Carbon nanotube yarns are micron-scale fibers comprised by tens of thousands of carbon nanotubes in their cross section and exhibiting piezoresistive characteristics that can be tapped to sense strain. This paper presents the details of novel foil strain gauge sensor configurations comprising carbon nanotube yarn as the piezoresistive sensing element. The foil strain gauge sensors are designed using the results of parametric studies that maximize the sensitivity of the sensors to mechanical loading. The fabrication details of the strain gauge sensors that exhibit the highest sensitivity, based on the modeling results, are described including the materials and procedures used in the first prototypes. Details of the calibration of the foil strain gauge sensors are also provided and discussed in the context of their electromechanical characterization when bonded to metallic specimens. This characterization included studying their response under monotonic and cyclic mechanical loading. It was shown that these foil strain gauge sensors comprising carbon nanotube yarn are sensitive enough to capture strain and can replicate the loading and unloading cycles. It was also observed that the loading rate affects their piezoresistive response and that the gauge factors were all above one order of magnitude higher than those of typical metallic foil strain gauges. Based on these calibration results on the initial sensor configurations, new foil strain gauge configurations will be designed and fabricated, to increase the strain gauge factors even more.

Carbon nanotube yarns have extraordinary mechanical, electrical and thermal properties that make them attractive for high-performance and multifunctional composite materials. They also exhibit a unique piezoresistive response when subjected to mechanical strain. This characteristic is of interest for sensing applications including strain measurement and damage detection when integrated in polymeric and composite materials. Thus, there is a need to understand the coupled mechanical and electrical behavior of the carbon nanotube yarns to fully comprehend the entire scope of their sensing applications. Of particular interest are their characteristics when used as piezoresistive strain sensors in structures that are subjected to dynamic loading including fatigue and impact, or quasi-static cyclic loading. This paper presents a study about the presence of hysteresis and other time-dependent effects in carbon nanotube yarns during quasi-static cyclic uniaxial tensile loading. By simultaneously measuring the resistance, the load and the displacement histories, any direct correlations between the mechanical and electrical characteristics of the carbon nanotube yarns are investigated including the effect of strain level, strain rate, and stress relaxation. It was observed that all these effects play a significant role in the piezoresistive response of the carbon nanotube yarns. In particular, a low strain rate appears to bring out a unique piezoresistive response that is not observed at higher strain rates. The underlying phenomena determining the piezoresistive responses are hypothesized and discussed in the context of strain rate and maximum strain level.

Carbon nanotube (CNT) yarns are fiber-like materials that exhibit excellent mechanical, electrical and thermal properties. More importantly, they exhibit a piezoresistive response that can be tapped for sensing purposes. The objective of this study is to determine experimentally the piezoresistive response of CNT yarns that are embedded in a polymeric medium while subjected to either tension or compression, and compare it with that of the free or unconstrained CNT yarns. The rationale is the need to know the piezoresistive response of the CNT yarn while in a medium, which provides a lateral constraint to the CNT yarn, thus mimicking the response of integrated CNT yarn sensors. The experimental program includes the fabrication of samples and their electromechanical characterization. The CNT yarns are integrated in polymeric beams and subjected to four-point bending, allowing the determination of their response under tension and compression. The electromechanical data from a combined Inductance–Capacitance–Resistance (LCR) device and a mechanical testing system were used to determine the piezoresistive response of the CNT yarns. At a strain rate of 0.006 min⁻¹, the gauge factor obtained under tension for a maximum strain of 0.1% is ~29.3 which is higher than ~21.2 obtained under compression. The CNT yarn sensor exhibited strain rate dependence with a gauge factor of approximately 23.0 at 0.006 min⁻¹, in comparison to 19.0 and 1.3, which were obtained at 0.0005 min⁻¹ and 0.003 min⁻¹, respectively. There is a difference of up to two orders of magnitude in the sensitivity of the constrained CNT yarn under bending with respect to that of the free CNT yarn under uniaxial tension. However, the difference becomes smaller when the constrained CNT yarn was tested under uniaxial tension. This data and information will be used for future modeling efforts and to study the phenomena that occur when CNT yarns are integrated in polymeric and composite materials and structures.

Carbon nanotube (CNT) yarns are fiber-like materials that exhibit excellent mechanical, electrical and thermal properties. More importantly, they exhibit a piezoresistive response that can be tapped for sensing purposes. The objective of this study is to determine experimentally the piezoresistive response of CNT yarns that are embedded in a polymeric medium while subjected to either compression or tension, and compare it with that of the free or unconstrained CNT yarns. The rationale for this study is the need to know the response of the CNT yarn while in a medium, which provides a lateral constraint to the CNT yarn thus mimicking the response of integrated CNT yarn sensors. The experimental program will include the fabrication of samples and their electromechanical characterization. The CNT yarns are integrated in polymeric beams and subjected to four-point bending, allowing the determination of their response under tension and compression. The electrical resistance data from an Inductance-Capacitance-Resistance (LCR) device is used with the data acquired from the mechanical testing system to determine the piezoresistive response of the CNT yarns. This data and information will be used for future modeling efforts and to study the phenomena that occur when CNT yarns are integrated in polymeric and composite materials and structures.

Carbon nanotube yarns are micron-scale fibers comprised by tens of thousands of carbon nanotubes in their cross section and exhibiting piezoimpedance characteristics that can be tapped to sense strain. This paper presents the details of novel foil-based strain gauge sensor configurations comprising carbon nanotube yarn. The strain gauge sensors were designed considering parametric studies that maximize the sensitivity of the sensor to mechanical loading. The fabrication details of the strain gauge sensors that exhibit the highest sensitivity, based on the modeling results, are described including the materials and procedures used in the first prototypes. Details of the calibration of the strain gauge sensors are also provided and discussed in the context of the electromechanical characterization when bonded to metallic specimens. This characterization included studying their response under monotonic and cyclic loading. It is shown that these strain gauge sensors consisting of carbon nanotube yarn are sensitive enough to capture strain. Previous modeling results indicated the potential of the strain gauges to exhibit gauge factors higher than those of metallic foil strain gauges.

In this paper, a numerical model is proposed to predict the mechanical response of Carbon Nanotube (CNT) yarns under uniaxial tensile loading. The model implements a Finite Element (FE) modeling technique to formulate the idealized structure of randomly formed configurations of CNT yarns. In the model, the bundles of the CNT yarn are simulated as bar elements randomly connected to the ends of each other. A set of transverse and diagonal members are introduced to capture the coupling effects of the frictional forces between the adjacent bundles. The FE model is loaded in a step-by-step monotonically-increasing pattern. At each step, the internal forces of the elements are checked against a failure criterion and the elements with internal forces higher than the set strength value are removed. The model is validated by comparing its results with the results of an experimental study conducted elsewhere. A very good correlation is achieved between the FE model results and the benchmark test results.