbate/aggravate the existing challenges, suggesting that much work has still to be done in SHM. The article focuses on the current challenges and barriers preventing the development of proper reliability metrics for SHM, analyzing the main differences with respect to POD methodologies for NDE. It was found that the development of POD curves for SHM systems requires a higher level of statistical expertise and their use in the literature is still limited to few studies. Finally, the discussion extends beyond POD curves towards new metrics such as Probability of Localization (POL) and Probability of Sizing (POS) curves, reflecting the diagnosis paradigm of SHM.

At present, in the automotive field, the noise generated by belt drives is evaluated by using microphones in the proximity of the belt, crankshafts, idlers and so on. Such a method can be misleading, since it may easily include the contributions of other noise sources present during the measurement. Moreover, a large amount of data is needed in order to test various layouts and various running conditions. We present a method for the analysis and prediction of the noise generated by belt drives which consists of two distinct phases in this paper. For simplicity, a two-pulley belt drive has been considered and the results have been validated at the meshing frequency, at which, as has been shown in the existing literature, the phenomenon of noise generation is mainly concentrated. In the first stage of the work, the acoustic power generation of the belt drive being tested was measured by means of acoustic intensity techniques. Subsequently, an acoustic prediction was performed by using vibration data obtained with a scanning laser Doppler vibrometer (SLDV) as inputs for a boundary element code. The SLDV was used because of its capability of measuring in-operation data on the running belt, which would not have been possible using traditional contact sensors (accelerometers and so on). The results obtained in the two phases were finally compared in order to evaluate the relation between the vibratory behaviour and the total acoustic radiation determined experimentally. The experimental and numerical data agree fairly well, adding precious information on the noise generation mechanisms and showing the feasibility of modelling the vibro-acoustic behaviour of belt drives and the possibility of a totally numerical procedure. In particular the implementation of an entirely numerical procedure using, for example, data generated through the use of codes for the dynamic characterization of mechanical systems (multi-body mechanical models and so on) seems foreseeable. In the final section of the present work, the uncertainty arising from the measurement processes of the investigation method presented is also discussed.