Multi-scale Structural Health Monitoring using Wireless Smart Sensors

Tremendous progress has been made in recent years in the wireless smart sensor (WSS) technology to monitor civil infrastructures, shifting focus away from traditional wired methods. Successful implementations of such WSS networks for full-scale SHM have demonstrated the feasible use of the technology. Much of the previous research and application efforts have been directed toward single-metric applications. Multi-metric monitoring, in combination with physics-based models, has great potential to enhance SHM methods; however, the efficacy of the multi-metric SHM has not been illustrated using WSS networks to date, due primarily to limited hardware capabilities of currently available smart sensors and lack of effective algorithms. This research seeks to develop multi-scale WSSN strategies for advanced SHM in cost effective manner by considering: (1) the development of hybrid SHM method, which combine numerical modeling and multi-metric physical monitoring, (2) multi-metric and high-sensitivity hardware developments for use in WSSNs, (3) network software developments for robust WSSN, (4) algorithms development to better utilize the outcomes from SHM system, and (5) fullscale experimental validation of proposed research. The completion of this research will result in an advanced multi-scale WSS framework to provide innovate ways civil infrastructure is monitored.Financial support for this research was provided in part by the National Science Foundation under NSF Grants No. CMS-0600433 and CMMI-0928886.Ope

Soft Elastomeric Capacitor for Angular Rotation Sensing in Steel Components

The authors have previously proposed corrugated soft elastomeric capacitors (cSEC) to create ultra compliant scalable strain gauges. The cSEC technology has been successfully demonstrated in engineering and biomechanical applications for in-plane strain measurements. This study extends work on the cSEC to evaluate its performance at measuring angular rotation when installed folded at the junction of two plates. The objective is to characterize the sensor’s electromechanical behavior anticipating applications to the monitoring of welded connections in steel components. To do so, an electromechanical model that maps the cSEC signal to bending strain induced by angular rotation is derived and adjusted using a validated finite element model. Given the difficulty in mapping strain measurements to rotation, an algorithm termed angular rotation index (ARI) is formulated to link measurements to angular rotation directly. Experimental work is conducted on a hollow structural section (HSS) steel specimen equipped with cSECs subjected to compression to generate angular rotations at the corners within the cross-section. Results confirm that the cSEC is capable of tracking angular rotation-induced bending strain linearly, however with accuracy levels significantly lower than found over flat configurations. Nevertheless, measurements were mapped to angular rotations using the ARI, and it was found that the ARI mapped linearly to the angle of rotation, with an accuracy of 0.416∘

Monitoring impact damage in composites with large area sensing skins

The effect of low energy impacts can seriously impair the operational life span of composites in the field. These low-energy impacts can induce a permanent loss in the toughness of the composite without any visible indication of the material’s compromise. The detection of this damage utilizing nondestructive inspection requires dense measurements over much of the surface and has been traditionally achieved by removing the part from service for advanced imaging techniques. While these methods can accurately diagnose the damage inflicted internally by the impacts, they accrue non-trivial opportunity costs while the structure is inspected. To enable the capabilities of in-service monitoring of the composite, the novel soft elastomeric capacitor was investigated as a sensing solution. The sensor is made of three layers comprised of a styrene-ethylene-butylene-styrene (SEBS) matrix, a commercially available elastomer. These layers consist of a titania filled center layer that forms the dielectric of the capacitor and two highly conductive outer layers doped with carbon black. This simple formation allows for a capacitor that has extremely robust mechanical properties. The soft elastomeric capacitor functions by taking up deformations on the surface of the composite that is transduced into a measurable change in capacitance. This study provides an electro-mechanical model for impact damage and experimentally investigates the efficacy of these sensors for use in damage detection given their promising characteristics; that being that the sensor geometry can be arbitrarily large allowing for much fewer sensors than traditional sensor networks employed for this task at a much lower cost than installing traditional in-situ sensing solutions. To investigate these properties a set of impact trials were undertaken on a drop tower using small samples of glass fiber reinforced plate, of random orient and short fiber, with a soft elastomeric capacitor mounted directly opposite the impact site. The impactor head was only allowed one contact with the sample before being intercepted. The testing range for the samples ranged from well below the yield strength of the glass fiber reinforced plate to the ultimate strength of the plate. Experimental results reported a square root relation between the impact energy given to the plate when inducing plastic deformations and the sensor’s measured change in capacitance

Experimental validation of textured sensing skin for fatigue crack monitoring

Automatic fatigue crack detection using commercial sensing technologies is difficult due to the highly localized nature of crack monitoring sensors and the randomness of crack initiation and propagation. The authors have previously proposed and demonstrated a novel sensing skin capable of fatigue crack detection, localization, and quantification. The technology is based on soft elastomeric capacitors (SECs) that constitute thin-film flexible strain sensors transducing strain into a measurable change in capacitance. Deployed in an array configuration, the SECs mimic biological skin, where local damage can be diagnosed over large surfaces. Recently, the authors have proposed a significantly improved version of the SEC, whereby the top surface of the sensor is corrugated in diverse non-auxetic and auxetic patterns. Laboratory investigations of non-auxetic patterns have shown that the use of corrugation can increase the sensor’s gauge factor, linearity, and signal stability when compared to untextured sensors, while numerical analyses of auxetic patterns have shown their superiority over non-auxetic corrugations. In this paper, we experimentally study the use of corrugated SECs, in particular with grid, diagrid, reinforced diagrid, and re-entrant hexagonal honeycomb-type (auxetic) patterns as a significant improvement to the untextured SEC in monitoring fatigue cracks in steel specimens. Results show that the use of corrugation significantly improves sensing performance, with both the reinforced diagrid and auxetic patterns yielding best results in terms of signal linearity, sensitivity, and resolution, with the reinforced diagrid having the added advantage of a symmetric pattern that could facilitate field deployments