Abstract
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.
Highlights
This paper summarizes the fabrication and calibration results on foil strain gauges comprising configuration of the material that maximizes its sensitivity to external loading
Foil strain gauge sensor concepts comprising unidirectional and parallel carbon nanotube yarns are being developed and the first experimental results including their fabrication and calibration details are described in this paper
The modeling of the piezoresistive response of the strain gauges had indicated that their sensitivity would be enough to measure strain and that their gauge factors could potentially exceed those of metallic foil strain gauges
Summary
Strains can be measured by sensors that rely on the piezoresistive effect [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33], the frequency shift of a resonator’s fundamental mode [34,35,36], the piezoelectric effect [37,38], the capacitance change [39,40,41,42,43], the optical properties changes [44,45,46,47,48,49], and other effects [50,51,52]. Semiconductor strain gauges rely on the piezoresistive effect of silicon or germanium and measure changes in resistance with respect to applied stresses [5,6,7] They are accurate, repeatable, and have a gauge factor that depends mostly on the effect of the piezoresistive part ranging between 200 and 500 according to the doping concentration and lattice orientation [7]. Piezoresistivity-based foil strain gauge sensors are usually made of a piezoresistive membrane layer attached to a flexible substrate This flexible structure acts as a compliant mechanism that translates an input force into local strain and stress in the piezoresistive layer so that changes in electrical resistivity can be monitored and correlated to strain using the piezoresistivity effect.
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