Glass Fibre Reinforced Composite Samples Inspected using THz Spectroscopy

Abstract

Glass fibre reinforced composites (GFRP) are recently very popular in many industrial branches like: marine, wind energy or civil engineering. From the Structural Health Monitoring (SHM) point of view it is very important to know behaviour of elements during their exploitation. One of recently investigated monitoring techniques are methods based on fibre optic sensors (e.g. fibre Bragg grating sensors) embedded into composite elements during their manufacturing process or into adhesive layer in composite joints. It is possible due to fibre optic advantages like: small size, weight and high corrosion resistance. The sensors can be used for monitoring both internal strain due to mechanical loading as well as strain induced e.g. by moisture influence. One of the problems is that embedding results in local discontinuities due to fibre optic occurrence that can leads to a damage origin. One of the non-destructive methods allow to observe and evaluate the internal structure of non-conductive materials is terahertz time-domain spectroscopy (THz-TDS). The technique can be used for detection and localisation of different damage types like voids, delaminations or mechanical damage. It can be also used for determination of the arrangement of fibre optics embedded into material. The THz method allows detection and localisation of material structural disintegration which occurrence influences on absorption or refractive index of examined material. The paper presents an application of THz spectroscopy for observation and evaluation of the internal structure of thin composite samples with embedded fibre Bragg grating (FBG) sensors and with inclusions (e.g. water intrusion, dust) that were introduced during their manufacturing process as well as a complex composite sample with honeycomb filler.