Measurements of temperature during fatigue of a thermoplastic polymer composite using FBG sensors

摘要

It is well known that cyclic fatigue induces hysteretic heating and temperature increase in polymer composite materials. Optical sensing technology is well developed to perform efficient in-field monitoring of the thermo-mechanical behaviour of engineering composite structures in sectors ranging from automotive to aerospace, where the implications of thermal fatigue are important. In this paper an experimental method and its results for temperature measurements in a glass/polypropylene composite beam subjected to cyclic bending at 6 Hz are reported. Since the sensors are sensitive to both temperature and strain, they are placed on the surface and in the centre of the specimen during processing, thus minimizing the effects of the mechanical strain during loading. Temperature increases of 9 degrees C both inside and on the specimen surface are recorded with fibre Bragg grating (FBG) sensors and verified by full-field temperature analysis using infrared thermography on the surface and thermocouples on both the surface and through the thickness of the composite. Discrimination between relatively large dynamic strain (responsible for similar to 280 pm Bragg wavelength shift oscillations), birefringence (responsible for similar to 70 pm), and temperature variations (9 degrees C leads to a Bragg wavelength shift of similar to 240 pm) is achieved by suitable integration of embedded FBGs and optical data processing. The achieved temperature resolution is 1 degrees C. The method outlined in this paper can be applied in various experimental configurations for temperature measurements in polymeric materials.