Continuous monitoring of composite structures

The possibility of using FBG optical fibre sensors as a monitoring technique for composite structures is the subject of study at Sirris. Interesting results have been gathered.

Fibre Bragg Grating (FBG) optical fibres enable the real-time control of the stresses in composite structures (aircraft, aerospace, cars, boats, buildings, bridges, pipelines, etc.). The optical fibres are linked to a data acquisition system enabling continuous monitoring of stress evolution by modifications of the output light signals, modifications due to the deformations undergone by the fibre Bragg gratings.

The measurement results enable the structure's state of damage to be calculated, its lifetime to be determined and the optimum maintenance frequency to be decided. They can also be used to optimise product design.

Compared with conventional methods (strain gauges), a significant advantage of optical fibre sensors is that it is possible to instrument several areas with a single fibre, providing a considerable gain in on-board connections. 

The operation which consists in embedding the fibres in the composite and especially in linking them to the acquisition system is however quite delicate. Sirris has developed relevant solutions as part of the European Mediatic project (FEDER) coordinated by Multitel. 

In the first part of the project, Sirris had developed a specific very thin connector which can be embedded in the material with reduced overthickness. It enables easy connection of the internal optical fibre to an external fibre linked to the data acquisition system via the composite edge with no risk during the operation of trimming the parts. 

Sirris then produced digital simulations with a view to better understanding the results obtained on specimens instrumented with particular FBG sensors. 

The pre-industrialisation study of the connection concept continues with the finalising of component injection and assembly of the semi-shells to be embedded in the material. This assembly must be accurate, clean and sealed.

The sensor is then put into the composite employed by drape forming and autoclave curing.

In some cases, optical fibre monitoring is interesting, but it is not possible, e.g. metal plate, or not appropriate, e.g. occasional temporary inspection, to embed the optical fibre in the material. 

The idea then is to develop measurement patches that could be stuck to or unstuck from any surface as required. Sirris has undertaken to define the concept and determine the optimum parameters via a theoretical approach based on experimental designs.