Space-Filling Curve Resistor on Ultra-Thin Polyetherimide Foil for Strain Impervious Temperature Sensing

Abstract

Monitoring process parameters in the manufacture of composite structures is key to ensuring product quality and safety. Ideally, this can be done by sensors that are embedded during production and can remain as devices to monitor structural health. Extremely thin foil-based sensors weaken the finished workpiece very little. Under ideal conditions, the foil substrate bonds with the resin in the autoclaving process, as is the case when polyetherimide is used. Here, we present a temperature sensor as part of an 8 µm thick multi-sensor node foil for monitoring processing conditions during the production and structural health during the lifetime of a construction. A metallic thin film conductor was shaped in the form of a space-filling curve to suppress the influences of resistance changes due to strain, which could otherwise interfere with the measurement of the temperature. FEM simulations as well as experiments confirm that this type of sensor is completely insensitive to the direction of strain and sufficiently insensitive to the amount of strain, so that mechanical strains that can occur in the composite curing process practically do not interfere with the temperature measurement. The temperature sensor is combined with a capacitive sensor for curing monitoring based on impedance measurement and a half-bridge strain gauge sensor element. All three types are made of the same materials and are manufactured together in one process flow. This is the key to cost-effective distributed sensor arrays that can be embedded during production and remain in the workpiece, thus ensuring not only the quality of the initial product but also the operational reliability during the service life of light-weight composite constructions.

Comparison of Different Cure Monitoring Techniques

“…Combining the strain gauge with an interdigitated electrodes sensor can improve the information yield from the curing reaction. The next step is to use the already developed monolithic multisensor node consisting of a temperature sensor, strain gauge and interdigital electrodes [ 81 ] for monitoring the curing process of composites.…”

“…For the dielectric measurement, a dielectric sensor with interdigitated electrodes is used. The sensor design and manufacturing as part of a multiple sensor node were described in [ 81 ]. Both electrodes consist of 15 fingers each with 100 width, 3.75 length and 30 distance between them.…”

Reducing the Weakening Effect in Fibre-Reinforced Polymers Caused by Integrated Film Sensors

“…For the production of the filigree sensor structures, fabrication processes were used as described more in detail in [27]. Figure 2a shows a sensor node designed for monitoring the progressing curing of epoxy resins.…”

“…Figure 2a shows a sensor node designed for monitoring the progressing curing of epoxy resins. The sensor node contains a thermal sensor that eliminates strain direction sensitivity and largely suppresses the influence of the magnitude of the strain by a suitable arrangement of the conductor path [27]. In addition, the sensor node contains two strain gages in half-bridge connection with mutually orthogonal orientation and a coplanar capacitive sensor with interdigitated electrodes.…”

Polyetherimide Reinforced Smart Inlays for Bondline Surveillance in Composites

“…The inlay fabrication depicted in Figure 1 is based on the process described earlier 89 [14]. A major change however, is that the bottom substrate layer was produced by 90 spin coating of a liquid 10 wt% PEI precursor based on polymer pellets diluted in 91 trichlorethanol [20] at a spin speed of 1000 rpm on a 4 inch glass wafer. This was 92 followed by hot plate curing for 2 min at 150 • C. After cooling a second polymer layer 93 was applied in the same manner, before final curing was conducted at 220 • C for 10 min 94 (see Figure 1a).…”