Large-Scale Capacitance Sensor for Health Monitoring of Civil Structures (2010)

In this paper, a new type of sensing technique for damage localization on large civil structures is proposed. Specifically, changes in strain are detected using a capacitance sensor built with a soft-stretchable dielectric polymer with attached stretchable metal film electrodes. This sensor is sensitive to a change in strain that causes a change in its capacitance. Fixed to a structure, small changes in the strain can be monitored directly by measuring the change in the capacitance of the unit. This technology allows for simple yet highly accurate measurements of cracks occurring due to deformation, aging, or other structural failures. The proposed system deploys a layer of dielectric polymer on the surface of a structural element, and regularly monitors any change in the structural state. The smart material is composed of inexpensive silicone elastomers, which make the monitoring system a promising application for large surfaces. Tests on concrete beams show that the sensor is highly sensitive to strains induced by flexural cracks, and is capable of detecting localized micro-fractures over its covered area. Introduction Structural health monitoring (SHM) is expected to play a predominant role in infrastructure and building management to cope with aging structures and the recent heightened public awareness concerning safety. It has been shown that the difference in risk between infrastructures and air or automobile travel is of an order of magnitude higher, because: 1) infrastructure play a key role in society and has a paramount economical value; 2) infrastructure is not heavily tested; and 3) infrastructure has a long life span, and is designed based on load assumptions and predictions (Karbhari, 2009). The field of structural health monitoring has recently attracted much attention (Song, et al. 2007). Emphasis has been placed on non-destructive evaluation, leading to a wide variety of nondestructive testing techniques such as pulse-echo, dynamic response, and acoustic emission (Bungey et al. 2006). However, the application of these methods is very often temporary and localized because of the technical constraints due to equipment limitations and costs associated with these various strategies. Consequently, most of these methods, despite the fact that they are well understood and provide an acceptable accuracy, are difficult to implement for a large-scale structure. As a result, monitoring of civil structures is commonly done by visual inspection, which can be costly and time consuming. Various SHM techniques have been developed for civil structures over the last decade. SHM is an integrated process consisting of sensing and data processing that leads to damage localization. The main problem in locating damage, which becomes more difficult as the number of degree-of-freedom and degree of indeterminacy increases. Some SHM techniques focus on damage localization, and typically include static sensing strategies, such as static strain gauges. These techniques are typically precise at localizing damages, but need to be deployed over the entire monitored system (analogous to 5WCS M-179