Spatially resolved self-sensing of strain and damage in carbon fiber cement Sihai Wen Æ D. D. L. Chung Received: 16 March 2005 / Accepted: 29 September 2005 / Published online: 2 May 2006 Ó Springer Science+Business Media, LLC 2006 Abstract Spatially resolved self-sensing of strain and damage has been shown in carbon fiber cement under flexure by three-point bending. This involves measurement of the one-dimensional distribution of the DC electrical resistance by the use of surface electrical contacts on the bottom (tension) and top (compression) surfaces. For a span of 290 mm, a spatial resolution of 5 mm has been attained. The bottom surface resistance, which increases reversibly with strain and increases irreversibly with damage, is a more effective indicator of strain and damage (in combination) than the top surface resistance, the ob- lique resistance or the through-thickness resistance for spatially resolved self-sensing. For sensing without spatial resolution, the oblique resistance is the most effective indicator. For sensing with distinction between strain and damage, the top surface resistance is the most effective indicator. Introduction Self-sensing refers to the ability of a structural material to sense itself (such as its damage and strain) without embedded or attached sensors. The advantages of self- sensing over the use of embedded or attached sensors are low cost, high durability, large sensing volume and absence of mechanical property loss. Applications include traffic monitoring, building monitoring, weighing, homeland security, structural vibration control, and structural health monitoring and hazard mitigation. The self-sensing of strain [1–17] and damage [13–21] has been shown in cement containing discontinuous car- bon fiber. This ability stems from the reversible change of the electrical resistivity with strain (a phenomenon known as piezoresistivity) and the irreversible change of the resistivity with damage. Although this ability has been shown under tension, compression and flexure, prior work has been limited to self-sensing without spatial resolution, i.e., self-sensing to obtain information on the overall specimen rather than information on various parts of a specimen. Spatially resolved sensing is practically impor- tant, due to the need to determine the location of damage or strain. The effectiveness of spatially resolved sensing depends on the electrical resistivity of the material. A low resistivity will cause the current applied at a particular location in the specimen to spread, thus limiting the spatial resolution. Thus, a self-sensing material of high resistivity is desirable for providing spatial resolution. The resistivity of carbon fiber cement decreases by or- ders of magnitude at the percolation threshold, which is the volume fraction above which the fibers contact one another and form a continuous conduction path [22]. For cement paste containing carbon fiber and silica fume, which is for enhancing the fiber dispersion, the percolation threshold is between 0.5 and 1.0 vol.% [22]. Most prior work on the self-sensing behavior of carbon fiber cement uses a fiber content of 0.5 vol.%, which is just below the percolation threshold. Percolation is not required for the self-sensing behavior. This work also uses a fiber content of 0.5 vol.%. Because of the low fiber content, the electrical resistivity of the cement-based material is quite high. The high value is favorable for spatially resolved self-sensing. S. Wen Æ D. D. L. Chung (&) Composite Materials Research Laboratory, University at Buffalo State University at New York, Buffalo, NY 14260-4400, USA e-mail: ddlchung@buffalo.edu J Mater Sci (2006) 41:4823–4831 DOI 10.1007/s10853-006-0028-5 123