Self-monitoring structural materials

Self-monitoring (or intrinsically smart) structural materials, including concrete containing short carbon fibers, and polymer-matrix and carbon-matrix composites containing continuous carbon fibers, were reviewed. Each material is capable of monitoring its own reversible strain and damage through the effects of these on the electrical resistance of the material. This capability is valuable for structural control and structural health monitoring. Among these three materials, the concrete gives the highest strain sensitivity or gage factor (up to 700), while the carbon-carbon composite gives the highest damage sensitivity (i.e., sensitivity even to the damage after the first cycle of tensile loading within the elastic regime). The origin of the self-monitoring ability differs among the three materials. For the concrete, it is related to slight fiber pull-out during strain and fiber and matrix fracture during damage. For the polymer-matrix composite, it is related to the increase in the degree of fiber alignment and reduction of fiber pre-stress during tension in the fiber direction and to fiber fracture and delamination during fatigue. For the carbon-carbon composite, it is related to dimensional changes during strain and fiber and matrix fracture during damage.