Investigation of surface textured sensing skin for fatigue crack localization and quantification

The timely discovery and monitoring of fatigue cracks in steel structures is an important task in order to ensure structural integrity. However, off-the-shelf strain sensors are small and their deployment is too spatially localized to successfully locate new crack formation or growth within acceptable confidence. A solution is the use of large-area electronics capable of covering large surfaces. The authors have previously developed a sensing skin technology based on a soft elastomeric capacitor (SEC) that consists of a highly compliant, low-cost, and scalable strain gauge that transduces surface strain into a measurable change in capacitance. In prior work, the SEC was fabricated using three flat layers that formed a parallel plate capacitor. An improvement to that design has recently been proposed, in which the dielectric layer is textured using a corrugated pattern, and preliminary work showed a substantial improvement in sensing performance due to the added in-plane stiffness and decrease in the sensor's transverse Poisson's ratio. This paper extends preliminary work by studying the textured sensor's capability to quantify and discover a fatigue crack, and to study the general sensing performance in terms of linearity, sensitivity, resolution, and accuracy. Four specific corrugation patterns are investigated: a symmetric grid, a diagonal diagrid, a reinforced diagrid, and a non-symmetric re-entrant hexagonal honeycomb (auxetic) pattern. Experimental results show that the use of a texture results in a significant increase in sensing performance, with auxetic and reinforced diagrid patterns outperforming the other patterns. In particular, the auxetic and reinforced diagrid patterns allowed the discovery of a 0.28 mm and 0.31 mm fatigue crack, compared to a 0.53 mm crack for the untextured SEC, and resulted in up to 106% increase in sensitivity, 113% in linearity, 319% in resolution, and 582% in accuracy, compared to untextured SECs.