This article describes research that investigated the ability of a carbon nanotube (CNT) sensor to detect and monitor fatigue crack initiation and propagation in metal structures. The sensor consists of a nonwoven carrier fabric with a thin film of CNT that is bonded to the surface of a structure using an epoxy adhesive. The carrier fabric enables the sensor to be easily applied over large areas with complex geometries. Furthermore, the distributed nature of the sensor improves the probability of detecting crack initiation and enables monitoring of crack propagation over time. Piezoresistivity of the sensor enables strains to be monitored in real time and the sensor, which is designed to fragment as fatigue cracks propagate, directly measures crack growth through permanent changes in resistance. The following laboratory tests were conducted to evaluate the performance of the sensor: (1) continuous crack propagation monitoring, (2) potential false positive evaluation under near-threshold crack propagation conditions, and (3) crack re-initiation detection at a crack-stop hole, which is a commonly used technique to arrest fatigue cracks. Real-time sensor measurements and post-mortem fractography show that a distinguishable resistance change of the sensor occurs due to fatigue crack propagation that can be quantitatively related to crack length. The sensor does not show false positive responses when the crack does not propagate, which is a drawback of many other fatigue sensors. The sensor is also shown to be remarkably sensitive to detecting crack re-initiation.

Strengthening of deteriorating structural members by fiber-reinforced polymers (FRPs) is an increasingly common and validated technique; however, concerns over means to evaluate the long-term durability of these retrofits exist. This paper explores a novel approach to overcome this concern through the use of a novel self-sensing composite material. Specifically, the objective of this paper is to provide a proof of concept for an integrated strengthening and sensing methodology for structural steel members achieved via infusing more-traditional composites with carbon nanotubes (CNTs). To assess the strengthening and sensing capabilities of the CNT-based composite, a set of unidirectional tensile tests were conducted. The experimental results show stiffness increases and strain reductions due to the application of the CNT-based sensing composites that were in close agreement with both analytical and finite-element models. The sensing aspect was also validated by a corresponding linear change in resi...