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.
