ABSTRACT
When compared to metals, composite materials exhibit some interesting advantages as for instance, a higher stiffness-to-weight ratio, lower density and corrosion resistance. In addition, they exhibit a gradual stiffness degradation during in-life operation and not a sudden loss of properties as it happens for a metallic component when reaching its end-life. Furthermore, composite materials allow also the integration of sensors during their production. The latter can then be used during the whole lifetime of the component in order to gain information on its structural health. A desirable sensor technology which can be employed for this purpose is represented by fibre optics, which thanks to their small dimensions (≈ 200 μm) and flexibility can be successfully embedded in between composite plies. This will allow for sensing the “real strain” to which the structure is subjected inside the composite laminate. However, questions arise when an optical fibre is embedded in a composite layup, e.g. what is the distortion created by the sensor in the surrounding composite layers? Will this distortion affect the structural integrity of the composite? Is the strain sensed by the fibre optic an artefact caused by the presence of the sensor itself ? In this study, these questions are addressed via an experimental approach, where fibre optics were embed-
2 EXPERIMENTAL SET-UP
2.1 Sample production
Two different laminates having both a [902,02]2 s lay-up were produced by an autoclave process. The first one was a carbon-fibre UD prepreg M55 J-M18 by Hexcel®, which was cured for 2 hours at 180 °C, imposing a vacuum level of -80 kPa and an external pressure of 7 bars. The second laminate was produced with MTM28 glass-fibre UD prepreg layers by Umeco® and was cured for 2 hours at 120 °C, using vacuum level of -80 kPa and 5 bars of additional pressure. In the mid-plane of each laminate, optical fibres of different diameters (the outer diameter was ranging from 106 μm to 195 μm) were aligned with the reinforcing fibres direction and embedded between two 0° layers. For the GFRP laminate the fibre optics were carrying also a grating, which was positioned in the centre of the plate, while for the CFRP dummy fibres were used instead. After autoclave manufacturing, the laminates were cut to coupon size according to the ISO 527-4 standard. Due to the limitations imposed by the micro-CT setup (which will be explained in more detail in subparagraph 2.3) the carbon fibre samples had the following dimensions t x w x l = 2.5 mm x 12 mm x 250 mm, while the glass fibre samples were 5 mm x 19 mm x 250 mm. Aluminium tabs were glued to the specimens ends, allowing an overall gauge length of approx. 150 mm.
