Skip to main content

Brillouin Distributed sensor for Aeronautical Structures

Periodic Reporting for period 2 - BRIDAS (Brillouin Distributed sensor for Aeronautical Structures)

Reporting period: 2018-01-01 to 2019-03-31

The fundamental objective of the Project was the development of a portable optoelectronic interrogation unit, realizing distributed temperature&strain measurements in optical fibres over a maximum length of 300 m and with a spatial resolution of 5 mm. The sensor technology was intended to provide support in the activities related to the evaluation of quality and structural health of composite parts for the aeronautical industry, such as complete fuselage, wings, vertical or horizontal tail plane of passenger commercial aircrafts.
The technology chosen for fulfilling the Project requirements is the Brillouin Optical Frequency Domain Analysis (BOFDA). According to this method, the Brillouin backscatter from a sinusoidally-modulated optical pump wave is detected in amplitude and phase, through a vector network analyser, over a range of modulation frequencies. Compared to the conventional Brillouin optical time-domain analysis (BOTDA), BOFDA offers a much better spatial resolution, thanks to the pre-activation of the acoustic wave involved in the scattering process.
The activities carried out during the Project have been focused on the design, optimization and realization of the interrogation unit. The reazlied equipment has been tested in aerospace scenarios, such as manufacturing, in-life service and real industrial demonstrator. The tests have been carried out at the Topic Manager facilities in Getafe (Spain). The accuracy of the measurements provided by the prototype has been assessed against conventional sensors such as thermocouples, strain gauges and fibre Bragg gratings (or even Rayleigh-based distributed sensing technology). The results confirm that the developed prototype represents a good solution for high-resolution distributed temperature and strain sensing in optical fibres. However, more work is needed in order to reduce the acquisition time and make it compliant with the aeronautic standards.
Given the Project requirements, the BOFDA technology was chosen for the realization of the interrogation unit. For the latter, a Vector Network Analyzer (VNA) operating over the frequency range 300 kHz - 20 GHz was chosen, allowing a spatial resolution as high as 5 mm. The VNA is based on the PXIe architecture, therefore being ideally suited for applications requiring the integration in a compact and low-weight prototype.
While spatial resolution and sensing range constraints were easily met using the BOFDA technology, a major concern, since the beginning of the Project activities, was the further requirement of a sub-second acquisition time. Several strategies have been adopted in order to reduce the acquisition time, by acting both on the hardware and software of the developed prototype. From the hardware point of view, the optimization regarded the choice of a low-phase noise laser source, as laser phase noise was early recognized as one of the main noise sources. Furthermore, a fast polarization-diversity scheme was adopted, for polarization management of the Brillouin acquisitions. As regards signal processing, a novel processing technique for the extraction of temperature&strain profiles has been developed. The technique removes the distorting terms affecting BOFDA measurements, while adding no relevant complexity in the reconstruction of the temperature (or strain) distributions from the acquired data.
The prototype has been assembled in a compact rack unit, controlled by an external PC through Ethernet connection and supplied from a single +12V power supply. The prototype has been also thermally tested, in order to verify its capability to operate for long periods and over an extended temperature range.
The prototype has been finally tested in the laboratory, in order to assess its performance. The results confirmed that the prototype was compliant with the Project’s requirements.
During the second part of the Project, a few experimental tests were carried out at the Topic Manager facilities in Getafe (Spain). Temperature tests have been carried out in the typical conditions of composite manufacturing, up to 200°C, as well as in in-service conditions, with temperatures down to -50°C. The temperatures provided by the prototype were compared to those provided by FBGs and thermocouples. Further tests have been carried out over composite plates subjected to tensile and compressive loads. In such cases, the accuracy of the developed unit was assessed against strain measurements provided by conventional FBGs and foil gauges.
As regard dissemination of the results, a special section was created on the Research Gate social networking site, in order to facilitate dissemination of the results of the Project and discussion with researchers. Furthermore, the technologies exploited for the development of the Prototype have been disseminated to master’s degree and PhD students at University of Campania and Rey Juan Carlos University. Some of the experimental results have been presented during the conference I3S MDPI, held in Naples in May 2019. For this contribution, a permission from the Topic Manager was achieved in advance. Some of the results will be also presented at the ISROS 2019 conference to be held in Airbus facilities in Toulouse (France), in November 2019.
Compared to state-of-art Brillouin interrogators, the assembled prototype has a much better spatial resolution (down to 5-mm, against the typical 1-m spatial resolution of commercial interrogators). This aspect is crucial when dealing with aircraft structures, as it provides structural health monitoring (SHM) and damage detection capabilities.
The socio-economic strategic impact of the SHM technology has two aspects for aerospace, not only in Europe: 1) SHM is an enabling technology for high-performance, environment-friendly and economic aircraft operation by better exploiting available weight reduction potentials of new structural materials and design philosophies without compromising the aerospace safety requirements; 2) SHM significantly contributes to improve safety of aircraft operation by means of more affordable and efficient integrated monitoring technologies instead of time-consuming and costly conventional procedures (disassembly, reassembly, checks, etc.). Especially the first aspect steps beyond the potential of new maintenance concepts, because it is driving innovative approaches in the design of aircraft structures. A self-sensing structure, giving warnings about potential, existing or foreseeable breaches in structural integrity safety limits, does not need to be dimensioned to the current standards. The full structural performance of modern designs and materials is made available by SHM for exploitation at minimized risk. The second aspect is critical to the feasibility of future maintenance concepts that are currently developed. In fact, sensor technologies are playing a key role in structural health monitoring of civil structures: A self-sensing structure does not need preventive (and statistically useless) inspection and maintenance but allows optimized and reasonably plannable operation and therefore reduced down-time.
Front view of the assembled Protptype