Monitoring of carbon fibre composites with miniaturized flexible interdigtial sensors

The focus of this work is the monitoring of fibre composites and adhesives with a miniaturized flexible Sensor. Monitoring of the curing process is one of the relevant parameters to optimize the production process and ensure the quality of the composite. The presented sensor measures the curing process locally in the material based on impedance spectroscopy. This enables the monitoring of specific critical points in a part. With this method it is also possible to distinguish between the curing process of different adhesives and resins of fibre composites. Using different mechanical test it can be shown that the sensor does not influence the mechanical properties of the material significantly. An inlay of polyimide with 5 I m thickness and holes in the substrate has the smallest influence on the mechanical properties. It has a net-shaped structure with all the dimensions in the range of the fibres diameter. The sensors are fabricated on this substrate and can therefore stay in the fibre composite without downgrading the mechanical stability. Hence the sensors can also be used for lifetime monitoring of the fibre composite. Here it is shown that measuring the moisture diffusion inside the composite is possible with the interdigital sensor. The changes of the impedance can be correlated to the mass change which is created by moisture uptake. Using multiple sensors the distribution of the moisture in the composite can be determined due to the local measurement of the sensors. In addition it is shown that the presented sensors can be used to analyse the internal configurations of shape memory polymers. These materials are a combination of thermoset and thermoplastic which should be covalent bonded to have a shape memory behaviour. Measuring the impedance with the interdigital sensor covalent bonded mixtures can be distinguished from those which are not covalent bonded. An external stimuli, here temperature, is used to be able to deform the material. This change in internal structure can also be measured with the sensor. Furthermore the curing process of adhesives can be monitored with interdigital structures. In many cases an adhesive is used to bond to parts together. Therefore one of these parts can be used to screenprint the interdigital structure on top, which is quite fast and cost effective. This is the reason why also printed sensors are investigated. With the printed sensors it is possible to monitor influence of the environmental conditions on the curing process of adhesives