JEC COMPOSITES MAGAZINE - Issue #44 - October 2008 - Special Asia - 81

with a strain resolution of 1 µm/m and a spatial resolution of 1 centimeter along an optical fiber up to 70 meters long. These technologies have already been tested for pipelines, dams or bridges (for example, Saint-Jean Bridge in Bordeaux monitored by CEA-LIST). Our applications to composite materials The composite materials under consideration are laminates with long carbon or glass fibers reinforcement and epoxy matrix. During processing, the resin is in a liquid state and polymerizes under room or high temperature. This step may involve shrinkage and internal stresses which lead to assess what happens before the composite part reaches its solid state. It is then necessary to develop in-situ measurement techniques for temperature, strains and degree of cure in order to help understanding the manufacturing process, and sensors based on optical fibers are suitable candidates: versatile, few intrusive and compatible with a strong bonding to resins employed in thermoset composites. Two applications tested in our labs are reported below. advantage of using the end of an optical fiber just located there as a "spy" inside a mould. Indeed, the end of an optical fiber is an interface which separates the constitutive glass and the outside firstly made of air and secondly replaced by resin during the manufacturing process. Then, when resin reaches this interface, the refractive index varies suddenly, and leads to a severe change in the reflected light power. This principle has been used for monitoring an RTM injection (figure 3a). A set of 12 sensors placed inside the preform allowed to identify the properties of the flow front all along the injection of resin and highlighted the presence of a preferred channel at the edge of the mould (cf figure 3b). This kind of sensor could be used to detect resin in the vents of a mould, and consequently help the process control of multi-point injections. In addition, an analysis of the optical index of the resin versus its degree of cure has been run, and allows to follow the resin glass transition [Fibre optics sensors applied to resin transfer moulding (RTM) in aeronautics: Composite materials process optimization; Xavier ADURIZ and al. Proceedings of SPIE, the International Society for Optical Engineering ISSN 0277-786X CODEN PSISDG ]. However, the relationship between thermal, cure and optical properties must be previously established in order to decouple corresponding effects and to avoid wrong analyses. Strain measurement profiles in composite parts The internal structure of a composite part involves heterogeneities (waviness, fiber tows, stacking sequence, and reinforcement fibers) which can be bigger or smaller than an optical fiber which would be embedded. Then, strains to be measured inside a composite material by means of an optical fiber lead to key remarks: • It is no more possible to evaluate a strain state at the scale of the reinforcement fibers because the optical fiber is already ten times bigger. • A strain state analysis at the ply scale becomes possible if we consider the periodical structure of a FBGS which is around 0.5 micrometers compared to the thickness of the plies in a composite stacking sequence, or the tows width which both have and order of magnitude around 100 to 3,000 micrometers. The interrogation technique can no more be the only shift in the Bragg wavelength but the OLCR explained above. • In the heart of the composite part, temperature and strain histories are highly locally coupled, and lead to transient gradients, which are almost impossible to de-correlate accurately. • During curing of the composite part, a stress transfer is occurring between the resin with an increasing stiffness and the optical fiber which remains twenty times stiffer than the resin at any stage. The intrusivity of the sensor is then obvious, and any interpretation of the strain level without caution would be hazardous, especially if the strain and temperature fields are not homogeneous. Fig. 3: a) Instrumented RTM mould; b) Spatial resin flow front characterized by means of refractiveindex optical fibre sensors Monitoring of resin flow front during RTM injection An idea to sense the presence of resin at one point is to take No44 October 2008 / JEC Composites Magazine

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