Moisture diffusion in carbon fiber composites changes the mechanical properties of the composite. Therefore, a monitoring method of the actual content of moisture in the composite is important. However, at the moment there are no online methods established. A common method is the measurement of the mass changes due to water uptake. This method is not suitable for online monitoring of a real composite part in service. We demonstrate that miniaturized flexible interdigital sensors are suitable for moisture measurement inside the carbon fiber composite. These sensors are directly integrated inside the composite. It was already demonstrated that these can be successfully used for resin-curing monitoring as a primary application. Here we demonstrate that the same sensors are also suitable for moisture measurement inside the material. In order to do so, we expose samples with and without integrated sensors to hot-wet conditions and measure the dielectric changes with the sensors and the mass gain. The moisture concentration and the measured admittance can be directly correlated to each other. This demonstrates that the sensors can be used for moisture measurement as a secondary application. In addition, it is shown that the sensors have the potential to measure the moisture locally inside the material.

Monitoring adhesives during manufacturing and their lifetime has become increasingly important due to the variety of materials and applications. Impedance spectroscopy is a suitable method for online monitoring of the curing process. We present a miniaturised interdigital structure to monitor the curing process of the adhesives using impedance spectroscopy. Compared to other sensors, our sensor is ultrathin, so that it disappears in the adhesive joint. Therefore, it can remain in the joint and be used for lifetime monitoring. In addition it is suitable for thin adhesive layers due to its fine grid. We demonstrate that the impedance of the sensor embedded in the adhesive gives insight into the curing mechanisms. Therefore monitoring of a dispersion and an epoxy is shown. In addition, the curing cycles can be reliably controlled using this monitoring method. The permittivity of the adhesives is extracted from the impedance measurements, applying analytical models of the electrical field of the interdigital structure.

We present a foil based miniaturized impedance sensor combined with a temperature sensitive structure to be integrated in Carbon-Fiber-Reinforced-Polymer (CFRP). Commercially available sensors are too big and they weaken the CFRP. In contrast the influence of our miniaturized flexible sensor can be neglected due to its size (5 µm thick); it is in the range of the fibers’ diameter. The sensor can remain in the produced part without disruption and even be used for structural health monitoring. Our sensor will enable automated control of the curing process and improve the fabrication. This is the first time impedance and temperature sensing is combined in such a small size.

We present a cost-efficient screen printed sensor for online cure monitoring of adhesives on semi-finished parts. It can be manufactured and used directly in the production line. A printed interdigital sensor is used for this purpose. We investigate the influence of environmental condition and modified adhesive properties on the sensor’s signal. The sensor can successfully detect insufficient curing, for example, due to excessive humidity or improper mixing ratio of binder and hardener. Therefore, our sensor will enable a cost-effective automated control of the curing of adhesives in the production lines of the factories of the future.

Monitoring of fibre composites is an important task to spread the application of fibre composites. Cure monitoring of the polymer matrix of fibre composites during the fabrication can be realized with interdigital sensors. The measured impedance can be directly related to the curing process. However wiring the sensor may disrupt the material, therefore sensing should be done wireless. As many fibre composites are processed under high temperature and high pressure conditions no electronics can be used inside the material. Here we present the design, fabrication, and characterization of a wireless sensory system using a chipless strategy for online cure monitoring. For this purpose, different combinations of sensors and readers are designed, consisting of a planar inductor-capacitor (LC) resonator, where capacitance is an interdigital capacitor, and an inductor as antenna operating wirelessly through inductive coupling. The designs are realized onto flexible polyimide substrate using microfabrication technology. To characterize the coupling effect, the response of the reader antenna has been carried out when sensor is placed in its vicinity. Furthermore, the curing process of adhesive is demonstrated and results are discussed. It is shown that the curing process of a thermoset polymer can be monitored by a flexible chipless LC resonator wirelessly.

Online monitoring methods for the production of carbon fiber reinforced polymers (CFRP) enable a fast and reliable manufacturing of CFRP parts for example for automotive and aerospace applications. With our miniaturized flexible dielectric sensor we realized online monitoring without harming the structural integrity of the produced part. We use the method of impedance spectroscopy where the changes of the dielectric parameters during curing are measured. The changes can directly be related to the curing degree. However during the curing strain can appear inside the part which is transferred to the flexible substrate of the sensor. Here we demonstrate that strain can also be measured with the sensor but the changes of the capacitance related to dielectric changes during the curing are several times higher. Therefore we can state that small strain, which can appear, does not influence our impedance measurement during curing.