Strain sensing and damage monitoring using carbon fiber polymer-matrix composites

Strain and damage sensing is a basic ability of smart structures. In this work, the strain/damage sensing ability of polymer-matrix composites containing short and continuous carbon fibers was studied, due to the importance of continuous carbon fiber composites as structure materials and the applicability of short carbon fiber composites as coatings.; Epoxy containing 5.5% short carbon fibers was found to be a piezoresistive strain sensor with strain sensitivity (reversible fractional resistance change {dollar}rmDelta R/Rsb0{dollar} per unit strain) 6-23 under tension and 39-31 under compression within the elastic deformation regime. The reversible {dollar}rmDelta R/Rsb0{dollar} was positive under tension and negative under compression, but the irreversible {dollar}rmDelta R/Rsb0{dollar} was positive under both tension and compression. Both reversible and irreversible portions of {dollar}rmDelta R/Rsb0{dollar} increased in magnitude with increasing stress/strain amplitude. The reversible portion was due to piezoresistivity, while the irreversible portion was due to damage, probably fiber-matrix interface weakening.; Measurement of the change in electrical resistance (R) under static and cyclic loading was conducted on continuous carbon fiber epoxy-matrix composites. The {dollar}0spcirc{dollar} electrical resistance change {dollar}rmDelta R/Rsb0{dollar} for unidirectional (0) and crossply (0/90) composites decreased in tension at low strains ({dollar}<{dollar}0.6%) due to the reduction of residual compressive stress caused by the shrinkage of matrix during curing and mismatch in coefficients of thermal expansion of carbon fiber and epoxy matrix during cooling. The {dollar}rmDelta R/Rsb0{dollar} through the thickness increased upon 0{dollar}spcirc{dollar} tension for (0) and crossply composites due to the increase in the degree of 0{dollar}spcirc{dollar} fiber alignment and consequent decrease in the chance of adjacent fiber layer contacts. In compression along the fiber direction, the 0{dollar}spcirc{dollar} {dollar}rmDelta R/Rsb0{dollar} increased and {dollar}rmDelta R/Rsb0{dollar} perpendicular to fiber layers decreased for the (0) composite. These effects are almost totally reversible when the longitudinal strain was reversible. These observations provide a basis for a carbon fiber composites to monitor its own strain.; Breakage of 0{dollar}spcirc{dollar} continuous fibers in a composite and delamination of the composite caused irreversible increases of 0{dollar}rmspcirc Delta R/Rsb0{dollar} and {dollar}rmDelta R/Rsb0{dollar} through the thickness respectively, thus providing a mechanism for damage sensing. The resistance increased in spurts or continuously depending on the extent of fatigue damage. Therefore, real-time structural health monitoring was demonstrated simultaneously with strain sensing in (0) and (0/90) composites.; For the (90) composite, the 0{dollar}spcirc{dollar} resistance increased reversibly upon 0{dollar}spcirc{dollar} tension and decreased reversibly upon 0{dollar}spcirc{dollar} compression, due to the change in proximity between adjacent 90{dollar}spcirc{dollar} fiber.