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Development of Integrated MEasurement Systems

Periodic Reporting for period 2 - DIMES (Development of Integrated MEasurement Systems)

Reporting period: 2020-01-01 to 2021-09-30

The aim of the project was to develop advanced integrated testing methods that have the capability to detect a crack or delamination in a metallic or composite structure and have the potential to be deployed as part of an on-board structural health monitoring system for passenger aircraft. The project incorporated a new philosophy for monitoring damage in which the disturbance to the strain field in the structure caused by the damage is used to identify significant damage and to track its propagation. This approach had been demonstrated to be at least as effective in composite structures as traditional non-destructive evaluation techniques and, in CS2 project INSTRUCTIVE using infrared technology, it had been shown to be capable of identifying smaller cracks in metallic structures than any other available technique. In this project, these innovations were amalgamated with established point sensors, including strain gauges and fibre Bragg gratings, to demonstrate an integrated testing method. The objectives were designed to mature technologies from TRL 4 to 6 that are likely to have a disruptive impact on the structural health monitoring of next generation large passenger aircraft. The objectives were: a) to develop a robust and innovative concept for integrating a diverse set of sensors and data acquisition systems for detecting and monitoring damage in an aircraft assembly; b) to produce an integrated system of sensors and data acquisition systems deployed on a test bench representing an aircraft assembly, and; c) to conduct prototype demonstration and evaluation tests of the integrated system and test bench using independent systems. The primary outcomes are demonstrations, both on a test bench section of aircraft and in ground tests at an aircraft manufacturer, of an integrated measurement system for ‘on-line’ detecting and monitoring damage based on a diverse range of sensor systems consisting of visible and infrared cameras, strain gauges and fibre Bragg gratings. The initiation and propagation of damage was successfully detected and monitored.
The DIMES project has been able to complete all of its deliverables and to achieve its objectives following a nine-month no cost extension due to the COVID-19 pandemic. The consortium has developed a strong working relationship with its Topic Manager, Airbus, at whose facilities in France and UK major demonstrations have been performed. An addition, demonstration of the DIMES integrated system in a full-scale ground test on an aircraft has been initiated at the Canadian National Research Council's Aerospace Laboratories in Ottawa using a remote installation approach developed during the project. From a technical perspective, the project delivered (i) the outcomes from a rational-decision-making process for an integrated measurement system for detecting and monitoring damage in aircraft structures; (ii) a set of robust and innovative conceptual designs that integrate a diverse set of sensors and data acquisition systems for detecting and monitoring damage in an aircraft assembly (project objective [a]); (iii) prototypes of an integrated system of sensors and data acquisition systems deployed on a test-bench representing an aircraft assembly (project objective [b]); and (iv) prototype demonstrations and evaluation tests of the integrated system and test bench using independent measurement systems to verify the outcomes (project objective [c]). Four papers have been presented at three conferences and one archived journal paper has been submitted and a number are being prepared. A series of posts on an engineering blog have been published and a set of video shorts has been released together with frequent tweets. Knowledge exchange workshops have been held at Airbus in Filton in January 2019 and September 2021 and another at Airbus in Toulouse in March 2020.
Farrar and Worden [Phil Trans R Soc A, 365:303-315, 2007] identified five questions that need to be answered in order to describe the damage state of a system: (i) Is there damage? (ii) Where is the damage? (iii) What kind of damage is present? (iv) How severe is the damage? (v) How much useful life remains? These questions are increasingly more difficult to answer. Many structural health monitoring (SHM) systems cannot progress beyond the first two questions, while some non-destructive evaluation (NDE) approaches can address all, except the final question. SHM is associated with 'on-line' damage identification, whereas NDE is usually performed 'off-line', in the sense that the component is taken out of service, which allows a wider range of techniques to be deployed including radiography, thermal imaging and ultrasound. In the DIMES project, these concepts have been integrated in a system that uses non-destructive testing and inspection approaches to perform SHM and thereby deliver answers to four of the five questions identified by Farrar and Worden - its implementation built around small, low-cost devices represents a significant advance on the state-of-the-art.

Prior work in the Clean Sky 2 INSTRUCTIVE project had shown that representative flight cycle loading can be used to generate a thermal signal which is sufficient to detect the initiation of damage in metallic components based on the thermoelastic effect. At the same time, compact low-cost microbolometer systems have become available for thermoelastic stress analysis (TSA); so that it is viable to translate TSA into the SHM domain by combining these developments to allow whole-field detection of damage during flight-cycle loading without any special surface preparation. In the DIMES project, this capability has been delivered in an instrumented test benches at EMPA, that includes a section of an Airbus wing and fuselage panels, and installed in full-scale ground tests on aircraft structures at Airbus in Toulouse and Bristol. This represents a substantial advance in the state-of-the-art relative to prior studies which have used flat or near-flat reinforced panels about 1x1 metre to demonstrate individual components of the technology.

The development of an automated system, which integrates data acquisition from a diverse set of sensors with a user interface and allows continuous operation during a structure test, has resulted in state-of-the-art instrumentation that will disrupt current approaches to ground tests on aircraft structures. And, the incorporation of multi-interface options using Commercial-Off-The-Shelf (COTS) sensors and of remote monitoring of the system, is likely to create a market in the worldwide aerospace industry that could be further developed for other industries, such as the power generation industry. The demonstration in an industrial environment of the multi-faceted technologies in the integrated measurement system is likely to provide a boost to further fundamental research on sensor technology, data acquisition and processing methodologies and development of more efficient design prototyping. Consequently, the research represents a significant and generic advance in the technology and methodologies used to test prototype structures and perform on-line structural health monitoring which is likely to be of benefit to the European aerospace industry, in the first instance, but subsequently to a wider range of industrial sectors.
Sensor module (55 x 65 mm) of integrated measurement system, view of a visible spectrum camera mount
Exemplar output from graphical user interface (GUI) showing live data readout from the optical and p
DIMES project team and the wing-based test bench at EMPA, see
DIMES technical coordinator (left) presenting during a session chaired by the Topic Manager (right)
Installation of fibre Bragg grating sensor chain on the internal cockpit frame by Airbus personnel i