Abstract
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.
Highlights
Metal structures exposed to cyclic loading, including aerospace structures, bridges, and vibrating machinery, are susceptible to fatigue cracking
The fabric acts as a carrier for the electrically conductive sensing network, which has been utilized as a sensor in the bondline of metal–composite hybrid structures in the authors0 prior work
This article documents the effectiveness of a carbon nanotube (CNT)-based sensor that is flexible and can
Summary
Metal structures exposed to cyclic loading, including aerospace structures, bridges, and vibrating machinery, are susceptible to fatigue cracking. Monitoring fatigue cracks in metal structures is critically important for preventing failure by making repair decisions before a crack reaches its critical length, leading to fracture. The literature shows that material fatigue contributes to the majority of in-service mechanical failure of metal structures [1,2,3]. In the United States (U.S.), an estimated 3% of the gross national product [4,5,6] is spent on replacing, repairing, inspecting, and/or monitoring fatigue damaged structures. Conventional fatigue repair techniques, including drilling crack-stop holes [7,8], adding bolted steel plates [9,10], and adhesively bonding fiber-reinforced polymer (FRP) sheets [11,12,13], often fail to reliably arrest a fatigue crack, and may result in crack re-initiation, which may trigger catastrophic failure
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