Flow front and cure monitoring with fibre optic sensors for thick CFRP laminates

During vacuum infusion processes such as VARI and VAP curved CFRP laminates exhibit flow front deviations in thickness direction. The flow path induced by curved structures varies depending on the layer. Flow lengths through outer layers are increased, leading to a flow front distortion. This can result in porosity and even in dry spots especially in thick laminates. An integrated sensor system measuring both, flow front arrival and curing, with negligible effect on the laminate textile structure, allows to generate process understanding which is used for an optimized process design. These aspects are provided by fibre optic sensors due to their small size and fibrous structure. In this study two fibre optical sensor types are investigated to measure flow front arrival in thickness direction within a vacuum infusion process and matrix curing in neat resin experiments. The changing pressure situation at the flow front is used to detect resin arrival with fibre Bragg grating sensors. Simultaneously a Fresnel reflectometer is applied to detect flow front arrival by means of refractive index change of the surrounding medium. In a first set of experiments the influence of compaction and sensor position on the sensor responds during flow front arrival is investigated. In a second experiment Fresnel reflectometers are exploited to measure matrix curing in isothermal neat resin experiments. This is followed by fibre optical glass transition temperature measurements and the accuracy evaluation of the obtained results. Complementary differential scanning calorimeter measurements are used as a reference for the fibre optic curing experiments.