JEC COMPOSITES MAGAZINE - Issue #44 - October 2008 - Special Asia - 82

KNOW-HOW Control …/… Potential of in-situ measurements by optical fibres Despite all these drawbacks, it remains possible to analyze some key data measured by embedded optical fibers for characterizing the manufacturing process or a loading case of a composite part. As a preliminary work, a FBGS has been embedded in a resin tensile coupon in which two holes have been drilled in order to apply a well known strain gradient along the grating [X. Chapeleau and al. Determination of strain distribution and temperature gradient profiles from phase measurements of embedded fibre Bragg gratings, Journal of Optics A: Pure and Applied Optics, pp. 775-781, 2006.]. Experimental and numerical strain profiles (figure 4a) show a very good agreement, and let us admit that the sensor is promising. Then, a composite plate [0,90]4S has been manufactured by wet layup of 16 unidirectionnal glass reinforcement and room temperature cure epoxy resin. The final laminate is 5 millimeters thick, and contains a FBGS embedded across the laminate. This experiment aims to assess the difficulty to embed a FBGS elsewhere than parallel to the plies and the surprizing strain profiles which come out (figure 4b): Indeed, strains are almost constant in the center of the specimen, but a strain gradient appears near the edges and reveals that during hardening, the FBGS does not traduce the full shrinkage of the resin because of its high stiffness compared to the one of the resin during and after curing. Conclusions Sensors based on optical fibers offer a wide range of opportunities to monitor composite manufacturing processes with many possible parameters (temperature, pressure, strain or degree of cure) to be measured with a good sensitivity and high measurement ranges. However, even if they seem very easy and versatile to use, they refer to very delicate physical phenomena to handle, and difficult practical steps to solve in order to be inserted in composite parts. Their potential is obvious and already brings relevant assets for scientific and technological investigations. To pursue the development of optical fiber sensors, some key issues are: • To use the relevant sensor and the corresponding interrogation technique for a reliable result. Some measurement techniques based on optical fibers are already on shelf, but many of them are still under development and require improving their accuracy and reliability. Fig. 4: Strain profiles identified by layer peeling of an FBGS embedded in a pure epoxy resin sample (a) and across the width of a composite laminate (b) • To develop a reliable know-how on the insertion of optical fiber based sensors in composite parts. Still now, many experiments fail because practical problems are underestimated. • To take all the heterogeneities into account in both the strain and the temperature fields. Indeed, a simplistic analysis would lead to wrong interpretations. Contacts: Pascal.Casari@univ-nantes.fr Cyril.Lupi@univ-nantes.fr JEC Composites Magazine / No44 October 2008

http://www.cel.eu/

JEC COMPOSITES MAGAZINE - Issue #44 - October 2008 - Special Asia

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #44 - October 2008 - Special Asia