Reinforcing carbon fibers as sensors: The effect of temperature and humidity

We investigated how temperature and relative humidity affects the electrical resistance of reinforcing carbon fibers in polymer composites. We used hybrid composites in which a tight, carbon fiber roving was laminated inside glass fiber-reinforced epoxy specimens. The electrical resistance of the carbon fibers was monitored continuously while the temperature or relative humidity was varied. The specific resistance was calculated in order to compare the different effects acting on the resistance of the carbon fiber. We found a relationship between temperature or relative humidity and specific resistance. As a result, carbon fibers can be used in different applications (for example in cure process monitoring). On the other hand, the effects should be compensated for when the resistance of the carbon fiber is measured in a health monitoring application (e.g. as an elongation sensor).


INTRODUCTION
Fiber-reinforced polymer composites are widely used in all areas of life, for example in the automotive and the energy industry or in sporting goods. The advantages of composites are low density, high modulus and strength, good resistance to chemicals and high temperature and good workability [1]. In recent years, hybrid composites have been extensively researched. Hybrid composites have two different reinforcing materials or matrices. The main reason is that these composites can combine the advantages of the components, for example, the high modulus of carbon fiber and the flexibility of glass fiber, and cost has to be taken into account as well. A leading industry is wind energy production: to improve energy generation, bigger turbines, therefore longer blades are needed. The blades have high loads (centripetal force, vibration, etc.), which limits blade size for a given material. The most common reinforcing material used to be glass fiber because of its low price and relatively good mechanical properties. A stiffer material with lower density, for example carbon fiber, enables larger structures, but to reduce cost, engineers combine glass and carbon reinforcing fibers [2].
Another advantage of hybrid composites is that they can be engineered to avoid catastrophic failure-instead, failure can be gradual (pseudo-ductility). In this kind of material, different failure methods can occur simultaneously: fragmentation of the high-modulus, low-strain reinforcement (carbon fiber), delamination between the layers of different modulus/strain (glass and carbon fiber) layers and failure of the high-strain reinforcement (glass fiber) [3]. In application areas, where high impacts, for example collisions can be expected, non-catastrophic failure means operational and life safety.
Most polymer composite applications are structural, where the mechanical properties of the reinforcing fibers are exploited. Some important, and rapidly growing fields, such as industry 4.0 or autonomous and zero emission transportation require lighter, more compact products that can collect more data throughout their lifetime. These requirements can be met with multifunctional materials, which combine two or more functions [4]. In our article, we use the definition created by Ferreira et. al. [5]: "a material is multifunctional when it has an additional function (e.g. electrical, biological, etc.) besides its structural property". Carbon materials (fibers, nanotubes) have good electrical properties, which can be used for additional functions, such as resistive heating [6], electromagnetic shielding, energy storage or health monitoring [7].
A health monitoring sensor can be created if the resistance of the reinforcing carbon fiber is measured; it is called the electrical resistance change method (ERCM) [8]. Carbon fibers used as a built-in sensor have many advantages: this configuration does not need any external sensors, therefore it is easier to manufacture and more cost-efficient, it does not have the "size-effect", which means that the fibers do not cause inhomogeneity, and carbon fiber can sense the state of the base material locally.
ERCM is widely used to monitor strain and failure in composites. Todoroki [9] made multifunctional carbon fiber-reinforced specimens, where the fibers are used for strain measurement. He measured the electrical resistance of the specimens during a tensile test and perceived similar behavior to that of conventional strain gages. The gage factor in his experiment was 2.6, which means the 1% of elongation would result in a 2.6% change in resistance.
Damage sensing is crucial in structural parts. With ERCM, it is possible to detect fiber or matrix failure, or delamination. Vavouliotis et al. [10] measured the electrical resistance of carbon fiber-reinforced composites to predict failure during fatigue loading. They tested carbon fiber-reinforced epoxy specimens with a cyclic load. Based on their results, they found good correlation between physical changes in the composite structure and change in electrical resistance, therefore, their measuring arrangement made it possible to predict the fatigue life of the composite. In their experiment, they compensated for the effect of temperature: they measured the temperature, calculated the change in resistance based on the temperature change and corrected the measured resistance value with it. They used an experimental value for the linear temperature coefficient.
They compared the original and corrected resistance values and showed that there is no significant difference.
With the spread of ERCM, it is important to understand what else has an effect on resistance besides elongation and failure. The environment (pressure, temperature, humidity) of the self-sensing structure has an impact on the electrical resistance of carbon fibers. The effect of temperature on the resistance of carbon materials has been known for a long time, but it was considered a disadvantage of the material. Schulte and Baron [11] mentioned in their paper that the heat generated by fatigue causes a change in resistivity, which can lead to a misinterpretation of ERCM results. Heat can also be generated by the Joule effect: as current flows in a resistor (voltage measurement, lightning strikes, etc.), atomic-scale collisions generate thermal energy in the material. This effect can be reduced by improving the conductivity of the material but should be taken into account [12]. In a recent research project, the researchers exploited the heat dependence of the resistance of carbon fiber: they used the interlaminar interface of two carbon fiber felts laid perpendicularly on each other as a sensor for temperature and thermal damage [13]. Takashi and Hahn [14] investigated the electrical resistance of a graphite fiber-reinforced polymer in different directions.
They measured a negative thermal coefficient of resistivity (which means a decreasing electrical resistance with increasing temperature) in the longitudinal, transverse and through-thickness directions.
Other environmental effects and relative humidity can also be determined by ERCM. Wang et al. [15] measured the contact resistance of an overlapping carbon fiber felt to estimate relative humidity. They explained this effect with the water uptake of the epoxy matrix, which causes the matrix to swell, thus increasing the distance between the carbon fiber layers.
The aim of this paper is to investigate the relationship between the temperature or relative humidity and the electrical resistance of reinforcing carbon fiber.

EXPERIMENTAL SETUP
In our experiment, we used continuous carbon fiber tow (Sigrafil C T24-5.0/270-E100, SGL Carbon Group, Germany) for resistance measurement ( Table   1). The carbon fiber products of SGL Carbon are used for automotive composite parts manufactured in Germany.

Number of filaments 24000
Fineness of yarn dry tex (g/1000m) 1600 Density g/cm 3 1.79 Filament diameter µm 6.9 Tensile strength GPa 5 Tensile modulus GPa 270 Elongation at break % 1.9 Single filament resistivity µΩm 14 Table 1. Material data of the Sigrafil C T24-5.0/270-E100 carbon fiber [16] The carbon fiber yarn was laminated in a glass fiber-reinforced composite to insulate it from the environment electrically. After cutting the specimens (250x40x3 mm), we made electric contact by winding wires around the carbon fibers. We chose four-wire resistance measurement, therefore two wires were fitted at each end of the fibers. In this arrangement, the electric current flowed from A to D and the voltage was measured between B and C ( Figure 1.) with an Agilent 34970A data logger (Agilent Technologies, USA).    During evaluation, we examined the heating and cooling phases separately, both between 0 °C and 100 °C. This range is the same as during the calibration of a metallic thermometer. To compare different specimens with different initial resistance, we calculated specific resistance by Equation (1): where the 0 reference point was the measured resistance value at 0 °C and was the measured resistance value at temperature . Figure 3 shows the specific resistance as a function of temperature; the heating and cooling phases are shown separately. a b

Figure 3. Specific resistance as a function of temperature during heating (a) and cooling (b)
Based on the results, the temperature-specific resistance diagram can be approximated with a straight line. The approximation is similar to the first order Equation (2) of the temperature dependence of metal resistance thermometers: in another form is Equation (3): where is the linear temperature coefficient. is the steepness of the linear trend In our experiments, the values of were negative, which means that the carbon fiber has a negative temperature coefficient (NTC). The values are almost the same during heating and cooling and are close to the values published earlier [14,17]. These results have two implications. On the one hand, carbon fiber can be used as a temperature sensor in various applications. For example, the crosslinking of a thermosetting matrix generates heat, which is a good indicator of the degree of cure. For demonstrating purposes, we created an experiment where the epoxy described before was cured in a container ( Figure 4). During the crosslinking process, the resistance of the impregnated carbon fiber roving was monitored as described before. A change in resistance caused by a change in temperature, on the other hand, has a negative effect when the carbon fiber is used as a sensor. We performed a tensile test (Z250 tensile tester, Zwick, Germany) on the specimens prepared for this test. We measured elongation (from crosshead travel) and resistance ( Figure 5).

Figure 5 Specific resistance as a function of elongation during the tensile test
The gauge factor (GF) at the starting, linear period (until around 1% elongation), which is the relationship between elongation and specific resistance, was = 0.7117 [−] in our experiment. The change in resistance just before breaking is the same as a change a 60 °C temperature decrease would cause. This means that the effect of temperature should be compensated for either during measurement (e.g. with a Wheatstone bridge) or during evaluation: the measured change in resistance should be corrected based on the linear temperature coefficient and external temperature.

Relative humidity dependence
In our RH experiment, specific resistance was calculated based on the initial resistance at the beginning of the test according to Equation (4): where is the initial resistance of the sample and is the resistance at % relative humidity. Figure 6 shows the measured specific resistance as a function of RH at different temperatures.    Our results show that elongation has an order of magnitude larger impact than temperature and two orders of magnitude larger than humidity.
All of our experiments were continuous tests, where only the change in temperature and change in humidity were monitored. Static tests could result in other relationships between electrical resistance and environmental properties, as the specimen would saturate with moisture, which would result in a nonlinear relationship between resistance and temperature.
As a result, carbon fibers can be used as temperature or humidity sensors in multifunctional composites, especially detecting changes in these parameters.
However, multiple factors act on the resistance of carbon fibers at the same time, and these should be taken into account: for instance, temperature, humidity, mechanical load or pressure. These effects should be separated or compensated during electrical resistance measurement.