Abstract
Recently, a mechanical crack-based strain sensor with high sensitivity was proposed by producing free cracks via bending metal coated film with a known curvature. To further enhance sensitivity and controllability, a guided crack formation is needed. Herein, we demonstrate such a ultra-sensitive sensor based on the guided formation of straight mechanical cracks. The sensor has patterned holes on the surface of the device, which concentrate the stress near patterned holes leading to generate uniform cracks connecting the holes throughout the surface. We found that such a guided straight crack formation resulted in an exponential dependence of the resistance against the strain, overriding known linear or power law dependences. Consequently, the sensors are highly sensitive to pressure (with a sensitivity of over 1 × 105 at pressures of 8–9.5 kPa range) as well as strain (with a gauge factor of over 2 × 106 at strains of 0–10% range). A new theoretical model for the guided crack system has been suggested to be in a good agreement with experiments. Durability and reproducibility have been also confirmed.
Highlights
The resistance of the metal layer dramatically increased due to the opening of the cracks[15] because there was no conductivity between disconnected lips of the crack
Along with the high strain sensitivity, the crack-based sensor was demonstrated to be highly repeatable since the resistance variations of the sensor while loading up to 10% strain and unloading back to 0% strain at a sweeping speed of 10 mm/min closely match during the experiment
Graphs of the resistance of the patterned crack sensor loaded to strain 10% and unloaded to strain 0% are overlapped, which illustrates that some hysteresis occurs only at sufficiently high strains
Summary
The resistance of the metal layer dramatically increased due to the opening of the cracks[15] because there was no conductivity between disconnected lips of the crack. For non-straightened cracks, the gauge factor was demonstrated to be about 200, relatively low compared to that of the 20 μm pattern gap sample (over 2 × 106). The simulation result shows that narrower pattern gaps generate a wider distribution of high stress that stimulates appearing cracks everywhere within the gap distance. This is not the case for the 20 μm gap where the crack originates at the very tips of the gap (Fig. 4c, right plate).
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