Fibre optical temperature sensor for distributed sensing in composite moulds

The temperature is the driving parameter in thermoset resin curing processes during composite manufacturing. In larger parts the temperature distribution can be uneven due to heat conduction effects or exothermic resin reaction. For that reason, composite moulds for manufacturing of wind turbine rotor blades are equipped with a growing number of mould-integrated heating circuits. In order to allow independent temperature control, each circuit has to be equipped with at least one temperature sensor. Until today, this is realised by integrating electrical single-point sensors into the composite mould. Their major drawback is the high application effort during mould manufacturing. Therefore, we want to develop a multi-point temperature sensor which can be applied in large composite moulds. This sensor has to be robust and flexible enough to withstand the lamination process. We developed a fibre optical temperature sensor based on quasi-distributed sensing with fibre Bragg gratings (FBG). The novelty is the carbon fibre reinforced plastic (CFRP) tube that encapsulates the FBGs. We performed sensor calibrations to determine the sensor sensitivity with bare FBGs and with FBGs in CFRP tube. At last, we integrated a calibrated multi-point fibre optical temperature sensor in an electrically heated composite mould to monitor temperature distribution during composite manufacturing. This paper reports about the new sensor design, its mechanical properties and temperature sensitivity, and the application in a CFRP mould. The design we found is adequate for distributed temperature measurements in composite moulds. The characteristic curve of the sensor is linear in the investigated temperature regime and shows small hysteresis. The proposed sensor provides an opportunity to increase the number of temperature measurement points in composite moulds. We see the potential to reduce mould manufacturing costs by raising the sensor density. This allows more precise temperature monitoring which can be used for optimised process control by composite manufacturers.